DC motor control

Discussion in 'General Electronics Chat' started by bug13, May 21, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys

    How tell if a small DC motor is blocked?
    And how do I hold the position of the motor at the blocked position?

    The DC motor I am dealing with is a small 3V-6V brushed DC motor, like the one you can see in a small RC car.

    The motor is likely to be controlled by a H-bridge.

    Thanks
     
  2. timescope

    Member

    Dec 14, 2011
    298
    44
    What is a blocked motor and blocked position ?

    Timescope
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    measure stall current & clamp with solenoid.
     
    bug13 and timescope like this.
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    [​IMG]

    hope my (hopeless) drawing will explain it better.

    motor driving wheel -> wheel attached to rope -> rope attached to weight.

    so the motor will pull the weight up, to the horizontal dash line, something will block the weight go any further. how do I detect that?

    and how do I keep the motor at that position.
     
  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    what do you mean by clamp with solenoid, can you explain it a bit more please?
     
  6. timescope

    Member

    Dec 14, 2011
    298
    44
    A solenoid has a coil and a metal plunger that moves when current flows through the coil. This could be used as a brake to hold the mechanism at the desired position.

    The current through a motor that is blocked will be much higher than the normal load current. You can tell if a motor is blocked by measuring the current through the motor.

    A better method would be to use a sensor to detect the position of the weight and use it's signal to stop the motor and energize the brake.

    Timescope
     
    bug13 likes this.
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    If the wheel had a toothed wheel attached, then a solenoid operated paul would require little power to hold. Sensor might be IR beam break. or a low value resistor in series with motor with comparator can give an adjustable current limit. Your drawing is worth 100 words.
     
    bug13 likes this.
  8. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Is there a way to hold the motor in position without mechanical means? will PWM the motor at a certain duty cycle work?
     
  9. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,001
    745
    Yes , short the terminals of the motor together at the desired position, and the DC braking effect will stop it moving.
     
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    You could use two sensors set up to detect the desired stopped position only when they are both activated Then use a pwm circuit to hold the motor position. Additional sensors could be used to detect when its getting close to the desired position.

    Will the motor be geared down ???
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Use a worm gear. This will lock the position of the load even when there is no power to the motor.
     
    bug13, tubeguy and shortbus like this.
  12. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    I don't know at the moment, it's not up to me I think.
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    Saw a nice worm reduced motor @ Electronic Goldmine, G18238, $ 16.95
     
    bug13 likes this.
Loading...