Series 988 Enhanced Software Features
Contents
1. rate Too much energy is introduced and the set point is over shot In most long lag time systems the process value may never settle out to an acceptable error Curve C represents a single control system tuned to minimize overshoot This results in unacceptable heat up rates with the final value taking hours to reach Curve B shows a cascade system that limits the energy introduced into the system time i Two controllers in one allowing an optimal heat up rate with minimal over shoot output 1 out This drawing shows two controllers configured as a cascade system The second controller generates Outer loop Controller Inner loop Controller the internal set point The Series 988 effectively combines both controllers into a single package The primary controller measures the process in the outer or primary loop with input 1 and compares the value to the desired set point The WATLOW Series 988 Enhanced Software Features 3 4 difference between the set point and the process temperature generates an internal percent output value for the second or inner loop controller This value cannot be seen by the operator This internal percent int output generates the internal set point for the secondary or inner loop The secondary loop uses this set point and the value of input 2 typically attached to the heater sheath to control the heat source tem2. will never fully cool down Set to the maximum desired heat source temperature The inner loop can be auto tuned by setting AYE to P 1d While auto tuning the inner loop will be controlled in an ON OFF mode at a set point equal to At SP x cHe Once the inner loop PID B has completed auto tuning we can then auto tune the outer loop PID A The outer loop will generate the set point for the inner loop This is done by comparing the value of the input 1 sensor to the process set point performing the control algorithm by using the values of P _ dA then generating a set point between l2 and He WATLOW Series 988 Enhanced Software Features The outer loop can be auto tuned by setting _AUE to P dA While auto tuning the outer loop will be controlled in an on off mode at a set point equal to AE SP x _ SP I Ina heating application make sure the set point is set at a value above ambient temperature In most cases the auto tuning feature will tune for acceptable control If not you must then manually tune the outer loop Before beginning manual tuning record the values of Pb 1A and E TA generated by the auto tuning feature The auto tune for the outer loop will not generate a value for c A 1A because rate derivative in the outer loop seems to cause instability in most systems Start manual tuning by setting to Q00 Enter the desired process set point and let the system stabili
3. Series 988 Enhanced Software Features Includes 986 987 988 and 989 is FLU ato 4 co 2 Ta a s a L t2 DEV san P ve sores ee 1 8 DIN Microprocessor Based Temperature Process Controller C 96 Reglstered Company Winona Minnesota USA O LOW Watlow Controls 1241 Bundy Boulevard P O Box 5580 Winona Minnesota USA 55987 5580 Phone 507 454 5300 Fax 507 452 4507 http www watlow com 2 Table of Contents Cascade oa ee aan ed eed 3 Differential 22444 i5ocp 0c9 aee224 7 D al PID ccc oc tik ea ee we ewe OF 9 DUDIOK vases tes nee esas aes 11 Rat n gist ary yeast do tate cee he Se Oe 12 Technical Assistance If you encounter a problem with your Watlow controller review all of your configuration information for each step of the setup to verify that your selections are consistent with your applications If the problem persists an Application Engineer can discuss your application with you Before calling please have the complete model number and user s manual available You can get technical support by dialing 507 494 5656 7 a m to 7 p m Central Standard Time or e mail to wintechsupport watlow com We Value Your Feedback Your comments and suggestions on this manual are welcome Please send them to Technical Writer Watlow Controls P O Box 5580 Winona MN 55987 5580 or call 507 454 5300 or fax 507 452 4507 The Series 988 Advanced Software Features manual is cop
4. er way with a 4 to 12mA signal This feature reduces the overall system cost by using a single output to act as two outputs Fluid Sample Container het water temperature iranemater cold wal er Sample Application The system outlined above uses a three way valve for heating and cooling a fluid sample Coils surround the container holding the fluid When the temperature needs to be raised the signal to the valve will be between 12 and 20mA sending hot water through the coils When cooling is required the signal will be between 12 and 4mA sending cold water through the coils WATLOW Series 988 Enhanced Software Features 11 12 Ratio Requirements Ratio control requires enhanced software Two analog inputs are required to monitor the process and at least one output adjusts the controlled part of the process Overview This feature allows the Series 988 to control one process as a ratio of another process This is especially useful in applications that mix two materials whether steam paint or food ingredients Input 2 of the controller measures the part of the process that is either uncontrolled or controlled by another device The part of the process controlled by the 988 will be maintained at a level equal to the quantity measured at input 2 multiplied by the ratio term set by the user Input 1 monitors the controlled part of the process mixed pairt input 1 flow transmitter flow comro
