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Improved PID code

The method of implementing automatic reset described in using a positive feedback loop was first used with pneumatic analog controllers. It can easily be implemented digitally.

There are several advantages of this algorithm implementation. Most important, it eliminates the problems that cause the output to pull away from a limit inappropriately. It also allows the use of external feedback when required.

Variables:
Input          Process input
InputD         Process input plus derivative                     
InputLast      Process input from last pass, used in deriv. calc.
Err            Error, Difference between input and set point
SetP           Set point                                    
OutPutTemp     Temporary value of output
OutP           Output of PID algorithm
Feedback       Result of lag in positive feedback loop.
Mode           value is ‘AUTO’ if loop is in automatic
Action         value is ‘DIRECT’ if loop is direct acting

The PID emulation code:

 1.           IF Mode = ‘AUTO’ THEN

2.              InputD=Input+(Input-InputLast)*Derivative *60  derivative.

3.              InputLast = Input

4.              Err=SetP-InputD             Error based on reverse action.

5.              IF Action = ‘DIRECT’ THEN Err=0 – Err  Change sign if direct.

6.              ENDIF

7.              OutPutTemp = Err*Gain+Feedback Calculate the gain time the error and add the feedback.

8.              IF OutPutTemp > 100 THEN OutPutTemp =100  Limit output to between

9.              IF OutPutTemp < 0 THEN OutPutTemp =0       0 and 100 percent.

10.          OutP = OutPutTemp                             The final output of the controller.

11.          Feedback=Feedback+(OutP - Feedback)*ResetRate/60

12.       ELSE

13.          InputLast=Input   While loop in manual, stay ready for bumpless switch to Auto.

14.          Feedback=OutP

15.       ENDIF

If external feedback is used, the variable “OutP” in line 11 is replaced with the variable containing the external feedback.