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Originally Posted by BradAMiller
Maybe that was a little confusing...
I meant that as the actual speed reaches the setpoint speed, the error term goes to zero.
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Much better
:-)
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Suppose there was only a proportional term, then the output value from the PID controller would go towards zero (proportional constant times zero is zero, similar for the other terms).
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It's not similar for the other terms. The I term can be non-zero at the setpoint. In fact, that's one of the purposes for the I term, to provide a non-zero output at the setpoint (if required).
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That's what the feedforward term attempts to fix.
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Yes, feedforward is one way to address that issue.
Another is to use the I term, when controlling speed, like you would use the P term when controlling position.
Yet another is to integrate the output of the PID.