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Synchronizing two motors for Pulley

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  • Synchronizing two motors for Pulley

    I had some thoughts about synchronizing two motors for our pulley system:
    1. Y-cable power and encoder
    2. PID Control

    Are there other ways that it would be doable? I would like to use PID but I'm a bit wary of how it'll perform because we have a cross bar that should be level throughout the entire lifting process.

  • #2

    You CAN Y-Cable the motors, but NOT the encoders. The signals from the two encoders will clash with each-other.

    That said.... it's perfectly reasonable to Y-Cable the motors (allowing for any possible direction reversals) but only use ONE encoder.
    We do this all the time for 4WD wheels. The single encoder measures the speed/position of BOTH motors (since they are mechanically interconnected) and by driving both motors with the same power, they ballence the load and give approx 2X torque.

    You definitely need to put the Motor Controller into either RUN_USING_ENCODER or RUN_TO_POSITION mode, and verify that both motors are running in the correct direction before mechanically interconnecting them.
    I suggest getting it working with 1 motor and encoder, and then adding the second motor.



    • #3
      Ideally in situations where 2 motors need to be synchronized I like to see a mechanical linkage. i.e. they share a common shaft or chain hooks them together, etc. Then a Y cable on the power and a single encoder channel from one motor works well.

      If the design precludes that - then a Y cable on just the power works. But if one of the motors is "tired" then it won't go as fast/far as the other one. We've had poor luck with tetrix motors - sometimes when stressed they don't burn out completely but don't really work as well as they used to.

      If the motors aren't mechanically linked - it is better to put each motor on it's own power cable and motor controller with it's own encoder. They might not stay perfectly synchronized but with using RUN_USING_ENCODER or RUN_TO_POSITION they will stay pretty close.


      • #4
        With more testing, without PID or mechanical link, it seems that the bar can't come down due to a cross bar causing one motor to take on a lot of the load. Since I'm planning to use the pulley only in TeleOp, how exactly would I use RUN_USING_ENCODER? Would I just use really small distances like 100 ticks?