Steppers vs Servos

I have some thoughts and questions regarding the stepper-vs-servo debate.

Steppers are often used on small, simple machines since their open
loop nature permits simpler electronics and controller design. Yet
there is always the risk of missing steps under high loads, which I
get the impression leads people to use rather larger motors than would
be needed with closed-loop servos.

Case in point, I recently used a nice CNC'd bridgeport and noted a 19
lb/in rating on its servos - which works out to only 304 oz/in. I
don't know how the performance of steppers vs servos compares to their
relative torque ratings, but it seems like people routinely use 470
oz/in steppers on machines a lot smaller than bridgeports. So I'm
assuming the closed loop nature of the servo system permits these
smaller motors.

The question I then ask is this - why not close the stepper loop?
Surplus rotary encoders are only a few bucks, and the Kulaga/Mauch
interface boards are cheap too. EMC apparently counts the pulses it
has sent to the stepper to determine the current position - why not
substitute an encoder reading instead? Is there a reason why this
would't work?

Or maybe I should ask this: per unit of useable machine moving torque,
are steppers and their drivers more or less expensive than basic
brushed servo motors? If it turns out they aren't any cheaper (only
more visible on the web) then I guess there is no reason to talk about
a closed loop stepper system. But if they are cheaper, why not a
closed servo loop with a stepper as the actuator?

Hoping someone can shed some light on this,

Chris

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
30 Griswold Street, Cambridge, MA 02138
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