Re: [Emc-users] Y axis following error woes...

[dave <den...@ch...>]

On Mon, 2009-11-30 at 11:58 -0600, Jon Elson wrote:
> Pat Lyons wrote:
> > Yea, I'd say it falls into the <15Hz category.
> >
> > I was thinking about finding a way to dampen the Table's mechanical response
> > by using the gibbs (I'm not sure thats the right name for em, but im
> > reffering to the screws that are used to apply pressure to the dovetails on
> > the ways)
> >   
> (Technically, the gibs are the bars that ride against the inside of the 
> dovetail, not the
> screws that apply the pressure to them.)
> Well, adding lots of friction is not a good approach.  OK, with analog 
> velocity
> servos, you have a bunch of control loops within the servo amp, and then 
> another
> control loop outside it (EMC).  So, the interactions get complicated.  
> If the torque loop
> or the velocity loop in the servo amp are unstable, then EMC certainly 
> can't fix it.
> 
> > Funny you mention FF2 as 0.0002, because i think the highest I have mine
> > (Xaxis) is like 0.0005.  My FF values in all instances are below 1.  I
> > managed to get it to settle a little with the error, but the biggest trouble
> > seems to be the initial error, which spikes-  I would slowly increase D and
> > would end up with much resonance as a result...   I was thinking, this could
> > also be due to an improper backlash measurement I've made (i suspect this
> > because the worst spikes occur when changing direction...
> >   
> I have never liked the way D works on the EMC PID.  It seems like more D 
> should just
> make the response sluggish, but it causes oscillation instead.  I think 
> this is because
> D only looks one sample interval behind.  But, certainly too much D does 
> cause instability.
> With a velocity servo amp doing all the work (meaning loop gain) you 
> don't need a lot
> of P gain.
> 
> If you have backlash compensation turned on, definitely try again with 
> it turned off.
> I don't use backlash comp on my systems, as it just makes movement messy.
> 
> Jon

Jon, 
Any progress on filtering D? Some commercial PID's apparently allow one
to adjust rolloff on D. 
I've never  found D or I as useful as I hope they are. If I can't fix it
with P, FF1 and FF2 then I probably won't be able to fix it. I suspect
the accel and decel spikes cannot really be fixed on systems with
backlash. I've never had a system tight enough to prove that. ;-)

Dave
> 
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