8 PID Control Function
Contents
1. 9 to be 1 to prevent from detected value seriously over the target value In this process if S3 2 bit8 changes from 0 to 1 it means the auto tune is successful and the optimal parameters are got if S3 2 is always 0 until S3 2 bit7 changes from 1 to 0 it means the auto tune is completed but the parameters are not the best and need to be modified by users l Every adjustment percent of current target value at auto tune process finishing transition stage S3 10 This parameter is effective only when S3 9 is 1 If doing PID control after auto tune small range of overshooting may be occurred It is better to decrease this parameter to control the overshooting But response delay may occur if this value is too small The defaulted value is 100 which means the parameter is not effective The recommended range is 50 80 Cutline Explanation Current target value adjustment percent is 2 3 S3 10 67 the original temperature of the system is 0 C target temperature is 100 C the current target temperature adjustment situation is shown as below Next current target value current target value final target value current target value x 2 3 So the changing sequence of current target is 66 C 88 C 96 C 98 C 99 C 100 C A target value current target value current target value 2 current target value 1 current value of the system l The stay times of the current target value at aut2. tune control bit auto tune finish If auto tune turns to be manual mode and auto tune is not permitted close auto tune control bit Soft components function comments D4000 7 auto tune bit D4002 8 auto tune successful sign MO normal PID control M1 auto tune control M2 enter into PID control after auto tune
3. If the target value is 100 PID operation zone is 10 then the real PID s operation zone is from 90 to 110 Death R egion S3 7 If the detected value changed slightly for a long time and PID control is still in working mode then it belongs to meanless control Via setting the control death region we can overcome this condition See graph below output value next value eurrent value last value time t Suppose we set the death region value to be 10 Then in the above graph the difference is only 2 comparing the current value with the last value It will not do PID control The difference is 13 more than death region 10 comparing the current value with the next value this difference value is larger than control death region value it will do the PID control with 135 8 4 Auto Tune M ode If users do not know how to set the PID parameters they can choose auto tune mode which can find the optimal control parameters sampling time proportion gain Kp integral time Ti differential time T D automatically l Auto tune mode is suitable for these objectives temperature pressure not suitable for liquid level and flow l Users can set the sampling cycle to be 0 at the beginning of the auto tune process then modify the value manually in terms of practical needs after the auto tune process is completed l Before doing auto tune the system should be under the no control steady state Take the temperature for example the det
4. Overshoot It s usually used in heat up control eva Sey a eae fi fi J Positive Movement Along with the i ame fadin E aner eane increase of the measures definite value PY outputvalue MY will also increase Current target value resident Count E TA ae used in cool control Read From FLO trite To FPLC E 8 3 10 Registers and their functions For PID control instruction s relative parameters ID please refer to the below table D S3 2 mode setting bit0 0 Negative 1 Negative bitl bit6 not usable bit7 0 Manual PID 1 auto tune PID bit8 1 auto tune successful flag bit9 bit14 not usable bit15 0 regular mode 1 advanced mode S3 4 Integration time TI oO 32767 100ms 0 is taken as no integral Differential time TD O 32767 10ms 0 is taken as no differential S3 6 PID operation zone OL 32767 PID adjustment band width ee S3 7 control death zone OL 32767 PID value keeps constant in S3 8 PID auto tune cycle full scale AD value 0 3 1 0 varied value S3 9 PID auto tune 0 enable overshoot S3 10 current target value adjustment percent in auto tune finishing transition stage S3 11 current target value resident count in auto tune finishing transition stage S3 12 occupied by PID S3 39 operation s internal process Below is the ID of advanced PID mode setting S542 Ouputuppertinit ave 326703267 Cid ssas ouput lowertinit valve 327670 30767 SSCS 8 3 2 Paramet
