DoN. Nichols wrote:
Bob Engelhardt wrote:

Four things are needed:

1) A servo motor with a tach generator ...
2) A servo amplifier ...
3) An encoder ... on the leadscrew, ...
4) A controller (the computer, ...

OK, but my application is so simple (degenerate, really), that it seems
that I could do with less. I just want to control the leadscrew speed,
not the position of the carriage, and the speed would be set manually
(e.g., with a pot). Doesn't my requirements collapse your list to just
#1 & 2?

... Specifically, could I use a
Baldor 2500 rpm, 1.65hp PMDC brush-type motor? ...

The servo motors which I have used are rather specialized, and I
don't think that the treadmill motor will be sufficient. ...
The reason for this construction is to minimize the rotating
mass, so it can get from one speed to another (including stop) as
quickly as possible.

Again, my requirements would not include quick speed changes.

How do I find a controller for 1000:1 range?

A servo amplifier. It is not called a "controller" for that application.

Ah! Knowing the terminology makes a BIG difference. What are the
chances of my discovering that without RCM?


And, can I really get full torque at very low speeds, say 2 1/2 rpm?

You can get full torque from a true servo motor and amplifier
at as little as 1/10 RPM. It depends on the tach generator to tell it
what speed is is currently producing, so the amplifier can provide it
with more or less current as needed to produce a stable speed.

Now that brings up something else I've wondered about. I've heard
before how servo amps control the motor speed with voltage and motor
torque with current. Makes sense conceptually, but how does the amp
control the current independently of the voltage? My intuition is that
you set the voltage for the speed you want & as the motor is loaded
(more torque needed), it just draws more current to supply the torque to
keep the motor running at the voltage-selected speed. That does require
a low impedance source - is that the complication that brings in
separate control of the current?


Going to eBay and searching for servo amplifier, I find:
260279721259 (an AC servo amp by Fanuc -...

$400!

350092690530 (... a mix of servo motors and ... amplifiers ...

$600!

260278680985 (these look like rather impressive amplifier specs,
especially given the buy-it-now price. ...

$47 (shipped). That's better. I just want to drive a lead screw, for
crying out loud.

Good Luck,
DoN.

Thanks for tutorial! I feel like I've taken a significant step up the
learning curve.

Bob

--
Nota for President