Re: Digest Number 12

Mike Romine wrote:

> From: Mike Romine <mromine@...>
>
> Jon wrote:
>
> > A dedicated
> > card (currently ISA) with quadrature encoder counters and servo DACs
> > interfaces to the motion hardware.
>
> Who makes this card? Is this the 'servo2go' card I have heard mentioned
> on this list?

Yes, exactly. their URL is : http://www.servotogo.com/ but it didn't work just
now when I tried it.

>
> > I don't know what jerk control is - do you mean backlash compensation?
>
> Jerk control is limiting the rate of change of acceleration. I was not
> thinking of the right thing when I wrote that as jerk control is part of
> the feedback loop (you feedback 3rd derivative of delta position), not
> feed forward. Actually, it sounds like the EMC control system needs
> some work if the PID filter is not working and you are using feed
> forward to compensate (feed forward is an open loop compensator).

Yes, it does. Their PID doesn't keep history, it just uses error from the
last sampling period. Real PID needs you to compute the integral term
from a history of some previous samples, at least 5 or 10. many also use
several previous samples to build a trend for the differential term. These
things usually require a rolling buffer of previous error samples.

Still, I'm cutting metal with accuracy approaching the machine's basic
accuracy, so I have no complaint about the CNC. Tool deflection
is probably my most significant error, now!

Jon