Re: Is this servo usable for position control?

--- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:

> > Assuming it is usable forward and reverse, then I can tell which
> > direction it is going depending on what polarity voltage I am

telling

> > it to use.
>
> No, a common misconception. A position servo is a 4-quadrant
> device, so not only does the servo amp supply power to the motor
> when accelerating, but the amp DRAWS power from the servo when
> decelerating. There is a point near zero speed where the voltage on

the

> armature is opposite to the direction of motion, because the

resistance

> of the armature is dropping more voltage than the back EMF from
> its velocity. So, your idea sounds good, but in fact won't work.
>
> There is a reason that servos have used some form of quadrature

position

> measurement since the 1960's.
>
> Jon

Thanks for your patient explications, Jon. Is there a good primer on
the web? (Servos for Dummies?)

From the C&H description, it seems as if it's not even possible to
stop these motors (other than by removing power).

"Variable speed 24 VDC servo motor with built in tachometer
and drive electronics. Speed control is accomplished by pulse width
modulation of a 1 kHz. TTL level input signal. Typical performance is
as follows: with a 50% duty cycle 1 kHz input signal the no load speed
is 1300 rpm. No load current is 0.4 amp. With a 20 oz-in load, the
output speed is 890 rpm, input current is 1.5 amps. Varying the pulse
width of the input signal at 20 oz-in load, the output speed can be
set anywhere between 890 and 120 rpm. Requires 24 VDC for motor power
and 5 VDC for logic power."

Grant