Re: Servo Amp designs

I was poking through my reference stack the other night and came
across an interesting digital servo amp design using a PIC microcontroller.
There are a couple of interesting things about it.

It uses a digital encoded on the motor for both position and velocity
information. In a rather slick little trick, it takes the quadrature
output of the encoded and uses a PLD to turn it into a stream of
up or down pulses (with selectable 1x or 4x resolution). These
go directly to the PIC's internal counters.

The PIC uses its builtin PWM hardware to generate the motor drive
signals which, in the app note, go to an integrated H bridge
driver.

Using the PIC for interface as well, it gets driven through a
standard RS232 port using various commands (absolute or relative
moves, velocity, etc). The code incorporates a tunable PID
algorithm, velocity ramps, etc. Using a 16Mhz clock, it can
run a PID calculation every .25 milisecond.

The hardware count is pretty low, maybe 8-10 ICs and a handful of
discrete parts, although, as a trade off, the code is fairly
involved. With a little additional tweaking, it might be
the basis of a very useful servo amp (use a different H-Bridge,
make the PID parameters loadable from the interface, etc).

Then, of course, there is the problem of integrating the interface
with a G code interpreter.

I'm rather interested in building one of these and seeing how
well a run of the mill DC motor with an encoder would function.
I have a number of 12V motors that have reasonable speed and
torque characteristics for small CNC work, although I wouldn't
put them on a Bridgeport.

Given the state of my project queue, that ain't gonna happen
this year.

The design was in the PIC Application Handbook, developed around
'93 if anyone is interested.

If anyone runs across any other public servo amp designs, I'd
like to hear about them.

Paul

--
Paul Amaranth | Rochester MI, USA
Aurora Group, Inc. | Software Development
paul@... | Unix / C / Tcl-Tk