My reading of the OP's post is that he is
not trying to tune the system without a load. He is
observing the effect of the P, I, and D gains in order to confirm that they do what he would expect. And he is questioning what he is observing.
Quote:
We've been testing different PID values using BDC-Comm, initially with motor power disconnected. We're trying out different values of P, I and D, and watching the motor output voltage as we move the arm. Our P and I experimentation gives us sensible results, but D seems almost non-functional. We test by setting P and I to zero, choosing a value for D, and the moving the arm and watching the motor voltage.
BDC-Comm testing with motor connected shows that if I have all three terms set to 0, and stop action set to coast then the arm can easily be moved (obviously). I'd expect that I could then use a D value as a brake, resisting any motion.
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