On 2008-09-02, Bob Engelhardt wrote:
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?
Yes -- as long as you did not need to accurately tie the
leadscrew speed to the spindle speed (such as for threading). If you
just want a nice finish, yes, the servo amplifier and the servo motor
with built-in tach would be sufficient.
... 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.
O.K. You could probably get away with less -- but you still
need a good tach generator sharing the motor's shaft to get stable
speed control.
And are you planning to put ths on a ball screw? You get more
reliable speed with that than with half nuts. At really slow speeds you
will have stick-slip behavior with Acme threads.
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?
You could probably find it eventually -- searching on "servo"
first, and reading a lot. :-)
Also -- in some of the eBay auctions they were called "drives".
The ones which I am most familiar with are capable of producing voltages
from -40V to +40V and -20A to +20A. The label says "Aerotech 4020 DC
Servo Controller", so "controller" is used at least sometimes to
describe them. My first pair of these came from a hamfest with a couple
of servo motors. Later I got others from eBay auctions at quite
reasonable prices.
If you wind up getting those, I have a scanned copy of the
manual (several Xeroxed pages, nothing bound) which I can let you have.
Ebay auction 130075041135 has a pair of them - but I think you
won't like the $575.00 buy-it-now price. :-)
Ebay auction 250180874877 has one of the motor with their label,
going for $275.00. It appears to be new. Note the two pairs of brush
caps -- one near the middle, and one near the bottom end.
The shaft at the bottom end would be used for mounting an encoder
if that was the way you want to go.
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?
Voltage will track current -- at a constant speed and load.
While feeding a constant voltage, if you increase the load the current
increases and the speed drops a little. While feeding a constant
current, the speed will vary widely with load.
So -- in comes the servo amplifier. You could look at it as
controlling the output voltage or the output current, but it does not
really matter. Here is what happens:
1) You feed a speed command voltage (between 0 and 10V DC
typically) to the amplifier.
2) The motor feeds a voltage generated by the tachometer generator
to the amplifier.
3) The amplifier compares the command voltage and the tach feedback
voltage, and if the tach voltage is lower than the command
voltage, it increases the voltage/current to the motor. If the
tach voltage is higher than the command voltage, the amplifier
reduces the voltage/current to the motor.
(Really -- there are adjustments to both inputs so you can
select what command voltage will give you the full speed from the motor
(typically 10V DC for full speed, but other options are possible -- and
negative command voltages cause the motor to run backwards.)
Now -- the motor brushes and the tach brushes have to be
oriented the same way when connecting to the servo amplifier. If you
reverse one or the other, you won't get stable speed at all. Instead it
will shoot to maximum speed forward or reverse. This is a good reason
to test the wiring *before* you bolt it to the leadscrew. :-)
Now -- reason for wanting the motor to be as low in inertia as
possible -- a change in load or command speed will cause the motor to
get a surge of voltage to catch it up -- and this can cause it to
overspeed, then drop below, then overspeed a bit less, eventually
reaching the desired speed.. There are adjustments, for damping so the
ramp-up and ramp-down are properly adjusted to the total inertia of the
whole load, but the less inertia in the motor itself, the easier it is
to stabilize the speed.
Going to eBay and searching for servo amplifier, I find:
260279721259 (an AC servo amp by Fanuc -...
$400!
Note also that this is an AC servo motor, so it requires a more
expensive AC servo motor -- not the DC ones which I have been talking
about.
350092690530 (... a mix of servo motors and ... amplifiers ...
$600!
But plenty to play with to set up for what you want to do plus
lots of other projects. It looks like about $50.00 of that lot is all
that you need for a single axis -- think of the rest as spares or
experimental items.
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.
You ideally want to drive a ballscrew for the slow speed end of
things, and for the ability to use smaller servo motors.
That amplifier looks like a nice one -- but remember that you
will also have to make or buy a power supply to feed it. Up to 80V
depending on what your motor will accept, and up to 15 A continuous, and
30 A peak. The Aerotech which I mentioned above has the advantage of
built-in power supply, simplifying the setup.
Remember to avoid a "brushless" servo motor. It won't work with
this (or any DC servo amp).
Good Luck,
DoN.
Thanks for tutorial! I feel like I've taken a significant step up the
learning curve.
The best thing to do is pick up a small motor to experiment with
and you can use it with the servo amp which you have liked the price of
above.
Good Luck,
DoN.
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