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

Hi David, Bailey,

I might be mistaken, but I think we're missing something here. The PWM
signal that can drive a servo motor does NOT have a direct 1:1
relationship to the distance (or velocity) desired. The PWM signal is
changed to control acceleration/deceleration, and to meet load
requirements. A "cheap" or inefficient motor will require more "juice"
(current) to achieve the same goals as an efficient one. Hence, the PWM
signal will be DIFFERENT for these two motors to accomplish the same
task. The steppers move a finite distance for a finite number of steps,
and will rotate at a finite rate for a finite step rate.

Otherwise, yeah, a PIC would be a good way to do it!

My .02 USD.

Alan KM6VV


David Kott wrote:

>
> On Tue, 8 Oct 2002, Howard Bailey wrote:
>
> > Anybody know of a circuit or IC that is capable of taking a PWM signal as
> > used for RC angular servos and outputs step and direction signals with
> > output adjustable for scaling? Basically, I would have a PWM signal for an
> > angular servo and would like to use it to run a stepper through a gear
> > reducer to increase resolution and torque. I couldn't find much on the web
> > on this so any information would be greatly appreciated.
>
> Eyup... that would be a PIC microcontroller running some custom microcode.
>
> The devil's in the details.
>
> How fast would the stepper have to run?
>
> 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.).
>
> 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.
>
> 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).
>
> What is the ultimate precision you require for your application?
>
> 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?
>
> Kind Regards,
>
> David Kott
>
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>