Re: Cheaper servos

Phil Plumbo wrote:

> From: psp@... (Phil Plumbo)
>
> I haven't heard much discussion about sources for servos. What are people
> using out there?
>
> I remember a Circuit Cellar column in the early days of Byte wherein Steve
> Ciarcia described a neat hack for controlling DC Motors via rotary encoders
> for a robotics project. He was enthusiastic about their cheapness compared
> to steppers of the day ( on a $/in-lb basis). IIRC he built his own encoder
> using a gray-coded paper wheel.
>
> Fellow listers, I submit that we don't need expensive servo motors to
> control our brand of basement CNC, just your common DC brush motor, perhaps
> from the ubiquitous cordless drill, or auto power window winder, or
> windshield wiper motor. DC motors are cheap and plentiful, and we can use
> them for our machine tool conversions.
>
> What say y'all?

Sorry, but I think you're nuts! 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. My servo system
moves smoothly down below .01"/minute, and up to 105"/minute.
With a little higher DC voltage, it could go faster. Some motors
salvaged from appliances might be acceptable, but unless you test it,
you won't know. Other motors clearly are unacceptable.

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