Re: Low cost Servo Controller

George,
I am happy to hear your offer. I have added my old post at the end.

Bertho
====================================

> From: George Potter <gpotter@...>
>
> Bertho,
>
> Could you please send me your earlier post ..... (off the list is fine) .....
>
> If you (and / or anyone else interested) can help define what we want in a
> servo amplifier, I can probably help design one that can be reproduced
> with limited resources (by the ones building it that is).
>
> Regards,
> George Potter

==================================
My old post:
Subject: Servo-PWM-DSP
Date: Fri, 10 Sep 1999 18:52:05 -0400
From: Bertho Boman <boman@...>

I have been thinking about stirring up this issue for a while and then I saw David Howland's nice post about an I-O board so I guess it is time to do it.

A "traditional" servo system has a "controller" that outputs typically +/-10 VDC to the analog servo Amplifier. The servo amp internally monitors motor current (torque) and gets a velocity and acceleration feedback from a
tachometer attached to the motor. Internally in the servo amp, the feedback loops are tweaked for stability and accuracy. The actual position of the motor or table is fed back to the "controller" from an encoder (shaft or linear).

When designing a hobby system, the "controller" is the easy part. A PC with an analog output board and an input card to read the encoders will basically meet our requirements. There are lots of surplus brush DC motors around that
can be attached to a ball screw or other loads. Similarly, attaching a shaft encoder or a linear DRO is straightforward and mostly a question of finding the right surplus one and how to
mechanically mount it.

I see very little discussion in the mail list about using tachs. They are little harder to find but again but that is not a problem and the mounting is a mechanical issue again. But why so little discussion? Further, I have seen
no comments at all about servo amplifier design and tuning. Has everyone found a perfect sources and there are no problems???? Or maybe everyone is using steppers??

After this long introduction, I finally have arrived at the meat of the subject:

A proper servo amp is difficult to design, difficult to tune and requires a tach for best performance.

With today's very fast processors, it is possible, and it is being done, to measure the time between each encoder pulse, calculate the actual speed and use that value in a DSP filter for the servo amp. That removes the need for
the tach. With DSP, the servo tuning can be done in software instead of with screwdrivers.

Now for the final step: Why not go digital all the way? If we used a switching amplifier, class-D or PWM, there would be no need for the analog difficult portion of the servo amp, just a smoothing filter on the out put to keep
the RFI out of the motors. (I forgot about the current sense, but that is not a big deal to add a little A-D to measure it.)

A DSP-PWM servo amp basically would have a small digital control section with a pair of heavy duty output switching devices. This might be reasonable as a hobby project and it would be a lot easier than an analog servo amp.

How does EMC fit into this. It is my impression that it will put out the +/- voltage in servo mode and read the encoders through an I-O card. Is it actually using the DSP calculations internally and if so, what specs are required
for the servo amps since if that case, the internal amplifier feed back loop is not needed?

Comments?

Bertho Boman
Vinland Corporation