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

----- Original Message -----
From: "Mariss Freimanis" <mariss92705@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Wednesday, November 24, 2004 1:35 AM
Subject: [CAD_CAM_EDM_DRO] Re: Closed Loop PC Based CNC


>
>
> 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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