PID Algorithm Code

The PID algorithmn in basic code suitable for implementation in Visual Basic or convertable to any programming language.

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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.

block diagram of PID algorithm

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

Err Process input

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 =100Limit 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.