Re: Low cost Servo Controller

Steve Carlisle wrote:

> From: "Steve Carlisle" <quatsein@...>
>
> I have seen a few LM628-629 based 3 axis boards that
> claim linear and circular interpolation. Computer Source
> is one that comes to mind.
>
> Steve Carlisle
>

I have a LM628 multi-axis board from Tech 80. I think they know more about the LM628/9 than anyone else out there including
National.

On this board any axis with more than a certain user specified following error will send an interrupt
to the host. The handler you write can adjust target accel, velocity, or timing to maintain sync. Doing
so will slow the system down somewhat. Maximum update rate is 1 kHz even though motor update
is 4kHz. This is due to latency of the LM628.

The multi LM628/9 boards are still considered fairly high performance as far as servo update time
but lack feedforward capability unless you trick them with software changing PID parameters on
the fly.

I suspect EMC with the STG card will outperform LM628/9 systems which is why i'm here. The
big cnc router I am building uses very high feed rates and following error is a big issue.

I have a comment on the servo power amp-
About a year ago I designed and built a large analog power amp (current limit is set at 20 amps!)
and it took a month of weekends to get it going. But companies like Copley Controls sell a 12 amp
h-bridge pwm amp for about $275. They will accept analog or pwm input I believe. I think I will just
buy them rather than roll my own.

It makes more sense for me to be doing things like ball screw end machining right now. This often
costs $300+ per screw from the manufacturer. To that end I bought a used Boxford lathe with a Themac tool post grinder friday.
Now I don't have to go to my friends shop 30 miles away to get
the work done. I'll need the stuff for maintenance anyway. See how easy it is to justify buying tools?
The other justification is even simpler- I want them.

Well, the sun is up so I'll go out and finish removing the horrible paint job on the lathe so I will be
able to shoot new paint this week. My idea of a really good time.

Leslie M.Watts
Illinois Tool Works
Corporate Advanced Technology group
(847) 657-4559
http://www.netcom.com/~leswatts/leswp.html