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Ideas for sensors measuring the elbow angle

  1. Aug 30, 2014 #1
    Hello there!

    I'm working for about 2 months to build a system for getting digitally values about the angle of the elbow. The project is designated to create a large usage tool and that's why it's preferably to use mechanical staff that are comfortable for the 'user' and to not look very peculiar. It is not compulsory to get directly the angle because if I get for instance the distance between hand and shoulder it's easy afterwards to transform it in angle. I have some options using mouse encoders but implemented on the body but they look too strange and its too much mechanic.
    If you have any suggestions/ideas please type them here, any response is welcome !
  2. jcsd
  3. Aug 30, 2014 #2

    Simon Bridge

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    Welcome to PF;
    How? Don't you need to know the length of the forearm and upper arm?

    Wrap wire in a coil around a rubber cord, stretch it from the wrist to shoulder - the coils separate as the rubber extends, changing the inductance of the coil. Research how to measure inductance electronically.

    You could always just use a laser range-finder... the bottom line is that there are too many variables: could you narrow it down a bit?
  4. Aug 30, 2014 #3


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    A potentiometer rotating with the elbow could be used as a rotary encoder, returning a voltage related to angle of angle of rotation.
  5. Aug 30, 2014 #4


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  6. Aug 31, 2014 #5
    When I get the max distance between wrist and shoulder automatically I can calculate the lengths of the forearm and upper arm because there is a constant ration between them (maybe it's the gold ratio)
    I don't know how precise is the method with the coil around the rubber cord. Is not possible to make only two 'bracelets' one at the wrist and one at the shoulder ?
    The 'user' can tilt downward and upward the arm. The laser range finder is working when I rotate the arm?

    This is one of the first options I have, although the arm is a bit too loaded with mechanic things..

    Maybe 1-6 grades error is acceptable.
  7. Aug 31, 2014 #6

    Simon Bridge

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    Or maybe not.
    It's like any other ratio in the human body - varies from individual to individual.
    If you plot forearm+upper-arm length vs forearm length, you get a wide scatter.

    It's used in NZ as a quick way to measure the circumference of tree trunks. It's as accurate as you want it to be.

    Or whatever you want - the details of the design are up to you.

    Operator aims the device, unless auto-aiming apparatus is included - you could also just use a tape-measure.

    The acceptable uncertainty values is up to you and depend on what you want to use the data for.
  8. Aug 31, 2014 #7


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    Consider two mutually perpendicular elliptical coils about the upper arm, fed in quadrature, sine and cosine.
    Then a pick-up coil about the wrist like a bangle.
    The phase of the signal in the pick-up coil will be proportional to the elbow angle.
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