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Chapter 8 - AutomationDirect

1. Read data only when alarm enable bit transitions from 0 to 1 Read data only on PLC Mode change Read on the fly means that the content of V memory can be changed while the PID loop is in operation 8 20 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation a S PID Mode Setting 1 Bit Descriptions Addr 00 The individual bit definitions of the PID Mode Setting 1 word Addr 00 are listed in the following table PID Mode Setting 1 Description Read Write Bit 1 Manual Mode Loop Operation request write 01 request Automatic Mode Loop Operation request write 01 request Cascade Mode Loop Operation request write 01 request Bumpless Transfer select write Mode Mode II Direct or Reverse Acting Loop select write Direct Reverse Position Velocity Algorithm select write Position Velocity PV Linear Square Root Extract select write Linear Sq root Error Term Linear Squared select write Linear Squared Error Deadband enable write Disable Enable Derivative Gain Limit select write off On Bias Integrator Freeze select write off On Ramp Soak Operation select write off On PV Alarm Monitor select write off On PV Deviation alarm select write off On PV rate of change alarm select write off On Loop with CPU Loop Independent mode of CPU mode 0 1 2 3 4 5 6 7 8 Loop mode is independent from CPU mode when set writ
2. DL05 Micro PLC User Manual 6th Edition Rev C 8 43 Chapter 8 PID Loop Operation Auto Tuning Procedure The auto tuning feature for the DLO5 loop controller will only run once each time it is enabled in the PID table Therefore auto tuning does not run continuously during operation this would be adaptive control Whenever there is a substantial change in loop dynamics such as mass of process size of actuator etc the tuning process will need to be repeated in order to derive new gains required for optimal control WARNING Only authorized personnel fully familiar with all aspects of the process should make changes that affect the loop tuning constants Using the loop auto tuning procedures will affect the process including inducing large changes in the control output value Make sure you thoroughly consider the impact of any changes to minimize the risk of injury to personnel or damage to equipment The auto tune in the DLOS is not intended to be used as a replacement for your process knowledge Once the physical loop components are connected to the PLC auto tuning can be initiated within DirectSOFT 5 see the DirectSOFT 5 Programming Software Manual and it can be used to establish initial PID parameter values Auto tuning is the best guess the CPU can do after some trial tests E The loop controller offers both closed loop and open loop methods The following sections describe how to use the auto tuning feature and what
3. Common Data Format Unipolar data format Bipolar data format Forward Acting Reverse Acting Transfer Mode amp 12 bit data format Bumpless 15 bit data format Bumpless ll 16 bit data format Select the Algorithm Type Chose either Position or Velocity The default algorithm is Position This is the choice for most applications which include heating and cooling loops as well as most position an level control loops A typical velocity control will consist of a process variable such as a flow totalizer in a flow control loop Enter the Sample Rate The main tasks of the CPU fall into categories as shown to the right j i The list represents the tasks done when the CPU is in Run Mode on nd each PLC scan Note that PID loop calculations occur after the ladder logic task Um The sample rate of a control loop is simply the frequency of the PID E calculation Each calculation generates a new control output value With om the DL05 CPU you can set the sample rate of a loop from 50 ms to PLC Pregam _ 99 99 seconds Most loops do not require a fresh PID calculation on Scan i every PLC scan Some loops may need to be calculated only once in Pes 1000 scans I Enter 0 05 sec or the sample rate of your choice for each loop and the Doris CPU automatically schedules and executes PID calculations on the appropriate scans EE Y DL05 Micro PLC User Manual 6th Edition Rev
4. 0 High 0 Yellow 0 0 Low Alarm hysteresis LowLow D Value Monitor Rate of Change Monitor Limit Alarms Checking this box will allow all of the PV limit alarms to be monitored once the limits are entered The PV absolute value alarms are organized as two upper and two lower alarms The alarm status is false as long as the PV value remains in the region between the upper and lower alarms as shown below The alarms nearest the safe zone are named High Alarm and Low Alarm If the loop loses control the PV will cross one of these thresholds first Therefore you can program the appropriate alarm threshold values in the loop table locations shown below to the right The data format is the same as the PV and SP 12 bit or 15 bit The threshold values for these alarms should be set to give an operator an early warning if the process loses control High high Alarm Loop Table High Alarm V 16 XXXX High high Alarm PV cu one a V 15 XXXX High Alarm V 14 XXXX Low Alarm Low Alarm e Low low Alarm a V 18 XXXX Low low Alarm NOTE The Alarm dialog can be left as it first appears without alarm entries The alarms can then be I setup in the DirectSOFT 5 PID View DL05 Micro PLC User Manual 6th Edition Rev C 8 35 Chapter 8 PID Loop Operation If the process remains out of control for some time the PV will even
5. The P I and D gains are 4 digit BCD numbers with values from 0000 to 9999 They contain an implied decimal point in the middle so the values are actually 00 00 to 99 99 Some gain values have units Proportional gain has no unit Integral gain may be selected in seconds or in minutes and Derivative gain is in seconds Gain Proportional Gain This is the most basic gain of the three Values range from 0000 to 9999 but they are used internally as xx xx An entry of 0000 effectively removes the proportional term from the PID equation This accommodates applications which need integral only loops Reset Integral Gain Values range from 0001 to 9998 but they are used internally as xx xx An entry of 0000 or 9999 causes the integral gain to be 0 effectively removing the integrator term from the PID equation This accommodates applications which need proportional only loops The units of integral gain may be either seconds or minutes as shown in the above dialog Rate Derivative Gain Values which can be entered range from 0001 to 9999 but they are used internally as XX XX An entry of 0000 allows removal of the derivative term from the PID equation a common practice This accommodates applications which require only proportional and or integral loops Most control loops will operate as a PI loop 8 32 DL05 Micro PLC User Manual 6th Edition Rev C Chap
6. 0 014 Tpc Sample Rate 0 03 Tpc Auto tuning error If the auto tune error bit bit 13 of loop Mode Alarm status word V 06 is on please verify the PV and SP values are at least 5 of full scale difference as required by the auto tune function NOTE If your PV fluctuates rapidly you probably need to use the built in analog filter see page 8 53 or create a filter in ladder logic see example on page 8 54 DL05 Micro PLC User Manual 6th Edition Rev C 8 47 Chapter 8 PID Loop Operation Use Direct SOFT 5 Data View with PID View The Data View window is a very useful tool which can be used to help tune your PID loop You can compare the variables in the PID View with the actual values in the V memory location with Data View Open a New Data View Window A new Data View window can be opened in any one of three ways the menu bar Debug gt Data View gt New the keyboard shortcut Ctrl Shift F3 or the Data button on the Status toolbar By default the Data View window is assigned Datal as the default name This name can be changed for the current view using the Options dialog The following diagram is an example of a newly opened Data View The window will open next to the Ladder View by default E amp DitectSOFT 5 Programming pid Ladder View Ele Edt Search View Tools PLC Debug Window Help SE oe A A a Datal IB Ladder View On SP1 L l WAIN geux DE
7. 6th Edition Rev C 8 69 Chapter 8 PID Loop Operation es PID Example Program Program Setup for the PID Loop After the PID loop or loops have been setup with DirectSOFT 5 you will need to edit your RLL program to include the rungs needed to setup the analog I O module to be used by the PID loop s The following example program shows how an analog combination module FO 2AD2DA 2 is setup This example assumes that the PID table for loop 1 has a beginning address of V2100 All of the analog I O modules used with the DLO05 are setup in a similar manner Refer to the DL05 DLO06 Options Manual for the setup information for the particular module that you will be using DirectSOFT Set up a F 2AD2DA 2 to use both inputs and outputs The inputs and outputs will be the raw data in binary format 0 4095 Input Ch 1 V2000 input CH 2 V2001 Output Ch 1 V2010 output Ch 2 V2011 _FirstScan SPO LC Store the analog input data in the V memory location for PID loop 1 PV since PID loop 1 begins with v2100 On 1 2 1 I Store PID loop 1 control output data in the V memory location for analog output Ch 1 _on 8P1 t LD K8282 OUT Data format and no of channels Format v7700 LDA 02000 OUT InPointer H V7701 LDA 02010 OUT OutPointer q V7702 ILD Ch 1 Input Data v2000 OUT Loop 1 PY v2103 LD Loop 1 Output 3 Program continued on next page 8 70 DL05 M
8. Yellow Deviation An alarm for High High and Low Low PV deviation from the setpoint Orange Deviation may also be specified When the PV is further from the setpoint than the programmed Yellow or Orange Deviation Limit the corresponding alarm bit is activated Rate of Change This alarm is set when the PV changes faster than a specified rate of change limit PV Alarm Hysteresis The PV Limit Alarms and PV Deviation Alarms are programmed using threshold values When the absolute value or deviation exceeds the dern the alarm status becomes true Real world PV signals have some noise on them which can cause some fluctuation in the PV value in the CPU As the PV value crosses an alarm threshold its fluctuations will cause the alarm to be intermittent and annoy process operators The solution is to use the PV Alarm Hysteresis feature DL05 Micro PLC User Manual 6th Edition Rev C 8 13 Chapter 8 PID Loop Operation SRR Loop Operating Modes The DLO5 loop controller operates in one of two modes either Manual or Automatic Manual In manual mode the control output is determined by the operator not the loop controller While in manual mode the loop controller will still monitor all of the alarms including High High High Low Low Low Yellow deviation Orange deviation and Rate of Change Automatic In automatic mode the loop controller computes the control output based on the programmed parameters stored in V memory All
9. turning off until the PV value has returned through the threshold by the hysteresis amount Alarm Programming Error The PV Alarm threshold values must have certain mathematical relationships to be valid The requirements are listed below If not met the Alarm Programming Error bit will be set as indicated to the right PID Mode and Alarm Status V 06 Bit 15 14 131211109 87 65 4 32 1 0 i Alarm Programming Error PV Absolute Alarm value requirements Low low Low High High high PV Deviation Alarm requirements Yellow Red Loop Calculation Overflow Underflow Error This error occurs whenever the output reaches its eer PID Mode and Alarm Status V 06 upper or lower limit and the PV does not reach the setpoint A typical example might be when a BigeqzdaTiuaQ IU RLS B T valve is stuck the output is at its limit but the PV has not reached setpoint Loop Calculation Overflow Underflow Error h NOTE Overflow underflow can be alarmed in PID View The optional C more IO panel see the automationdirect com website can also be set up to read these error bits using the PID Faceplate templates 8 38 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation nN Ramp Soak R S Ramp Soak is the last dialog available in the PID setup The basic PID does not require any entri
10. 00029 07680 06 Loop Tile With both windows positioned in this manner you are able to see where the PID values have been set and see the process that it is controlling In the diagram below you can see the current SP PV and Output values along with the other PID addresses Refer to the Loop Table Word Definitions page 8 20 for details for each word in the table This is also a good data type reference for each word in the table Scale the time axis of the viewing The trend can be cleared and window by using this input box restarted from the left at anytime Ii DirectSOFT 5 Programming rtd example PID View 8 x File Edit Search View Tools PLC Debug window Help 8x Process Variable and Setpoint trends are color coded WWIE Read Write New Open ene 3 amp 2 A A Status Data Value le Info Sitar x EE CDer z word The loop name area turns red whenever there is an overflow error P 715 BIAS 760 5 OUT565 B 7 F8 v2100 001 Loop Num 1 RTD test Ei ro Settings Variables Alarms 10 x100 OFF P ieee spfeso M Rocjo T Limit Alarms im Nee uo PME High High o 12 l Gain 3 00 LE Dev Alarms High 0 MS c ias H3 Reset 2000 P em Io Rea fo Low 0 Tm Min Dut 565 p Rate 0 00 sec Yellow f0 Low Low 0 15 D hel Autotun
11. Refer to the Ramp Soak Flag Bit Description table on page 8 59 when adding the control rungs to your program similar to the ladder rungs below For the example below PID parameters begin at V7000 The Ramp Soak bit flags are located at V7033 Start RIS X100 B7033 0 13 MM str Hold R S X101 B7033 1 14 MM tr Resume R S X102 B7033 2 15 Iel ET Enable R S X107 B7000 11 OUT NOP END DL05 Micro PLC User Manual 6th Edition Rev C 8 61 Chapter 8 PID Loop Operation i Test the Profile After the Ramp Soak program has been developed in RLL test the program Check your profile by using PID View If there are any changes to be made in the profile they are made in the PID Setup R S profile Make the chages in Program mode then start the Ramp Soak process again 8 62 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation Cascade Control Introduction Cascaded loops are an advanced control technique that is superior to individual loop control in certain situations As the name implies cascade means that one loop is connected to another loop In addition to Manual open loop and Auto closed loop Modes the DL05 also provides Cascaded Mode NOTE Cascaded loops are an advanced process control technique Therefore we recommend their use only for experienced process control engineers When a manufacturing process is complex
12. User Manual 6th Edition Rev C Chapter 8 PID Loop Operation a S PID Loop Feature Specifications Number of loops Selectable 4 maximum CPU V memory needed 32 words V locations per loop selected 64 words if using ramp soak PID algorithm Position or Velocity form of the PID equation Control Output polarity Selectable direct acting or reverse acting Error term curves Selectable as linear square root of error and error squared Loop update rate time between PID calculation Minimum loop update rate 0 05 seconds for 1 to 4 loops Loop modes Automatic Manual operator control or Cascade control Ramp Soak Generator Up to 8 ramp soak steps 16 segments per loop with indication of ramp soak step number PV curves Select standard linear or square root extract for flow meter input Set Point Limits Specify minimum and maximum setpoint values Process Variable Limits Specify minimum and maximum Process Variable values Proportional Gain Specify gains of 0 01 to 99 99 Integrator Reset Specify reset time of 0 1 to 999 8 in units of seconds or minutes Derivative Rate Specify the derivative time from 0 01 to 99 99 seconds Rate Limits Specify derivative gain limiting from 1 to 20 Bumpless Transfer Automatically initialized bias and setpoint when control switches from manual to automatic Automatically set the bias equal to the control output when control switches from manual to automatic Step Bias Provides proportional bias adjustment
13. are different for each mode In Manual Mode the loop is not executing PID calculations however loop alarms are still active With regard to the loop table the CPU stops writing values to location V 05 control output for that loop It is expected that an operator or other intelligent source is manually controlling the output by observing the PV and writing data to the control output as necessary to keep the process under control The drawing below shows the equivalent schematic diagram of manual mode operation Input from Operator Manual E Control Output V 05 Loop Calculation no In Automatic Mode the loop operates normally and generates new control output values It calculates the PID equation and writes the result in location V 05 every sample period of that loop The equivalent schematic diagram is shown below Input from Operator Manual Loop d Calculation Auto Control Output V 05 DL05 Micro PLC User Manual 6th Edition Rev C 8 27 Chapter 8 PID Loop Operation C a a amaaa In Cascade Mode the loop operates as it does in Automatic Mode with one important difference The data source for the SP changes from its normal location at V 02 to using the control output value V 05 from another loop So in Auto or Manual modes the loop calculation uses the data at V 02 In Cascade Mode the loop calculation reads the control output from another loop s parameter table V 05 Anot
14. derivative action This bump is not present in the second form of the equation DL05 Micro PLC User Manual 6th Edition Rev C 8 9 Chapter 8 PID Loop Operation a The DLO5 also combines the integral sum and the initial output into a single term called the bias Mx This results in the following set of equations Mx M Mx Kixe Mx 4 M Ke xe Kr PV PV Mx The DL05 by default will keep the normalized output M in the range of 0 0 to 1 0 This is done by clamping M to the nearer of 0 0 or 1 0 whenever the calculated output falls outside this range The DL05 also allows you to specify the minimum and maximum output limit values within the range 0 to 4095 in binary if using 12 bit unipolar NOTE The equations and algorithms or parts of in this chapter are only for references Analysis of these equations can be found in most good text books about process control Reset Windup Protection Reset windup can occur if reset action integral term is specified and the computation of the bias term Mx is Mx Kix ep Mx For example assume the output is controlling a valve and the PV remains at some value greater than the setpoint The negative error e will cause the bias term Mx to constantly decrease until the output M goes to 0 closing the valve However since the error term is still negative the bias will continue to decrease becoming ever more negative When the PV finally does come back down below th
15. effect of the filter A value of 1 results with no filtering The filtered value will be placed in V2100 See Chapter 5 page 138 for more detailed information C100 Filter Over Time Binary 1 FILTERB Filter Freq Timer Filter Freq Time 0 01 sec Raw Data Binary Filter Divisor 1 100 Filtered Value Binary DL05 Micro PLC User Manual 6th Edition Rev C 8 55 Chapter 8 PID Loop Operation ee Ramp Soak Generator Introduction Our discussion of basic loop operation noted the setpoint for a loop will be generated in various ways depending on the loop operating mode and programming preferences In the figure below the ramp soak generator is one of the ways the SP may be generated It is the responsibility of your ladder program to ensure only one source attempts to write the SP value at V 02 at any particular time Setpoint Sources Operator Input _ Seas Setpoint V 02 Loop Ramp soak generator T Ladder Program eee d Another loop s output cascade Calculation Process Variable Control Output If the SP for your process rarely changes or can tolerate step changes you probably will not need to use the ramp soak generator However some processes require precisely controlled SP for these applications The terms ramp and soak have special meanings in the SP process control industry and refer to desired setpoint SP values in temperature control app
