How to make linear motor to extend/retract by itself?

Discussion in 'General Electronics Chat' started by ericsim, Mar 31, 2013.

  1. ericsim

    Thread Starter New Member

    Mar 31, 2013
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    Hi everyone,

    I am new here. I have two linear motor which I would like to make them to extend and retract by itself. What is the simplest way to make it work? I just want it to turn on and after that it will extend to one position and touches the limit switch, it will retract and touches another limit switch, and it goes like that until I turn off the power. I am mechanical engineering student and I am not familiar with this. Please help :confused:

    Thanks,
    Eric
     
  2. Chris Mc

    New Member

    Mar 28, 2013
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    DPDT relay (double pole double throw)
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    The relay will change polarity, but how do you see it being automated at the end points?

    Ken
     
  4. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Also, please post the operating voltage and current requirements of the linear motor.

    What is the approximate travel time between limits ?
     
  5. ericsim

    Thread Starter New Member

    Mar 31, 2013
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    Hi all,

    Thanks all for the swift reply.

    I am using linear motor with the specification:
    Voltage: 24VDC (But I have used only 12VDC on it)
    Speed: 12mm/s
    Current: 3A max

    The speed is very much not important for me. I will be placing a potentiometer for user to control the speed if necessary. The travel time will be very slow as I will be using this linear motor to simulate the stretching and bending motion of a human leg for physiotherapy purpose. The travel length should be about 200mm but one end of the limit switch can be adjusted to suit the height of the user.

    I have thought of using DPDT relay but I can't figure out how to make the relay to stay on reversed polarity after it touches the limit switch (assume bottom) to another end (assume top) and vice versa as AFAIK if the limit switch is not touched, it will revert back to normal polarity.

    I have browsed through and found this: http://forum.allaboutcircuits.com/showthread.php?t=37345&highlight=motor+reverse (see post #2)

    I am not familiar with the circuit but from there, I can see that by pressing S1 the motor will turn on direction and by pressing S2 the motor will turn another direction. If the user release the button, the motor will stop moving. But what if I want to make it to do this automatically without pressing the button? Can I just directly remove the S1 and S2?

    Oh I forgot to add one more thing. When the user power on the linear motor, it will automatically extend without pressing any button. I guess this can be done with just altering the circuit, right?

    Thanks,
    Eric
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    First, a potentiometer in series with a motor to control the speed is a really bad idea. A variable voltage power supply is better. A PWM (pulse width modulated) power supply is best.

    Second, the push button switches in that circuit would be your limit switches.

    Third, if there is a person attached to your device, you might want to think of all that things that could go wrong, and design safety devices/circuits to limit potential injury.

    Ken
     
  7. ericsim

    Thread Starter New Member

    Mar 31, 2013
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    Hi KMoffett,

    Thanks for your advise.

    May I know what is the potential problem of putting a potentiometer in series with motor? I am sorry I am not familiar with electronic components. :D

    If I change the push button (S1 and S2) to limit switches (NO/NC?), how do I make the linear motor to extend by default? (assuming the linear motor is in between both limit switches and it is not touching any of them).

    I have thought of placing a on/off switch at the power source so that if anything goes wrong, user can turn off the switch to cut off the power.

    Thanks,
    Eric
     
    Last edited: Mar 31, 2013
  8. crutschow

    Expert

    Mar 14, 2008
    12,993
    3,227
    Below is a circuit using two SPDT limit switches and one DPDT relay that should do what you want. The relay is unenergized to extend the motor and energized to retract the motor. Thus the motor will always extend when the power is first applied.

    Note that the relay coil voltage rating should match the motor supply voltage.

    D1 insures that the relay will stay latched for the short time that the Extend limit switch takes to change state as the motor reverses.

    D2 eliminates the relay coil transient that can damage the contacts. If the relay seems to take too long to de-energize than add a small resistor in series with D2 (say 100Ω to start).

    A potentiometer would have to be a high power rheostat to control the speed of a motor which tend to be large and expensive. Better to use a variable electronic control. PWM is the most efficient way to do that such as this circuit.

    Continuous Revers.gif
     
  9. ericsim

    Thread Starter New Member

    Mar 31, 2013
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    Thanks for your reply Crutschow! I really appreciate your help :) I have bought the required item online and will begin working on my project once I have received the components.
     
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