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Beam Curvature Measurement with a Capacitive Sensor

  1. Apr 30, 2012 #1

    I am trying to measure the center deflection of a simply supported beam with a capacitive sensor. The beam's surface that is the sensor's target is originally flat. After applying equal forces on the two edges of the beam, its center curves upwards (away from the capacitive sensor) thus creating a concave surface. I am not sure how to correct the sensor's reading to compensate for the curvature of the beam.

    The beam's specs are: 5mm x 5mm cross-section, 30mm length. Equal forces of about 800-1000N are applied on each edge. The center moves up about 15um. The beam is made of high strength aluminum. The supports are placed in between the loading location and the center of the beam. Thus, its center moves up when the edges are pushed down.

    The capacitive sensor has a target electrode of 5mm in diameter. It's range is 500um and provides a voltage output of 1 volt per 50um. Its resolution goes down to 3 decimal places.

    I'd really appreciate any suggestions.
    Thank you.
  2. jcsd
  3. May 2, 2012 #2
    I would say your largest problem right now is the 5mm sensing area. The electric field from the electrode is somewhat conical so the spot size is about 130% of sensing area diameter. This means that the field is wrapping over the edges of your beam and the sensor is already not operating in its calibrated condition (you have errors).

    Also, smaller tips are less affected by radiused surfaces because the surface is flatter in smaller areas. So the large tip is making this worse.

    Of course this is all dependent on your desired resolution/accuracy. 3 decimal places is only 1/1000. Is this Lion Precision equipment? We have a sensor that matches those specs, and it can have resolution as high as 10 nm.

    I can't speak to the 5mm sensor, but we have data showing that a 2mm sensor has about a 2% sensitivity error when measuring a 1 inch sphere. Your bend radius is larger, but so is your sensor size. There is also an offset error, curved surfaces appear further away, since your target is changing shape you will also have that error source. Fortunately for you, 15µm isn't very far so you may still be within your desired accuracy.
    Last edited by a moderator: May 21, 2012
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