Hi Neil,

What you describe is a textbook case of a problem which integral control
solves. PID or still possibly just PI should work well for your
application, without special gain switching or anything. I would still
recommend a very simple feedforward control input to reduce the burden on
the integrator.

Sean


On Wed, Dec 31, 2014 at 1:36 AM, Neil <picdude3@narwani.org> wrote:

> Meaning that if the motor is say in the center of its travel and I set
> the speed to zero, it won't stay there.  I need to keep some power on it
> to counter the spring pulling it off to one side.  This raised the
> question of whether once I tune P,I (and possibly D), would it work for
> both directions, or would the difference in control input to the motor
> mean tuning differently for each direction.  I don't have access to the
> system again for a bit, but I'll setup a similar test system so I can
> experiment.
>
> Cheers,
> -Neil.
>
>
>
>
>
> On 12/31/2014 12:20 AM, Sean Breheny wrote:
> > 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 t=
o
> > 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 basi=
c
> > model of your system and predict the control effort needed to achieve t=
he
> > trajectory you desire, and then apply this pre-programmed control profi=
le
> > 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 overshoo=
t
> > 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 almo=
st
> > trivial (iterative process where you first determine the highest integr=
al
> > gain you can tolerate with P=3D0, then back off on that and increase P =
to
> > gain response speed).
> >
> > Sean
> >
>
>
--=20
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