Hi all,

I'm trying to make what is essentially a servo for a one-off project for=20
a friend... I'm driving a window motor (I can control speed=20
bidirectionally) with a controller/driver and there's a potentiometer=20
attached for feedback.  For now, I'm using an Arduino to get it running,=20
but I'll get a custom PCB made for a PIC-based circuit later.  BUT...=20
there's a one-sided force (spring) on the load the motor is moving.

For an initial test I tried setting the motor speed proportional to how=20
far off it was from it's target, but when it gets close, the spring load=20
is too much for it.  Changing the force so it's a bit more at close to=20
target position helps, but I think I'm just going to use PID.  My=20
concern is that with the spring force acting only one way, the PID=20
parameters would be different from one side to the next.  Essentially to=20
stay in a fixed position, I'd need a continuous non-zero speed on the motor=
..

How is this (easily) handled?  Do I do separate cases for one direction=20
vs. the other?  Or do I empirically determine how much speed to send it=20
to counter the spring force and add that value before sending to the=20
motor controller?  Of course, this would assume the spring it mostly=20
constant force for the full range of movement.  But what if it's not?

Cheers,
-Neil

--=20
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