Friction brake

It's off-topic, but I wonder if anyone here has got any sound experience of
using friction brakes to accurately control the torque on a small
continuously driven rotor.

The application is a bit like a drum brake on a car, but it runs
continuously and needs to give very consistent friction levels over long
periods of time, independent of rotational speed.
In a way, it's a bit like a miniature dynamometer.

Regards,
John H

 

The application is a bit like a drum brake on a car, but it runs

Use a DC motor running as a generator as your brake. Wire the motor
to a variable rheostat and you'll have very accurate control of the
resistance offered. If you can afford to use a more sophisticated
approach you can use MOSFET's and digital control to present a load to
the motor-generator. This allows precise digitally controlled
torque. Be sure to heat-sink adequately.

-Martin

 

The unit is going to be built by the thousand for $50 cost price each, so
that sort of approach is not feasible. There is a competitor's unit which
occupies a niche of the market, selling for $1000 each which uses a servo
motor with closed loop control, but most customers can't afford it.

John H

 

The unit is going to be built by the thousand for $50 cost price each, so

What power level are we talking about here? Not much if you are going
for a $50 MSRP.

-Martin

 

Have you looked at switched reluctance motors.

 

It's for individual threadlines in a textile application, maybe 20-40W each.

 

A gearbox and a propeller?

 

.....speed dependent....

John

 

No problem.

The continously driver rotor you mention in your original post, is
this driven by a larger DC motor? How fast?

Funny enough, I have bit of a background in the textile industry as my
parents were in it when I as a kid. I'm not anywhere near that
industry now.

The problem with your brake-based torque control is that too many
variables are involved in order to make it work reliably and
consistently. You didn't say what degree of accuracy, stability,
repeatability, etc. you expect to deliver for $50.

If you can do this electronically $50 is not a horrible budget.
Mechanically? Well, it depends on a great many variables.

-Martin

 

The motor isn't a bad approach and if the application only requires
very small motors, (ie.one per thread) this may not be as expensive as
one may think. Otherwise, I have seen cardboard spools used with
adjustable spring tension fairly successfully in the printing
industry. But, small DC motors with individual potentiameters could
be as inexpensive as $5 per axis, (estimate with lots of assumptions)
and is considerably more elegant.

Edt