5. eristics This need can arise from a variety of circumstances such as significant set point changes controlling at 250 then controlling at 750 operating a furnace with half a load versus a full load of steel changing the speed of a conveyor through a curing oven or using different materials in an extruder Series 988 controllers can be configured to switch between PID A and PID B based on a process value a set point value or the event input status e At PID 2 Crossover Selection Global Menu select what will cause the switch Crossover Process Value input 1 PIDs will switch based on the crossover process value Crossover Set Point 1 Value PIDs will switch at the crossover set point value PID A used below the crossover point and PID B above no crossover e At _E I Event Input 1 or Event Input 2 select _P 1d PID Ais used when the event input switch is open PID B when the event input switch is closed Note One event input is standard on all units a second event input is an option WATLOW Series 988 Enhanced Software Features 9 Figure 9 10 Test chamber controlled with dual PID sets Teat Chamber Tamper ure Prescure Sample Application A test engineer needs to control the temperature in a test chamber that can be operated at normal atmosphere or under vacuum conditions If he tunes the controller for normal atmospheric conditions when he reaches the portion of his test that re
6. ion measures the heater sheath Input 1 the outer loop measures the lube oil temperature before it leaves the tank The external set point is 125 By setting range high 2 to 250 the set point for the heater sheath will be limited thus extending the lube oil life 6 WATLOW Series 988 Enhanced Software Features Differential Requirements Two inputs and the enhanced software option are required Overview Differential control allows the Series 988 to control one process at a difference to another process Input 2 acts as a remote set point input However the displayed set point indicates the desired difference between input 1 and input 2 The set point that input 1 will use is determined by the equation internal set point input 2 differential set point The lower display shows the differential set point which can be adjusted with the increment up arrow and decrement down arrow keys Please note that while in the differential control mode the internal set point for input 1 cannot be viewed and must be calculated with the equation WATLOW Series 988 Enhanced Software Features 7 Figure 9 8 Freezer display case with differential control ambier ail temperature glasa tempe rature sensar Die play Case Sample Application An application using differential control is to maintain glass temperature of a freezer display case above dew point at a differential to the ambient air temperature A therm
7. lledilow transmiter c pigmen Mixing Tank uncomrolled flow ci unmixed pair met orized valve Figure 9 12 Mixing tank with ratio con trol Sample Application Blue pigment must be added to paint at a ratio of one part per 100 to cre ate a mixed paint of the desired color The uncolored paint flows into the mixer in an uncontrolled stream that is set manually and sensed by input 2 A motorized valve controls the flow of pigment which is monitored by the flow sensor to input 1 The flow rate of the uncolored paint determines the set point for the motorized valve that controls the pigment flow If an operator needs to change the rate of flow for the uncolored paint the set point will shift accordingly to maintain the correct ratio in the mixing tank The application engineer set up this feature in software by choosing ratio as the control parameter in the Global Menu The set point value displayed was then a ratio value He entered 0 01 to maintain an input l input 2 ratio of 1 100 WATLOW Series 988 Enhanced Software Features
8. ocouple at input 2 senses the outside air temper ature and adjusts the internal set point to maintain the glass temperature 10 degrees higher Substituting values we have glass temperature ambient 10 In this application the system uses two type J thermocouples one to sense glass temperature input 1 and one to sense the ambient air tem perature input 2 To configure the controller first enable input 2 set _ fre to ____J To enable the differential control algorithm set the control prompt TAEL in the Global Menu to differential gd 1 FF Press the DISPLAY key The lower display will read O indicating no differential between input 1 and input 2 Adjust the set point to 10 The internal set point for input 1 is now equal to the input 2 value plus 10 which will maintain the boiler water temper ature 10 degrees higher than the outside air temperature 8 WATLOW Series 988 Enhanced Software Features Dual PID Sets Requirements The Series 988 controller needs the enhanced software option to use dual PID sets Overview Standard software units have a single set of PID parameters Units with enhanced software can use two independent sets of heat cool PID para meters PID A and PID B P 1d To enable dual PID enter the Global Menu and set the algorithm prompt to dual PID P d This second set of PID parameters enables the controller to switch between two sets of PIDs to compensate for changes in the system charact
