RE: I-LPT & step/dir servo amps

Jon -

Yeah, as soon as I sent that message, I remembered a key issue. I've read
that the encoder resolution on a servo system should be at least 5-10 times
the desired positional resolution, because of the losses due to of
dithering, hunting, etc. (I think I read that in Tech 80's "Designing with
Motion Handbook"). That roughly agrees with your .000025" encoder
resolution, which I would guess is using 5 TPI screws, 2:1 reduction, and a
1000 cpr encoder wheel (with quadrature, of course).

What if the step/dir frequency was limited to the desired positional
resolution multiplied by the max feedrate, and the step/dir servo amp
handled the closing of the loop using the higher resolution of the encoder?
I imagine you could setup the step/dir servo amp to increment its counter
the equivalent of .0001" for each pulse from the PC. A pulse rate of 80,000
steps/sec from the PC would then equal 8"/sec, or 480 ipm. So, maybe this
way the PC max pulserate and the servo amp's update rate are not so closely
coupled. For example, in 3D contouring, the CNC control is spitting out
point-to-point moves extremely fast. The CNC's max blockrate would most
likely be the speed bottleneck. (I usually hear claims of around 2000
blocks/sec for standard newer CNC controls). To get good speed you then
have to increase your average move size to something like .004" by loosening
the scalloping tolerances in your CAM program. The closed-loop feedback
system in the meantime is doing everything it can to get the actual motor
position to match the commanded position. So what's the difference whether
you close the loop at the servo amp or at the PC?

Sorry if this is a bit long-winded, but it's all work-in-progress up in the
grey matter, and I'm just trying to make sense of it all.

Carlos Guillermo

> -----Original Message-----
> From: Jon Elson [mailto:jmelson@...]
> Sent: Sunday, October 17, 1999 12:42 AM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO] I-LPT & step/dir servo amps
>
>
> From: Jon Elson <jmelson@...>
>
>
>
> Carlos Guillermo wrote:
>
> > What would it take to match the performance of a "standard"
> servo system,
> > using step/direction output? I've looked into this a little,
> and it seems
> > one issue is closing the loop with an equivalent, high-performance PID
> > algorithm. Another issue is resolution - you would need a fairly high
> > frequency step/direction source, I suppose. Would that
> frequency need to be
> > any faster than a good servo loop update rate (for an equivalent machine
> > resolution)?
>
> It certainly could. Assuming a .000025" encoder resolution (I
> have that on
> my Z axis) and a top speed of 120 IPM, that is 80000 (full) steps/sec for
> that one axis alone.
>
> Jon
>
> > Welcome to CAD_CAM_EDM_DRO@...,an unmoderated list for
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