Electric motor - one rotation then stop

[Klench]

Hi all

Can anyone suggest the best way to get a 12V DC electric motor (70RPM) to rotate roughly 360 degrees and then stop?

So the process would be;

1. Power switched on.
2. Motor rotates once, then stops.
3. Power switched off.
4. Repeat

Thanks in advance

 

[AnalogKid]

A double-pole relay acting as a latch, triggered by a limit switch.

ak

 

[Bluejets]

Difficult to achieve with any accuracy.
Assume a gearbox is attached to the motor to give the 70RPM.
If that is the case the motor would have quite a bit of over-run once the power was removed.
One could "short" the motor terminals to give an amount of dynamic braking after the power was turned off by the limit switch mentioned above.

 

[Klench]

Thanks all for the feedback. The motor is actually already set to 70RPM

I'm a complete novice here, but will investigate how to set up using the technologies mentioned.

 

[davenn]

you would be much better off using a stepper motor

 

[newjeeper369]

what is the application for this motor and type of function? does it need to be accurate or roughly one rotation?

 

[Klench]

It's intended as a metered dose type of application, e.g. power on via switch, motor starts and opens a spring-loaded latch using a cam. The latch releases material, then closes when the cam has moved past. What I'd like to avoid is situation where the latch stays open and all the material comes out at once, hence the idea to have a rotating cam that rotates 360 degrees then stops until the trigger switch is clicked again.

It would be great to turn off the process via switch, e.g. so if the latch opened at 12 o'clock, a 'stop' switch could be similarly pressed at 9 o'clock, but I don't know how to do that while at the same time allowing the process to start again, e.g. it seems to me that if a 'stop' switch was used, the circuit is then broken.

Thanks

 

[BobK]

The stop switch would be a normally closed switch which opens when pressed. The start switch would be a normally open switch wired directly across the stop switch. When you press the start switch, and hold it long enough for the motor to move past the stop switch, it will then run until the rotation is complete and stop switch is pressed again.

Bob

 

[AnalogKid]

No information on the top secret motor current spec, so I'm guessing 1 A.

First pass at a Dose Motor controller. SW1 is a limit switch that is activated by a cam on the motor shaft. The motor opens the switch at the end of a cycle. When power is applied, the RC timer shorts out the switch for a little under 200 ms (according to LTSpice), long enough for the motor to rotate off of the switch it so it can close. By the time the motor gets around to the switch again, Q2 is off, so when the motor opens the switch, it halts. A power off time of 1 s resets everything.

ak

 

[Klench]

Thanks all, it's much appreciated

 

[AnalogKid]

Not done yet...

 

[Gryd3]

Have you thought of using a spring loaded solenoid to move the latch instead?
Apply power to open the latch, power removed, the spring returns the latch closed.
If the circuit fails, you won't have the latch half-open

Pay attention to AnalogKid though. He's newer on here, but knows his stuff

 

[AnalogKid]

More stuff...

This is a minimalist approach, courtesy of that thin sheet of glass in a MOSFET. Again, the circuit pulses the motor until it comes off the limit switch. The MOSFET pulse is approx. 200 ms.

Step 3 in post #1 is "Power is switched off." Since the way this is done is a secret, I guessed at the worst case, that power is mechanically disconnected with a switch and the Vcc net goes to a high impedance. In this case C1 could take a long time to discharge enough for the next timing cycle to be accurate. R2 resets the timer circuit in about 400 ms.

ak

 

[Colin Mitchell]

Simply use a cam and micro switch. The cam is long enough to allow for the over-run. That's all you need.

 

[AnalogKid]

How do you reset a microswitch?

ak

 

[Colin Mitchell]

Put your switch across the micro switch

 

[BobK]

Put your switch across the micro switch

Yep, as I said in post #8.

Bob

 

[CDRIVE]

Bob, you and Colin are taking the fun out of complicating things.

Chris

 

[BobK]

Bob, you and Colin are taking the fun out of complicating things.

Chris

Yep, that is called ... engineering!

Bob