Pid Tuning and Pid Algorithms - the Odds are Against You!

By: Abhinav Sidana

Amazingly there are no industry standards that define how a PID controller should work, the kind of algorithm it uses or what the various adjustable constants are called.



This can result in you upgrading a controller to a new model or vendor, copy the exact P, I and D constants over, and be left with a totally out-of-control process. This is because you live in a world where all manufacturers implement the PID control algorithm with different degrees of functionality.



This means that it is essential to identify and understand what controller type you are tuning. The differences that we need to know, because they will affect the tuning, are:



1. Algorithm Type: Ideal, Series or Parallel



2. Proportional Units = Gain or Percentage



3. Integral Units: seconds per repeat, repeats per second,minutes per repeat or repeats per minute



Derivative Units: seconds or minutes



That means that there are 3 * 2 * 4 * 2 = at least 48 different combinations of algorithms and units that any single controller can implement.What are the odds that the controller you are using uses the same units and algorithms as the fellow who wrote the tuning method you are using? Less than landing Zero on a roulette table!



This means that you must know:



* which of the 3 Algorithms your tuning rules are designed for



* and which particular units it gives you the answers in



It is then essential that you convert the tuning constants so they are compatible with your algorithm and your units.



A good PID Tuning technique will walk you step-by-step through:



* How to find out the exact algorithm your controller uses.



* How to convert your calculated P I and D constants to your controller's specific units.



* How to convert your constants to the units that your controller uses.

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