16. into the accumulator SUB Subtract the Loop Table PV value from the raw V2003 analog input MUL V1400 The MUL and DIV instructions have the combined effect of multiplying by a decimal D number between 0 1 and 1 0 ADD Add the adjusted difference between the raw analog V2003 input and the PV back to the PV T Replace the previous Loop Table PV value with the OU V2003 new filtered analog value in the Loop Table V2200 V2003 LD If the raw analog input is less than the current PV V2003 value in the PID Loop Table V2003 load the current PV value into the accumulator SUB Subtract the raw analog input the from Loop Table PV V2200 value MUL V1400 The MUL and DIV instructions have the combined effect of multiplying by a decimal DIV number between 0 1 and 1 0 K10 ADD Add the adjusted difference between the raw analog input V2200 and the PV back to the analog input value OUT Replace the previous Loop Table PV value with the V2003 new filtered analog value in the Loop Table END END coil marks the end of the main program 8 54 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation Use the DirectSOFT 5 Filter Intelligent Box Instruction For those who are using DirectSOFT 5 you have the opportunity to use Intelligent Box IBox instruction IB 402 Filter Over Time in Binary decimal This IBox will perform a first order filter on the Raw Data on a defined time interval The equation
17. is New Old Raw Old FDC where o New New Filtered Value Filter Over Time Binary Old Old Filtered Value FILTERB IB 402 FDC Filter Divisor Constant Filter Freq Timer TO Wi Raw Raw Data Filter Freq Time 0 01 sec n n Raw Data Binary TAO The Filter D e Filter Divisor Constant is an integer 1n Filter Divisor 1 100 the range K1 to K100 such that if it equaled K1 then no filtering is performed x e Filtered Value Binary The rate at which the calculation is performed is specified by time in hundredths of a second 0 01 seconds as the Filter Freq Time parameter Note that this Timer instruction is embedded in the IBox and must NOT be used any other place in your program Power flow controls whether the calculation is enabled If it is disabled the Filter Value is not updated On the first scan from Program to Run mode the Filter Value is initialized to 0 to give the calculation a consistent starting point Since the following binary filter example does not write directly to the PID PV location the BCD filter could be used with BCD values and then converted to BIN FilterB Example Following is an example of how the FilterB IBox is used in a ladder program The instruction is used to filter a binary value that is in V2000 Timer T1 is set to 0 5 seconds the rate at which the filter calculation will be performed The filter constant is set to 3 0 A larger value will increase the smoothing
18. occurs in open and closed loop auto tuning The controls for the auto tuning function use three bits in the PID Mode 2 word V 01 as shown below DirectSOFT 5 will manipulate these bits automatically when you use the auto tune feature within DzrectSOFT 5 Or you may have your ladder logic access these bits directly for allowing control from another source such as a dedicated operator interface The individual control bits allow you to start the auto tune procedure select PID or PI tuning and select closed loop or open loop tuning If you select PI tuning the auto tune procedure leaves the derivative gain at 0 The Loop Mode and Alarm Status word V 06 reports the auto tune status as shown Bit 12 will be on 1 during the auto tune cycle automatically returning to off 0 when done Auto Tune Function Start Auto Tune Auto Tune 0 to 1 transition Active O PID tuning Auto Tune Auto Tuning 1 open PI tuning Error Auto Tuning Controls O closed loop Status 1 open loop PID Mode 2 Setting V 01 Loop Mode and Alarm Status V 06 Bit 15 141312111098 76543210 Bit 15 1413 1211109 8 76543210 8 24 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation i Open Loop Auto Tuning During an open loop auto tuning cycle the loop controller operates as shown in the diagram below Before starting this procedur
19. or control output a SP value error or other causes If the operator responds to a PV Rate of Change Alarm quickly and effectively the PV absolute value will not reach the point where the material in process would be ruined The DL05 loop controller provides a programmable PV Rate of Change Alarm as shown below The rate of change is specified in PV units change per loop sample time This value is programmed into the loop table location V 21 Loop Table PV slope ce PV slope excessive V421 XXXX PV Rate of Change Alarm PV Td PID Mode and Alarm Status V 06 Pa rate of change alarm ae Bit 15 141312111009 87 65 43 2 1 0 Sample time Sample time PV Rate of Change Alarm As an example suppose the PV is the temperature for your process and you want an alarm whenever the temperature changes faster than 15 degrees minute The PV counts per degree and the loop sample rate must be known Then suppose the PV value in V 03 location represents 10 counts per degree and the loop sample rate is 2 seconds Use the formula below to convert our engineering units to counts sample period 15 degrees x 10 counts degree 150 Alarm Rate of Change 5 counts sample period 1 minute 30 loop samples min 30 From the calculation result you would program the value 5 in the loop table for the rate of change The PV Rate of Change Alarm can be independent
20. out of the lower V memory range The starting address in V7640 is out of the upper V memory range The number of loops selected in V7641 is greater than 8 The loop table extends past straddles the boundary at V7377 Use an address closer to V1200 NOTE As a quick check if the CPU is in Run mode and V7642 0000 there are no programming errors Establishing the Loop Table Size and Location On a PROGRAM to RUN mode transition the CPU reads the loop setup parameters as pictured below At that moment the CPU learns the location of the loop table and the number of loops it configures Then during the ladder program scan the PID Loop task uses the loop data to perform calculations generate alarms and so on There are some loop table CPU Tasks V Memory Space User Data READ So Ladder TS LOOP Program J a DATA lee a CONFIGURE ny MONITOR PID Loo za IN Setup Parameter READ V7640 V7641 2 at powerup DirectSOFT Programming Software Chapter 8 PID Loop Operation nS KAASA parameters the CPU will read or write on every loop V Memory User Data calculation The Loop Table contains data for only the number of loops V2000 LOOP that are selected The address for the table is stored in V7641 i V2037 32 words Each loop configuration occupies 32 words 0 to 37 octal in i V2040 LOOP 2 the loop table Y v2077 32 words F LOOP 3 F
21. output of the filter xi is the current input to the filter yi 1 is the previous output of the filter k is the PV Analog Input Filter Factor Creating an Analog Filter in Ladder Logic A similar algorithm can be built in your ladder program Your analog inputs can be filtered effectively using either method The following programming example describes the ladder logic you will need Be sure to change the example memory locations to those that fit your application Filtering can induce a 1 part in 1000 error in your output because of rounding If your process cannot tolerate a 1 part in 1000 error do not use filtering Because of the rounding error you should not use zero or full scale as alarm points Additionally the smaller the filter constant the greater the smoothing effect but the slower the response time Be sure a slower response is acceptable in controlling your process SP1 Load the filter constant into the accumulator To mimick I ID the built in filter use values between 1 and 10 This is equivalent to the 0 1 to 1 0 range of the built in filter Output the constant to a convenient memory location that Ao is not in use V2200 V2003 If the raw analog input V2200 in our example is greater gt us than or equal to the current PV value in the PID Loop Table E 00 V2003 load the raw analog input
22. range will appear Also entering the number of PID loops 1 to 8 will set the total V memory range for the number of loops entered After the V memory address has been entered the necessary PID parameters for a basic loop operation for each loop can be setup with the dialogs made available Set PID Table Address Table Start Address Z EF Number of loops 12 4 firs Memory Range Y2000 V2077 jw Re read PID data from PLC Update and Exit Cancel NOTE Have an edited program open then click on PLC gt Setup gt PID to access the Setup PID dialog DL05 Micro PLC User Manual 6th Edition Rev C 8 19 Chapter 8 PID Loop Operation i M M Loop Table Word Definitions These are the loop parameters associated with each of the four loops available in the DL05 The parameters are listed in the following table The address offset is in octal to help you locate specific parameters in the loop table For example if a table begins at V2000 then the location for the reset integral term is Addr 11 or V2011 Do not use the Word in the first column to calculate addresses Address Offset Description Format the tye Addr 0 PID Loop Mode Setting 1 bits Yes Addr 1 PID Loop Mode Setting 2 bits Yes Addr 2 Setpoint Value SP word binary Yes Addr 3 Process Variable PV word binary Yes Addr 4 Bias Integrator Value word binary Yes Addr 5 Control Output Value word binary Y
23. the PID loop independent of the CPU mode then you must take certain steps to make it possible to make loop parameter changes You can temporarily make the loops follow the CPU mode by changing bit 15 to 0 Then you will be able to place the loop into Manual Mode using DirectSOFT 5 After you change the loop s parameter setting s just restore bit 15 to a value of 1 to re establish PID operation independent of CPU DL05 Micro PLC User Manual 6th Edition Rev C 8 29 Chapter 8 PID Loop Operation You may optionally configure each loop to access its analog I O PV and control output by placing proper values in the associated loop table registers in your RLL program The following figure shows the loop table parameters at V 36 and V 37 and their auto transfer role to access the analog values directly Setpoint V 02 Error Loop Control output V 05 gt Calculation Process Variable V 03 Loop Table vs V2036 OF XX _ Slot channel number for PV V2037 OF XX Slot channel number for Output seeeeeee OF XX Eo Channel number 1 to 4 Option Module Slot E When these loop table parameters are programmed directly a value of 0F02 in register V2036 directs the loop controller to read the PV data from channel 1 of the analog input A value of 0000 in either register tells the loop controller not to access the corresponding analog value directly In that case ladder logic must be used to tra
24. 5 Micro PLC User Manual 6th Edition Rev C 8 65 Chapter 8 PID Loop Operation i M Time Proportioning Control The PID loop controller in the DLO5 CPU generates a smooth control output signal across a numerical range The control output value is suitable to drive an analog output module which connects to the process In the process control field this is called continuous control because the output is on at some level continuously While continuous control can be smooth and robust the cost of the loop components such as actuator heater amplifiers can be expensive A simpler form of control is called time proportioning control This method uses actuator which are either on or off no in between Loop components for on off based control systems are lower cost than their continuous control counterparts In this section we will show you how to convert the control output of a loop to time proportioning control for the applications that need it Let s take a moment to review how alternately turning a load on and off can control a process The diagram below shows a hot air balloon following a path across some mountains The desired path is the setpoint The balloon pilot turns the burner on and off alternately which is his control output The large mass of air in the balloon effectively averages the effect of the burner converting the bursts of heat i
25. C 8 25 Chapter 8 PID Loop Operation Igi MM M Select Forward Reverse It is important to know which direction the control output will respond to the error SP PV either forward or reverse A forward direct acting control loop means that whenever the control output increases the process variable will also increase The control output of most PID loops are forward acting such as a heating control loop An increase in heat applied will increase the PV temperature A reverse acting control loop is one where an increase in the control output results in a decrease in the PV A common example of this would be a refrigeration system where an increase in the cooling input causes a decrease in the PV temperature The Transfer Mode Choose either Bumpless I or Bumpless II to provide a smooth transition of the control output from Manual Mode to Auto Mode Choosing Bumpless I will set the SP equal to the PV when the control output is switched from Manual to Auto If this is not desired choose Bumplessll The characteristics of Bumpless I and II transfer types are listed in the chart below Note that their operation also depends on which PID algorithm you are using the position or velocity form of the PID equation Note that you must use Bumpless Transfer type I when using the E velocity form of the PID algorithm Transfer R Manual to Auto Auto to Cascade TransferType Select Bit 3 PID Algorithm Transfe
26. D tutorial Powerpoint slide show This a Windows Power Point documentation The Power Point Viewer can be downloaded from our website if you do not have Power Point to view the tutorial DL05 Micro PLC User Manual 6th Edition Rev C 8 71 Chapter 8 PID Loop Operation M LLL Troubleshooting Tips Q The loop will not go into Automatic Mode A Check the following for possible causes A PV alarm exists or a PV alarm programming error exists The loop is the major loop of a cascaded pair and the minor loop is not in Cascade Mode Q The Control Output stays at zero constantly when the loop is in Automatic Mode A Check the following for possible causes The Control Output upper limit in loop table location V 31 is zero The loop is driven into saturation because the error never goes to zero value and changes algebraic sign Q The Control Output value is not zero but it is incorrect A Check the following for possible causes The gain values are entered improperly Remember gains are entered in the loop table in BCD while the SP and PV are in binary If you are using DirectSOFT 5 PID View will display the SP PV Bias and Control output in decimal BCD converting it to binary before updating the loop table Q The Ramp Soak Generator does not operate when I activate the Start bit A Check the following for possible causes The Ramp Soak enable bit is off Check the status of bit 11 of loop parameter ta
27. I terms or just the P term Parts of the PID equation may be eliminated by choosing appropriate values for the gain Kc reset Ti and rate Td yielding a P PI PD I and even an ID and a D loop Eliminating Integral Action The effect of integral action on the output may be eliminated by setting Ti 9999 or 0000 When this is E done the user may then manually control the bias term Mx to eliminate any steady state offset Eliminating Derivative Action The effect of derivative action on the output may be eliminated by setting Td 0 most loops do not require a D parameter it may make the loop unstable Eliminating Proportional Action Although rarely done the effect of proportional term on the output may be eliminated by setting Kc 0 Since Kc is also normally a multiplier of the integral coefficient Ki and the derivative coefficient Kr the CPU makes the computation of these values conditional on the value of Kc as follows Ki Kc Ts Ti if Kc 0 Ki Ts Ti if Kc 0 I or ID only Kr Kc Td Ts if Kc 0 Kr Td Ts if Kc 0 ID or D only Velocity Form of the PID Equation The standard position form of the PID equation computes the actual actuator position An alternative form of the PID equation computes the change in actuator position This form of the equation is referred to as the velocity PID equation and is obtained by subtracting the equation at time n from the equation at
28. Operation Error Deadband Control With error deadband control no control action is taken if the PV is within the specified deadband area around the setpoint The error deadband is the same above and below the setpoint Once the PV is outside of the error deadband around the setpoint the entire error is used in the loop calculation e 0 SP Deadband_Below_SP lt PV lt SP Deadband_Above_SP e P PV otherwise The error will be squared first if both Error Squared and Error Deadband is selected Derivative Gain Limiting When the coefficient of the derivative term Kr is a large value noise introduced into the PV can result in erratic loop output This problem is corrected by specifying a derivative gain limiting coefficient Kd Derivative gain limiting is a first order filter applied to the derivative term computation Y as shown below Ts Ya Yai Nu PV Yeu D Kd Position Algorithm Mx Ki e Mx 1 M Kcxe Kr Y Y j Mx Velocity Algorithm AM Kc x e 6 4 Kix ep Kr Y 2 Y 4 Yo 5 DL05 Micro PLC User Manual 6th Edition Rev C 8 15 Chapter 8 PID Loop Operation ee Ten Steps to Successful Process Control Controllers such as the DLO5 PLC provide sophisticated process control features Automated control systems can be difficult to debug because a given symptom can have many possible causes We recommend a careful step by step approach to bringing new contr
29. PID Loop OPERATION In This Chapter DLOS PID Control ates ke zik 3 RC LORS ER ORC OC Kee RE EOS 8 2 Introduction to PID Control 4 5 uo oia P0 ER ORE Cb ea ee 8 4 Introducing DLOS PID Control 5 5 RR RR 8 6 PID Loop Oper ANON sce eee i tot ne e D ene OEC eee 8 9 Ten Steps to Successful Process Control 00006 8 16 PID OOO SUD s uod dcic6 ero 189 Doce ee cta dob cd E oes o EM debes e d 8 18 PID Loop Tuning ilu kr RRRERRERR RU ERR ERR 8 40 Using the Special PID Features 34 99 on ee CU Eo Ee 8 51 Raimp Soak Generator sexa saa Vode ves Rcx WX EX e ace x E n 8 56 DirectSOFT 5 Ramp Soak Example 4 5 oom nn 8 61 Cascade Control i hctepisbesse RR ERE IRE E S 8 63 Time Proportioning Control 64 65 c0ne cere oe DIC A ORTA 8 66 Feedforward Control ss ce ics eene rk RR RETE REE ES 8 68 PID Example Progra Lau ed io oec be Rn tb o teo doe ord 8 70 Troubleshooting Tips isse ck RR RR ER REX ERE n 8 72 Glossary or PID Loop Terminology esce Sar eee EE 8 74 Bibliography 224 4 cies ee kex s REX EA REX e ip Y ee eee eS 8 76 Chapter 8 PID Loop Operation E DLO5 PID Control DL05 PID Control Features Along with control functions discussed in this manual the DL05 PLC features PID process control capability The DL05 PID process control loops offer the same features offered in much larger PLCs The primary features are Up to 4 PID loops individual programmable sample rates Manual Automatic and Cascade loop operation
