Dot Physics

Potentiometer for angle measurements

There is something I am working on that will need some type of angle sensor. I am going to use a potentiometer. First, who came up with that name? Isn’t this name the same as a voltmeter? Something is wrong with that name. How about we just call it a variable resistor or something like that?

These things are pretty easy to find if you have some old electronic stuff. Here is one I found in our “junk room”.

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This one was used in a lab as a variable resistor with plugs built in. The normal potentiometer has three connections; one for each end and one for the variable position in the middle. For this device, only two are used (the middle one and one of the ends).

All I am going to do is see how the resistance changes with the angle. I really just wanted to see if this works, so I am just approximating the angle and look at the resistance at 0, 90, 180, and 270 degrees (measured with a multimeter). Here is the data that I get:

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Looks pretty linear. This means that I can measure the resistance and get the angle by using the following function:

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Simple enough. Hopefully, it will be useful also (to me). If you have your own potentiometer, I doubt it will have the same parameters as this one. You should probably do something similar if you want to use it.

Comments

  1. #1 Gray Gaffer
    August 12, 2010

    This may be of interest re the history:

    http://tinyurl.com/3cm224

    I don’t know if this is still done, but in my day at HS and college (UK, 1963 – 1967) wire resistance bridges and galvanometers were standard lab fare for measuring voltages. Which I recommend you do instead of measuring resistance (which is indirect anyway). “Potentiometer” derives from using them as a method of measuring voltage potentials. See http://tinyurl.com/2b32xgo for the really gory details of how that went.

    “variable resistor” – this is how you are actually using by ignoring the third terminal, but not an accurate description of what it is. There are actually two terminal devices which really are “variable resistors”, typically with a screw adjustor at one end instead of the volume control style, used for trimming circuit behaviours.

    You perhaps should mention that you luckily found a linear pot for this project. All too often what one finds by random choice is a log pot – these are used for audio volume controls. You would not have got that nice linear resistance plot with one.

  2. #2 Eric Juve
    August 12, 2010

    Actually the two terminal version of the variable resistor is referred to as a rheostat, the three terminal version is a potentiometer. Not to be picky….

  3. #3 Adam_Y
    August 12, 2010

    First, who came up with that name? Isn’t this name the same as a voltmeter? Something is wrong with that name. How about we just call it a variable resistor or something like that?

    Because the way you are supposed to use the three terminal devices is by measuring the potential and not the resistance. As Eric Juve pointed out if you use them as two terminal devices then they are variable resistors.

  4. #4 Keith Barkley
    August 13, 2010

    You are lucky the one you found was not an audio taper version!

  5. #5 Stephen Matthews
    January 5, 2011

    Hello,

    Could anybody please suggest a low cost variable resistor that would provide a linear output for measuring angles.

    The device is for extreme outdoor purposes and repeatably needs to define angle measurements within one degree.

    I appreciate your help.

    Kind regards

    Stephen Matthews

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