Re: CNC question

Morris Dovey wrote:
...
your O/P didn't give much to go on with regard to your experience with
servos, control systems, or machine design.


Good point. My Ph.D. work focused on RF electromagnets, but I've taken
undergrad and graduate courses in control systems and linear systems
theory. However, I never took a robotics course. I do have a fair bit
of experience with design and construction of various instrumentation.
I've done plenty of computerized data acquisition and programming
thereof.

I bought a four-axis driver PCB because I couldn't make one for less
than the price I paid; and because my interest isn't really in circuit
design and PCB fabrication.

It may be the same for me. If I can control the motors directly with
my data acquisition card and some simple amplifiers I may be able to
implement almost everything in software. That would definitely be
cheaper (considering that I already have the card).


Writing the software is turning out to be an "interesting" exercise,
since it's not enough to simply drive the steppers from point-to-point
at constant speed. Every movement needs to take into account the
friction and mass component of each axis as well as changes in
direction to avoid problems of lost steps (which won't be a problem
with servos) and overshoot. With servos, you trade off my problem of
lost steps at the beginning of a move for undershoot problems at the
end of the move.

I had thought about using stepper motors for ease of use factor, but my
hesitation involves lost steps, as you mention. Do the motors provide
any sort of feedback that they've missed a step?

As I understand, analog servo controllers use proportional control and
are, therefore, subject to undershoot near the end of a move. That's
why I was thinking PID. I'd just have to make sure the derivative term
was big enough to avoid overshoot. PID's so simple to implement in
analog, I'm surprised the default controllers don't use it.


I'm building my own linear bearings and using 3/8"-12 Acme threaded
rod with shop-built delrin followers for mechanical drive. Most of the
machine body will be constructed of baltic birch ply.

The machine will be approximately 24"x16"x14" with a 12" x 12" x 4"
workspace - subject, of course, to revision at any time.


That's really similar to what I was thinking, with overall dimensions
varying accordingly, of course.

By the way, I don't have my mind made up 100% for servos versus
steppers. You chose steppers. In your opinion, is that a better way
to go overall, or did you decide based on price/availability.

Thanks, Morris.

Josh

.

Relevant Pages

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