30. PV Cascade Mode An operational mode of a loop in which it receives its SP from another loop s output Continuous Control Control of a process done by delivering a smooth analog signal as the control output E Control Output The numerical result of a PID equation which is sent by the loop with the intention of nulling out the current error Derivative Gain A constant that determines the magnitude of the PID derivative term in response to the current error Direct Acting Loop A loop in which the PV increases in response to a control output increase In other words the process has a positive gain Error The difference in value between the SP and PV Error SP PV Error Deadband An optional feature which makes the loop insensitive to errors when they are small You can specify the size of the deadband Error Squared An optional feature which multiplies the error by itself but retains the original algebraic sign It reduces the effect of small errors while magnifying the effect of large errors Feedforward A method of optimizing the control response of a loop when a change in setpoint or disturbance offset is known and has a quantifiable effect on the bias term Integral Gain A constant that determines the magnitude of the PID integral term in response to the current error Major Loop In cascade control it is the loop that generates a setpoint for the cascaded loop Manual Mode An operational mode of a loop it which t
31. The R S generator ends the profile when it finds ramp slope 0 Ramp Soak Table Flags The individual bit definitions of the Ramp Soak Table Flag Addr 33 word is listed in the following table Ramp Soak Flag Bit Description Read Write Bit 1 0 Start Ramp Soak Profile write 01 Start 1 Hold Ramp Soak Profile write 01 Hold 2 Resume Ramp soak Profile write 01 Resume 3 Jog Ramp Soak Profile write 01 Jog 4 Ramp Soak Profile Complete read Complete 5 PV Input Ramp Soak Deviation read Off On 6 Ramp Soak Profile in Hold read Off On T Reserved read Off On 8 1 Current Step in R S Profile read decode as byte hex Ramp Soak Generator Enable The main enable control to permit ramp soak PID Mode 1 Setting V 00 generation of the SP value is accomplished with bit 11 in the PID Mode 1 Setting V 00 word as shown Bit 15 14131211098 76543210 to the right The other ramp soak controls in V 33 4 Ramp Soak shown in the table above will not operate unless this Generator Enable bit 1 during the entire ramp soak process Ramp Soak Controls The four main controls for the ramp soak generator are in bits Ramp Soak Settings V 33 0 to 3 of the ramp soak settings word in the loop parameter Bit 15 14 13 12 1109876543210 table DirectSOFT 5 controls these bits directly from the J 4 f jog
32. This will D be necessary after application startup if mode changes are part of the application Use the rung shown to the right to SET the mode bit do not use Go to Auto Mode an OUT coil On a 0 1 transition of X0 the rung sets the Auto bit xo B2000 1 equal to 1 The loop controller resets it IT SET Operator panel interface the operators panel to ladder logic using standard methods then use the logic to the right to set the mode bit Since mode changes can only be requested the PID loop controller will decide when to permit mode changes and provide the loop mode status It reports the current mode on bits 0 1 and 2 of the Loop Mode Alarm Status word location V 06 in the loop table The parallel request monitoring functions are shown in the figure below The figure also shows the two possible mode dependent SP sources and the two possible Control Output sources Input from Operator Manual Control Output c d from another loop ascade Control Output p Setpoint Error Term Loop Calculation Normal Source e Auto Cascade Auto Manual Process Variable Mode Select a PID Mode m Control PID Mode 1 Setting V 00 Loop Mode and Alarm Status V 06 Bit 15 141312111098 7 65 4 32 1 0 Bit 15 14 131211 109 8 7 65 4 3 21 0 Mode Reques
33. V3000 Ramp Soak 1 loop table specifies the starting location of the 32 words ramp soak table In the example to the right the loop parameter tables for Loop 1 and 2 occupy contiguous 32 word blocks as shown Each has a pointer to its V3600 Ramp Soak 2 y ramp soak table independently located elsewhere Se words in user V memory Of course you may locate all the tables in one group as long as they do not overlap DL05 Micro PLC User Manual 6th Edition Rev C 8 57 Chapter 8 PID Loop Operation i M The parameters in the ramp soak table must be user defined the most convenient way is to use DirectSOFT 5 which features a special editor for this table Four parameters are required to define a ramp and soak segment pair as pictured below Ramp End Value specifies the destination SP value for the end of the ramp Use the same data format for this number as you use for the SP It may be above or below the beginning SP value so the slope could be up or down we don t have to know the starting SP value for ramp 1 Ramp Slope specifies the SP increase in counts units per second It is a BCD number from 00 00 to 99 99 uses implied decimal point Soak Duration specifies the time for the soak segment in minutes ranging from 000 1 to 999 9 minutes in BCD implied decimal point Soak PV Deviation optional specifies an allowable PV dev
34. XX Red Deviation Alarm Yellow Deviation Alarm Yellow Red Deviation Alarm Red The thresholds define zones which fluctuate with the SP value The green zone which surrounds the SP value represents a safe no alarm condition The yellow zones lie outside the green zone and the red zones are beyond those The PV Deviation Alarms are reported in the two bits in the PID Mode and Alarm Status word in the loop table as shown to the right We highly recommend using ladder logic to monitor these bits The bit of word instructions make this 2s M Red Deviation easy to do Additionally you can monitor PID alarms using yellow Deviation DirectSOFT 5 The PV Deviation Alarm can be independently enabled and disabled from the other PV alarms using bit 13 of the PID Mode 1 Setting V 00 word Remember the alarm hysteresis feature works in conjunction with both the deviation and absolute value alarms and is discussed at the end of this section PID Mode and Alarm Status V 06 Bit 15 14 13121110098 765 4 321 0 8 36 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation PV Rate of Change Alarm An excellent way to get an early warning of a process fault is to monitor the rate of change of the PV Most batch processes have large masses and slowly changing PV values A relatively fast changing PV will result from a broken signal wire for either the PV
35. a WlnLBwzESLeZxmESeL 4px E ler ar fme ei dE Mobt Element Staus Edis m VA SP from DIP 0 4 _On SP1 4 PY from a 4 20mA analog V2600 BIN our Setpoint 3002 LD PY from 4 20mA z le For Help press F1 v2500 input then converteredtof 45 Sw2 BIN IOUT Process Variable v3003 ziv For Help press F1 Offline 100023 07680 06 0004003001 The Data View window can be used just as it is shown above for troubleshooting your PID logic and it can be most useful when tuning the PID loop 8 48 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation Open PID View The PID View can only be opened after a loop has been setup in your ladder program and ree the programming computer is connected to the PLC online PID View is opened by selecting it from the View submenu on the Menu bar View gt PID View The PID View can also be opened by clicking on the PID View button from the PLC Setup toolbar if it is in view EZ DirectSOFT 5 Programming pid Ladder View BEE File Edt Search View Tool PLC Debug Window Hep v LadderView Cul t x Ph Nw Stage View E Find ie amp d om EM Ho E Mnemonic View Cross Reference View x 77 p NK t
36. alarms are monitored while in automatic Cascade Cascade mode is an option with the DL05 PLC and is used in special control applications If the cascade feature is used the loop will operate as it would if in automatic mode except for the fact that a cascaded loop has a setpoint which is the control output from another loop Special Loop Calculations Reverse Acting Loop Although the PID algorithm is used in a direct or forward acting loop controller there are times when a reverse acting control output is needed The DL05 loop controller allows a loop to operate as reverse acting With a reverse acting loop the output is driven in the opposite direction of the error For example if SP gt PV then a reverse acting controller will decrease the output to increase the PV Mx Kixe Mx 4 M Kcx e Kr PV PV _ Mx Square Root of the Process Variable Square root is selected whenever the PV is from a device such as an orifice meter which requires this calculation Error Squared Control Whenever error squared control is selected the error is calculated as e SP PV x ABS SP PV A loop using the error squared is less responsive than a loop using just the error however it will respond faster with a large error The smaller the error the less responsive the loop Error squared control would typically be used in a PH control application 8 14 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop
37. amp Soak Settings V 33 Bit 15 14 13 1211109 8 765 4 32 1 0 Current Profile Step 2 digit hex Value 01 to 10 hex or 1 to 16 decimal Ramp Soak Table Error V 35 Bit 15 14 131211109 87 65 4 3 21 0 Starting Address set in reserved system V memory Starting Address set out of V memory upper range Starting Address set out of V memory lower range It s a good idea to test your ramp soak profile before using it to control the process This is easy to do because the ramp soak generator will run even when the loop is in Manual Mode Using DirectSOFT 5 s PID View will be a real time saver because it will draw the profile on screen for you Be sure to set the trending timebase slow enough to display completed ramp soak segment pairs in the waveform window 8 60 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation nS DirectSOFT 5 Ramp Soak Example The following example will step you through the Ramp Soak setup Setup the Profile in PID Setup The first step is to use Setup PID in DirectSOFT 5 to set the profile of your process Open the Setup PID window and select the R S tab and then enter the Ramp and Soak data Loop 1 SP PV Output Tuning Alarms E Tables Locstion vpn V5037 henge Ramp Soak Enable Program the Ramp Soak Control in Relay Ladder
38. an actuator connected to a 4 20mA current output module to control a valve With DirectSOFT 5 additional ladder logic programming both time proportioning eg heaters for temperature control and position actuator eg reversible motor on a valve type of control schemes can be easily implemented This chapter will explain how to set up the PID control loop how to implement the software and how to tune the loop The following block diagram shows the key parts of a PID control loop The path from the PLC to the manufacturing process and back to the PLC is the closed loop control PLC System Loop Configuring and Monitoring t Error Term i Loop Calculation Control Output External Disturbances Process Variable Manufacturing Process Setpoint Value p Bi L DL05 Micro PLC User Manual 6th Edition Rev C 8 7 Chapter 8 PID Loop Operation Ei Process Control Definitions Manufacturing Process the set of actions that adds value to raw materials The process can involve physical changes and or chemical changes to the material The changes render the material more useful for a particular purpose ultimately used in a final product Process Variable The controlled variable part of the process that you wish to control It may be temperature pressure level flow composition den
39. and contains a lag time from control input to process variable output even the most perfectly tuned single loop around the process may yield slow and inaccurate control It may be the actuator operates on one physical property which eventually affects the process variable measured by a different physical property Identifying the intermediate variable allows us to divide the process into two parts as shown in PROCESS the Intermediate Process D Control input Process A Variable Process B Variable PV following figure The principle of cascaded loops is simply that we add another process loop to more precisely control the intermediate variable This separates the source of the control lag into two parts as well The diagram below shows a cascade control system showing that it is simply one loop nested inside another The inside loop is called the minor loop and the outside loop is called the major loop For overall stability the minor loop must be the fastest responding loop of the two try a factor of 10 for a better response time We do have to add the additional sensor to measure the intermediate variable PV for process A Notice the setpoint for the minor loop is automatically generated for us by using the output of the major loop Once the cascaded control is programmed and debugged we only need to deal with the original setpoint and External External Disturbances Disturbances Outp
40. ation on cascaded loops you must use the same data range 12 15 bit and unipolar bipolar settings on the major and minor loop To prepare a loop for Cascade Mode operation as a minor loop you must program its remote Setpoint Pointer in its loop parameter table location V 32 as shown below The pointer must be the address of the V 05 location control output of the major loop In Cascade Mode the minor loop will ignore the its local SP register V 02 and read the major loop s control output as its SP instead E Major Loop Auto mode Minor Loop Cascade Mode Loop Table Loop Table V 02 XXXX SP V 02 X V 03 XXXX PV V 03 XXXX PV V 05 XXXX Control Output V 05 XXXX Control Output b V 32 XXXX Remote SP Pointer When using DzrectSOFT 5 s PID View to watch the SP value of the minor loop DirectSOFT 5 automatically reads the major loops control output and displays it for the minor loops SP The minor loops normal SP location V 02 remains unchanged Now we use the loop parameter arrangement above and draw its equivalent loop schematic shown below Process Variable Major loop Minor Cascaded loop 7 1 Cascade SUR F Control Output V 05 N Control AE Remote bs Setpoint Loop Output _ SP Local SP Calculation 9 4 Remember that a major loop goes to Manual Mode automatically if its minor loop is taken out
41. better loop performance is still needed The term feed forward refers to the control technique involved shown in the diagram below The incoming setpoint value is fed forward around the PID equation and summed with the output Feedforward path Setpoint 5 Loop 5 Control Output Calculation Process Variable E In the previous section on the bias term we said that the bias term value establishes a working region or operating point for the control output When the error fluctuates around its zero point the output fluctuates around the bias value Now when there is a change in setpoint an error is generated and the output must change to a new operating point This also happens if a disturbance introduces a new offset in the loop The loop does not really know its way to the new operating point the integrator bias must increment decrement until the error disappears and then the bias has found the new operating point Suppose that we are able to know a sudden setpoint change is about to occur common in some applications We can avoid much of the resulting error in the first place if we can quickly change the output to the new operating point If we know from previous testing what the operating point bias value will be after the setpoint change we can artificially change the output directly which is feedforward The benefits from using feedforward are e The SP PV error is reduced during
42. ble location V 00 It must be set 1 The hold bit or other bits in the Ramp Soak control are on The beginning SP value and the first ramp ending SP value are the same so first ramp segment has no slope and consequently has no duration The ramp soak generator moves quickly to the soak segment giving the illusion the first ramp is not working The loop is in Cascade Mode and is trying to get the SP remotely The SP upper limit value in the loop table location V 27 is too low Check your ladder program to verify it is not writing to the SP location V 02 in the loop table A quick way to do this is to temporarily place an end coil at the beginning of your program then go to PLC Run Mode and manually start the ramp soak generator Q The PV value in the table is constant even though the analog module receives the PV signal A Your ladder program must read the analog value from the module successfully and write it into the loop table V 03 location Verify the analog module is generating the value and the ladder is working Q The Derivative gain doesnt seem to have any affect on the output A The derivative limit is probably enabled see section on derivative gain limiting 8 72 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation SS Q The loop Setpoint appears to be changing by itself A Check the following for possible causes The Ramp Soak generator is enabled and is generating
43. ch occur in the closed loop auto tuning cycle The auto tune function examines the direction of the offset of the PV from the SP The auto tune function then takes control of the control output and induces a full span step change in the opposite direction Each time the sign of the error SP PV changes the output changes full span in the opposite direction This proceeds through three full cycles Closed Loop Auto Tune Cycle Wave Limit Cycle Method Xo YA SP D 1 a mcs URN A a PV i i 1 i i 1 Process Wave Output Value k i i i gt E 3 To PID Cycle gt zl PID Cycl y Auto Tune Cycle A yeg Auto Tune Start Auto Tune End bw Calculation of PID parameter Mmax Output Value upper limit setting Mmin Output Value lower limit setting This example is direct acting When set to reverse acting the output will be inverted When the loop tuning observations are complete the loop controller computes To bump period and Xo amplitude of the PV Then it uses these values to compute Kpc sensitive limit and Tpc period limit From these values the loop controller auto tune function computes the PID gains and the sample rate according to the Zeigler Nichols equations shown below Kpc 4M z X0 Tpc 0 M Amplitude of output PID Tuning PI Tuning P 0 45 Kpc P 0 30 Kpc 0 60 Tpc l 1 00 Tpc D 0 10 Tpc D 0 Sample Rate
44. conditions The quality of a loop s performance may generally be judged by how well the PV follows the SP after a SP step change It is important to keep in mind that understanding the process is fundamental to getting a well designed control loop Sensors must be in appropriate locations and valves must be sized correctly with appropriate trim PID control does not have typical values There isn t one control process that is identical to another Manual Tuning vs Auto Tuning You may enter the PID gain values to tune your loops manual tuning or you can rely on the PID processing engine in the CPU to automatically calculate the gain values auto tuning Most experienced process engineers will have a favorite method the DL05 will accommodate either preference The use of auto tuning can eliminate much of the trial and error of the manual tuning approach especially if you do not have a lot of loop tuning experience However performing the auto tuning procedure will get the gains close to optimal values but additional manual tuning can get the gain values to their optimal values E WARNING Only authorized personnel fully familiar with all aspects of the process should make changes that affect the loop tuning constants Using the loop auto tune procedures will affect the process including inducing large changes in the control output value Make sure you thoroughly consider the impact of any changes to minimize the risk of injury to pers
