Re: Servo Amp designs

Jon,

It seems like I could rig a tach to the back end of my servo motors if I
wanted , just as long as I got some kind of tach output eh? Then I could
use your servo amps?

How much would a set of 4 servo boards go for? Or the kits? I am
interested.

I'm trying to build my own small machine and need a source for smaller
ballscrews, about 12" travel zero backlash, 3 of them. Any ideas. I'm
trying to do it on the cheap so surplus would be ok. I even have a 36" or
so travel super precision ground ball screw and nut I could trade....

Michael

At 02:27 PM 5/21/99 -0500, you wrote:

>From: Jon Elson <jmelson@...>
>
>
>
>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
>
>
>------------------------------------------------------------------------
>How many communities do you think join ONElist each day?
>http://www.onelist.com
>More than 1,000! Create yours now!
>------------------------------------------------------------------------
>welcome to CAD_CAM_EDM_DRO@..., an unmodulated list for the

discussion of shop built systems in the above catagories.

>
>

--
michael leonard tilse - draemr@...
tujunga.california.usa.terra.sol.milkyway