This programming example shows a method for programming a Proportional Integral Derivative (PID) controller on a S7 PLC. The example is already a fully functioning program, needing only for the user to tie the actual inputs and outputs to appropriate variables to be a working
controller. This program is suitable for simple PID applications.
For complex PID applications, Siemens offers the SIMATIC S7 Standard Control software package, which offers numerous features that this applications tip lacks. These features include alarming, scaling, deadband control, feed-forward control, range limiting, ramp/soak steps, and an integrated scheduler. The Standard Control package includes a Windows-based configuration tool that greatly simplifies configuring and tuning a PID loop.
To prepare a user to make these programming changes, the text will explain the basics of the PID controller implemented in the sample code. Below is a brief outline for the rest of this document:
1. What does the example program do?
2. Where do you use a PID controller?
3. Auto Mode vs. Manual Mode
4. What does a PID controller do, and how?
5. What are the Sample, Gain, Rate, and Reset?
6. How is the Error figured?
7. How is the Proportional term figured?
8. How is the Integral term figured?
9. How is the Derivative term figured?
10. What if the final Output is too high?
11. What should the user add to make the program work for his system?
12. Adjusting the Reset, Rate, Gain, Sample time and Mode during run-time
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