need help on controlling an electrical winch

Discussion in 'The Projects Forum' started by davis103, Jun 9, 2008.

  1. davis103

    Thread Starter Member

    Jun 9, 2008
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    Hello every body:

    Here is my concern:

    I have chosen a motor to exert a force on an object. the motor is an electric winch (12 v and about 6 amps drawing current for 150 lbs up to 20amps). Well the key of this project is to get the motor to apply only 150 lbs, then hold the force for 1 second and then release the force. This procedure will be repeated for a certain amount of time.
    Here it is how i think i can do it:

    1. have a swith to be an input to microcontroller
    2. then the output of controller will be high when switch is pressed
    3. To drive the current i think a mosfet can do it with the motor's specs
    4. Try to use current sensing circuit as a feedback to microcontroller to check if force is 150lbs

    My main question is how do i limit the current through the motor for a desired force. For instance, lets say i want 100 lbs which is proportional to current through motor and in order to produce 100lbs i need 4 amps then my current limit will be 4 amps. The winch will keep drawing current until the supply voltage is reached.

    Any suggestions, improvements, Is the feedback good or not?

    I appreciate every comment
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    PWM is just fine for controlling the speed of the winch, but the methodology is designed to allow full torque over the speed range of the motor - not to allow one to limit the pull on the hook. You will have to control current to reduce torque.

    Running the current passer as a limiting resistor means the FET/s will have to be heavily heat sinked. Will the motor withstand the stall current? To limit the pull means that the motor will be operated with the armature stalled. This can build heat very quickly. You may need to blow air on the motor as well as the heat sink.

    If you have to exert the 150 lbs accurately, you will need to use some other way to sense the force than motor current. Brush noise alone make this necessary. You might think of a load cell arrangement or an S cell. Or a spring scale with an optical sensor.
     
  3. davis103

    Thread Starter Member

    Jun 9, 2008
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    Well the load cell sensors are way too expensive and this why i am trying to use some other method

    The heat problem it is something i have not taken innto account. Your information is extremely usefull.

    I think if i come up with a mechanical design of stoping the motor with a locking mechanism when it reaches the desired force (150 lbs) would take care of the heat problem. Right?
    Any suggestions on other way of measureing force. The price on load cell sensors are about 100 dollars for one up to 150 lbs in digikey website. This is way too expensive

    I greatly appreciate your quick response
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    The actual torque exerted by the winch motor once it is stalled may be less than it was before stalling. Thus, if it stalls at 150 pounds, you can probably keep it stalled with considerably less than 150 pounds, depending on the design.

    A clutch is one way around that.

    Another option might be to use a spring mechanism in the system so the spring actually maintains the pull while the motor is stopped. For the winch, you may want to consider a helical gear winch. That is, one that is self braking. In that design, the winch would run until the correct tension is attained, then the winch stays off while the spring maintains tension. As a more concrete example, imagine a spring-loaded arm that is under the line. As tension builds, the arm is pulled down until the tension is 150 pounds. The motor stops, and the arm maintains the tension. John

    Edit: I meant worm gear
     
    Last edited: Jun 10, 2008
  5. davis103

    Thread Starter Member

    Jun 9, 2008
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    What do you suggest for sensing the Force applied
     
  6. jpanhalt

    AAC Fanatic!

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    How accurate does it need to be? Something as simple as a limit switch a certain position on the spring-loaded tensioner arm might work. John
     
  7. davis103

    Thread Starter Member

    Jun 9, 2008
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    It has to be within a 8% error.
     
  8. davis103

    Thread Starter Member

    Jun 9, 2008
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    Also how do i certainly know if the force applied is accurate. If i use a limit switch i can not see if it is doing it or not. In other words by having a force sensor i can feed this back into microcontroller and then display it based on electrical signal. The thing is that most of Force sensors are made to measure compression force not tension like a weight sensor
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    You can get unmounted strain gauges from Omega Engineering and make your own load cell.
     
  10. RiJoRI

    Well-Known Member

    Aug 15, 2007
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    And don't forget "Engineering Samples" to get one or two pieces from a manufacturer!

    --Rich
     
  11. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    A fish scale (for big fish) works too. John
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    For single point sensing. If you're trying to run a servo loop to hold a tension, a load cell of some sort will be needed. It might be possible to modify a spring scale to give an electrical output proportional to displacement...
     
  13. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    That was the purpose of the spring-loaded lever (once calibrated) and microswitch. Two microswitches could be rigged to increase or decrease tension, as needed.

    There are still lots of details that are not clear, such as, whether a servo loop to decrease tension is needed, what the budget is, what the winch line is (how much stretch), and is the pulled object moved or static. I am sure there are others details as well.

    John
     
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