PID Tuning and PID AlgorithmsThe odds are against you!

by : Ashish K Arora

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

?This can result in you upgrading acontroller 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 ina world where all manufacturers implement the PID control algorithm withdifferent degrees of functionality.

?This means that it is essential to identifyand understand what controller type you are tuning. The differences that weneed to know, because they will affect the tuning, are:

1.???????????? AlgorithmType: Ideal, Series or Parallel

2.???????????? ProportionalUnits = Gain or Percentage

3.???????????? IntegralUnits: seconds per repeat, repeats per secondArticle Search,minutes per repeat or repeats perminute

?Derivative Units: seconds or minutes

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

?This means that you must know:

?*????????????? whichof the 3 Algorithms your tuning rules are designed for

*????????????? andwhich particular units it gives you the answers in

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

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

?*????????????? Howto find out the exact algorithm your controller uses.

*????????????? Howto convert your calculated P I and D constants to your controller's specificunits.

??? How toconvert your constants to the units that your controller uses.