45. ctronic sensors such as thermocouples pressure VFDs etc These signals may be used in mathematically derived control systems In addition the DL05 has built in PID control algorithms that can be implemented The basic function of PID closed loop process control is to maintain certain process characteristics at desired setpoints As a rule the process deviates from the desired setpoint reference as a result of load material changes and interaction with other processes During this control the actual condition of the process characteristics liquid level temperature motor control etc is measured as a process variable PV and compared with the target setpoint SP When deviations occur an error is generated by the difference between the process variable actual value and the setpoint desired value Once an error is detected the function of the control loop is to modify the control output in order to force the error to zero E The DL05 PID control provides feedback loops using the PID algorithm The control output is computed from the measured process variable as follows Let K proportional gain T Reset or integral time Tq Derivative time or rate SP Setpoint PV t Process Variable at time t e t SP PV t PV deviation from setpoint at time t or PV error Then M t Control output at time t t M t Kc e t 1 T f e x dx Ty d dt e t M The analog input module receives the
46. d no loop calculations occur However note that output modules including analog outputs turn off in PLC Program Mode So actual manual control is not possible when the PLC is in Program Mode The only time the CPU will allow a loop mode change is during PLC Run Mode operation As such the CPU records the modes of all 4 loops as the desired mode of operation If power failure and restoration occurs during PLC Run Mode the CPU returns all loops to their prior mode which could be Manual Auto or Cascade On a Program to Run mode transition the CPU forces each loop to return to its prior mode recorded during the last PLC Run Mode You can add and configure new loops only when the PLC is in Program Mode New loops automatically begin in Manual Mode Loop Mode Override In normal conditions the mode of a loop is determined by the request to V 00 bits 0 1 and 2 However some conditions exist which will prevent a requested mode change from occurring A loop that is not set independent of PLC mode cannot change modes when the PLC is in Program mode A major loop of a cascaded pair of loops cannot go from Manual to Auto until its minor loop is in Cascade mode In other situations the PID loop controller will automatically change the mode of the loop to ensure safe operation Aloop which develops an error condition automatically goes to Manual If the minor loop of a cascaded pair of loops leaves Cascade Mode for a
47. e DL05 Micro PLC User Manual 6th Edition Rev C 8 2 1 Chapter 8 PID Loop Operation EY PID Mode Setting 2 Bit Descriptions Addr 01 The individual bit definitions of the PID Mode Setting 2 word Addr 01 are listed in the following table PID Mode 2 Word Description Read Write Bit 0 Bit 1 Input PV and Control Output Range Unipolar Bipolar select See Notes 2 and 3 write unipolar bipolar Input Output Data Format select See Notes 2 and 3 write 12 bit 15 bit Analog Input filter write off on SP Input limit enable write disable enable Integral Gain Reset units select write seconds minutes Select Auto tune PID algorithm write closed loop open loop Auto tune selection write PID PI only rate 0 Auto tune start auto tune See Note 1 cancel done PID Scan Clock internal use read i Input Output Data Format 16 bit select See Notes 2 and 3 write not 16 bit select 16 bit tos ec pie for input and output write same format separate formats ur oa Unipolar Bipolar select write unipolar bipolar Output Data Format select See Notes 3 and 4 write 12 bit 15 bit Output data format 16 bit select See Notes 3 and 4 write not 16 bit select16 bit Reserved for future use read write force start 0 1 2 3 4 5 6 7 8 9 a a c N ow NOTE 1 Bit 7 can be used to cancel Autotune mode by setting it to 0 NOTE 2 I
48. e place the loop in Manual Mode and ensure the PV and control output values are in the middle of their ranges away from the end points PLC System Process Variable Response Step Function Open Loop Auto T uning Control Setpoint Value 5 Error Term Loop Output Manufacturing Calculation Process Process Variable L d NOTE In theory the SP value does not matter in this case because the loop is not closed However the requirement of the firmware is that the SP value must be more than 5 of the PV range from the actual PV before starting the auto tune cycle for the DLO5 12 bit PV should be 205 counts or more D below the SP for forward acting loops or 205 counts or more above the SP for reverse acting loops When auto tuning the loop controller induces a step change on the output and simply observes the response of the PV From the PV response the auto tune function calculates the gains and the sample time It automatically places the results in the corresponding registers in the loop table The following timing diagram shows the events which occur in the open loop auto tuning cycle The auto tune function takes control of the control output and induces a 10 of span step change If the PV change which the loop controller observes is less than 296 then the step change on the output is increased to 2096 of span When Auto Tune s
49. e Li H7 Dutput Action Control Method hal c Pl C Closed Loop Limit Cycle Ws xl c C Pn Ninani nnn Sten Resnonsel Zi Mon Apr 10 12 58 51 2006 Value 255 og lFor Help press F1 OK Online DSCeL 06 MR 00030 07650 06 ES 8 50 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation nS Using Other PID Features It s a good idea to understand the special features of the DLO5 and how to use them You may want to incorporate some of these features for your PID How to Change Loop Modes The first three bits of the PID Mode 1 word V 00 request PID Mode 1 Setting V 00 the operating mode of the corresponding loop Note These Bit 15 141312111098 76543210 bits are mode change requests not commands certain far conditions can prohibit a particular mode change see next Gsccays f Manual page Automatic The normal state of these mode request bits is 000 To request a mode change you must SET the corresponding bit to a 1 using a one shot The PID loop controller automatically resets the bits back to 000 after it reads the mode change request Methods of requesting mode changes are DirectSOFT 5 s PID View this is the easiest method Use the drop down menu or click on one of the radio buttons and the appropriate bit will get set Ladder program ladder logic can request any loop mode when the PLC is in Run Mode
50. e SP the valve will stay closed until the error is positive for long enough to cause the bias to become positive again This will cause the process variable to undershoot One way to solve the problem is to simply clamp the normalized bias between 0 0 and 1 0 The DL05 CPU does this However if this is the only thing that is done then the output will not move off 0 0 thus opening the valve until the PV has become less than the SP This will also cause the process variable to undershoot The DL05 CPU is programmed to solve the overshoot problem by either freezing the bias term or by adjusting the bias term 8 10 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation nS Freeze Bias If the Freeze Bias option is selected when setting up the PID loop discussed later then the CPU simply stops changing the bias Mx whenever the computed normalized output M goes outside the interval 0 0 to 1 0 Mx Kix e Mx M Kc x e Kr PV PV j Mx M 0 fM lt 0 M M f0 lt MS lt 1 M 1 FM gt 1 Mx Mx Sf0 lt M S lt 1 Mx Mx otherwise Thus in this example the bias will probably not go all the way to zero so that when the PV does begin to come down the loop will begin to open the valve sooner than it would have if the bias had been allowed to go all the way to zero This action has the effect of reducing the D amount of overshoot Adjusting the Bias The n
51. e isn t a PID instruction that can be used in RLL such as a block to setup the PID loop control Instead the CPU reads the setup parameters from system V memory locations These locations are shown in the table below for reference only they can be used in a RLL program if needed Address Setup Parameter Data type Ranges Read Write Loop Parameter V1200 V7340 i V7640 Table Pointer Octal V10000 V17740 write V7641 Number of Loops BCD 1 8 write V7642 Loop Error Flags Bit 0 or 1 read NOTE The V memory data is stored in SRAM memory If power is removed from the CPU for an extended period of time the PID Setup Parameters will be lost It is recommended to use the MOV instruction which places the data in non volatile memory when setting up the parameters in the ladder program PID Error Flags V7642 OOT eea PID Error Flags Bit 15 14 131211109 8 7 654 321 0 The CPU reports any programming errors of the setup parameters in V7640 and V7641 It does this by setting the appropriate bits in V7642 on program to run mode transitions If you use the DzrectSOFT 5 loop setup dialog box its automatic range checking prohibits possible setup errors However the setup parameters may be written using other methods such as RLL so the error flag register may be helpful in those cases The following table lists the errors reported in V7642 Error Description 0 no error 1 error The starting address in V7640 is
52. ed during closed loop auto tuning If the Independent format had been checked previously make the Data format selections here NOTE The SP PV dialog can be left as it first appears for basic PID operation Set Control Output Limits Another dialog that will be available in the PID setup will be the Output dialog The control output address V 05 determined by the PID loop table beginning address will be in view Enter the output range limits Upper Limit and Lower Limit that will meet the requirement of the process and which will agree with the data format that has been selected For a basic PID operation using a 12 bit output module set the Upper Limit to 4095 and leave the Loop 1 Loop 2 Doc Configure sPiPV i Output Gute Address W2005 Range z Upper Limit 4095 4095 Lower Limit 0 0 2 bit c c T Auto transfer to 1 0 module ME T crema T d Lower Limit set to 0 Check the box next for Auto transfer to I O module if there is a need to send the control output to a certain analog output module as in the case of using the Loop Mode independent of CPU Mode otherwise the PID output signal cannot control the analog output when the PLC is not in RUN Mode If the Auto transfer to I O module feature is checked all channels of the module must be used for PID control outputs If Independent format has been previously chosen the Output Data Format will need to be setup here that is select Unipolar or Bipola
53. ent error PV Absolute Alarm A programmable alarm that compares the PV value to alarm threshold values PV Deviation Alarm A programmable alarm that compares the difference between the SP and PV values to a deviation threshold value Ramp Soak Profile A set of SP values called a profile which is generated in real time upon each loop calculation The profile consists of a series of ramp and soak segment pairs greatly simplifying the task D of programming the PLC to generate such SP sequences Rate Also called differentiator the rate term responds to the changes in the error term Remote Setpoint The location where a loop reads its setpoint when it is configured as the minor loop in a cascaded loop topology Reset Also called integrator the reset term adds each sampled error to the previous maintaining a running total called the bias Reset Windup A condition created when the loop is unable to find equilibrium and the persistent error causes the integrator reset sum to grow excessively windup Reset windup causes an extra recovery delay when the original loop fault is remedied Reverse Acting Loop A loop in which the PV increases in response to a control output decrease In other words the process has a negative gain Sampling time The time between PID calculations The CPU method of process control is called a sampling controller because it samples the SP and PV only periodically Setpoint SP The desired value for the proces
54. ert P Benedict Publisher John Wiley and Sons ISBN 0 471 89383 8 A d Industrial Instruments amp Controls Handbook Fourth ition Author Editor in Chief Douglas M Considine Publisher McGraw Hill Inc ISBN 0 07 012445 0 pH Measurement and Control Second Edition Author Gregory K McMillan Publisher Instrument Society of America ISBN 1 55617 483 7 Eorammaie Controllers Concepts and Applications First ition Authors C T Jones and L A Bryan Publisher International Programmable Controls ISBN 0 915425 00 9 Fundamentals of Programmable Logic Controllers Sensors and Communications Author Jon Stenerson Publisher Prentice Hall ISBN 0 13 726860 2 Process Control Third Edition Instrument Engineer s Handbook Author Editor in Chief Bela G Liptak Publisher Chilton ISBN 0 8019 8242 1 Process Measurement and Analysis Third Edition Instrument Engineer s Handbook Author Editor in Chief Bela G Liptak Publisher Chilton ISBN 0 8019 8197 2 8 76 l DL05 Micro PLC User Manual 6th Edition Rev C
55. es Addr 6 Loop Mode and Alarm Status bits Addr 7 Sample Rate Setting word BCD Yes Addr 10 Gain Proportional Setting word BCD Yes Addr 11 Reset Integral Time Setting word BCD Yes Addr 12 Rate Derivative Time Setting word BCD Addr 13 PV Value Low low Alarm word binary Addr 14 PV Value Low Alarm word binary Addr 15 PV Value High Alarm word binary Addr 16 PV Value High high Alarm word binary Addr 17 PV Value deviation alarm YELLOW word binary Addr 20 PV Value deviation alarm RED word binary Addr 21 PV Value rate of change alarm word binary Addr 22 PV Value alarm hysteresis setting word binary Addr 23 PV Value error deadband setting word binary Addr 24 PV low pass filter constant word BCD Addr 25 Loop derivative gain limiting factor setting word BCD Addr 26 SP value lower limit setting word binary Addr 27 SP value upper limit setting word binary Addr 30 Control output value lower limit setting word binary Addr 31 Control output value upper limit setting word binary Addr 32 Remote SP Value V Memory Address Pointer word hex Addr 33 Ramp Soak Setting Flag bit Addr 34 Ramp Soak Programming Table Starting Address word hex Addr 35 Ramp Soak Programming Table Error Flags bits Addr 36 PV direct access channel number word hex Addr 37 Control output direct access channel number word hex l N N A co N
56. es to be made in order to operate the PID loop Ramp Soak will be discussed in another section Setup PID 1 Loop 1 Loop 2 sev Output Tuning Alarms RIS Complete the PID Setup Once you have filled in the necessary information for the basic PID setup the configuration should be saved The icons on the Setup PID dialog will allow you to save the configuration to the PLC and to disk The save to icons have the arrow pointing to the PLC and disk The read from icons have the arrows pointing away from the PLC and disk An optional feature is available with the Doc tab in the Setup PID window You enter a name and description for the loop This is useful if there is more than one PID loop in your application x Loop 1 Loop 2 Doc Configure SP PV Output Tuning Gm Title Loop Title Copy r Comment Add your personalized documentation for this loop Help e Save to disk 2 Save to PLC NOTE It is good practice to save your project after setting up the PID loop by selecting File from the menu toolbar then Save project to disk In addition to saving your entire project all the PID parameters are also saved DL05 Micro PLC User Manual 6th Edition Rev C 8 39 Chapter 8 PID Loop Operation i PID Loop Tuning Once you have set up a PID loop it must be tuned in order for it to work The goal of loop tuning is to adjust the loop gains so the loop has optimal performance in dynamic
57. et value to the stable value such as 38 Be careful with this adjustment since the oscillation can destroy the process The control output response should be optimal now without a Derivative gain The example recorded values are Proportional gain 9 7 and Integral gain 38 seconds Note that the error has been minimized Minimum Oscillations Shortest response time The foregone method is the most common method used to tune a PID loop Derivative gain is almost never used in a temperature control loop This method can also be used for other control loops but other parameters may need to be added for a stable control output Test your loop for a high PV of 80 and again for a low PV of 20 and correct the values if necessary Small adjustments of the parameters can make the control output more precise or more unstable It is sometimes acceptable to have a small overshoot to make the control output react quicker 8 12 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation na The derivative gain can be helpful for those control loops which are not controlling temperature For these loops try adding a value of 0 5 for the derivative gain and see if this improves the control output If there is little or no response increase the derivative by increments of 0 5 until there is an improvement to the output trend Recall that the derivative gain reacts with a rate of change of the error
58. f the Open Loop Test page 8 40 shows the PV reading is correct and the control output has the proper effect on the process you can follow the closed loop tuning procedure see page 8 46 In this step the loop is tuned so the PV automatically follows the SP Step 9 Run Process Cyde If the closed loop test shows the PV will follow small changes in the SD consider running an actual process cycle You will need to have completed the programming which will generate the desired SP in real time In this step you may want to run a small test batch of product through the machine watching the SP change according to the recipe WARNING Be sure the Emergency Stop and power down provision is readily accessible in case AN the process goes out of control Damage to equipment and or serious injury to personnel can result from loss of control of some processes Step 10 Save Parameters When the loop tests and tuning sessions are complete be sure to save all loop setup parameters to disk DL05 Micro PLC User Manual 6th Edition Rev C 8 17 Chapter 8 PID Loop Operation i M PID Loop Setup 8 18 DLO05 Micro PLC User Manual 6th Edition Rev C Some Things to Do and Know Before Starting Have your analog module installed and operational before beginning the loop setup refer to the DL05 06 Option Modules User Manual DO OPTIONS M The DLO05 PLC gets its PID loop processing instructions from V memory tables Ther
59. f the value in bit 9 is 0 then the values in bits 0 and 1 are read If the value in bit 9 is 1 then the values in bits 0 and 1 are not read and bit 9 defines the data format the range is automatically unipolar B NOTE 3 If the value in bit 10 is 0 then the values in bits 0 1 and 9 define the input and output ranges and data formats the values in bits 11 12 and 13 are not read If the value in bit 10 is 1 then the values in bits 0 1 and 9 define only the input range and data format and bits 11 12 and 13 are read and define the output range and data format T NOTE 4 If bit 10 has a value of 1 and bit 13 has a value of 0 then bits 11 and 12 are read and define the output range and data format If bit 10 and bit 13 each have a value of 1 then bits 11 and 12 are not read and bit 13 defines the data format the output range is automatically unipolar 8 22 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation nS Mode Alarm Monitoring Word Addr 06 The individual bit definitions of the Mode Alarm monitoring Addr 06 word is listed in the following table Mode Alarm Bit Description Read Write Bit 1 Manual Mode Indication read Manual Automatic Mode Indication read Auto Cascade Mode Indication read Cascade PV Input LOW LOW Alarm read On PV Input LOW Alarm read On PV Input HIGH Alarm read On PV Input HIGH HIGH Alarm read On PV Input YELLOW Deviation Alarm read On PV Input RED Deviati
