elbow movement

Im working on my own olaf the snowman animatronic.   Im using PVC for the arms.  I want to make one of his arms wave at the elbow.   I think i have a handle on it  but coudl use a bit of advice.   Im cutting a 1" pvc end into a U shape  and then I have heated adn fabricated the lower arm end so that it fits into the us end.   my question is  how to make it wave without sprining back.


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    edited October 2014
    Many of the folks doing simple animatronics for Halloween yard displays/haunts use 12 volt windshield wiper motors to do simple repetitive motion such as a wave.

    The motor is typically hidden inside the body of the character or behind it, and using a cam mechanism you can convert its rotational motion into linear motion for something like a wave.

    I would lock the shoulder in place and use it as a rigid base, and then have the forearm attached with a basic hinge.  Then you can use a length of latex tubing (or elastic/bungee cord) like a muscle to pull the arm to the outer extent of the wave motion.  (where the hand is at the furthest point away from the body during the wave)

    Then you can attach some strong monofilament (fishing line) near to the forearm and have it attach to a cam on a motor that is either geared down to rotate slowly, or powered by lower voltage to slow it down.  During one half of the cam rotation it will pull the line, moving the arm in towards the body, and during the other half of the rotation the line would slowly slack, allowing the latex tubing to pull the arm back to its furthest extent. 

    You can adjust the range of the wave motion by how far down the forearm the line is attached and the length of the line, and the speed can be adjusted by changing the voltage for the motor.  Variation in the length of the cam arm on the motor will also change speed/range of motion.

    If you want to get a bit more complex you can do the entire mechanism without the line being exposed, so everything is completely hidden.

    Here is an image that should help you visualize this concept.

    Best of luck!

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