RE: [CAD_CAM_EDM_DRO] PWM signal to step/direction signals

You could have an RC servo turn an encoder, from which you would get two
output channels. These in turn could drive an H-bridge which would drive
a bipolar stepper.

Regards,
JJ

Be Kind, Be Careful, Be Yourself

> -----Original Message-----
> From: Howard Bailey [mailto:bogmstr@...]
> Sent: Wednesday, October 09, 2002 2:20 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] PWM signal to step/direction signals
>
>
> Thanks to all who replied both on and off list.
>
>
> > Eyup... that would be a PIC microcontroller running some
> custom microcode.
>
> I came across a limited version of the PICBasic compiler that
> will work for
> the 16F84 and has the handy command Pulsin which measures the
> width of a PWM
> signal. From the examples in a book based on this compiler,
> it doesn't seem
> too difficult to output step and direction signals from the
> PIC. It seems
> like it could work with the following steps: 1 measure the
> pulse, 2 compare
> to last pulse and determine direction, 3 subtract the smaller
> pulse width
> from the larger pulse, 4 set direction bit, 5 loop to send
> step pulses,
> accelerating if needed, 6 repeat.
> >
> > The devil's in the details.
>
> A very welcome reminder.
> >
> > How fast would the stepper have to run?
>
> I honestly don't know yet. I am still in the preliminary hair brained
> scheme phase.
> >
> > If all you want is a low-current, TTL-level step and
> direction signal
> > based on the PWM duty cycle of a servo controller output, it's a
> > straight-forward implementation. Then, you'd take this
> output, and run it
> > into your preferred stepper motor driver (Gecko, et. al.).
>
> Geckodrives were exactly what I had in mind. I was actually
> planning to use
> one to drive a servo so I could have a closed loop system and
> let the Gecko
> deal with the encoder feedback.
> >
> > Developing the software to run on an $8 microcontroller to
> do this is
> > about 40 man-hours worth of work. It's not dissimilar to some
> > motion-control projects I've spun in the past.
>
> I have plenty of time and a willingness to learn.
> >
> > A normal RC servo driver (bar some ringers) runs at a base frequency
> > between 50Hz and 400Hz with a typical pulse width of 0.6 mS
> to about 2.5
> > mS.
> >
> > This pulse width is proportional to some angular rotation
> between 0 and 90
> > degrees (or thereabouts).
>
> Looks like something between 50 and 60 HZ would be good enough.
> >
> > What is the ultimate precision you require for your application?
>
> See hair brained scheme phase above, but +/- .5 deg on the
> output of a gear
> head would probably do. Without the reduction ratio yet, I
> can't calculate
> maximum number of step pulses needed between each 50Hz input
> pulse, but I
> will be aware of the need to keep the step pulses well within the time
> limit.
> >
> > 1 degree is relatively simple to implement. 0.018 degrees
> would be about
> > the best a 20 MHz microcontroller could generate.
> >
> > Does that meet your requirements?
>
> It seems promising and the accuracy should not be a problem
> as long as I can
> keep the step pulses within their alloted time.
> >
> > Kind Regards,
> >
> > David Kott
> >
> Thank you for a comprehensive and informative answer.
>
> Best Regards,
>
> Howard Bailey
>
>
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