Quote:
Originally Posted by drakesword
I have noticed that with position control that PID works correctly. When using speed on the other hand it isn't quite where it should be. Maybe the PID in the speed loop isnt looking at speed properly . . . meh.
Our solution was to tune the loop to be "quiet" as in no chatter on the drive train at low and high speeds. In the robot code once output stabilized we calculated the error and just increased our set-point by the same percentage.
Eventually we got the output we wanted . . . eventually.
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Assuming you've only implemented P, I is effectively 'automatic reset'.
What you're doing by shifting the set point is a 'manual reset'.
With just 2 speeds it's not so bad, but if you have a lot of speeds you'd have a heck of a time making the changes because...if I'm right...the algorithm doesn't account for the way the motors actually respond to the change in output. It basically thinks the response is more linear than it really is. I expect this to a small extent...but to this extent it makes me wonder if the feedback is really correct...especially in the case of the older Jaguar hardware.
However, that is what the I does. It's an 'automatic reset'. Basically the I recognizes that when the loop repeats it didn't get to it's desired output the last time(s) and compensates by 'pushing' the output. The amount of 'pushing' is a combination of the gain (which they apparently let you set) and is scaled by time (repetition rate). However, I don't see as they specify the repetition rate for this code and usually it's something that is measured, not just programmed.
The I has a draw back in the sense that if the I 'pushes' too hard it overshoots the set point. In a drive train, that sort of thing might happen because you had a momentary impact and so the I climbed rapidly but then...suddenly...the resistance is lost and now the I is too high. So I is usually capped. As mentioned earlier in cases like that where the disturbance (the impact) happens often you might find that 'reseting' the I is a good idea because if you let I climb to a maximum it'll stay there for a bit till the last few operations even it out. If you don't reset the I in situations were repeated disturbances of noise (especially at a fundamental frequency) you'll get into a situation where the control appear to become erratic or less responsive until the loop figures out that the integration needs to reset.