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Unread 07-02-2014, 17:25
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Re: Arm Position Control

In PID:

-P is the error of the position
-I is the accumulated error of position (steady state error)
-D is the velocity and often opposes the other two terms

A proper D gain will compensate for the velocity. With all of the gains set correctly, there will be a point where the error is so great that the sum of P, I, and D is larger than the motor's max output, so the negative D term will have no effect. As the arm slows down, the D term will compensate for the velocity (inertia) as the arm approaches the target.

When I calibrate PID loops by hand, I usually only end up with a PI controller. I have tried to add D manually with little success. When I calibrated automatically (using the ultimate stability method), I found that the D gains were many times what I was getting by hand, but the system worked beautifully.

On a side note, I autotuned a full PID loop on two VRC robots (relatively high intertia, fast gear ratio, more counterforce than gravity long arms) and found that a single PID gain set was adequate. On a cam driven linkage on our 2012 FRC robot, I needed to schedule the gains because the motion ratios changed significantly over the stroke.
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