Don Foreman wrote:
On Mon, 6 Jul 2009 03:05:21 -0400, "Buerste" wrote:

Imagine a simple fractional hp DC gearmotor with a torque limiter or torque
limiting clutch that powers a small machine at 60 or so RPM. Lets say the
machine jams and the torque limiting device kicks out. How can I cheaply,
easily sense this condition and turn off the motor?

This is a personal project and when I get it running it will amaze and
astound!

Several have suggested sensing drive and driven parts but left the
signal processing details as an exercise.

I'm in dialup-land just now so it'd be tedious to download a
datasheet, but I'm pretty sure the CD4046 CMOS phaselocked loop would
work well here. It would produce an output signal if the frequency of
(digital) inputs differ, as they would here. I'd suggest something
like Allegro hall-effect geartooth sensors with integral magnets
because they're so easy to apply. They could easily sense screwheads
on the drive and driven parts. The CD4046 costs less than a buck and
will accept supply voltages from 3 to 18 volts DC.

You'll find a datasheet at www.ti.com Search for CD4046B.

Ping me if you're interested in this approach. I'll be back in town
tomorrow.

How about this, Assuming a spring loaded ball type limiter, keyed to
input and output shafts. Leave the keys long, place a roller limit
switch (spdt) against each shaft so the keys trigger the switch.
Install one dpdt relay in the power line to the motor. Wire it so power
flows to the motor in the NC position.

The drive limit roller is wired so it passes power when the key rolls
under it. The driven side is wired NC from the output of the drive
switch, if it hits the key, it opens. This circuit goes to the relay
coil. Giving the driven side a longer dwell might be helpful.

When both are running normally, the drive switch tries to shut off the
motor every revolution, but the series wired driven switch stops the
signal from getting to the relay. If the limiter slips, the driven side
is out of line with the drive phase, and the relay kicks, then run the
second pole of that relay to latch it until reset.

2 switches (or sensors), 1 relay, 1 control transformer (unless motor
voltage matches relay coils), 1 reset switch (in latching loop).