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Originally Posted by KenWittlief
going back more than 2 loops is only useful for the intergral part - if the arm is not moving you keep adding a small part of the error - so the control signal can actually build up over several loops before the motor move enough to eliminate the error
but for the differential part you only want this reading and the last one - the D is like giving the system a swift kick for a loop or two, so it needs to be very responsive to the most recent action of the system.
There is another type of PID control that does keep track of several previous commands and positions- its an adaptive algorythm - one that responsed differently if the mass of the system changes (like if you pick up a ball with your arm) or it the friction of the system changes (like if you move from carpet to smooth surface) and it adjusts the gains of the feedback accordingly. But I think thats really too fancy for what we need.
so far on our drivetrain and our arm motion, we are only using proportional feedback this year - its works well so we dont want to get lost in complexity.
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ah, sorry, i do understand what a derivative is, just didn't think about the fact that previous values wont affect it.