60. fectively disabled Feedforward Example How do we know when to write to the bias term and what value to write Suppose we have an oven temperature control loop and we have already tuned the loop for optimal performance Refer to the figure below We notice that when the operator opens the oven door the temperature sags a bit while the loop bias adjusts to the heat loss Then when the door closes the temperature rises above the SP until the loop adjusts again Feedforward control can help diminish this effect Oven door PV Bias Closed Open PV excess Loo poo po Wm Closed First we record the amount of bias change the loop controller generates when the door opens or closes Then we write a ladder program to monitor the position of an oven door limit switch When the door opens our ladder program reads the current bias value from V 04 adds the desired change amount and writes it back to V 04 When the door closes we duplicate the procedure but subtracting desired change amount instead The following figure shows the results Oven door PV Bias Closed Open Closed Feed forward x X Feed forward The step changes in the bias are the result of our two feed forward writes to the bias term We can see the PV variations are greatly reduced The same technique may be applied for changes in setpoint DL05 Micro PLC User Manual
61. for large setpoint changes For position form of PID this inhibits integrator action when the control output reaches 0 or 100 speeds up loop recovery when output recovers from saturation Specify a tolerance plus and minus for the error term SP PV so that no change in control output value is made 0 05 to 99 99 seconds user programmable Bumpless Transfer II Freeze Bias Anti windup Error Deadband Alarm Feature Specifications PV Alarm Hysteresis Specify 1 to 200 word binary does not affect all alarms such as PV Rate of Change Alarm PV Alarm Points Select PV alarm settings for Low low Low High and High high conditions PV Deviation Specify alarms for two ranges of PV deviation from the setpoint value Rate of Change Detect when PV exceeds a rate of change limit you specify DL05 Micro PLC User Manual 6th Edition Rev C 8 3 Chapter 8 PID Loop Operation iY Introduction to PID Control What is PID Control In this discussion we will explain why PID control is used in process control instead of trying to provide control by simply using an analog input and a discrete output There are many types of analog controllers available and the proper selection will depend upon the particular application There are two types of analog controllers that are used throughout industry 1 The ON OFF controller sometimes referred to as an open l
62. g dialog but they can set later in the DirectSOFT 5 PID View Error Term Selection The error term is internal to the CPUs PID loop controller and is generated again in each PID calculation Although its data is not directly accessible you can easily calculate it by subtracting Error SP PV The PID calculation operates on this value linearly to give the result However a few applications can benefit from non linear control The Error squared methnod of non linear control exaggerates large errors and diminishes small error Error Squared When selected the squared error function simply squares the error term but preserves the original algebraic sign which is used in the calculation This affects the Control Output by diminishing its response to smaller error values but maintaining its response to larger errors Some situations in which the error squared term might be useful e Noisy PV signal using a squared error term can reduce the effect of low frequency electrical noise on the PV which will make the control system jittery A squared error maintains the response to larger errors Non linear process some processes such as chemical pH control require non linear controllers for best results Another application is surge tank control where the Control Output signal must be smooth DL05 Micro PLC User Manual 6th Edition Rev C 8 33 Chapter 8 PID Loop Operation i Enab
63. ge the cruise control by turning it ON then he manually brings the car to the desired cruising speed say 70 miles per hour Once the cruise speed is reached the SET button is pushed fixing the speed at 70 mph the setpoint Now the car is cruising at a steady 70 mph until it comes to a hill to go up As the car goes up the hill it tends to slow down The speed sensor senses this and causes the throttle to increase the fuel to the engine The vehicle speeds up to maintain 70 mph without jerking the car and it reaches the top at the set speed When the car levels out after reaching the top of the hill it will speed up The speed sensor senses this and signals the throttle to provide less fuel to the engine thus the engine slows down allowing the car to maintain the 70 mph speed How does this application apply to PID control Lets look at the function of P I and D terms Proportional is commonly referred to as Proportional Gain The proportional term is the corrective action which is proportional to the error that is the change of the manipulated variable is equal to the proportional gain multiplied by the error the activating signal In mathematical terms Proportional action proportional gain X error Error Setpoint SP Process Variable PV Applying this to the cruise control the speed was set at 70 mph which is the Setpoint The speed sensor senses the actual speed of the car and sends this signal to the cruise controller as
64. he PID calculations are stopped The operator must manually control the loop by writing to the control output value directly Minor Loop In cascade control the minor loop is the subordinate loop that receives its SP from the major loop On Off Control A simple method of controlling a process through on off application of energy into the system The mass of the process averages the on off effect for a relatively smooth PV A simple ladder program can convert the DL05 s continuous loop output to on off control 8 7 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation a PID Loop A mathematical method of closed loop control involving the sum of three terms based on proportional integral and derivative error values The three terms have independent gain constants allowing one to optimize tune the loop for a particular physical system Position Algorithm The control output is calculated so it responds to the displacement position of the PV from the SP error term Process A manufacturing procedure which adds value to raw materials Process control particularly refers to inducing chemical changes to the material in process Process Variable PV A quantitative measurement of a physical property of the material in process which affects final product quality and is important to monitor and control Proportional Gain A constant that determines the magnitude of the PID proportional term in response to the curr
65. her loop Cascaded loop Loop Control Output V 05 Cascade Calculation Loop Control Output Calculation Normal SP V 02 Oo Auto Manual Process Variable As pictured below a loop can be changed from one mode to another but cannot go from Manual Mode directly to Cascade or vice versa This mode change is prohibited because a loop would be changing two data sources at the same time and could cause a loss of control D dmm m Once the CPU is operating in the Run Mode the normal operation of the PID loop controller is to read the loop data and perform calculations on each scan of the RLL program When the CPU is placed in the Program Mode the RLL program halts operation and all PID loops are automatically put into the Manual Mode The PID parameters can then be changed if desired Similarly by placing the CPU in the Run mode the PID loops are returned to the operational mode which they were previously in i e Manual Automatic and Cascade With this selection you automatically affect the modes by changing the CPU mode CPU Modes Program Mode change Loop Mode Linking 0 loop follows PLC mode V PID Mode 1 Setting V 00 1 loop is independent of PLC mode C Manual Mode change Automatic Mode change If bit 15 is set to one then the loops will run independent of the CPU mode It is like having two independent processors in the CPU one is running the RLL program and the other is ru
66. hoot at the turn on point 71 OFF OFF 69 SETPOINT 67 ON ON ON TIME The ON OFF controller is used in some industrial control applications but is not practical in the majority of industrial control processes The most common process controller that is used in industry is the PID controller 8 4 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation a The PID controller controls a continuous feedback loop that keeps the process output control variable flowing normally by taking corrective action whenever there is a deviation from the desired value setpoint of the process variable PV such as rate of flow temperature voltage etc An error occurs when an operator manually changes the setpoint or when an event valve opened closed etc or a disturbance cold water wind etc changes the load thus causing a change in the process variable The PID controller receives signals from sensors and computes corrective action to the actuator from a computation based on the error Proportional the sum of all previous errors Integral and the rate of change of the error Derivative We can look at the PID controller in more simple terms Take the cruise control on an automobile as an example Let s say that we are cruising on an interstate highway in a car equipped with cruise control The driver decides to enga
67. iation above and below the SP value during the soak period A PV deviation alarm status bit is generated by the ramp soak generator Ramp End cup 3pm SP Value Soak PV Ramp Soak Table deviation V 00 XXXX Ramp End SP Value na o T V 01 XXXX Ramp Slope Slope E SP NS Soak v 02 XXXX Soak Duration duration V 03 XXXX Soak PV Deviation a segment becomes active The ramp segment becomes active when the previous soak segment ends If the ramp is the first segment it becomes active when the ramp soak generator is started and automatically assumes the present SP as the starting SP Description Description Ramp End SP Value Ramp End SP Value Ramp Slope Ramp Slope Soak Duration Soak Duration Soak PV Deviation Soak PV Deviation Ramp End SP Value Ramp End SP Value Ramp Slope Ramp Slope Soak Duration Soak Duration Soak PV Deviation Soak PV Deviation Ramp End SP Value Ramp End SP Value Ramp Slope Ramp Slope Soak Duration Soak Duration Soak PV Deviation Soak PV Deviation Ramp End SP Value Ramp End SP Value Ramp Slope Ramp Slope Soak Duration Soak Duration Soak PV Deviation Soak PV Deviation 4 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 8 58 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation a Many applications do not require all 16 R S steps Use all zeros in the table for unused steps
68. icro PLC User Manual 6th Edition Rev C V2105 OUT Ch 1 Output Dat V2010 Chapter 8 PID Loop Operation nS Example program continued Continuously read the setpoint value stored in V memory and convert it from BCD to binary and store itto Loop 1 SP LD On SP1 SP location 4 1 vi400 OUT Loop 1 SP v2102 Select PID loop 1 Manual Mode Loop 1 Manual Loop 1 Auto x0 x1 B2100 1 5 iri ar SET Select PID loop 1 Auto Mode Loop 1 Auto Loop 1 Manual X1 xO B2100 0 6 irt 4 SET 7 END M nor Note that the module used in this example was set up for binary format It could have been setup for BCD format In the later case the BCD data would have to be converted to binary format before being stored to the setpoint and process variable and the control output would have to be converted from binary to BCD before being stored to the analog output By following the steps outlined in this chapter you should be able to setup workable PID control loops For a step by step tutorial go to the Technical Support section located on our website www automationdirect com Once you are at the website click on Technical Support Home After this page opens find and select Technical and Application Notes located under the Using Your Products column When the Technical Notes page opens select General then locate the document number AN MISC 012 titled PI
69. ill cause the ramp soak generator to truncate the current segment step and go to the next segment Ramp Soak Profile Monitoring You can monitor the Ramp Soak profile status using other bits in the Ramp Soak Settings V 33 word shown to the right e R S Profile Complete 21 when the last programmed step is done Soak PV Deviation 1 when the error SP PV exceeds the specified deviation in the R S table e R S Profile in Hold 1 when the profile was active but is now in hold Ramp Soak Settings V 33 The number of the current step is available in the upper 8 bits of the Ramp Soak Settings V 33 word The bits represent a 2 digit hex number ranging from 1 to 10 Ladder logic can monitor these to synchronize other parts of the program with the ramp soak profile Load this word to the accumulator and shift right 8 bits and you have the step number Ramp Soak Programming Errors The starting address for the ramp soak table must be a valid location If the address points outside the range of user V memory one of the bits to the right will turn on when the ramp soak generator is started We recommend using DirectSOFT 5 to configure the ramp soak table It automatically range checks the addresses for you Testing Your Ramp Soak Profile Ramp Soak Settings V 33 Bit 15 14 131211109 87 65 4 3 2 1 0 R S Profile in Hold E Soak PV Deviation R S Profile Complete R
70. ion Control Method itj C FowadActing GP Cte or 18 zl 4 a gt For Help press F1 Fri Mar 1017 14 41 2006 Value 3 m For Help press F1 ok OnineDSCBLO6 Program 0029 07680 06 DL05 Micro PLC User Manual 6th Edition Rev C 8 u9 Chapter 8 PID Loop Operation The two views are now ready to be used to tune your loop You will be able to see where the PID values have been set and see the process that it is controlling i DirectSOFT 5 Programming pid PID View 8 x Ele Edt Search View Tools PLC Debug Window Heb aT r EDIT AZ ue w md A il umts x Ladder View PID View 4 x amp eme wono z D Wise Setup 10 Element Status Edits 4 Sec Div SE 1 a 20 5 I gt SPO z _ ro Hak PV 2 BlASO Reset ol HE 4 10 5 OUT aaa ol x 7 F 8 Loop Num T 4j Loop Tile a ast 3 Settings Variables Alarms T PLC Mode Pogam cpi Roc T L Alm Ti Mode Manua y Highhigh f0 12 Gain 0 00 p F DevAlms High 0 Sec Bias 0 13 Reset 0 00 Reef Low o 14 p C Mn ow o Rate 000 sec Yelow o Lowiow o 16 Autotune E 17 Output Action Control Method C Forward Acting Cr C Closed Loop Limit Cycle J 42H 18 j iS Fri Mar 10 17 25 26 2006 Value 7 wy Data enor OK Online DSCBL 06 Program
71. its own ramp soak generator use is optional You may specify up to eight ramp soak steps 16 segments The ramp soak generator can run anytime the PLC is in Run mode Its operation is independent of the loop mode Manual or Auto Ramp soak real time controls include Start Hold Resume and Jog Ramp soak monitoring includes Profile Complete Soak Deviation SP minus PV and current ramp soak step number The following figure shows a SP profile consisting of ramp soak segment pairs The segments are individually numbered as steps from 1 to 16 The slope of each of the ramp may be either increasing or decreasing The ramp soak generator automatically knows whether to increase or decrease the SP based on the relative values of a ramp s end points These values come from the ramp soak table 15 16 13 14 Ramp Soak Soak Ramp Step SP Ramp Soak Table V Memory Space The parameters which define the ramp soak profile for a loop are in a ramp soak table Each User Data loop may have its own ramp soak table but it is F optional Recall the Loop Parameter table consists 2000 LOOP 2034 V2037 32 words N 8000 octal a 32 word block of memory for each loop and v2040 OORA together they occupy one contiguous memory v BRL V2074 area However the ramp soak table for a loop is y SEU OUR individually located because it is optional for each loop An address pointer in location V 34 in the
72. ivative gain values are automatically updated in loop table locations V 10 V 11 and V 12 respectively The sample time in V 07 is also updated automatically You can test the validity of the values the auto tuning procedure yields by measuring the closed loop response of the PV to a step change in the output The instructions on how to do this are in the section on the manual tuning procedure located prior to this auto tuning section 8 Closed Loop Auto Tuning During a closed loop auto tuning cycle the loop controller operates as shown in the diagram zi Process Variable Manufacturing below PLC System YN Response Limit cycle wave Closed Loop Auto T uning Control Setpoint Value Error Term Loop Output Calculation Process Variable Process L zl When auto tuning the loop controller imposes a square wave on the output Each transition of the output occurs when the PV value crosses over under the SP value Therefore the frequency of the limit cycle is roughly proportional to the mass of the process From the PV response the auto tune function calculates the gains and the sample time It automatically places the results in the corresponding registers in the loop table 8 46 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation i i The following timing diagram shows the events whi
73. le Deadband When selected the enable deadband function takes a range of small error values near zero and simply substitutes zero as the value of the error If the error is larger than the deadband range then the error value is used normally Freeze Bias The term reset windup refers to an undesirable characteristic of integrator behavior which occurs naturally under certain conditions Refer to the figure below Suppose the PV signal becomes disconnected and the PV value goes to zero While this is a serious loop fault it is made worse by reset windup Notice the bias reset term keeps integrating normally during the PV disconnect until its upper limit is reached When the PV signal returns the bias value is saturated windup and takes a long time to return to normal The loop output consequently has an extended recovery time Until recovery the output level is wrong and causes further problems PV 0 Reset windup Freeze bias enabled E Bias Recovery time Recovery time PV loss PV loss In the second PV signal loss episode in the figure the freeze bias feature is enabled It causes the bias value to freeze when the control output goes to its range limit Much of the reset windup is thus avoided and the output recovery time is much less For most applications the freeze bias feature will work with the loop as described above It is suggested to enable this featu
74. le Ramp End SP Value 34 Ramp End SP Value Ramp Slope 35 Ramp Slope Soak Duration 36 Soak Duration l CO NI NI GD A 01 BY BY CoO Co NINI Soak PV Deviation 37 Soak PV Deviation Ramp Soak Table Programming Error Flags Addr 35 The individual bit definitions of the Ramp Soak Table Programming Error Flags word Addr 35 is listed in the following table Further details are given in the PID Loop Mode section and in the PV Alarm section later in chapter 8 R S Error Flag Bit Description Read Write Starting Addr out of lower V memory range read Starting Addr out of upper V memory range read Reserved for Future Use Starting Addr in System Parameter V memory Range read Reserved for Future Use 8 24 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation aaa Configure the PID Loop Once the PID table is established in V memory configuring the PID loop continues with the DirectSOFT 5 PID setup configuration dialog You will need to check and fill in the data required to control the PID loop Select Configure and the following dialog will appear for this process Loop 1 Loop 2 Doc Configure SP PY Output Tuning Algorithm SP PV amp Output Format Loop Mode Position Velocity Common format Independent C of CPU mode Sample Rate 0 05 sec Independent format
