Re: [CAD_CAM_EDM_DRO] Re: true closed loop vs. hobby closed loop

Kevin R. Walsh wrote:

>--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
>>The Gecko drives are known to be within +/- 128 counts of commanded
>>position, but you don't know where. With software closed loop, the
>>CNC program DOES know what the error is at any time. EMC allows you
>>to have 2 terms for following error. One is a lower limit at
>>
>>
>standstill,
>
>
>>and can be set VERY small, perhaps 2-5 encoder counts. The other
>>
>>
>limit
>
>
>>is proportional to velocity. This allows you to have a larger
>>
>>
>following
>
>
>>error limit when moving fast, and especially at rapid feed rates,
>>
>>
>while
>
>
>>keeping a small limit at the low cutting feedrates, where it
>>
>>
>matters.
>
>I interpret this to mean that if I use smdromod.o and the Gecko
>drive, that I can set the following error to basically be a MAXIMUM
>of 128 counts. With 2000 ppr, and a .200" lead screw, that's 0.0128"
>of allowable error. Doing profiling I guess I could see that being a
>bit much.
>

Well, without some interface between the Dan Mauch card and the Gecko,
it would probably be unreliable. You can't expect ground to be the same at
the computer and the Gecko drive. An opto-coupler to repeat the encoder
signals would be a very good idea.

You could set the following error to be quite a bit less than the 128
counts,
or set up the sliding scale so it would be low at lower speeds. You
might try
for +/- 10 counts at zero speed, and 25 counts per inch/second of velocity.
But, my experience with Gecko 320s has been that the allowable error
band at idle is a lot larger than it is supposed to be.

>On the Gecko G201/G210 if I were to ground terminal 7 (DISABLE) then
>you should be able to drive the system manually, using the encoder to
>feed the DRO, just as you say, correct?
>
>

Oh, we're talking steppers here, I see. Yes, that is right. I can't
say anything
about loop stability with steppers and G201, and especially G210s, which
have another control loop inside them (the pulse multiplier).

>Sorry for the basic level questions, but I just got EMC working on my
>laptop this weekend (WOO HOO!) and I am now going about the business
>of getting my Bridgeport Series 1 CNC retrofitted with new drives
>(same old motors). Anyone have any luck fitting handle to the old
>Bridgeport CNC machines? I can easily get one on the Z drive. The Y
>drive I couls maybe see how I can modify the belt cover and put a
>handle in. What about the X?
>
>

This is a BOSS? The X leadscrew doesn't turn on a BOSS. The nut, buried
DEEP in the knee is what turns. I can see handwheels on this being almost
impossible! How about flexible shafts?

J