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DoN. Nichols DoN. Nichols is offline
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Default servo motor control ??

According to :
I posted this note on comp.robotics.misc but of course it soon got
lost in all the crap that someone is posting to various groups.

So I thought I would give it a try here.


[ ... ]

I'm aware of hobby style giant servos. They might work but not sure if
they would be beefy enough.

Industrial servo motors seem to be quite different from hobby stuff. I
have one that's rated 128 VDC, has 5 wires running to the motor and a
bunch more off the attached encoder. No real idea what's going on
here.


O.K. There are two major classes of industrial servo motors.
The DC ones (which I believe that you have), and the AC ones. I'll skip
the AC ones, because I don't really know enough about them.

The DC ones, however, are typically permanent magnet field, with
a pair of wires going to the commutator to get power into what is
typically a basket of wires woven in the shape of a rotor without iron.
There may be stationary iron inside that, but it does not rotate, so the
inertial mass of the rotor is at a minimum.

In addition, there is either a separate set of similar (though
perhaps lighter gauge) windings connected through a second commutator to
a second pair of wires, or (sometimes) simply a second set of brushes at
90 degrees to the pair which bring power into the motor.

Those extra wires are the tachometer (tach) feedback. The motor
generates a voltage proportional to its speed and feeds out those extra
wires.

The motor is driven through a high-power amplifier, with a DC
voltage generated by the amplifier to drive the motor, while the output
of the tach generator is summed with an input command voltage. As long
as the motor's speed (and thus the output of the tach) is below the
input speed command voltage, the amplifier outputs quite a bit of power.
It produces less as the two get closer together in value, and when they
are identical it produces just enough output to maintain the speed.

If the command voltage is very low (for example 0.0001V when the
amplifier is configured to give maximum RPM at 10V input), the motor
will be turning so slowly that you won't even notice it -- unless you
put some tape on the motor's shaft as a flag -- then you can see it
moving very slowly.

Note that if the command voltage is below zero, the motor will
turn the other direction.

So -- you can select whatever speed you want.

The encoder can help tell a computer what the shaft angle
happens to be at the moment -- and there is often an index encoder which
produces a signal only once per revolution.

[ ... ]

But I don't really like this solution as there is no automatic
centering feature.


For this -- you need some kind of position encoder -- either
limit switches, or a potentiometer whose output is proportional to the
angle of the steering. These servo motors are intended to rotate
leadscrews at precise speeds -- not to return to a zero center as model
aircraft servos do.

Any other ideas? What do those guys like Mythbusters do for remote
control full size cars?


The serious industrial servo motors, with a computer to control
them, I suspect.

Enjoy,
DoN.
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