75. lications In the figure to the right the setpoint increases during the ramp segment It remains steady at one value during the soak segment Complex SP profiles can be generated by specifying a series Ramp slope value changes The ramp soak generator can greatly reduce the amount of programming required Soak of ramp soak segments The ramp segments are specified in SP units per second The soak time is also programmable in minutes Time It is instructive to view the ramp soak generator as a dedicated function to generate SP values as shown below It has two categories of inputs which determine the SP values generated The ramp soak table must be programmed in advance containing the values that will define the ramp soak profile The loop reads from the table during each PID calculation as necessary The ramp soak controls are bits in a special loop table word that control the real time start stop functionality of the ramp soak generator The ladder program can monitor the status of the ramp soak profile current ramp segment number Ramp soak table gt Ramp soak Ramp soak controls Generator Setpoint Loop 8 56 DL05 Micro PLC User Manual 6th Edition Rev C 265 Calculation Process Variable Control Output gt Chapter 8 PID Loop Operation 2 Now that we have described the general ramp soak generator operation we list its specific features Each loop has
76. lter is controlled by using register V 24 in the loop parameter table the filter constant The data format of the filter constant value is BCD with an implied decimal point 00X X as follows The filter constant has a valid range of 000 1 to 001 0 DirectSOFT 5 converts values above the valid range to 001 0 and values below this range to 000 1 Values close to 001 0 result in higher roll off frequencies while values closer to 000 1 result in lower roll off frequencies We highly recommend using DirectSOFT 5 for the auto tuning interface The duration of each auto tuning cycle will depend on the mass of your process A slowly changing PV will result in a longer auto tune cycle time When the auto tuning is complete the proportional and integral gain values are automatically updated in loop table locations V 10 and V 11 respectively The derivative is calculated if you autotune for PID and updated in loop table location V 12 The sample time in V 07 is also updated automatically You can test the validity of the values the auto tuning procedure yields by measuring the closed loop response of the PV to a step change in the output The instructions on how to do this are in the section on the manual tuning procedure DL05 Micro PLC User Manual 6th Edition Rev C 8 53 Chapter 8 PID Loop Operation i The algorithm which the built in filter follows is yi k xi yi 1 yi 1 yi is the current
77. ly enabled and disabled from the other PV alarms using bit 14 of the PID Mode 1 Setting V 00 word The alarm hysteresis feature discussed next does not affect the Rate of Change Alarm DL05 Micro PLC User Manual 6th Edition Rev C 8 37 Chapter 8 PID Loop Operation E MM PV Alarm Hysteresis The PV Absolute Value Alarm and PV Deviation Alarm are programmed using threshold values When the absolute value or deviation exceeds the threshold the alarm status becomes true Real world PV signals have some noise on them which can cause some fluctuation in the PV value in the CPU As the PV value crosses an alarm threshold its fluctuations cause the alarm to be intermittent and annoy process operators The solution is to use the PV Alarm Hysteresis feature The PV Alarm Hysteresis amount is programmable from 1 to 200 binary decimal When using the PV Deviation Alarm the programmed hysteresis amount must be less than the programmed deviation amount The figure below shows how the hysteresis is applied when the PV value goes past a threshold and descends back through it Alarm threshold i Hysteresis Loop Table PV V 22 XXXX PV Alarm Hysteresis Alarm 1 0 The hysteresis amount is applied after the threshold is crossed and toward the safe zone In this way the alarm activates immediately above the programmed threshold value It delays
78. modes Two types of bumpless transfer available Full featured alarms Ramp soak generator with up to 16 segments Auto Tuning The DL05 CPU has process control loop capability in addition to ladder program execution Up to four loops can be selected and configured All sensor and actuator wiring connects directly to DL05 analog I O modules All process variables gain values alarm levels etc associated with each loop reside in a Loop Variable Table in the CPU The DL05 CPU reads process variable PV inputs during each scan Then it makes PID loop calculations during a dedicated time slice on each PLC scan updating the control output value The control loops use a Proportional Integral Derivative PID algorithm to generate the control output This chapter describes how the loops operate and how to configure and tune the loops PP Analog Output DLO5 PID Loop Calculations Manufacturing Process amp Analog Input P DirectSOFT 5 programming software release 5 or later is used for configuring analog control loops in the DL05 DirectSOFT 5 uses Intelligent boxes IBoxes to help you set up the individual loops After completing the setup you can use DirectSOFT 5 s PID Trend View to tune each loop The configuration and tuning selections you make are stored in the DLO05 s V memory RAM The loop parameters also may be saved to disk for recall later 8 2 DLO05 Micro PLC
79. n 16 bit data format is selected 8 26 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation SY Setpoint V 02 Loop Control Output V 05 Calculation PID Mode 2 Setting V 01 Process Variable V 03 Bit 15 14131211109 87 65 4 321 0 7 Data formats LSB Bit 15 141312111098 76543210 00 12 bit unipolar 0 to OFFF 0 to 4095 Select data format using 01 E 12 bit bipolar 0 to OFFF 8FFF to 8001 f 0 to 4095 4095 to 4095 bits 0 and 1 10 15 bit unipolar 0 to 32767 11 E 15 bit bipolar Oto 7FFF FFFF to 8001 E ian bit 0 to 32767 32767 to 32767 sign bi Magnitude plus sign and the control output device This selects the data format for both the SP and the PV Loop Mode Loop Mode is a special feature that allows the PID loop controller to perform closed loop control while the CPU is in the Program Mode Careful thought must be taken before using this feature called Independent of CPU mode in the dialog Before continuing with the PID setup a knowledge of the three PID loop modes will be helpful The data format determines the numerical interface between the PID loop and the PV sensor D The DLO5 provides the three standard control modes Manual Automatic and Cascade The sources of the three basic variables SD PV and control output
80. nning the process loops lt Bit 15 14 13 12 1 109876543210 8 28 DL05 Micro PLC User Manual 6th Edition Rev C a Chapter 8 PID Loop Operation Having the ability to run loops independently of the RLL program makes it feasible to make a ladder logic change while the process is still running This is especially beneficial for large mass continuous processes that are difficult or costly to interrupt The independent of CPU is the feature used for this If you need to operate the PID loops while the RLL program is halted in Program Mode either select the Independent of CPU mode in the dialog or edit your program to set and reset bit 15 of PID Mode 1 word V 00 in your RLL program If the bit is set to a zero the loop will follow the CPU mode then when the CPU is placed in the Program Mode all loops will be forced into the Manual Mode When Independent of CPU mode is used you should also set the PV to be read directly from an analog input module This can easily be done in the PID setup dialog SP PV The SP PV dialog has a block entitled Process Variable There is a block within this block called Auto Transfer From from analog input with the information grayed out Checking the box to the left of the Auto Transfer From will high light the information There are two choices to select for the auto transfer The first choice is O Module If this is selected
81. nsfer the value between the analog input and the loop table I NOTE When auto transfer to from I O is used the analog data for all of the channels on the analog module cannot be accessed by any other method i e pointer or multiplex SP PV Addresses An SP PV dialog will be made available to setup how the setpoint SP and the process variable PV will be used in the loop If this loop is the minor loop of a cascaded pair enter that control output address in the Remote SP from Cascaded Loop Output area It is sometimes desirable to limit the range of setpoint values allowed to be entered To activate this feature check the box next to Enable Limiting This will activate the Upper and Lower fields for the values to be entered Set the limits around the SP value to prevent an operator from entering a Loop 1 Loop 2 Doc Configure SE BV Setpoint Variable Process Variable Address V2002 Address V2003 Square root Auto Transfer From Remote Setpoint from Cascaded Loop Output Enable Limiting 8 30 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation setpoint value outside of a safe range The Square root box is only checked for certain PID loops such as a flow control loop If the Auto transfer from I O module is selected a first order low pass filter can be used by checking the Enable Filter box The filter coefficient is user specified The use of this filter is recommend
82. nto a continuous effect slowly changing balloon temperature and ultimately the altitude which is the process variable Time proportioning control approximates continuous control by virtue of its duty cycle the ratio of ON time to OFF time The following figure shows an example of how duty cycle approximates a continuous level when it is averaged by a large process mass e period Desired Effect On Off on T Control Off If we were to plot the on off times of the burner in the hot air balloon we would probably see a very similar relationship to its effect on balloon temperature and altitude 8 66 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation On Off Control Program Example The following ladder segment provides a time proportioned on off control output It converts the continuous output in V2005 to on off control using the output coil YO SP Loop V2005 Time YO P P NEL rocess Calculation Proportioning V i QE T U py continuous on off The example program uses two timers to generate On Off control It makes the following assumptions which you can alter to fit your application The loop table starts at V2000 so the control output is at V2005 The data format of the control output is 12 bit unipolar 0 FFF The time base one full cycle fo
83. ny reason its major loop automatically goes to Manual Mode 8 52 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation PV Analog Filter A noisy PV signal can make tuning difficult and can cause the control output to be more extreme than necessary as the output tries to respond to the peaks and valleys of the PV There are two equivalent methods of filtering the PV input to make the loop more stable The first method is accomplished using the DLO5 s built in filter The second method achieves a similar result using ladder logic The DLO5 Built in Analog Filter The DLO5 provides a selectable first order low pass PV input filter We only recommend the use of a filter during auto tuning or PID control if there is noise on the input signal You may disable the filter after auto tuning is complete or continue to use it if the PV input signal is noisy 5 Loop Control Output Calculation Unfiltered UM 0 E id uL Process Variable Filtered fy D 9 71 PV LS PID Mode 2 Setting V 01 Bit 1514131211098 765 432 1 0 Loop Table PV filter V 24 XXXX Filter constant enable disable Bit 2 of PID Mode Setting 2 provides the enable disable control for the low pass PV filter 0 disable 1 enable The roll off frequency of the single pole low pass fi
84. o real to BCD instruction BCD Convert the number in the accumulator to BCD format This satisfies the timer preset format requirement Output the result to V1400 In our example this is the een location of the timer preset for the second timer TO The second fast timer also counts in increments of 01 TMRF T1 seconds so its range is variable from 0 to a maximum V1400 of 1000 ticks or 10 seconds This timer s output T1 turns off the output coil YO when the preset is reached Ti TAI KO YO The N C T1 contact inverts the T1 timer output The ro control output is on at the beginning of the 10 second time IZ OUT interval YO turns off when T1 times out The STRNE contact prevents YO from energizing during the one scan when TO resets T1 YO is the actual control output END END coil marks the end of the main program DL05 Micro PLC User Manual 6th Edition Rev C 8 67 Chapter 8 PID Loop Operation i MMMMMMMMMMMMMM MMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMMmMmmImImI Feedforward Control Feedforward control is an enhancement to standard closed loop control It is most useful for diminishing the effects of a quantifiable and predictable loop disturbance or sudden change in setpoint Use of this feature is an option available to you on the DL05 However it s best to implement and tune a loop without feedforward and adding it only if
85. of Cascade Mode 8 64 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation nS Tuning Cascaded Loops When tuning cascaded loops you will need to de couple the cascade relationship and tune the minor loop using one of the loop tuning procedures previously covered Once this has been done have the minor loop in cascade mode and auto tune the major loop see Step 4 1 If you are not using auto tuning then find the loop sample rate for the minor loop using the method discussed earlier in this chapter Then set the sample rate of the major loop slower than the minor loop by a factor of 10 Use this as a starting point 2 Tune the minor loop first Leave the major loop in Manual Mode and you will need to generate SP changes for the minor loop manually as described in the loop tuning procedure 3 Verify the minor loop gives a critically damped response to a 10 SP change while in Auto Mode Then we are finished tuning the minor loop In this step you will need to get the minor loop in Cascade Mode and then the Major loop in Auto Mode We will be tuning the major loop with the minor loop treated as a series component its overall process Therefore do not go back and tune the minor loop again while tuning the major loop 5 Tune the major loop following the standard loop tuning procedure in this section The response of the major loop PV is actually the overall response of the cascaded loops together 8 DL0
86. ol loops online Step 1 Know the Recipe The most important knowledge is how to produce your product This knowledge is the foundation for designing an effective control system A good process recipe will do the following Identify all relevant Process Variables such as temperature pressure or flow rates etc which need precise control Plot the desired Setpoint values for each process variables for the duration of one process cycle Step 2 Plan Loop Control Strategy This simply means choosing the method the machine will use to maintain control over the Process Variables to follow their Setpoints This involves many issues and trade offs such as energy efficiency equipment costs ability to service the machine during production and more You must also determine how to generate the Setpoint value during the process and whether a machine operator can change the SP Step 3 Size and Scale Loop Components Assuming the control strategy is sound it is still crucial to properly size the actuator and properly scale the sensors Choose an actuator heater pump etc which matches the size of the load An oversized actuator will have an overwhelming effect on your process after a SP change However an undersized actuator will allow the PV to lag or drift away from the SP after a SP change or process disturbance Choose a PV sensor which matches the range of interest and control for our process Decide the resolution of c
87. on Alarm read On PV Input Rate of Change Alarm read On Alarm Value Programming Error read Error Loop Calculation Overflow Underflow read Error Loop in Auto Tune indication read On Auto Tune error indication read 14 15 Reserved for Future Use Ramp Soak Table Flags Addr 33 The individual bit definitions of the Ramp Soak Table Flag Addr 33 word is listed in the following table Ramp Soak Flag Bit Description Read Write Bit 1 Start Ramp Soak Profile write 01 Start Hold Ramp Soak Profile write 01 Hold Resume Ramp soak Profile write 01 Resume Jog Ramp Soak Profile write 01 Jog Ramp Soak Profile Complete read Complete PV Input Ramp Soak Deviation read On Ramp Soak Profile in Hold read On Reserved read Current Step in R S Profile read decode as byte hex 0 1 2 3 4 5 6 7 8 NI oD A Go PO T RH Bits 8 15 must be read as a byte to indicate the current segment number of the Ramp Soak generator in the profile This byte will have the values 1 2 3 4 5 6 7 8 9 A B C D E F and 10 which represent segments 1 to 16 respectively If the byte 0 then the Ramp Soak table is not active DL05 Micro PLC User Manual 6th Edition Rev C 8 23 Chapter 8 PID Loop Operation Ramp Soak Table Location Addr 34 Each loop that you configure has the option of using a built in Ramp Soak genera
88. onnel or damage to equipment The auto tune in the DLO5 is not intended to be used as a replacement for your process knowledge Open Loop Test Whether you use manual or auto tuning it is very important to verify basic characteristics of a newly installed process before attempting to tune it With the loop in Manual Mode verify the following items for each new loop Setpoint verify that the SP source can generate a setpoint Put the PLC in Run Mode and leave the loop in Manual Mode then monitor the loop table location V 02 to see the SP value s If you are using the ramp soak generator test it now Process Variable verify that the PV value is an accurate measurement and the PV data arriving in the loop table location V 03 is correct If the PV signal is very noisy consider filtering the input either through hardware RC low pass filter or using the filter in this chapter Control Output if it is safe to do so manually change the output a small amount perhaps 10 and observe its affect on the process variable Verify the process is direct acting or reverse acting and check the setting for the control output inverted or non inverted Make sure the control output upper and lower limits are not equal to each other Sample Rate while operating open loop this is a good time to find the ideal sample rate procedure given earlier in this chapter However if you are going to use auto tuning the auto tuning proced
89. ontrol you need for the PV such as within 2 C and make sure the sensor input value provides the loop with at least 5 times that resolution at LSB level However an over sensitive sensor can cause control oscillations etc The DL05 provides 12 bit and 15 bit unipolar and bipolar data format options and a 16 bit unipolar option This selection affects SP PV Control Output and Integrator sum Step 4 Select I O Modules After deciding the number of loops PV variables to measure and SP values you can choose the appropriate I O module Refer to the figure on the next page In many cases you will be able to share input or output modules or use a analog I O combination module among several control loops The example shown sends the PV and Control Output signals for two loops through the same set of modules AutomationDirect offers DLO5 analog input modules with 4 channels per module that accept 0 20mA or 4 20mA signals Also analog input and output combination modules are now available Thermocouple and RTD modules can also be used to maintain temperatures to a 10th of a degree Refer to the sales catalog for further information on these modules or find the modules on our website www automationdirect com 8 16 DLO05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation SY DLO5 CPU Input V memory Digital Mod
