motor driver with capacitor as timer

Discussion in 'The Projects Forum' started by Sergio83, May 2, 2010.

  1. Sergio83

    Thread Starter New Member

    May 2, 2010
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    Hi all,

    first, thanks for this site, it helped me a lot to better understand basic components and rule of electricity, even if i'm still a rookie.

    I'm trying to do a project about a motor driver.

    I'm going to use a DC motor with brushes.
    I've to supply the circuit with an AAA battery of 1,5V. If is not enough i can think of using 2 batteries in order to reach 3V supply.

    What i want to achieve is:

    <
    -The user press only one button for a time 'T' which can vary every time.
    Pressing that button the engine turns clockwise.

    -When the user release the button, the motor turns anticlockwise for the same time 'T' the user pushed the button before.
    Then everything stop untill the user press the button again.

    >

    In that way the engine will go back the same turns it went forward. (actually even if is not that precise it doesn't matter)

    I thought to do that without IC such as PICs, but to use a capacitor as memory/timer.
    I would like to do that circuit as much easy as possible, in order to get the cheapest and smallest system i can, even if not that precise.

    I came out with that idea:

    [​IMG]

    For the weak knowledge i have about electricity it looks like it can work, but i need an expert look.

    Can it work?

    I bet i have to fix something.
    If it is completly wrong i'll try to use transistors and H bridge.


    Thanks for your attention.

    Sergio

    P.S. for the button i will use that double throw micro button:
    [​IMG]
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    No,it cannot work.

    What is the application the motor is used in?

    It would be much easier and accurate if you can place to limit switches on the positions you want the motor to stop. By pushing the first button, the motor moves forward until it reaches the limit switch. By pushing the second button, the motor reverses until it reaches the second limit switch.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hello Sergio,

    Unfortunately, it's not going to work like that.

    The switch you are using looks like it's a pretty low-current device; those are generally rated for 30v, 100mA. It would not last very long controlling an electric motor.

    It's very difficult (if not impossible) to get a DC motor to run one direction for awhile, and then the same number of turns in the other direction simply by using timer(s). It's a lot easier if you use limit switches, or if you count the number of turns the motor has actually completed.

    Suddenly reversing the direction of current flow through a motor is very hard on it, and the switching device used. It's sort of like throwing it against a block wall.

    The quickest safe way to stop a DC motor is to short it's terminals together, and wait for it to stop - then reverse the direction of current.
     
  4. Sergio83

    Thread Starter New Member

    May 2, 2010
    6
    0
    Thanks for your answers.

    What do you exactly mean for limit switches?
    i googled it, and i guess they are switches that work by physical contact and that are quite big.

    In that case they are not good for this project since i want to make something quite small, like a mobile.
    Even the engine i'm using is really small and low power. (it's the same motor u can find inside a electric razor).

    You should also consider that i dont want the engine to have a limit. The user could make the motor turns for 100 turns, and the next time for 45 turns. The important thing is the motor to go back more or less the same turns.
    Instead with the limit switches it probably has always the same amount of turns.

    I'll tell you what's the use for:
    The motor has to twist a thread for depilation purposes. It is something like this machine.

    I already made a prototype where the user act on two buttons, one for the clockwise movement and one for anticlockwise movement. The motor works great but i would like to make the anticlockwise movement automatic.

    Even if there's no precision or the circuit is not accademically correct, i don't care is enough that it doesn't explode. The object is not thought to be an high-tech device, like something you can buy at a 99p store.

    Anyway i'm doing that for my Industrial Design thesis, i would like to make a real prototype.
     
    Last edited by a moderator: May 3, 2010
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Will this do? Switch is DPDT , when pushed, C charges via R. Time should be less than R X C. Motor runs while sw is held, when released, motor runs in opoaite direction untill C discharges giving almost = run times.Comparator should be high impedance input & operate singleended. Shaver motor runs great on 4.5V, not great on 1.5V, no load about 150mA, stalled .5A.
     
    Sergio83 likes this.
  6. Sergio83

    Thread Starter New Member

    May 2, 2010
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    Thanks a lot for your help Bernard, i haven't seen your post till now. I will try to study your scheme, I really hope to understand it, since it looks exactly what i need. Thanks also for the advise about the 4.5V.
    Are the H rectangles about H-bridge?

    By the way I was here to ask for this other scheme, which is probably less correct, but much simpler, and if it works almost the same, i'll be quite happy with it:

    [​IMG]

    Do you think is that a more reasonable solution than the previous i made?

