Re: Cheaper servos

>Sorry, but I think you're nuts!

Well, Jon, you're not the only one...no hard feelings. I wanted to generate
some discussion. I'm happy to play the role of the guy that asks the stupid
questions to that end, if you all won't mind.

>Servo motors are designed to have minimal
>'torque ripple' and 'velocity ripple', meaning that with a constant DC voltage
>applied, they provide constant torque if the speed is held constant, and
>turn at constant speed when free to turn. Cordless drill motors, etc.
>are not built this way, and won't give smooth motion.

I see, so we rely on the motor characteristics as well as positional
feedback? My recollection of the gist of the Byte article was that with
positional feedback, even cheap motors can give accurate and repeatable
positioning. For a robot. My question is really, "if OK for a robot, why not
for a home shop machine tool?"

I'd like to know more about how a DC motor is constructed to have these
desirable ripple characteristics.

>My servo system
>moves smoothly down below .01"/minute, and up to 105"/minute.
>With a little higher DC voltage, it could go faster.

Smooth is good, and the range is impressive... I'm still not clear on why,
with good positional feedback, we can't use cheaper motors.

>Some motors
>salvaged from appliances might be acceptable, but unless you test it,
>you won't know.

Do you have any suggestions as to which motors might possibly be acceptable?

>I am using 1000 cycle shaft encoders. I can't imagine making one
>of these out of paper! The 'starburst' pattern of stripes has to be
>centered on the axis of rotation to an extreme degree of precision
>to get accurate position readout.
>Jon

It would be nuts to use paper for this app, for sure. I mentioned that only
to jog the memory about the particular article, and to recall Ciarcia's
spirit of "Sure, it can be done".

Thanks for the reply

Phil