90. oop controller 2 The PID controller sometimes called a closed loop controller Regardless of type analog controllers require input signals from electronic sensors such as pressure differential pressure level flow meter or thermocouples As an example one of the most common analog control applications is located in your house for controlling either heat or air conditioning the thermostat You wish for your house to be at a comfortable temperature so you set a thermostat to a desired temperature setpoint You then select the comfort mode either heat or A C A temperature sensing device normally a thermistor is located within the thermostat If the thermostat is set for heat and the setpoint is set for 69 the furnace will be turned on to provide heat at normally 2 below the setpoint In this case it would turn on at 67 When the temperature reaches 71 2 above setpoint the furnace will turn off In the opposite example if the thermostat is set for A C cooling the thermostat will turn the A C unit on off opposite the heat setting For instance if the thermostat is set to cool at 76 the A C unit will turn on when the sensed temperature reaches 2 above the setpoint or 78 and turn off when the temperature reaches 74 This would be considered to be an ON OFF controller The waveform below shows the action of the heating cycle Note that there is a slight overshoot at the turn off point also a slight unders
91. or example consider an application with 4 loops and V2000 sree has been chosen as the starting location The Loop Parameter LOOP 4 will occupy V2000 V2037 for loop 1 V2040 V2077 for S IDE loop 2 and so on Loop 4 occupies V2140 V2177 Determine the block of V memory to be used for each PID loop Besides being the beginning of the PID parameter memory block the first address will be the start of loop 1 parameters Remember there are 32 words 0 to 37 octal needed for each loop Once you have determined the beginning V memory address to be used you can setup and store the PID parameters either directly in your RLL program or by the using PID Setup in DirectSOFT 5 NOTE Whether one or more loops are being setup this block of V memory will only be used for the I PID loop parameters do not use this block of memory for anything else in your program Using DirectSOFT 5 is the simplest way to setup the parameters The PID parameters can be setup either offline or online while developing the user program The parameters can only be entered in PID setup when the PLC is in the Program mode Once the parameters have been entered and saved for each loop changes made through the PID setup can be made but only in Program Mode You can type the beginning address in the PID Table Address dialog found when the PID Setup is opened in DirectSOFT 5 This can be seen in the diagram below After the address has been entered the memory
92. ormal action of the CPU is to adjust the bias term when the output goes out of range as shown below Mx Kix ep Mx M Kc x e Kr PV PV ji Mx M 0 if M lt 0 M M Sf0 lt M lt 1 M 1 if M gt 1 Mx Mx if0 lt M lt 1 Mx M Kc x ep Kr PV PV p1 otherwise By adjusting the bias the valve will begin to open as soon as the PV begins to come down If the loop is properly tuned overshoot can be eliminated entirely If the output went out of range due to a setpoint change then the loop probably will oscillate because we must wait for the bias term to stabilize again The choice of whether to use the default loop action or to freeze the bias is dependent on the application If large step changes to the setpoint are anticipated then it is probably better to select the freeze bias option see page 8 34 DL05 Micro PLC User Manual 6th Edition Rev C 8 ll Chapter 8 PID Loop Operation ee Step Bias Proportional to Step Change in SP This feature reduces oscillation caused by a step change in setpoint when the adjusting bias feature is used Mx Mx SP SP if the loop is direct acting Mx Mx SP SP if the loop is reverse acting Mx 0 if Mx lt 0 Mx Mx if 0 x Mx 1 Mx 1 Gf M gt 1 Eliminating Proportional Integral or Derivative Action It is not always necessary to run a full three mode PID control loop Most loops require only the P
93. predictable setpoint changes or loop offset disturbances Proper use of feedforward will allow us to reduce the integrator gain Reducing integrator gain gives us an even more stable control system Feedforward is very easy to use in the DLO5 loop controller as shown below The bias term has been made available to the user in a special read write location at PID Parameter Table location V 04 Parameter Table location V 04 Loop Calculation kp n Setpoint Error T C lO etpoin rror Term I ie OOO BER if caw 3 ontrol Output Process Variable kd D 8 68 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation To change the bias operating point ladder logic only has to write the desired value to V 04 The PID loop calculation first reads the bias value from V 04 and modifies the value based on the current integrator calculation Then it writes the result back to location V 04 This arrangement creates a sort of transparent bias term All you have to do to implement feed forward control is write the correct value to the bias term at the right time see the following example once at the moment when the new bias operating point is to occur If ladder logic writes the bias NOTE When writing the bias term one must be careful to design ladder logic to write the value only value on every scan the loop s integrator is ef
94. process variable in analog form along with an operator entered setpoint the CPU computes the error The error is used in the algorithm computation to provide corrective action at the control output The function of the control action is based on an output control which is proportional to the instantaneous error value The integral control action reset action provides additional compensation to the control output which causes a change in proportion to the value of the change of error over a period of time The derivative control action rate change adds compensation to the control output which causes a change in proportion to the rate of change of error These three modes are used to provide the desired control action in Proportional P Proportional Integral PI or Proportional Integral Derivative PID control fashion 8 6 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation eT Standard DL05 06 analog input modules are used to interface to field transmitters to obtain the PV These transmitters normally provide a 4 20mA current or an analog voltage of various ranges for the control loop For temperature control thermocouple or RTD can be connected directly to the appropriate module The PID control algorithm residing in the CPU memory receives information from the user program primarily control parameters and setpoints Once the CPU makes the PID calculation the result may be used to directly control
95. r Action Transfer Action Position Forces Bias Control Output Forces Major Loop Output Bumpless 0 Forces SP PV Minor Loop PV Transfer Velocity Forces SP PV iud rs T Bumpless Position Forces Bias Control Output none Transfer II Velocity none none The transfer type can also be selected in a RLL program by setting bit 3 of PID Mode 1 V 00 setting as shown PID Mode 1 Setting V 00 lt Bumpless Transfer I II Select Bit 15 14 131211109 8 7 65 4 3 21 0 SP PV amp Output Format This block allows you to select either Common format or Independent format Common format is the default and is most commonly used With this format both SP PV and Output will have the same data structure Both will have the same number of bits and either bipolar or unipolar If Independent format is selected the data structure selections will be grayed out The reason for this is that they become independently selectable in the SP PV and the Output dialogs Common Data Format Select either Unipolar data format which is positive data only in 12 bit 0 to 4095 15 bit 0 to 32767 or 16 bit 0 to 65535 format or Bipolar data format which ranges from negative to positive 4095 to 4095 or 32767 to 32767 and requires a sign bit Bipolar selection displays input output as magnitude plus sign not two s compliment The bipolar selection is not available whe
96. r format and the bit structure This area is not available and is grayed out if Common format has been chosen see page 8 26 WARNING If the Upper Limit is set to zero the output will never get above zero In effect there will be no control output DL05 Micro PLC User Manual 6th Edition Rev C 8 31 Chapter 8 PID Loop Operation i Enter PID Parameters Another PID setup dialog Tuning is for entering the PID parameters shown as Gain Proportional Gain Reset Integral Gain and Rate Derivative Gain Setup PID Loop 1 Loop 2 sP Pv Output ET 1 Gain Bias Gain Proportional Gain 10 00 Enor Squared Copy Setup Reset Integral Gain 0 00 l Enable sabad Help FreezeBias Units Seconds g C Minutes er g Rate Derivative Gain 0 00 sec Derivative to gain limiting EO E Recall the position and velocity forms of the PID loop equations which were introduced earlier The equations basically show the three components of the PID calculation E Proportional Gain P Integral Gain I and Derivative Gain D The following diagram shows a form of the PID calculation in which the control output is the sum of the proportional gain integral gain and derivative gain With each calculation of the loop each term receives the same error signal value Loop Calculation P Setpoint Error Term a Control Output gt m a D de Process Variable
97. r the On Off waveform is 10 seconds We use a fast timer 0 01 sec tick counting to 1000 ticks 10 seconds The On Off control output is YO The time proportioning program must match the resolution of the PID output 1 part in 1000 to the resolution of the time base of TO also 1 part in 1000 NOTE Some processes change too fast for time proportioning control Consider the speed of your process when you choose this control method Use continuous control for processes that change too fast for time proportioning control Also consider using a solid state switch for a longer switch life instead of a relay TO A fast timer 0 01 sec timebase establishes the primary TMRF TO time interval The constant K1000 sets the preset at 10 K1000 seconds 1 000 ticks The N C enabling contact TO makes the timer self resetting TO is on for one scan each 10 seconds when it resets itself and T1 TO LD At the end of the 10 second period TO turns on and V2005 loads the control output value binary from the loop table V 05 location V2005 BTOR The BTOR instruction changes the number in the accumulator to a real number Dividing the control output by 4 095 converts the DIVR 0 4095 range to 0 1000 which matchs the preset R4 095 time for TMRF TO RTOB This instruction converts the real number back to binary This step prepares the number for conversion to BCD There is n
98. ramp soak settings dialog However you must use ladder logic Resume to control these bits during program execution We Hold recommend using the bit of word instructions Start Ladder logic must set a control bit to a 1 to command the corresponding function When the loop controller reads the ramp soak value it automatically turns off the bit for you Therefore a reset of the bit is not required when the CPU is in Run Mode The example program rung to the right shows how an external Start R S Generator switch XO can turn on and the PD contact uses the leading yg T edge to set the proper control bit to start the ramp soak HTI SET profile This uses the Set Bit of word instruction DLO05 Micro PLC User Manual 6th Edition Rev C 8 59 Chapter 8 PID Loop Operation a _ The normal state for the ramp soak control bits is all zeros Ladder logic must set only one control bit at a time e Start a 0 to 1 transition will start the ramp soak profile The CPU must be in Run Mode and the loop can be in Manual or Auto Mode If the profile is not interrupted by a Hold or Jog command it finishes normally e Hold a 0 to 1 transition will stop the ramp soak profile in its current state and the SP value will be frozen Resume a 0 to 1 transition cause the ramp soak generator to resume operation if it is in the hold state The SP values will resume from their previous value Jog a 0 to 1 transition w
99. range Over damped PV response 5096 here Change the SP to the 60 point of the range The response may take awhile but you will see that there isn t any oscillation This response is not desirable since it takes a long time to correct the error also there is a difference between the SP and the PV 60 here 10 of SP range PV response SP 5096 here PV Increase the Proportional gain for example to 2 0 The control output will be greater and the response time will be quicker The trend should resemble the figure below DL05 Micro PLC User Manual 6th Edition Rev C 8 41 Chapter 8 PID Loop Operation i M 60 here Error 50 here Now return the Proportional gain to the stable response for example 9 7 The error SP PV should be small but not at zero Next add a small amount of Integral gain reset in order for the error to reach zero Begin by using 80 seconds adjust in minutes if necessary The error should get smaller Set the Integral gain to a lower value such as 50 for a different response If there is no response continue to decrease the reset value until the response becomes unstable See the figure below 60 here r3 Z E 10 of SP range SP 5096 hate PV Under damped PV response For discussion let us say that a reset value of 35 made the control output unstable Return the res
100. re by selecting it in the dialog Bit 10 of PID Mode 1 Setting V 00 word can also be set in RLL end of the data range If you have set limits on the control output other than the range i e 0 4095 for a unipolar 12bit loop the bias term still uses the end of range for the stopping point and bias freeze will not work NOTE The freeze bias feature stops the bias term from changing when the control output reaches the 8 34 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation Setup the PID Alarms Although the setup of the PID alarms is optional you surely would not want to operate a process without monitoring it The performance of a process control loop may generally be measured by how closely the process variable matches the setpoint Most process control loops in industry operate continuously and will eventually lose control of the PV due to an error condition Process alarms are vital in early discovery of a loop error condition and can alert plant personnel to manually control a loop or take other measures until the error condition has been repaired The alarm thresholds are fully programmable and each type of alarm may be independently enabled and monitored The following diagram shows the Alarm dialog in the PID setup which simplifies the alarm setup Loop 1 Loop 2 Configure SP PV Output Tuning Alarms eo Monitor Limit Alarms Enable PV Deviation Alarms High High Red 0
101. re in the shower and you turn up the hot water a little the time it takes before the water gets hot is the lag time Dead Time The time it takes for a change in the process to be recognized Composition analyzers and quality control are usually sources of significant dead time Loop Configuring Operator initiated selections which set up and optimize the performance of a control loop The loop calculation function uses the configuration parameters in real time to adjust gains offsets etc Loop Monitoring The function which allows an operator to observe the status and performance of a control loop This is used in conjunction with the loop configuring to optimize the performance of a loop minimize the error term 8 8 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation nS PID Loop Operation The Proportional Integral Derivative PID algorithm is widely used in process control The PID method of control adapts well to electronic solutions whether implemented in analog or digital CPU components The DL05 CPU implements the PID equations digitally by solving the basic equations in software I O modules serve only to convert electronic signals into digital form or vice versa The DLO5 uses two types of PID controls position and velocity These terms usually refer to motion control situations but here we use them in a different sense PID Position Algorithm The control output is calcula
102. s variable The setpoint SP is the input command to the loop controller during closed loop operation Soak Deviation The soak deviation is a measure of the difference between the SP and PV during a soak segment of the Ramp Soak profile when the Ramp Soak generator is active Step Response The behavior of the process variable in response to a step change in the SP in closed loop operation or a step change in the control output in open loop operation Transfer To change from one loop operational mode to another between Manual Auto or Cascade The word transfer probably refers to the transfer of control of the control output or the SP depending on the particular mode change Velocity Algorithm The control output is calculated to represent the rate of change velocity for the PV to become equal to the SP DL05 Micro PLC User Manual 6th Edition Rev C 8 75 Chapter 8 PID Loop Operation Bibliography Fundamentals of Process Control Theory Second Edition Author Paul W Murrill Publisher Instrument Society of America ISBN 1 55617 297 4 MM Application Concepts of Process Control Author Paul W Murrill Publisher Instrument Society of America ISBN 1 55617 080 7 PID Controllers Theory Design and Tuning 2nd Edition Author K Astrom and T Hagglund Publisher Instrument Society of America ISBN 1 55617 516 7 Fundamentals of Temperature Pressure and Flow Measurements Third edition Author Rob
103. se _ x MODE H fel ER MEL SP read fom operator Interface hen converted to IF Ele v Status Bar i EE EE on LD 2s LA T C rom DIP Ein E Options SP1 4F s E Themes H v2600 F2 ral R Color Setup AF n Boss BIN F3 5 di Zoomin 6 Al Zoom Dut pes 7 zs pur F2 Ira Setpoint i H 3002 10 PV from a 4 20mA analog input then convertered tot 4r Shif2 i EH Lo EI ud On PV from 4 20mA sua 13 n 14 5npI v2500 15 Mey BIN 44 16 1 1 Ten pr 31 x I Pci ais gt For Heb press Fl v3003 zl v Changes to this view type OK OnineDSCELOS NURI 00029707680 06 0004003001 The PID View will open and appear over the Ladder View which can be brought into view by clicking on its tab When using the Data View and the PID View together each view can be sized for better use as shown on the facing page G DirectSOFT 5 Programming pid PID View lexi Fie Edt Search View Took PLC Debug Window Hep 8x E EDIT MV ESI TESESuESSE bweDelid Status aa Vak je Mode Info Sman x Datal Ix Ladder View PID ET H e E z ol Loop THe Mox z Element Status Edis A Hi ix igi El 4H 3 F2 F4 H F3 8 3E 7 F2 A Loop Num 1 44 E ra Settings Variables p Aam H PLC Mode Program 4m 10 spo F ROC TI Mode Manual wp zu Id 1 H EJ mp r9 b aeo IE Reef fm n in Quo sl Rate 000 sec hel Autotune F Output Act