5. PID Control Function In this chapter we mainly introduce the applications of PID instructions for XC series PLC basic units including call the instructions set the parameters items to notice sample programs etc 8 1 Brief Introduction of The Functions 8 2 Instruction Formats 8 3 Parameter Setting 8 4 Auto Tune M ode 8 5 Advanced M ode 8 6 A pplication Outlines 8 7 Sample Programs 8 1 Brief Introductions of The Functions PID instruction and auto tune function are added into XC series PLC basic units Version 3 0 and above Via auto tune method users can get the best sampling time and PID parameters and improve the control precision The previous versions can not support PID function on basic units unless they extend analog module or BD cards PID instruction has brought many facilities to the users 1 The output can be data form D and on off quantity Y user can choose them freely when program 2 Via auto tune users can get the best sampling time and PID parameters and improve the control precision 3 User can choose positive or negative movement via software setting The former is used in heating control the later is used in cooling control 4 PID control separates the basic units with the expansions this improves the flexibility of this function 8 2 Instruction Forms 1 Brief Introductions of the Instructions Execute PID control instructions with the data in specified registers PID cont
6. ected temperature should be the same as the environment temperature To enter the auto tune mode please set bit7 of S3 2 to be 1 and turn on PID working condition If bit8 of S3 2 turn to 1 it means the auto tune is successful PID auto tune period value S3 8 Set this value in S3 8 during auto tune This value decides the auto tune performance in a general way set this value to be the AD result corresponding to one standard detected unit The default value is 10 The suggested setting range full scale AD result x 0 3 1 User don t need to change this value However if the system is interfered greatly by outside this value should be increased modestly to avoid wrong judgment for positive or negative movement If this value is too large the PID control period sampling time got from the auto tune process will be too long As the result do not set this value too large 1 1f users have no experience please use the defaulted value 10 set PID sampling time control period to be Oms then start the auto tune PID auto tune overshooting permission setting S3 9 If set 0 overshooting is permitted the system can study the optimal PID parameters all the time But in self study process detected value may be lower or higher than the target value safety factor should be considered here If set 1 overshooting is not permitted For these objectives which have strict safety demand such as pressure vessel set S3
7. ent and TD is the differential time coefficient The result of the operation 1 Analog output MV digital form of u t the default range is 0 4095 2 Digital output Y T MV PID output upper limit Y is the output s activate time within the control cycle T is the control cycle equals to the sampling time PID output upper limit default value is 4095 8 3 Parameters Setting Users can call PID instruction in XCP Pro software directly and set the parameters in the window see graph below for the details please refer to XCPPro user manual Users can also write the parameters into the specified registers by MOV instructions before PID operation PID Instruction Parameter Config x Target Value SY o Measure Value IPY mo Parameter neo Output T0 Mode Config m Farameter Config Common Mode f Advanced Mode Manual C Auto Sampling Time 0 ms Input Filter Constant fa fo Differential Increase 00 50 Proportion Gain KP 0 j E Output Upper Limit Value 4095 _ Integration Time TI fo 100ms i ue a0 z utp owe imit Value T Differential Time 1D 0 10ns Sers tatru Naas fE PID Limit Belt Value 0 5 r Direction Config Death Region 0 i ay x Negative Movement Positive Movement gt Hegative Movement Along with the increase Overshoot Config of the measures definite value FY 3 outputvalue MV will also reduce Enable Overshoot Disable
