A match made in heaven: The P + I Controller
In the previous lesson we saw how Integral action can overcome Proportional action's deficiencies in the face of Disturbances.
If we put Proportional and Integral Action together, we get the humble PI controller. The Diagram below shows how the algorithm in a PI controller is calculated.
The tricky thing about Integral Action is that it will really screw up your process unless you know exactly how much Integral action to apply.
A good PID Tuning technique will calculate exactly how much Integral to apply for your specific process - but how is the Integral Action adjusted in the first place?
Adjusting the Integral Action
The way to adjust how much Integral Action you have is by adjusting a term called “minutes per repeat”. Not a very intuitive name is it?
So where does this strange name come from? It is a measure of how long it will take for the Integral Action to match the Proportional Action.
In other words, if the output of the proportional box on the diagram above is 20%, the repeat time is the time it will take for the output of the Integral box to get to 20% too.
And the important point to note is that the “bigger” integral action, the quicker it will get this 20% value. That is, it will take fewer minutes to get there, so the “minutes per repeat” value will be smaller.
In other words the smaller the “minutes per repeat” is the bigger the integral action.
To make things a bit more intuitive for hairy arsed engineers like you and me a lot of controllers use an alternative unit of “repeats per minute” which is obviously the inverse of “minutes per repeat”.
The nice thing about “repeats per minute” is that the bigger it is - the bigger the resulting Integral action is.
Derivative Action – predicting the future
OK, so the combination of P and I action seems to cover all the bases and do a pretty good job of controlling our system. That is the reason that PI controllers are the most prevalent. They do the job well enough and keep things simple. Great.
But engineers being engineers are always looking to tweak performance.
They do this in a PID loop by adding the final ingredient: Derivative Action
Tomorrow: Derivative Action: Handle With Care!
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