RE: [CAD_CAM_EDM_DRO] Re: DC motor controller

Les wrote >>. Higher CPR encoders make it easier for the drive to
compensate.



This is a very good point for diy'ers. Many diy'ers look at the output speed
of programs like mach3 together with acceptable accuracy/resolution to
decide how many count they will select for their encoder. But resolution and
accuracy are not necessarily the same.

Many modern servo controllers have a divider to allow you to set one step
input pulse to correspond to several encoder output pulses. This allows you
to use a hi count encoder so the servo drive has very good accuracy and
several counts per step so it can see errors within each step and keep those
error small to much less than one step resolution. The divider then lets you
set the step resolution, and maintain speed with your step resolution being
pretty much your maintained accuracy even with a little over and undershoot
error. (cogging problems excluded in poor servo motors). Try selecting a
pulse count resolution of at least 4 times your step resolution.



Regs Mark



_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Leslie Newell
Sent: 12 November 2006 09:29
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: DC motor controller



Hi John,

The servo drive will have to work to reduce the effect of the cogging.
Modern drives are pretty good and can mask most of it. On my mill, if I
watch the pulleys very closely I can just see the motor speed varying
slightly at certain feed rates. It appears to have no discernible
effect on accuracy or surface finish so I don't worry about it. Higher
CPR encoders make it easier for the drive to compensate.

Les

John Hansford wrote:

> Hi Dave,Ron,
>
> Pictures...
> http://www.flickr <http://www.flickr.com/photos/26361144@N00/>

com/photos/26361144@N00/

>
> I took the CW to mean the nut would probably fly off if it ran
> the other way. I opened it ...Nothing flew apart.
>
> What's the Bad news...How _BAD_ could they be as servos?
>
> I wouldn't have noticed the very slight cogging if I weren't
> trying to find it.
>
>
> John
>

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