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Need help. non-contact measuring device.

  1. Oct 12, 2004 #1
    I am doing a project called "gravitational waves in the rubber universe" and am looking for a little help.
    The idea is to make a cone shape from latex and roll a ball-bearing around it (a bit like those donation boxes where you put in a coin and it rolls around in circles untill it falls in the hole at the middle). From this I need to measure the displacement of the latex at various points as the ball moves around.
    This is where I have the problem of what method to use. A few ideas I've had are:

    Laser: put a reflective foil on the rubber and shine the laser onto it and measure the change in reflected angle.

    Utrasound: Have an ultrasound generatror/reciever above the latex to measure the change in distance between it and the rubber.

    Anyone with a good idea or any improvements or information on the two ideas above please help :smile:
  2. jcsd
  3. Oct 12, 2004 #2


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    Staff: Mentor

    That's a hard one. It would probably just be better to do a 3-D simulation and do an animation of the results. Is that a possibility?

    If not, then you could draw a grid on the underside of the cone with a felt pen, and use a video camera underneath to watch the distortions in the grid. The ball will have to be heavy compared to the elastic force of the cone, and you will have to video tape a number of runs from different angles, but you should be able to use the overall video to calculate the forces involved at different heights in the spiral down the chute.

    Let us know what you figure out. -MIke-
  4. Oct 12, 2004 #3
    I like berkmans idea. I would pit a tapered grid on the latex before you roll into cone. The taper is so each of the resulting squares are the same size after forming the cone. Make the grid finer than the ball diameter so you can see the change easier.

    You can also just look at the cone in profile (just at the edge)and you will see the deflection there, which is what you are interested in.

    I think video and post analysis is your best bet. Not in situ measurements like a laser.

  5. Oct 12, 2004 #4


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    Homework Helper

    Assuming you can determine the undisturbed shape of the latex, I would focus on measuring the movement of the ball, not the latex.

    If the latex was transparent enough, maybe you could use a bright light (far enough away to not heat things up) source and 3 video cameras (x, y, and z axis), to measure the path, or regular cameras synchonised to take pictures at regular intervals, then a curve fit done afterwards. If you use video cameras, I'd recomment a "clap" stick (make a sharp fast sound), to synchronise the videos afterwards.
    Last edited: Oct 12, 2004
  6. Oct 13, 2004 #5
    Thanks for the feedback. Using a grid is definately a good idea. I will look into using video cameras, but the problem is I need numerical readouts of the dispalcement. However I may be able to feed the video data into a computer and by using points on the grid write a programme to give the numerical values as the point moves around.
    Will keep you posted on how it works out and any further problems.
  7. Oct 14, 2004 #6
    Numerical readouts... A digital milliamp meter would provide numbers if you wound a loop of wire near the underside of the latex. Two looong loops of 2 wires close together, each only touching once at the very end. One loop for bias, the other for contrast (like a metal detector). Connect a 9v battery to the bias loop, connect the contrast loop to the DVOM set to milliamps.
    As the weight & speed of the ball stretches the latex nearer to the loop, the inductance of the contrast loop would change, causing a deflection on the DVOM (multimeter).
    It would probably block the grid though...
  8. Oct 18, 2004 #7
    when tooling cribs set concentricity of a milling tool, one method they use is shadow graphing.
    the amplification is a matter of the distance between the light and the cone, and the distance between the cone and shadow graph screen.
    do your measurements on a percentage basis, set up an amplified grid on the screen accordingly (the cone will also move up due to displacement i think), consider 2d same as 3d due to consistency of material, use a heavy ball, and video for accuracy.
    good luck
  9. Oct 22, 2004 #8
    If you coat the underside of the latex with a grounded, conducting material to form an electrode (A), then at location x1 you can put a second, small
    electrode (Bx1) spaced about one mm away from A. Displacement of
    the latex at location x1 will cause a measurable change in the capacitance
    of A-Bx1. B electrodes could be spaced appropriately around the under
    surface of the latex cone at locations x1, x2, x3 ...... The data, being
    capacitance values at those locations, is in a form that is easy to record
    and analyze.
    This would be easier to do if you could construct your large surface
    with aluminized mylar as is done in active-control astronomical mirrors
    (cf the work of James Faller at the University of Colorado, Boulder) where
    the forces from an array of such capacitors is used to fine-tune the shape
    of a parabolic reflecting surface in order to overcome the image jitter from atmospheric effects.

    Jens Zorn, Physics Dept University of Michigan
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