Re: Closed Loop PC Based CNC

Gary,

No estimate at all. Right now it's just an interesting lab circuit
that will be turned into a product after a lot of development work.

Mariss

--- In CAD_CAM_EDM_DRO@yahoogroups.com, gary <garyclick1@c...> wrote:
> Any estimate for delivery? Test units?
>
> gary
>
> Mariss Freimanis wrote:
>
> >This probably doesn't satisfy your immediate needs but I am
working
> >on a true closed-loop step motor/encoder system that will be able
to
> >use any step motor drive.
> >
> >The "magic" will be in the step/direction controller.
> >
> >Step motors in many ways make ideal CNC motors; they have high
torque
> >at low speeds where work is being done and little torque at high
> >speeds (rapids) where little torque is demanded of the motor. The
> >motor stalls when it falls behind by two ordinal steps. Just short
of
> >that it is delivering 100% of its available torque for that speed.
> >
> >A step motor's "percentage of available torque demanded" is
> >sinusoidally related to the motor's "lag angle". This simply means
> >the motor's position falls behind from where it should be as
torque
> >increases. This "lag angle" can be easily measured with a motor-
> >mounted encoder.
> >
> >The step/direction controller would adaptively react to an
impending
> >overload by "backing up" the motor's speed-torque curve by
decreasing
> >the step pulse rate sent to the drive until a new stasis point is
> >established, accomodating the new load-point.
> >
> >This would not be a problem in 2-D and 3-D vector motion because
the
> >velocity ratios of the 2 or 3 axies would be maintained; only the
> >vector-sum velocity would change.
> >
> >The effect would be an "unstallable" step motor system. It would
> >accelerate at a rate limited only by the mechanicals to speed
limited
> >only by the load or controller parameters. As load would change,
the
> >motors would slow down or speed up as needed to adapt. An
increasing
> >load would ultimately slow the motor down to its holding torque
(zero
> >speed). Past that, it would misposition but the encoder would
allow
> >it to recover position once the overload was relieved.
> >
> >This past week I cobbled together a strange combination of a G320
> >(encoder inputs, summing junction and a PID loop), a G210 (a VCO
and
> >step motor drive) and a G2002 (a motion controller step/direction
> >source). It was enough to demonstrate the feasability of this
idea,
> >feeble and limited as it was.
> >
> >I am coding up the G2002 to move the encoder inputs, summing node
and
> >PID loop to reside within it. This will allow it to control an
> >ordinary step drive (G201) external to it when the driven motor is
> >encoder mounted. The results should be interesting because of the
> >goal.
> >
> >Mariss
> >
> >
> >
> >--- In CAD_CAM_EDM_DRO@yahoogroups.com, "nitewatchman5"
> ><garyclick1@c...> wrote:
> >
> >
> >>I have a Bench Mill currently running with stepper motors in an
open
> >>loop configuration. We are running production parts in fixtures
and
> >>after running several parts to several dozen parts, sizes and
> >>locations start to drift due to missed steps. Slowing traverse
> >>
> >>
> >rates
> >
> >
> >>down and extending longer acc/dec ramps helps but the problem is
> >>
> >>
> >still
> >
> >
> >>there depending upon cutting forces and the shear number of axis
> >>reversal moves. Additionally, slowing the machine to fit the
> >>speed/torque curve of the stepper motors increases the part cycle
> >>
> >>
> >time
> >
> >
> >>and the part cost directly.
> >>
> >>As a solution I want to take this machine to a true closed-loop
> >>
> >>
> >system
> >
> >
> >>using AC Servo Motors. While I have retro-fitted several large
> >>machines (25'+ travel) with Fagor and AB 9-Series controls,
> >>maintaining a PC based control presents some problems.
> >>
> >>A likely scenario would be to use a matched servo drive and motor
> >>combination such as the Yaskawa DR-1/SGM set for each axis. The
> >>encoder feedback from the motor for commutation can be redirected
> >>
> >>
> >to a
> >
> >
> >>Controller board in the PC for position error control. The
> >>
> >>
> >controller
> >
> >
> >>would output a +/-10VDC analog signal to the Servo Amps to control
> >>speed and direction. The calculated following error can then be
> >>intergrated into a PID Loop to control speed, torque and location.
> >>
> >>There are several interesting controllers on the market such as
the
> >>Galil, Delta Tau and Servo to Go. The Galil especially looks very
> >>powerful and is sometimes found on eBay for a reasonable price.
The
> >>big question however is the Front-End software to drive the
> >>controller, does for example EMC support these controllers?
> >>
> >>Advice would be appreciated, practical experience perferred -
> >>theoretical suggestion catalogued. If we can find no other
solution
> >>
> >>
> >we
> >
> >
> >>will refit the machine with a Fagor 8025M control.
> >>
> >>nitewatchman
> >>
> >>
> >
> >
> >
> >
> >
> >Addresses:
> >FAQ: http://www.ktmarketing.com/faq.html
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