Motor Zero speed detection circuit

Discussion in 'General Electronics Chat' started by devalvyas, Aug 17, 2009.

  1. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
    I am trying to build a circuit that can detect when the motor shaft comes to a halt and want to use this circuit for the same.

    The idea that I am toying around is this:

    I have a small contactor attached to the shaft of the motor. As the motor rotates this contactor touches two pins A and B there by giving pulse to the comparator through a trigger circuit.

    As soon as the connection is established the output of the comparator changes and remains at that state for some time say for 0.25 seconds, before the output can change back, the contactor touches the pins again and again triggers the comparator. This continues as long as the shaft is moving.

    But the moment the shaft stops, the comparator no-longer gets any pulse for 0.25 seconds and it changes state, hence i get to know that the motor has stopped rotating.

    The reason why I need the trigger circuit is that Even if the shaft comes to halt exactly at the point where it touches pin A and B, the comparator receives only one trigger through the trigger circuit and does not get second trigger for 0.25 seconds.

    My expected speed of rotation will be around 1500 rpm. this means that the contactor will touch pins A and B 1500 time in a minute or 25 times a second.
    The time between each pulse will be around 40 msec and the duration of the pulse will be for 4 msec only.

    Will this circuit work? or is there any better simpler way to do it? How do i select the component values? currently the component values are only for circuit testing, but actual component values for above mentioned time durations have to be worked out...
  2. mauro.laurenti

    Active Member

    May 8, 2009

    If you have a small DC motor with a permanent magnet, you can consider also to measure the voltage on the motor terminals, once you power off it.

    In fact, when the DC motor is turned off but still run, it will work as a dynamo, so you have a voltage on the terminals that you normally use to turn it on (called f.e.m, but I'm not sure if in English is call this way as well).

    When this voltage is 0, or better below a threshold, it means that the motor is not turning any longer.

    If you have an AC motor this technique does not work.

    If you have an encoder you can check its pulses.


  3. drc_567

    AAC Fanatic!

    Dec 29, 2008
    It would seem that the salient problem will be the friction contact between the rotating shaft and the stationary contact part. ... It is unlikely that the proposed contactor will withstand any acceptable time period of operation, without excessive wear and subsequent failure.

    If you were to somehow attach a small piece of bar magnet to the rotating shaft ... for example on the free end of the shaft, you could then employ a hall effect sensor as the stationary component. The HE sensor would deliver a pulse at each pass of the magnet on the shaft. This pulse could be employed in a missing pulse detector ... or some similar scheme. ... Depending on the motor shaft material, it may be possible to magnetize the shaft ... in an asymmetrical fashion ....rubbing it a number of times with a permanent magnet .... so that the HE sensor will produces a pulse as the sensitized section of the shaft passes by.
  4. devalvyas

    Thread Starter Active Member

    Nov 11, 2008
    I am using a ac motor...and am intetested in knowing the zero speed physically from outside the motor and not from the supply current or voltage to the motor...

    regarding the contact friction part, i was planning to use something like a carbon brush and if it lasts for a couple of years...its worth it...Hall effect sensor can also be used though..

    what i am currently not able toachieve is the missing pulse detector using comparator circuit...
  5. drc_567

    AAC Fanatic!

    Dec 29, 2008
    As I recall, the missing pulse detector circuit
    can be built using a 555 timer .
    A useful part for attaching a bar magnet segment to the shaft is a shaft collar ... which tightens using a set screw.
    I have used the hall effect sensor ... it is simple ... produces a strong, steady output pulse.
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    If you are not bothered about changing components, look at inductive sensors.
    These can be used for sensing gear teeth or any none-smooth (intermittent gap) ferrous part.

    They are basically a coil plus magnet, a bit like a guitar pickup in principle but in a slim tubular body for mounting in machinery.

    As the output is strictly caused by movement, it does not matter if the motor stops on or off the sensor.

    This is an example of the generic type, there are low cost ones about.
  7. dexterbateman

    New Member

    Aug 18, 2009
    The only thing that would work 100% would be an encoder or a resolver. Each one needs a controler to interfece with it and your program. The encoder is the cheaper choice.