Re: [CAD_CAM_EDM_DRO] Re: Closed Loop PC Based CNC

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
>>
>>
>
>
>
>
>
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