Re: [CAD_CAM_EDM_DRO] Yet Another Servo Question

Joe Vicars wrote:

> Ok, I just got back from a salvage trip, and I want to clear
> something up. Any permanant magnet DC brush motor can be converted to a
> position controlled servo by adding a quadrature encoder to the shaft.
> Resolution of the position control being only a function of the encoder
> resolution.
> Is this correct?

In the grossest sense, yes. Some brush motors meant for spinning fans or
whatever may have inclined brushes or offset brushes that make the motor
not perform well (or burn up brushes rapidly) when run in the opposite
direction. Another concern is torque ripple and velocity ripple - generally
called cogging. Turn the motor slowly by hand, and see if you feel
detent-like fluctuations in the drag. If so, that is not a good sign. Another
test is to connect the motor to a variable DC power supply, and apply
just enough voltage to break free from the static friction. If the motor
turns very smoothly, without speeding up and slowing down, that is
a good sign, too. A single D cell can also be used for this test.

Many DC servo amps require a DC tach for velocity feedback. Most
servo systems for motion control applications are actually velocity servos,
where the CNC control sends a velocity command to the servo amp, and
the amp's job is to make the motor move at that velocity. Newer
servo amps often derive velocity feedback from the encoder. To get
really smooth motion, you may need an excessively high resolution
encoder, which gets expensive.

Without any velocity feedback, it becomes VERY hard to obtain both
accurate positioning, rapid acceleration/deceleration AND smooth
motion. A simple position-only servo has to give up either performance
or accuracy to remain stable.

Finally, as there is substantial acceleration involved in some motions,
it requires a certain level of power output to achieve that. A motor that
can deliver the average torque to move at some speed may not be
able to deliver the torque to rapidly accelerate to that speed.
Note that even very short pulses of excessive current can demagnetize
the permanent magnets in a PM motor.

Jon