Hi Niel,

If you have integral working in your algorithm, this will take care of it.

As long as a position error exists, the I term will increase in value for e=
ach timestep and increase the PID output to turn on your motor against the =
spring.

In the other direction, without the spring opposing, the I term is 'out of =
the loop', and the proportional term does it all by itself (or maybe not, f=
or a tight seal)

What you are going to see, is P slew the motor until resistance is met, the=
 longer its not at the desired position, output increases, closing against =
the resistance.

I have used this on servo valves with rubber seats, where I always have to =
give the output term an increase, the longer it remains out of position.

Randy

> Date: Tue, 30 Dec 2014 21:37:48 -0500
> From: picdude3@narwani.org
> To: piclist@mit.edu
> Subject: [EE] Asymmetrical motor/servo PID control?
>=20
> Hi all,
>=20
> 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.
>=20
> 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 mot=
or.
>=20
> 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?
>=20
> Cheers,
> -Neil
>=20
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