Neil,

I don't understand what you mean by "with the spring force acting only one
way, the PID
parameters would be different from one side to the next.  Essentially to
stay in a fixed position, I'd need a continuous non-zero speed on the
motor."


Can you please explain more about the setup?

It is a standard technique, which works very well, to begin with a basic
model of your system and predict the control effort needed to achieve the
trajectory you desire, and then apply this pre-programmed control profile
and perform PID only on the deviation of the actual device from the planned
trajectory. This is called feedforward control can allow you to use higher
PID gains for more precise control without the large amount of overshoot
you would get otherwise.

Based on your description, I suspect that this can be done with feedforward
and a simple PI controller (no D term needed). If so, tuning it is almost
trivial (iterative process where you first determine the highest integral
gain you can tolerate with P=3D0, then back off on that and increase P to
gain response speed).

Sean


On Tue, Dec 30, 2014 at 9:37 PM, Neil <picdude3@narwani.org> wrote:

> Hi all,
>
> I'm trying to make what is essentially a servo for a one-off project for
> a friend... I'm driving a window motor (I can control speed
> bidirectionally) with a controller/driver and there's a potentiometer
> attached for feedback.  For now, I'm using an Arduino to get it running,
> but I'll get a custom PCB made for a PIC-based circuit later.  BUT...
> 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
> far off it was from it's target, but when it gets close, the spring load
> is too much for it.  Changing the force so it's a bit more at close to
> target position helps, but I think I'm just going to use PID.  My
> concern is that with the spring force acting only one way, the PID
> parameters would be different from one side to the next.  Essentially to
> 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
> vs. the other?  Or do I empirically determine how much speed to send it
> to counter the spring force and add that value before sending to the
> motor controller?  Of course, this would assume the spring it mostly
> constant force for the full range of movement.  But what if it's not?
>
> Cheers,
> -Neil
>
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