Re: Servo Amp designs

paul@... wrote:

> 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.

Ohh, there's the killer, right there. Just like all the other darn
beautiful one-axis chips (Nat Semi LM628/629, HP HCTL1200
etc.) when you need coordinated motion on 2 - n axes, it all falls
apart. Especially for contouring or engraving, where you're following
splines with a change in angle every few thousandths of an inch.
That's the reason why everyone seems to come up with some way
of using one processor to compute the trajectory for ALL the axes.

> 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.

Well, my PWM servo amps aren't exactly public domain, as I want to
retain the rights to make these as a commercial product in the future,
there are about 12 people who have bought my 'micro kits' for them.
That is a PCB and a few components that bould be hard to get in
small quantities, plus all documentation on them. (They do need
DC tachs, and take a +- 10 V velocity command from a DAC on
the CNC control.) It might be a neat project to make an small
board that converts quadrature encoder signals to emulate an analog
DC tach. I think it could be made with about 3 chips and half a
dozen discrete parts. But, it would be hard to make it as clean
as the DC tachs can. I can have the machine move down to a
feed rate of .01"/Minute or so before it starts to exhibit stick/slip
friction. That is about 4 encoder counts per second on my particular
setup, so I don't know how smoothly the system would respond
down there. But, that is not a real common situation, anyway.

Jon