    With the value you can see in the scheme, when the user push the engine runs for 2.5 second untill the charged capacitor stops the flow.
    When the user release the button the capacitor gives power to the engine for almost the same time (being a short time the difference are really small).

    Sergio
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It'll work fine if you can find a motor that will run on 400μA, or 2mW. The advantage of using a comparator is its high input impedance, same with H bridge vs electro-mechanical relay, about 800 mW or about as much as motor. Steve Bolt ?? published a simple H bridge for low V moror; maybe some one will remember more.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Put Steve Bolt in Find box, ref to H bridge and a pot full of others curtesy The Sgt.
     
  9. Sergio83

    Thread Starter New Member

    May 2, 2010
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    do you mean that I actually need a much bigger capacitor?

    Thanks for the steve bolt link
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If you want the motor to turn precisely the same number in one direction as the other, you will not be able to do so with a timer. You will need to count the turns in one direction, and then count the turns going in the other direction.

    This will involve at least a Hall-effect sensor and a binary counter, or something like a rotational encoder.

    A microcontroller will help to keep your parts count low, but the learning curve will be steep.

    A simple capacitor simply won't get the job done.
     
  11. Sergio83

    Thread Starter New Member

    May 2, 2010
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    I don't need much precision, but probably enough to don't allow me to use a timer.
    Let's say that the motor does 100turns per second. I don't want the system to has more than 5turns error.
    So i need a precision of 0,05 seconds.

    Isn't that ok for a timer?

    Maybe the motor can make once 7turns and onother time 4turns, even if I supllied power for 0,05 seconds in both times?

    ANYWAY

    I think i'll try to use a Flip-Flop now with a Hall Effect Sensor (but adding a wheel with magnet is gonna get the machine too complicated) or with a timer (if precision is enough).

    I'm starting to believe the same, but i can't understand why this calculation I made doesn't work:
    Knowing the initial Voltage (1,5V), the resistance of my Low Voltage pmDC Motor (10ohm) and the capacitance of my Back-Up Capacitor of 1F (yes one farad)...
    ...I calculate the Voltage respect to the time and so the Power supplied to the motor by the dischaging capacitor:
    [​IMG]

    Watching the Performance Data of my motor, i can say that with the Watt supplied by the capacitor the motor should run for few seconds:
    [​IMG]

    Instead the theory say something, and the pratic something else.
    I made an experiment with these components, but nothing happened, the motor was dead. Zero Turns.
    (No loads where applied to the motor)

    What did i do wrong?


    Ciao
    Sergio
     
    Last edited: May 15, 2010
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    There is no way, using a timer, you are going to get 5 revolution accuracy.

    You will need a smart brake that will count the amount of turns you want, learn the motor, and reduce the speed to the point where a mechanical breaking system can halt the motor at the stop point.

    If you use a high RPM motor geared down to 100rpm, this will be a much easier project.

    I think, if this is to be automated, you should count the revolutions and depending on how long the motor takes to slow down from full power, use 40 or 50 rpm to finish the last 10 revolutions or so.

    A timer just wont cut the cake.

    You are talking about RPM in excess or 9000. And you say:
    I dont need precision, and 5 turn precision. In other words. You NEED precision.

    Count the turns. adjust the speed when approaching the final turns to a speed where the break can stop the motor in 1 turn.
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Use an Up/ Down counter; when SW pressed, motor runs, count increases untill SW is released, motor reverses, count goes down to 0, motor stopps.
     
  14. retched

    AAC Fanatic!

    Dec 5, 2009
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    the thing is, when the motors power is cut, any flywheel inertia or shaft inertia will be different at the loaded(wound) end and the empty end. So with 9000 turns of wire on the bobbin, it will have a few more rotations till it comes to a stop then when it has unwound after the reverse.
     
  15. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    With a up/down counter, we can decode a low count to initiate a slowdown.
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
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    True. I suppose if it is a repetitive task. But if the end weight is going to change, adjustments will need to be continuously made
     
  17. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It would be nice to know what the maximum number of turns is required, how fast is the final drive, how much torque, & howmuch space is available???
     
  18. Sergio83

    Thread Starter New Member

    May 2, 2010
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    0
    hi guys, in the end i understood the precision i want is too much for allow me to solve the problem in an easy way.

    Since is becoming too complicated with sensors and stuff, I think i will leave the idea to use electricity and i'll go back to a mechanical solution.

    I wanted to go electrical because i thought was more simple, but is not.

    Thanks to make me understand where are the lacks in using eletricity for this task.


    Ciao
    Sergio
     
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