104. setpoints If this symptom occurs on loop Manual to Auto Mode changes the loop automatically sets the SP PV if set to bumless transfer mode 1 bumpless transfer feature Check your ladder program to verify it is not writing to the SP location V 02 in the loop table A quick way to do this is to temporarily place an end coil at the beginning of your program then go to PLC Run Mode Q The SP and PV values I enter with DirectSOFT 5 work okay but these values do not work properly when the ladder program writes the data A The PID View in DirectSOFT 5 lets you enter SP PV and Bias values in decimal and displays them in decimal for your convenience For example when the data format is 12 bit unipolar the values range from 0 to 4095 However the loop table actually requires these to be in hex so DirectSOFT 5 converts them for you Your ladder program must convert constant values from their BCD format when entered as Kxxxx to binary with the BIN instruction or you must enter them in the constant field Kxxxx as the hex equivalent of the decimal value D Q The loop seems unstable and impossible to tune no matter what gains I use A Check the following for possible causes The loop sample time is set too long Refer to the section near the front of this chapter on selecting the loop update time The gains are too high Start out by reducing the derivative gain to zero Then reduce the integral gain by increasing the integral
105. sity the ratio of two streams etc Also known as the actual value Setpoint This is the target for the process variable When all conditions of the process are correct the process variable will equal the setpoint Control Output The result of the loop calculation which becomes a command for the process such as the heater level in an oven This is sometimes referred to as control variable Error Term The algebraic difference between the process variable and the setpoint This is the control loop error and is equal to zero when the process variable is equal to the setpoint desired value A well behaved control loop is able to maintain a small error term magnitude Manipulated Variable This is what is used to effect the controlled variable For example the fuel used in a furnace might be manipulated in order to control the temperature Disturbance Something in the system that changes such that corrective action is required For instance when controlling a flow and the upstream pressure drops the control valve must open wider in order to keep flow constant The drop in upstream pressure is the disturbance Final Control Element This is the physical device used to control the manipulated variable Valves are probably the most widely used final control element Lag Time The time it takes for the process to respond to a change in manipulated variable This is also known as the capacitance of the system When you
106. t N Mode Monitoring Cascade Manual Cascade Manual Automatic Automatic DL05 Micro PLC User Manual 6th Edition Rev C 8 51 Chapter 8 PID Loop Operation ee Operator Panel Control of PID Modes Since the modes Manual Auto and Cascade are the most fundamental and important PID loop controls you may want to hard wire mode control switches to an operator s panel Most applications will need only Manual and Auto selections Cascade is used in special applications Remember that mode controls are really mode request bits and the actual loop mode is indicated elsewhere The following figure shows an operator s panel using momentary push buttons to request PID mode changes The panel s mode indicators do not connect to the switches but interface to the corresponding data locations Operator s Panel i i Manual Pus Auto O Mode Monitorin 1 1 Cascade 9 Y PID Mode 1 Setting V 00 T Loop Mode and Alarm Status V 06 Mode Request Bit 1514131211109 87 654 32 1 0 Bit 1514131211109 87 654 32 1 0 E PLC Modes Effect on Loop Modes If you have selected the option for the loops to follow the PLC mode the PLC modes Program Run interact with the loops as a group The following summarizes this interaction When the PLC is in Program Mode all loops are placed in Manual Mode an
107. tarts step change output m 10 During Auto Tune the controller output reached the full scale positive limit Auto Tune stopped and the Auto Tune Error bit in the Alarm word bit turned on When PV change is under 296 output is changed at 2096 Open Loop Auto Tune Cycle Wave Step PV 96 Tangent Rr Slope SP Process Wave A 0 0 0 0 0 a 000007 fe SEE Base Line LrRr i el PAE sec ag Time sec YA Step Change A m 10 Output Value i Auto Tune Cycle PID Cycle zd ra Bir PID Cycle Auto Tune Start Auto Ture End DL05 Micro PLC User Manual 6th Edition Rev C 8 45 Chapter 8 PID Loop Operation a When the loop tuning observations are complete the loop controller computes Rr maximum slope in sec and Lr dead time in sec The auto tune function computes the gains according to the Zeigler Nichols equations shown below PID Tuning PI Tuning P 1 2 Am LrRr P 0 9 Am LrRr 2 0 Lr 3 33 Lr D 0 5 Lr D 0 Sample Rate 0 056 Lr Sample Rate 0 12 Lr Am Output step change 10 0 1 20 0 2 We highly recommend using DirectSOFT 5 for the auto tuning interface The duration of each auto tuning cycle will depend on the mass of the process A slowly changing PV will result in a longer auto tune cycle time When the auto tuning is complete the proportional integral and der
108. ted so it responds to the displacement position of che PV from the SP error term PID Velocity Algorithm The control output is calculated to represent the rate of change velocity for the PV to become equal to the SP Position Form of the PID Equation Referring to the control output equation on page 8 6 the DL05 CPU approximates the output M t using a discrete position form of the PID equation Let Ts Sample rate Kc Proportional gain Ki Kc Ts Ti Coefficient of integral term Kr Kc Td Ts Coefficient of derivative term Ti Reset or integral time Td Derivative time or rate SP Setpoint PV Process variable at nth sample en SP PV Error at n sample M Value to which the controller output has been initialized Then M Control output at nt sample M Ke x e Ki ej Kr e e M isl This form of the PID equation is referred to as the position form since the actual actuator position is computed The velocity form of the PID equation computes the change in actuator position The CPU modifies the standard equation slightly to use the derivative of the process variable instead of the error as follows M Ke x e Ki gt e Kr PV PV M is These two forms are equivalent unless the setpoint is changed In the original equation a large step change in the setpoint will cause a correspondingly large change in the error resulting in a bump to the process due to
109. ter 8 PID Loop Operation NOTE You may elect to leave the tuning dialog blank and enter the tuning parameters in the DirectSOFT 5 PID View Derivative Gain Limiting The derivative gain rate has an optional gain limiting feature This is provided because the derivative gain reacts badly to PV signal noise or other causes of sudden PV fluctuations The function of the gain limiting is shown in the diagram below Loop Calculation P Proportional Control Output Setpoint Error Term Integral D z Derivative Process Variable Derivative Lo S gain limited Loop Table PID Mode 1 Setting V 00 V 25 00XX Derivative Gain Limit Bit 15 14 13 12 1 109876543210 A Derivative gain limit select The gain limit can be particularly useful during loop tuning Most loops can tolerate only a little derivative gain without going into uncontrolled oscillations If this option is checked a unit of 0 to 20 for Limit must also be entered NOTE When first configuring a loop it s best to use the standard error term until after the loop is tuned Once the loop is tuned you will be able to tell if these functions will enhance control The Error Squared and or Enable Deadband can be selected later in the PID setup Also values are not required to be entered in the Tunin
110. the Process Variable PV When the car is on a level highway the speed is maintained at 70 mph thus no error since the error would be SP PV 0 When the car goes up the hill the speed sensor detected a slow down of the car SP PV error The proportional gain would cause the output of the speed controller to bring the car back to the setpoint of 70 mph This would be the Controlled Output Integral this term is often referred to as Reset action It provides additional compensation to the control output which causes a change in proportion to the value of the error over a period of time In other words the reset term is the integral sum of the error values over a period of time Derivative this term is referred to as rate The Rate action adds compensation to the control output which causes a change in proportion to the rate of change of error Its job is to anticipate the probable growth of the error and generate a contribution to the output in advance DL05 Micro PLC User Manual 6th Edition Rev C 8 5 Chapter 8 PID Loop Operation Bj MM Introducing DLO5 PID Control The DLO5 is capable of controlling a process variable such as those already mentioned As previously mentioned the control of a variable such as temperature at a given level setpoint as long as there are no disturbances cold water in the process The DLO5 PLC has the ability to directly accept signals from ele
111. the Slot number F and the Channel Process Variable Address Y2003 Square root Enable I i Filter E Slot F l Factor t LN Merron Channel hn 2 po f 1 0 Module number will automatically appear Since there is only one option slot in the DLO5 the letter F represents the default option slot Also select the analog input channel of your choice The second choice is V Memory When this is selected the V memory address from where the PV is transferred from must be specified Whichever method of auto transfer is used it is recommended to check the Enable Filter Factor a low pass filter and specify the coefficient Output Address 2005 Range Upper Limit 0 4035 Lower Limit 0 0 Slot F m Channel Process Variable Address Y2003 Iw Auto Transfer From l Square root C VOModde Locator T Fiter Memon TAO E no You should also select the analog output for the control output to be transferred to This is done in the PID setup Output dialog shown here The block of information in this dialog is grayed out until the box next to Auto transfer to I O module is checked Once checked the slot number defaults to F like the input and the analog output channel can be selected NOTE To make changes to any loop table parameters the PID loop must be in Manual mode and the PLC must be stopped If you have selected to operate
112. time n 1 The velocity equation is given by AM M M 4 AM Kc x ep Kix PV 2 x PV PV 2 8 12 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation MM MH Bumpless Transfer The DL05 loop controller provides for bumpless mode changes A bumpless transfer from manual mode to automatic mode is achieved by preventing the control output from changing immediately after the mode change When a loop is switched from Manual mode to Automatic mode the setpoint and Bias are initialized as follows Position PID Algorithm Velocity PID Algorithm SP PV SP PV Mx M Control Output The bumpless transfer feature of the DLO5 is available in two types Bumpless I and Bumpless II see page 8 26 The transfer type is selected when the loop is set up Loop Alarms The DLO05 allows the user to specify alarm conditions that are to be monitored for each loop Alarm conditions are reported to the CPU by setting up the alarms in DirectSOFT 5 using the PID setup alarm dialog when the loop is setup The alarm features for each loop are PV Limit Specify up to four PV alarm points D High High PV rises above the programmed High High Alarm Limit High PV rises above the programmed High Alarm Limit Low PV fails below the Low Alarm Limit Low Low PV fails below the Low Low Limit PV Deviation Alarm Specify an alarm for High and Low PV deviation from the setpoint
113. time value and the proportional gain if necessary There is too much transfer lag in your process This means the PV reacts sluggishly to control output changes There may be too much distance between actuator and PV sensor or the actuator may be weak in its ability to transfer energy into the process There may be a process disturbance that is over powering the loop Make sure the PV is relatively steady when the SP is not changing DL05 Micro PLC User Manual 6th Edition Rev C 8 73 Chapter 8 PID Loop Operation Glossary of PID Loop Terminology Automatic Mode An operational mode of a loop in which it makes PID calculations and updates the loop s control output Bias Freeze A method of preserving the bias value operating point for a control output by inhibiting the integrator when the output goes out of range The benefit is a faster loop recovery Bias Term In the position form of the PID equation it is the sum of the integrator and the initial control output value Bumpless Transfer A method of changing the operation mode of a loop while avoiding the usual sudden change in control output level This consequence is avoided by artificially making the SP and PV equal or the bias term and control output equal at the moment of mode change Cascaded Loops A cascaded loop receives its setpoint from the output of another loop Cascaded loops have a major minor relationship and work together to ultimately control one
114. tor dedicated to that loop This feature generates SP values that follow a profile To use the Ramp Soak feature you must program a separate table of 32 words with appropriate values A DirectSOFT 5 dialog box makes this easy to do In the loop table the Ramp Soak Table Pointer at Addr 34 must point to the start of the ramp soak data for that loop This may be anywhere in user memory and does not have to adjoin to the Loop Parameter table as shown to the left Each R S table requires 32 words regardless of the number of segments programmed The ramp soak table parameters are defined in the table below Further details are in the section on Ramp Soak Operation in chapter 8 Addr Description Offset Description Ramp End SP Value 20 V Memory Space Ramp End SP Value Ramp Slope 21 Ramp Slope Soak Duration 22 Soak Duration Soak PV Deviation 23 Soak PV Deviation User Data 2000 o NM V2037 32 words A Ramp End SP Value 24 Ramp End SP Value LOOP 2 32 words Ramp Slope 25 Ramp Slope Soak Duration 26 Soak Duration Soak PV Deviation 27 N2000 Ramp Soak 1 amp Soak PV Deviation 32 words Ramp End SP Value 30 Ramp End SP Value Ramp Slope 31 Ramp Slope Soak Duration 32 Soak Duration Soak PV Deviation 33 V2034 3000 Octal Soak PV Deviation Pointer to R S table Pointer to R S tab
115. tually cross one of the outer alarm thresholds named High high alarm and Low low alarm Their threshold values are programmed using the loop table registers listed above A High high or Low low alarm indicates a serious condition exists and needs the immediate attention of the operator The PV Absolute Value Alarms are reported in the four bits in PID Mode and Alarm Status V 06 the PID Mode and Alarm Status word in the loop table as shown to the right We highly recommend using ladder logic to monitor these bits The bit of word instructions make this High high Alarm ail easy to do Additionally you can monitor PID alarms using M DirectSOFT 5 Low low Alarm PV Deviation Alarms Bit 15 141312111098 765 4 32 1 0 The PV Deviation Alarms monitor the PV deviation with respect to the SP value The deviation alarm has two programmable thresholds and each threshold is applied equally above and below the current SP value In the figure below the smaller deviation alarm is called the Yellow Deviation indicating a cautionary condition for the loop The larger deviation alarm is called the Red Deviation indicating a strong error condition for the loop The threshold values use the loop parameter table locations V 17 and V 20 as shown Red Deviation Alarm Red Yellow Deviation Alarm Yellow Loop Table SP Green V 17 XXXX Yellow Deviation Alarm i V 20 XX
116. ule Output Loop 1 Data Channel 1 PV SP OUT Channel 1 Process 1 Loop 2 Data Channel 2 PV SP OUT Channel 2 Process2 1 Channel 3 Channel 4 Step 5 Wiring and Installation After selection and procurement of all loop components and I O module s you can perform the wiring and installation Refer to the wiring guidelines in Chapter 2 of this manual and to the D0 OPTIONS M manual The most common wiring errors when installing PID loop controls are Reversing the polarity of sensor or actuator wiring connections ncorrect signal ground connections between loop components Step 6 Loop Parameters After wiring and installation choose the loop setup parameters The easiest method for programming the loop tables is using DirectSOFT 5 5 0 or later This software provides PID Setup using dialog boxes to simplify the task Note It is important to understand the meaning of all loop parameters mentioned in this chapter before choosing values to enter Step 7 Check Open Loop Performance With the sensor and actuator wiring done and loop parameters entered we must manually and carefully check out the new control system using the Manual mode Verify that the PV value from the sensor is correct If it is safe to do so gradually increase the control output up above 0 and see if the PV responds and moves in the correct direction Step 8 Loop Tuning I
117. ure will automatically calculate the sample rate in addition to the PID gains 8 40 DL05 Micro PLC User Manual 6th Edition Rev C Chapter 8 PID Loop Operation Manual Tuning Procedure It is not necessary to try to obtain the best values for the P I and D parameters in the PID loop by trial and error Following is a typical procedure for tuning a temperature control loop which you may use to tune your loop Monitor the values of SP PV and CV with a loop trending instrument or use the PID View feature in DirectSOFT 5 see page 8 48 h NOTE We recommend using the PID View Tuning and Trending window to select manual for the vertical scale feature for both SP PV area and Bias Control Output areas The auto scaling feature would otherwise change the vertical scale on the process parameters and add confusion to the loop tuning process Adjust the gains so the Proportional Gain 0 5 or 1 0 1 0 is a good value based on experience Integral Gain 9999 this basically eliminates reset and Derivative Gain 0000 This disables the integrator and derivative terms and provides some proportional gain Check the bias value in the PID View and set it to zero Set the SP to a value equal to 50 of the full range Now select Auto Mode If the loop will not stay in Auto Mode check the troubleshooting tips at the end of this chapter Allow the PV to stabilize around the 50 point of the range 60 here 10 of SP
118. ut B Setpoint Loop B Setpoint Loop A Output A Process A Process B Calculation Calculation secondary primary Minor Major Loop Loop PV Process A PV Process B process variable at the system level The cascaded loops behave as one loop but with improved performance over the previous single loop solution One of the benefits to cascade control can be seen by examining its response to external disturbances Remember the minor loop is faster acting than the major loop Therefore if a DL05 Micro PLC User Manual 6th Edition Rev C 8 63 Chapter 8 PID Loop Operation Cascaded Loops in the DL05 CPU In the use of the term cascaded loops we must make an important distinction Only the minor loop will actually be in the Cascade Mode In normal operation the major loop must be in Auto Mode If you have more than two loops cascaded together the outer most major loop must be in Auto Mode during normal operation and all inner loops in Cascade Mode NOTE Technically both major and minor loops are cascaded in strict process control terminology Unfortunately we are unable to retain this convention when controlling loop modes Remember that all minor loops will be in Cascade Mode and only the outer most major loop will be in Auto Mode You can cascade together as many loops as necessary on the DL05 and you may have multiple groups of cascaded loops For proper oper

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