Digest Number 19

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In a message dated 5/21/99 7:00:18 PM Hawaiian Standard Time, 
tkulaga@... writes:

<< unless there's software out there to
 take advantage of the encoder feedback.  I haven't seen any, but then
 again, I haven't looked.  :)
 
 -Tom Kulaga >>

Tom,
   Thanks for the reply. I was looking at the Microkinetics Package. It does 
have a provision for encoder feed back. I realize there are two things we are 
looking at. One would be lost steps, the other is location error feed back. 
If I simply put encoders on my Acme lead screws, I could only keep track of 
steps but not error. This is where I was wondering if adding spars would be a 
cheaper and/or easier way of  accurately positioning the work rather than 
ballscrews.  I have come to the conclusion that I am barking up the wrong 
tree and need to get the ballscrews first. Peter

Tom Kulaga

#354

<PTENGIN@...> wrote:
  In looking over information from Mitutoyo, they offer BCD and several 
other output cards for the KC and other DRO systems.  I have one of these 
(KC) systems and was suspecting the output could be used to inteface to a 
CNC. Also, in the MSC catalog, Mitutoyo used to list a linear spar to digital 
output converter box. PeterOne big problem using the output of the linear encoders of the DRO to
control servos is that *any* backlash between the drive motor and the
workpiece can cause some pretty bad instability (for controls
newbies, the instability I'm talking about is a tendancy for the
motor to oscillate around the target position, buzzing like crazy in
the best case, or slamming back and forth in the worst).  There are
ways around it, but none of them are cheap, assuming you are faced
with the normal HSM budget.  And no, even zero-backlash ballscrews
won't be enough to eliminate the backlash unless you're running VERY
light loads at low speeds.  I had once guessed otherwise, but found
out fast that I was wrong.

If you plan to use steppers, well, you're running open-loop already,
but I guess you could use the linear encoders as feedback to correct
for missed steps.  This assumes that you'll be writing a custom
program to run the steppers, unless there's software out there to
take advantage of the encoder feedback.  I haven't seen any, but then
again, I haven't looked.  :)

-Tom Kulaga

Dan Mauch

#362

I would be willing to help build theprinted circuit card. But take a look at
the PIC chip from JR Kerr
They have a PIC-Servo chip that has PID filter and all in a $30 chip.
The web site is www.jrkerr.com  The pic servo may decrease the effort
substanially. I have tried a  single axis version of the chip and it is
fast. The only problem I ran into was that he sells a PIC-Step chip that
allows use of the parallel port and existing step and direction software.
The servo runs very slowly on a 486-33SX. The best I could get was about 200
RPM with a 200 line encoder. A 500 line encoder only yielded 100 RPM
It may have been due to the 486-33SX but by using the ISA bus it would run
and 2000 RPM. When I ran the pic servo using the RS232 port I was able to
get well over 5000 RPMs.
Dan

Show quoted text

-----Original Message-----
From: Tom Kulaga <tkulaga@...>
To: CAD_CAM_EDM_DRO@... <CAD_CAM_EDM_DRO@...>
Date: Friday, May 21, 1999 10:00 PM
Subject: Re: [CAD_CAM_EDM_DRO] Digest Number 19

From: Tom Kulaga <tkulaga@...>

<PTENGIN@...> wrote:
  In looking over information from Mitutoyo, they offer BCD and several
other output cards for the KC and other DRO systems.  I have one of these
(KC) systems and was suspecting the output could be used to inteface to a
CNC. Also, in the MSC catalog, Mitutoyo used to list a linear spar todigital
output converter box. PeterOne big problem using the output of the linear encoders of the DRO to
control servos is that *any* backlash between the drive motor and the
workpiece can cause some pretty bad instability (for controls
newbies, the instability I'm talking about is a tendancy for the
motor to oscillate around the target position, buzzing like crazy in
the best case, or slamming back and forth in the worst).  There are
ways around it, but none of them are cheap, assuming you are faced
with the normal HSM budget.  And no, even zero-backlash ballscrews
won't be enough to eliminate the backlash unless you're running VERY
light loads at low speeds.  I had once guessed otherwise, but found
out fast that I was wrong.

If you plan to use steppers, well, you're running open-loop already,
but I guess you could use the linear encoders as feedback to correct
for missed steps.  This assumes that you'll be writing a custom
program to run the steppers, unless there's software out there to
take advantage of the encoder feedback.  I haven't seen any, but then
again, I haven't looked.  :)

-Tom Kulaga

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Digest Number 19