Re: Servo Torque Drive?

Most DC servodrives control motor torque as part of the closed loop.
By analogy, this is what you do with the gas pedal and on your car.
The position of the gas pedal bears no direct relationship with your
car's speed. You watch the speedometer (velocity) and make
adjustments to the gas pedal (torque) to maintain the speed you want
(closed loop).

Most servo loops are PID. This stands for Proportional, Integral and
Derivative. Servodrives that require tachometers derive these signals
from the tach feedback voltage. Newer ones get the same info directly
from the encoder pulses. Both require quadrature encoders for
position feedback.

Using the same analogy, here's how PID works. Let's say you have to
stay side by side with another moving vehicle (command velocity). If
initially you are far behind, you will press the gas pedal harder
than if you are slightly behind (Proportional). As you draw near, you
note the rate of position difference change (velocity) between you
and the target vehicle. If it looks like you will rapidly overtake
him, you might use the brakes to lessen the velocity difference
(Derivative). Finally when you are side by side, you make very small
adjustments with the gas pedal to maintain your position (Integral).

It really makes no difference if the target vehicle is moving or
stopped; you would still do the same things to maintain your position
to be side by side.

PID serodrives do much the same thing to maintain exact position.


Mariss

--- In CAD_CAM_EDM_DRO@egroups.com, djr@n... wrote:
> Am I correct in my understanding that a torque servo drive is
> just like a basic speed control (but controls torque) for a servo
> motor and there are not any tach or encoder feedback connections to
> the driver? Does a step/direction driver use quadrature encoder
> feedback? If all of this is correct where does the tach come into
> play and what type of driver uses it?