RE: [CAD_CAM_EDM_DRO] Re: Servos and Controller boards: an apolog y

If anyone cares, I think you're BOTH right!

Andy Wander
Verrex Corporation


-----Original Message-----
From: caudlet [mailto:info@...]
Sent: Tuesday, April 13, 2004 9:21 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Servos and Controller boards: an
apology

<snip>Why don't we give that same encouragement to the poster who
started
this thread? I have no idea what Nick already knows about servos, but
He has asked us to help him "know" more. Are we going to answer his
question??? (What can YOU tell us about high current switching
layouts, mosfet drivers, and such so that WE DON'T have to get a T0-
220 in OUR forehead?!?!?)</Snip>

Your right Ballendo. What was I thinking? I made the assumption
that this person was not a power switching analog/digital design
engineer.

Nick: To my knowledge there are no "kits" or build-it-yourself
schematics of closed loop servo controllers suitable for CNC
control. I don't know the precise reason for the dirth of
information other than I suspect that those of us who have actually
done a design using descreet available components want to perpetuate
the concept that given the scope and complexity of the project it
makes sense to buy it commercially. Obviously this is self serving
since if you built your own then the guy that spent all of his time
and effort to build, test and market his design will go hungry.

I hereby encourage you to pursue the design and building of a DIY
Servo controller.

The first thing I would recommend is to get application notes from
the main suppliers of power FET's about gate drive techniques and
driving highly inductive loads. The acquisition of a high speed
digital storage scope will help you analyse the nanosecond voltage
transients. You will also need a high frequency current probe as
well. Build up a few of the examples and you will learn a lot from
the component failures. (Note: buy yourself a good set of googles).
If you already have done this type design I apologize for assuming
you haven't.

After you have mastered the power part of the equation it's time to
switch to the control side. This will be an excellent learning
experience for you.

There are some good books on feedback/loop design and
characteristics but I would recommend a solid background in higher
math prior to reading them. Possibly there are some self study math
courses you can acquire. You will want to investigate all of the
concepts then check the newer generation of chips. As electronics
has progressed more and more functions have been rolled into fewer
and fewer components. This integration will make your design
smaller and more cost effective but brings with it the challenge of
tiny circuits and multilayer PC boards and possibly the skills to
deal with PLA's and programmable controller (embedded controller)
chips. There is, of course, the option to use older analog and
digital chips but you will definitly need to go back and review the
technical books on proper analog and feedback loop design. Lacking
a few years experience of laying out sensitive feedback circuitry in
close proximity to high current switching components be prepared for
a lot of board revisions on your initial design.

Should you elect to use some of the newer SMT chips in your design
you will have to invest in the specialized soldering tools for that
technology or develop methods the average hobbiest can use.

You will probably need some mechanical test fixture to be able to
simulate varying loads and fault conditions prior to your final
design. At the end of this you will have something you can
contribute back to the group and then you can setup a forum to
answer all of the questions you will get when their copy of your
design built of perfboard and soldered with a hot rock doesn't work.

You don't of course have to contribute back anything but then that
leaves us exactly where we are: Stuck with low cost finished units
that have a warranty and work. You will rob someone of a learning
experience many times more valuable than the mere cost of the unit.

I would be glad to help you with your quest except I have my own
learning experience going with all of the other stuff involved with
CNC control and I am kinda busy.

I don't want to stiffle anyone from being the next Art or Mariss or
Ray H. Everything is doable. We often make the stupid mistake as
engineers of assuming that if something is already available cheap
that it doesn't make sense to DIY. Who knows maybe you will
discover a way to build a Servo Control for 12 bucks! We all await
you triumph!

Now if I can just talk somebody into explaining how to build your
own 68,000 MFD 100VDC filter cap (they are WAY too expensive IMO) we
will have one more piece of the true build it yourself machine.






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