Quote:
Originally Posted by FrankJ
... I generally found the process measurement value to be noisy enough to make the derivative term useless...
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Depending on the kind of noise (amplitude and frequency components relative to the control cycle), sometimes a small filter on the input can allow the derivative term to still improve the response time. Not as good as eliminating the noise, but might be better than not using the derivative at all.
Quote:
Originally Posted by FrankJ
...it is a lot easier to find PID controllers rather than PI controllers.
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It is about as easy to make a generic PID controller as PI, and then the user has the option of setting the D term to zero, resulting in a PI controller. So, from a marketing point of view, it makes sense to make the PID controller whether most people use the D or not.