Re: [Emc-users] Any PID tuning tips

[Eric H. J. <ejo...@aa...>]

Ray,

> You need to think of P a +- value around the setpoint.  The 
> larger the P the closer to that point it gets.  What you did 
> for setting P to minimize errors is a good start but as soon 
> as you begin to add in D and I you can increase the P.
> 
> I tend to start out raising P until I get some oscillation 
> (ringing) of the axis.  Not a lot of oscillation but it 
> should be noticeable.  Now add in a bit of I and D and you 
> should be able to see the ringing stop.
> Increase P again until the ringing returns and increase I and 
> D.  You should be able to repeat this several times.  Each 
> time you do you will see more stiffness when you try to 
> rotate the motor by hand.
> 
> You will get to a point where you are not able to make the 
> axis settle down by increasing I or D.  At that point back 
> off P a bit and then twiddle I and D until you see minimal 
> overshoot and a quick return to correct position.

Thanks for the feedback. For the most part that is pretty close to what I
was doing. In fact, if you happen to be familiar with Galil motion systems,
it has a tuning facility where one of the functions is an auto tune which
operates very similarly to what you describe, and what I was manually trying
to emulate here.

What I was finding was that after I reached a P value of about 325, the
error started getting worse rather than better. Of course I may well have P
and D well out of synch in moving each of them up. However, at one point I
did crank P up to 1000, and still had rather soft response on the drive.

Which is why I thought perhaps it was one or more of the non-PID parameters,
like acceleration or output_scale, which I had wrong. Pushing up
output_scale had much more effect on the stiffness of the motor than
cranking up P.

I also saw a reference in the Integrator's Handbook regarding the use of ff0
and ff1. I tried ff0 through the range of 4-7, but did not see any real
noticeable difference, at least in that range.

> If this is EMC2 you can start up halscope and watch some 
> signals.  I've found that connecting axis position to a ddt 
> block will let you watch change of position rather than position.

I wrote a wrapper around halcmd and with it, a little PID tuning utility.
Mainly the ability to manipulate P, I, D, ff0, ff1 and bias, monitor various
pins, and issue MDI commands. Monitoring change in position is a good point.

I will be taking another shot at it tomorrow.

Regards,
Eric