Re: [CAD_CAM_EDM_DRO] What makes a motor a servo?

In a message dated 5/6/2005 3:52:50 PM Eastern Daylight Time,
alex@... writes:

It has closed loop control so I'd say it's a servo, but I don't
understand what they gain by using a stepper instead of a DC motor- I
thought the only reason you would use steppers is so you can use open
loop control and avoid the complexity of an encoder and loop
controller. Surely this system combines the worst of both worlds?





It does present some problems. With a servo motor you control the speed
with either an analog voltage to the servo amplifier or pulse width modulation
to the motor. Actually many of the DC servo units did both since the
proprietary CNC controllers all output +/-10Vdc as the speed control. They just took
the analog value and used that to control the pwm. A stepper controls speed
based upon the number of pulses it sees in a given time and that is also how
a stepper is controlled as far as the distance travelled, the total number
of pulses sent to it. I have seen a FEW industrial machines utilizing a
stepper drive system AND an incremental encoder, to be honest these were pretty
much a disaster. It was extremely difficult to sync the encoder to the step /
direction pulses.

Most of those machines are now running AC brushless servos.

Hope this two cents worth is of some help

Randy A.


[Non-text portions of this message have been removed]