Re: [CAD_CAM_EDM_DRO] Re: Re: Re: GeckoDrives.. Can anyone Help?

I'm not an expert, but have built one of each.

The problem is not really the amp, but the design of the system. The
Geckodrives (which I love) are fed step and direction (position)
information. Even if you feed tach back to the
amp, your computer is still controlling position.

To do velocity, the computer would need the tach feedback, and then
output a velocity
command, which would be interperted by an amp that understood
_velocity_.

As long as a system is constrained to using the step and direction bits
for output,
you're probably limited to position control. (Unless you do something
like PWM I guess).

murph

Tom Eldredge wrote:

>
> Jon Elson,
>
> Thanks for clarifying yesterday's email. So the tach is the real missing
> link.
>
> Understanding what you do about this situation and the Gecko drives, do you
> think Mariss could add tach feedback to his amps also? If he, say for
> example, added DC tach feedback from the motor to the G340 amp, would we
> then be able to say we have true velocity control? Would this configuration
> completely bridge the gap?
>
> It seems I remember Mariss saying something about making a velocity drive
> sometime in the future. I wonder if this is what he is thinking of. I am
> going to do a search on his emails to see if my memory is showing its
> weakness again.
>
> I was under the impression that the multiplier on the G340 drives only
> allowed one to use the higher count encoders found in retrofit situations.
> It does not do anything for us in the way of smoothing out the motion does
> it?
>
> Thanks again for your valuable help, Jon.
>
> Tom Eldredge
> ----- Original Message -----
> From: Jon Elson <jmelson@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Tuesday, March 20, 2001 2:36 PM
> Subject: Re: [CAD_CAM_EDM_DRO] Re: Re: Re: GeckoDrives.. Can anyone Help?
>
> >
> >
> > Tom Eldredge wrote:
> >
> > > As I understand it, the Gecko drives partially bridge the gap between
> > > step/dir and servos, but do not give the velocity control that true
> servo
> > > systems provide.
> > >
> > > It seems as though in order to get true velocity control, the encoder
> > > feedback needs to reach the control earlier in the processing stage.
> >
> > No, the encoder is not enough. As I gave in an example yesterday,
> > I have 20000 encoder counts/inch, and can go down to .01 IPM
> > before the stick-slip friction becomes dominant. This is only 3.3 encoder
> > counts/second. That is not enough to give reliable velocity info to
> > whatever is controlling the motors. In my case, a DC tach gives feedback
> > to the servo amp up to a several KHz bandwidth no matter what the
> > velocity is. The servo amp then generates armature current to keep the
> > velocity constant.
> >
> > > Are people using 320/340 gecko drives with EMC and DeskNCrt? How are
> they
> > > typically hooking up the encoders--to the drives and the control with
> the
> > > camtronics board?
> >
> > I believe people are using the Gecko 320/340 with the encoders feeding
> > back ONLY to the Gecko drives, not an encoder counter for the CPU.
> >
> > > Is DeskNCrt just as good or complete as the Lunix based EMC?
> >
> > I can't answer that. I can make some comments about the interrupt
> response
> > time under RT-Linux, with EMC. I get less than 10 uS interrupt response
> jitter
> > on a 266 MHz Pentium-II. The CPU time required to complete 4 axes of
> > servo computations is about 50 uS on that CPU, indicating that you could
> > run a servo update rate of 10 KHz without killing the CPU.
> >
> > I understand that DeskNCrt uses the standard character screen, which is
> > a limitation. EMC uses a 'GUI' interface to the screen, that is
> user-configurable,
> > in color, has defined fields for various parts of the display. Ie. the
> position
> > display is in larger characters, and change color when the axis is at the
> limit
> > position or has been homed. There are other blocks that display state
> with
> > color or reversed video. There are slider bars to display feedrate
> override.
> > You can select files to run from a 'browser' type menu. There is an
> > additional script that can be loaded into the GUI to preview the toolpath.
> > This avoids the situation where a program contains an error that will stop
> > the run in the middle. Since ALL interpreter functions are actually used
> during
> > the preview, if it runs in preview, it WILL run when motion is enabled.
> > (This is called the 'backplot' option.)
> > The advantage of being able to hop into the text editor, get files through
> the
> > network, use a calculator program, etc. while EMC is running is a great
> advantage.
> >
> > Jon
> >
> >
> >
> >
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>
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