RE: servos

I may have been the one to strike the match on the topic and I have snipped out quite a bit of very useful data, however, it seems that the features and costs of servo system components are like a menu of performance, and the price you must pay if you want the performance.

Case in point of a tach to integrate motor speed into the performance - cost curve. Don't forget that the tach is required to improve performance for a price. What about a little less performance for less cost?

I have designed a digital PWM servo amplifier board. I am considering all cost effective computer interfacing means available so that I end up with a reasonable total cost for accomplishing what I set out to do.

I have started a list of possible computer interfacing methods with costs. Obviously the overall system performance may be limited by the system architecture one selects, and worse, the programming and construction time may be a little demanding, however, TV is a real waste of time. The most scary fear I have is the compatibility with application programs, but it's all on paper for the moment.

Computer Interfacing cards and aproximate price:
ISA SCSI-II Controller card $50
PCI USB I/O Card $40
Dual port ISA Bi-directional Printer Card $35

Any one of these three examples can control 3 servo motors through the properly designed external digital amplifier. I am playing on the bottom end, but the concept here is that you add whatever you want in the computer and glue it all together with software (even 486 assembly code as a macro), and you manage your costs based on what you need.

The problem I still face, is understanding how one might tie "odd ball low cost systems into CNC programs of interest. To that, I am looking at demos of programs and searching out the interfacing requirements (before I paint myself in a corner).

David Howland




-----Original Message-----
From: Jon Elson [SMTP:jmelson@...]
Sent: Friday, September 24, 1999 12:27 PM
To: CAD_CAM_EDM_DRO@onelist.com
Subject: Re: [CAD_CAM_EDM_DRO] servos

From: Jon Elson <jmelson@...>



"Arne Chr. Jorgensen" wrote:

> Jon is smart, he know that an old fashion tacho generator works,
> and what we are looking for is something that will behave like one.
> There is some PPL, freq->dc signal tachometer cicuits like ( LM2907)
> etc. But somewhere in the back of my mind, I have read something
> about designs that did not match an old fashion tacho. What I would
> like is something "stupid simple" - but it has to work properly. I
> wish I had some better knowledge about what was wrong with my own
> attempts, and Ray Henry has pointed out that there is other
> attempts, - that must have experienced something similar to what I
> have done.

Well, with a very nice DC tach available for US $7.95, and the problems
of a simulator using the encoder, I just don't see what the problem is.
Getting a bipolar pulse interval to DC converter (kind of like a freq-voltage
converter, but optimized for VERY low frequencies, too) to work just
looks troublesome to me. It needs to go from below 1 Hz to 40 KHz
or so, and have constant delay across those frequencies, and not give
bumps when pulses come in at the 1 Hz rate, but not have more than
a couple mS delay. I think that is HARD!!!!! Getting a couple KHz
bandwidth, with no ripple below 1 Hz input is a TOUGH PROBLEM!

<snip>

> How is the pico servos working ? How good are they.

Well, they have been working great for a full year, now. No failures,
quirks, or other problems! I'm VERY happy with the performance.
And, once I figured out the trick with the feedforward term, the error
is VERY small. Maybe I need to pull one of the graphs from EMC's
logging function out, and post it for you guys! Maybe that will convince
you how good it does. (I'll try to do this this weekend.)

Jon

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