Rotation detection?

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!
Reply to
Buerste
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use an appropriately sized breaker?
Reply to
Ignoramus18221
But once the limiter trips, will the motor draw enough overage to pop a breaker?
Reply to
Buerste
You could put a photo sensor on the gear motor and the part of the machine that stops turning when the terque limiter kicks out. Use these to set and reset a flipflop. Integrate the output of the flipflop. When the torque limiter kicks out the reset would no longer occur, and the voltage from the integrator will go to high. Use this to turn off the motor. Dan
Reply to
dcaster
You could put a photo sensor on the gear motor and the part of the machine that stops turning when the terque limiter kicks out. Use these to set and reset a flipflop. Integrate the output of the flipflop. When the torque limiter kicks out the reset would no longer occur, and the voltage from the integrator will go to high. Use this to turn off the motor. Dan Good idea.
Reply to
Buerste
'A missing pulse detector' not only detects a complete stop, but if the speed is off.
Reply to
Michael A. Terrell
Probably better if there are multiple reset pulses per revolution of the output and only one set pulse on the motor. That would assure the flipflop is reset shortly after being set regardless of the relation of the two shafts.
Dan
Reply to
dcaster
Better if there are multiple reset pulses per rev and only one set pulse per rev. That would ensure the f/f got reset right after it got set. Regardless of the phase between the two shafts.
Dan
Reply to
dcaster
This is a maybe, since I don't have details of the application. Use a PTC thermistor (fuse type) limiter for the motor. Let the PTC perform the torque limiting. Put the LED (properly current limited) of an optocoupler across the thermistor. When the thermistor goes high resistance, there will be enough voltage across the optocoupler input to turn it on. The output of the optocoupler goes back to your office, or wherever.
Kevin Gallimore
Reply to
axolotl
Seems like there's just one complicated solution after another so far in this thread, that all involve adding a bunch of parts. Here's one that uses *fewer* parts: Take out the torque limiting device; couple the motor straight through to the machine; take out the motor mounting bolts; fasten the motor in place with a bit of string or duct tape. When the machine jams, the motor spins and its power leads wind up and twist off, stopping the motor.
Reply to
James Waldby
simple electronic way. Have a magnet on the rotating part. Place a reed switch near enough that it closes every time the magnet comes by. Have a timer you set to two seconds. Reset the timer when you presss "start", and every time the reed switch is closed by the magnet. You can make this timer from a 555 chip, or just buy a timer. If the timer expires, drop out a relay to de-energize the motor.
Reply to
Bill Noble
You'll never be able to sell your brain for much...it's USED! Thanks Bill.
Reply to
Buerste
We had a machine at work where the motor was mounted to allow it to rotate against a spring. There was an adjustable limit switch so that at a certain torque, the spring would compress far enough to trip the switch and stop the motor. Simple and cheap.
RogerN
Reply to
RogerN
Or -- set up a retriggerable one-shot which times out a bit slower than the optical sensor pulses so it keeps getting reset as long as things are running at normal speed. When the shaft slows down, the one-shot finally reaches the end of its time and triggers the motor stop circuit. And it could be done with magnetic pickups as well if dust is a problem with a optical sensor. For that matter -- if there are steel or iron gears involved, a magnetic pickup near the teeth of the gear could be used to generate the pulses.
Enjoy, DoN.
Reply to
DoN. Nichols
The torque of a DC motor is proportional to current, and vise versa. You can limit torque by limiting current. If you want the motor to turn off the motor at some torque level, simply turn it off if current draw reaches a predetermined level. Any additional torque limiting would be redundant unless there are dynamic considerations you didn't mention.
Reply to
Don Foreman
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
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Search for CD4046B.
Ping me if you're interested in this approach. I'll be back in town tomorrow.
Reply to
Don Foreman
if you are going to build your own timer, I would strongly recommend the 555 - set the timer to 2 seconds, hit the reset with the reed switch I recommended, and wire the output to a 2n2222 in open collector mode to drive a relay (or if a small relay, drive directly, just don't forget the suppression diode) - minimal parts count, very cheap, easy to hook up - just follow the schematic in the application handbook
Reply to
Bill Noble
Had I bothered jumping in on this thread earlier this is pretty much what I would have recommended. National called it a "pulse omission detector" in their data sheets, although I don't know if Fairchild will have as rich a selection of circuits as the 1980 National Linear Databook.
Reply to
Tim Wescott
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).
Reply to
Stuart Wheaton
Two (or more) radially opposed balls between plates, the driven plate free to slide axially against a spring and adjusting nut. Cut a shallower circular groove or lip, slightly outside the ball recesses, that will retain them but let the plates turn independently, at a greater axial separation. Arrange your limit switch to detect the shifted, stalled plate.
The trick is resetting it. Perhaps have a short spiral track the ball follows out when tripped and back into place when either shaft is reversed. The noise of the ball dropping into the slightly deeper outer end of the spiral groove is the audible trip indicator.
jsw
Reply to
Jim Wilkins

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