9. perature Algorithm The following formulas show how the primary control sends a set point based on input 2 range high and range low values to the secondary con trol The secondary control uses this set point SP int to generate a percent output out to the heater 1 jnt PID Set A In1 SP 2 SPint CH2 rL2 jnt rL2 3 out PID Set B In2 SPintl The critical parameters are the range settings for input 2 of the inner loop controller The range high value rH2 is the maximum allowed set point for the secondary or inner loop The range low value rL2 is the mini mum allowed set point In a system controlling a heater this would be the maximum and minimum desired sheath temperatures of the heater Typically the range low term is set below the ambient temperature Otherwise the system could never fully cool down Setup When tuning a cascade system the inner loop must be tuned first Ina heating system the inner loop is comprised of the output device and the input 2 sensor which usually measures the heater sheath temperature The output device controls a power switching device which in turn switches the heater The set point for the inner loop is generated by the outer loop and will have a range between range low 2 and range high 2 CH2 Before tuning the inner loop you must make sure _ rt and _ He are set properly Set the value of slightly lower than the ambient tem perature otherwise the system
10. quires a vacuum he must stop the test and enter new PID parameters to maintain stable temperatures The system characteristics are so very different that one set of PIDs will not give satis factory results under both normal and vacuum conditions The Series 988 solves this problem with the dual PID option Auto tuning PID A under normal atmospheric conditions then auto tuning PID B under vacuum conditions establishes PID values for two sets of system characteristics A pressure switch connected to the event input tells the controller when to switch between PID A and PID B eliminating the need to change PID values manually 10 WATLOW Series 988 Enhanced Software Features NOTE Duplex applications require a special valve Figure 9 11 Fluid sample container with duplex control Duplex Requirements The duplex control feature requires enhanced software and a process out put Overview Certain systems require that a single process output control both heating and cooling outputs A Series 988 controller configured with enhanced software and a process output can function as two separate outputs With a 4 to 20mA output the heating output will operate from 12 to 20mA 0 to 100 percent and the cooling output will operate from 12 to 4mA 0 to 100 percent In some cases this type of output is required by the device that the 988 controls such as a three way valve that opens one way with a 12 to 20mA signal and opens the oth
11. yrighted by Watlow Winona Inc October 1999 with all rights reserved 1859 WATLOW Series 988 Enhanced Software Features Figure 9 3a System heat up profiles using three different control methods Figure 9 3b The cascade feature allows one Series 988 controller to internalize the func tions of two con trollers Cascade Requirements Cascade control requires enhanced software and two analog inputs input 1 to monitor the primary or outer loop and input 2 to monitor the secondary or inner loop At least one control output is required to control the process Overview Cascade control can handle a difficult process with minimal overshoot while reaching the set point quickly This minimizes damage to system components and allows for oversizing heaters for optimal heat up rates Heater life is also extended by reducing thermal cycling of the heater Systems with long lag times between the energy source heater steam etc and the measured process value cannot be controlled accurately or efficiently with a single control loop because a lot of energy can build up before a response is detected This can cause the system to overshoot the set point which could damage Bolt See a 2 the heater product or heat transfer B medium such as a heat transfer fluid f This graph illustrates a system with j a long lag time Curve A represents a single control system with PID parameters that allow a maximum heat up
12. ze Once the system stabilizes observe the value of in the Display Menu If the value fluctuates make the proportional band setting wider until the value stabilizes Make adjustment in 5 to 10 increments allowing time between adjustments for the system to stabilize Once has stabilized observe percent power in the display loop It should be stable 10 At this point the process temperature should also be stable but will exhibit droop stabilized below set point The droop can be eliminated with reset of integral Start with a setting of 0 01 allow 10 minutes for the process temperature to come up to set point If it has not increase the setting to 0 05 and wait another 10 minutes After this double the reset setting until process value equals the set point If the process becomes unstable the reset value is too large Decrease the setting until the process stabilizes WATLOW Series 988 Enhanced Software Features 5 outer loop thermocouple Ts ae inner loop thermocouple limit sensor Lube Oil Tank Figure 9 6 Lube oil i tank with cascade Sample Application control A Series 988 controller is used to heat lube oil to 125 F with a screw plug style heater To protect the oil from breaking down and maximize its life it is desirable to limit the maximum heater sheath temperature to 250 F The Series 988 is ordered with two thermocouple inputs Input 2 the inner loop in the cascade configurat
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