There are many ways to implement the PID algorithm digitally. Two are discussed here. In each case, there will be a section of code (in structured Basic, easily convertible to any other language) that will be executed by the processor every second. (some other scan rate may be used, change the constant 60 to the number of times per minute it is executed.) In each code sample there is an IF statement to execute most of the code if the loop is in the auto mode. If the loop is in manual mode only a few lines are executed in order to allow for bumpless transfer to auto. Also, while the control loop is in manual, the output (variable OutP) will be operator adjustable using the operator interface software.

The code shown here and on the next page can be translated into any computer language. For information about process control programming in Visual Basic and C#, some very good information can be found at HT Services site HERE >>>

Simple PID code.

One method of handling the integration and bumpless transfer to automatic mode is an algorithm that calculates the change in output from one pass to the next using the derivative of the PID algorithm, or: dOut/dt = gain x (dError/dt + ResetRate x Error + Derivative x d2Error/dt2 derivative of output = gain x (derivative of error + reset rate x error + second derivative of error) .

This program is run every second. If the control loop is in manual, the output is adjusted by the operator through the operator interface software. If the control loop is in Automatic, the output is computed by the PID algorithm. Each pass the output is changed by adding the change in output to the previous pass output. That change is the sum of:
* the change in error (Err-ErrLast)
* the error multiplied by the reset rate, and
* the second derivative of the error (Err-2*ErrLast+ErrLastLast) times the derivative.
The total is then multiplied by the gain.

This simple version of the PID controller work well in most cases, and can be tuned by the standard PID tuning methods (some of which are discussed later). It has �Parallel� rather than �Series� reset and derivative, and derivative is applied to the error rather than the input only.

Variables:

Input	Process input
Err	Error, Difference between input and set point
ErrLast	Error from previous pass
ErrLastLast	Error from pass before previous pass
OutP	Output of PID algorithm
Mode	string, set by operator. value is �AUTO� if loop is in automatic
Action	string, value is �DIRECT� if loop is direct acting
Derivative	Derivative value in minutes
Reset	reset rate in repeats per minute
SetP	Set Point, set by operator

Next: A Better Implementation