8. ers Description H M ovement Direction Positive movement the output value MV will increase with the increasing of the detected value PV usually used for cooling control Negative movement the output value MV will decrease with the increasing of the detected value PV usually used for heating control M ode Setting Common Mode The parameter s register zone is from 3 to 3 43 S3 to S3 11 needs to be set by users 3412 to 3 43 12 are occupied by the system users can t use them Advanced Mode The parameter s register zone is from 3 to 3 43 S3 to 3 11 and S3 40 to S3 43 need to be set by users S3 12 to S3 39 are occupied by the system users can t use them Sample Time S3 The system samples the current value according to certain time interval and compare them with the output value This time interval is the sample time T There is no requirement for T during AD output T should be larger than one PLC scan period during port output T value should be chosen among 100 1000 times of PLC scan periods PID Operation Zone S3 6 PID control is entirely opened at the beginning and close to the target value with the highest speed the defaulted value is 4095 when it entered into the PID computation range parameters Kp Ti TD will be effective See graph below output value Be PID operation area target value a iemel ael aein a a a a i a aia abah ei daa eas ae FID open completely time t
9. o tune process finishing transition stage S3 11 This parameter is valid only when S3 9 is 1 If entering into PID control directly after auto tune small range of overshoot may occur It is good for preventing the overshoot if increasing this parameter properly But it will cause response lag if this value is too large The default value is 15 times The recommended range is from 5 to 20 8 50 Advanced M ode Users can set some parameters in advanced mode in order to get the better effect of PID control Enter into the advanced mode please set 3 2 bit 15 to be 1 or set it in the XCP Pro software Input Filter constant It will smooth the sampling value The default value is 0 which means no filter Differential Gain The low pass filtering process will relax the sharp change of the output value The default value is 50 the relaxing effect will be more obviously if increasing this value Users do not need to change it l Upper limit and lower limit value Users can choose the analog output range via setting this value Default value lower limit output 0 Upper limit 4095 8 60 Application Outlines Under the circumstances of continuous output the system whose effect ability will die down with the change of the feedback value can do self study such as temperature or pressure It is not suitable for flux or liquid level Under the condition of overshoot permission the system will get the optimal PID parameters from self stud
10. rol PID 16 bits 32 bits instruction instruction Executing Normally ON normally closed Suitable XC2 XC30 XC5 XCM cites oni Lee AER ARON Hardware V3 0 or above Software V3 0 or above Condition Condition 2 Operands S set the ID Nr of the target value SV 16bits BIN S2 set the ID Nr of the tested value PV 16 bits BIN S3 set the first ID Nr of the control parameters 16 bits BIN the ID Nr of the operation resule MV or output port 16 bits BIN 3 Suitable soft components Operands D10 Functions sr X0 ae PID DO Actions S1 xo a l PID DO D10 D4000 D100 D4000 YO l S3 S3 43 will be occupied by this instruction so please don t use them as the common data registers This instruction executes when each sampling time interval comes To the operation result D the data registers are used to store PID output values the output points are used to output the occupy ratio in the form of ON OFF PID control rules are shown as below r t 0 Be controlled object e t r t c t 1 1 u t Kp e t 1 Ti e t dt TD de t dt 1 2 Here e t is warp r t is the given value c t is the actual output value u t is the control value In function 1 2 Kp is the proportion coefficient Ti is the integration time coeffici
11. y Under the condition of overshoot not allowed the PID parameters got from self study is up to the target value it means that different target value will produce different PID parameters which are not the optimal parameters of the system and for reference only l If the self study is not available users can set the PID parameters according to practical experience Users need to modify the parameters when debugging Below are some experience values of the control system for your reference u Temperature system P 2000 6000 I minutes 3 10 D minutes 0 5 3 u u u Flux system P 4000 10000 I minutes 0 1 1 Pressure system P 3000 7000 I minutes 0 4 3 Liquid level system P 2000 8000 I minutes 1 5 8 70 Program Example PID Control Program is shown below M8000 MOV ID100 D10 Move ID100 content into D10 M1 D4002 7 S convert PID mode to be auto tune at M2 the beginning of auto tune control starts or auto tune finish MO PID DO D10 D4000 YO start PID DO is target value D10 is Ml detected value from D4000 the zone M2 is PID parameters area output PID result via YO M2 1D4002 7 j Cc 8 PID control finish close auto tune PID D4002 8 D4009 KO M1 mode R if auto tune is successful and Wr ae Kl overshoot is permitted close auto
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