I need a design for a simple power supply to provide power to a DC motor
for use in a constant tension device. Need the design to vary the
voltage to the motor from a external pot that has an arm that rides on
the spool I am unwinding. The power supply would need to provide about
2 amps max to run the motor at something like about 0 - 12 volts. The
motor would provide tension on the spool that would vary as the size
changed. If someone has another suggestion I would be open to that
also. I looked at a device I found on the web from a company called
Magnetic Tech. that uses a magnetic brake that probably would work
however it is priced at almost $600/ unit. I have been using a
mechanical brake that places drag on the spool however this is not
constant tension and I have increased the speed of the unwinding and
this has caused me alot of problems.

Thanks -

Barry

Ned Simmons wrote in message ...
In article ,
says...
I need a design for a simple power supply to provide power to a DC motor
for use in a constant tension device. Need the design to vary the
voltage to the motor from a external pot that has an arm that rides on
the spool I am unwinding. The power supply would need to provide about
2 amps max to run the motor at something like about 0 - 12 volts. The
motor would provide tension on the spool that would vary as the size
changed. If someone has another suggestion I would be open to that
also. I looked at a device I found on the web from a company called
Magnetic Tech. that uses a magnetic brake that probably would work
however it is priced at almost $600/ unit. I have been using a
mechanical brake that places drag on the spool however this is not
constant tension and I have increased the speed of the unwinding and
this has caused me alot of problems.

What sort of torque and RPM of the spool are we talking
here? Is the unwind constant or start/stop? How accurately
do you need to control the tension? What's the diameter and
mass of the full spool?

For very light torques at constant speed, a small AC motor
connected to a DC voltage supply works pretty well. I've
used a KBIC motor control as the supply for this sort of
thing.

For somewhat higher torques, an AC torque motor (see
Bodine) and variac is a possibility.

A linear dancer with hi/lo limits and a DC motor set up to
start and stop to maintain the dancer between the limits is
good for close tension control at moderate speeds or with
intermittent feed. Tension is controlled by the weight of
the dancer and is independent of the diameter of the
material on the spool. Also avoids problems of non-
linearity of the diameter sensing mechanism.

Same as above with a rotary dancer is OK at lower speeds.

If you want to stick with the DC motor, look at the stuff
on the KB Electronics website. I think some of their
controls can be set up to control torque(current) instead
of speed(voltage). Since even the cheapest KBs have a
current limit pot, I'm sure with a little tinkering you
could get them to do what you want.

http://www.kbelectronics.com/

It would be very straightforward (but overkill) to do it
with an inexpensive servo amp (Copley or AMC) set to torque
mode.

Ned Simmons

Hi Guy's,
I aggree with Ned re. the dancer. The simplest way to drive
the 12v d.c. motor would be to connect a variac to the dancer & use
the output to drive a transformer/rectifier for the motor. If you
could find a plugpack (wallwart?) with a high enough current you would
only need that & the variac.
Hope this helps.
Ian Sutherland, Oz.

I made a machine for respooling sapphire fiber. Mytake-off tension control was
made with a standard shaded pole motor, a 12 volt DC power supply controlled
with an LM317 voltage regulator connected to a dancer pot. The 120 VAC shaded
pole motor acts as a very smooth viscous drag when supplied with a small amount
of DC. Try it on a bench test. About as simple as you can get.

Earle Rich
Mont Vernon, NH

OK, I'll bite:

Is this a modern or a ballet dancer?



Yours,

Doug Goncz, Replikon Research, Seven Corners, VA
Unpublished work Copyright 2003 Doug Goncz
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