[apalrd]

There are a couple ways you could do this:

-First off, look at how much friction there is in the arm. For a low-range-of-motion thing like a 180deg arm, I actually recommend a bit of friction. Our 2013 arm had an aluminum shaft pressed into a delrin block as the final bearing. We could have done ball bearings or needle bearings, but the added friction helped the control loops stabilize (and they had more than enough torque to do it, since we also hung with it).

-Second, look at your counterforce. Always counterforce. There are a lot of ways to do it.

-Third, look at your software. A basic PID control will probably work but not be optimal at all positions (As you said). There are a few ways to get around this:

-Gain Scheduling. You would have a no-gravity and peak-gravity gainset (kP,kI,kD) and blend them with the angle. This should work fairly well

-Feed Forward Gravity - You find the motor power to hold the arm at peak gravity (and assume it's 0 at no-gravity) and multiply it by sin or cos of theta, then add it in.

-In general, if you know exactly what you want from a control-loop steady state, you should add it in as a feed forward term. Then the PID only has to handle the slop.


-In the context of an arm, velocity PID is useless. It would not provide the desired response at all.