servo motor controller

Discussion in 'The Projects Forum' started by khalillabaki, Sep 28, 2016.

  1. khalillabaki

    Thread Starter New Member

    Sep 28, 2016
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    hello guys,

    i need to build an electronic circuit to control a servo motor
    the servo i have, doesn't have any electronic circuit in it, just a motor connected to a potentiometer via gears, so 5 wires come out of the servo (2 power wires for the dc motor and 3 wires for the potentiometer).
    i want to control the servo with a poentiometer.
    the input voltage for the system is 12vdc
    the motor is also 12v.
    can anyone please help me build this circuit board?

    thanks in advance
     
  2. nerdegutta

    Moderator

    Dec 15, 2009
    2,515
    785
    I think you need to add a picture or two of the motor, or at least a link to a datasheet.

    Oh... And welcome to AAC. :)
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    2,369
    If it is a typical DC motor do you need continuous rotation or just position within the range of the pot?
    If continuous, do you need accurate positioning and velocity or just velocity?
    Max.
     
  4. khalillabaki

    Thread Starter New Member

    Sep 28, 2016
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    i dont need continuous rotaion. The purpose of it is to control the throttle valve on an engine, with the potentiometer mounted on the gas pedale
     
  5. crutschow

    Expert

    Mar 14, 2008
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    3,232
    So then you need the circuit to control the motor so that the gas peddle pot position matches the servo pot position(?).

    But we do need more info on the servo to suggest a proper solution.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You could use a pot on the pedal and another on the valve and move the motor until coincidence occurs between pots.
    Have you used any micro processors or just logic IC's.?
    Max.
     
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    There is also another way of making a synchro with two small identical bi-polar stepper motors connected together, one will track the other, no power needed.
    Max.
     
  8. khalillabaki

    Thread Starter New Member

    Sep 28, 2016
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    the motor is 12v 1.4A rated current
    The potentiometer is 3k ohm

    Im not too into electronic circuit, i just need a circuit diagram so i can apply
     
  9. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,685
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    Now that we know it is for a throttle pedal, is it for a car? If so, what make and model? Is it a repair or are you creating something?

    I have attached a drawing for the connections that a Ford electronic throttle uses. By necessity and common sense, there are safety features. I wonder whether this project is appropriate as a DIY for someone with little experience?

    John
     
  10. khalillabaki

    Thread Starter New Member

    Sep 28, 2016
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    Yes it is for a home built car, i built it at my workshop. So im creating something not that complicated, just a servo motor to pull the throttle cable according to the position of the potentiometer on the gas pedale.
    Im making this system because i have a long distance between the throttle pedale and the throttle valve on the engine, so the cable has high friction and doesnt give accurate throttle response, and sometimes it stucks. In otherwords, im placing the servo near the engine, and connect it to the existing throttle cable but with a shorter distnace now.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Sounds like something a cruise control could be adapted for?
    Any wreckers where you are?
    Max.
     
  12. khalillabaki

    Thread Starter New Member

    Sep 28, 2016
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    Yes exactly its a cruise control servo, for BMW. It fits perfectly! I just need a circuit diagram to control it
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If you have nothing else but the motor and the pot for the pedal, you are going to need some kind of feedback element such as a pot on the throttle position, and use some kind of comparator/motor correction circuit for each pot position.
    Max.
     
  14. khalillabaki

    Thread Starter New Member

    Sep 28, 2016
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    No, the servo assembly has a motor and a potentiometer connected to it via gears (this is the feedback pot) i need an electronic circuit and a potentiometer on the gas pedale to control the servo. I need to know what pot size i need on the gas pedal as well as the circuit diagram for the controller
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It most likely could be done with a Picmicro and a PWM output module, but you state you are not very electronics literate?
    Max.
     
  16. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,931
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    If this is a full-size car,and not a model, I think this sounds very dangerous. What precautions do you have in mind for the throttle stuck in the fully open position when this circuit goes wrong?
     
  17. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Yeah, the probability of a home brew servo position control going haywire is rather high.

    Position servo's like this are far from fail-safe, in fact they love to slam to full scale when any part of the system fails.

    Shorted drive transistor? FULL THROTTLE!
    Flakey pot? FULL THROTTLE!
    Gear falls off? FULL THROTTLE!
    In all of these failure modes, you have a 50-50 chance of full throttle- scary.
     
    Last edited: Oct 2, 2016
  18. crutschow

    Expert

    Mar 14, 2008
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    How much current does the servo take.
    Does the motor need to be driven by one polarity to advance the throttle and the opposite polarity to retard the throttle?
     
  19. khalillabaki

    Thread Starter New Member

    Sep 28, 2016
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    0
    The motor takes app. 0.8A
    It does have to be driven in the two polarities. In case of circuit failure, the throttle valve has a return spring to idle position.
     
  20. Sensacell

    Well-Known Member

    Jun 19, 2012
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    Big spring?

    If the motor can overcome the spring to open the throttle, it has then NO UTILITY as a fail safe mechanism.

    The motor can easily fail FULL POWER.
     
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