Measuring linear position

Discussion in 'The Projects Forum' started by sully1071, Feb 22, 2012.

  1. sully1071

    Thread Starter New Member

    Nov 8, 2011
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    Im looking for a sensor to measure linear position between 0 and 1000mm. The sensor is for a college project so has to be as cheap as possible. I wanted to but a draw wire potentiometer but they seem to costing around €300 and my budget is under 100. I was loooking at making a string pot using a 10 turn pot, im not sure of the electronics? would it be a difficult build?

    or is there other options?
     
  2. luvv

    Member

    May 26, 2011
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    I'm sure someone will be by to tell you a pic could do exactly what you need.

    And they are 100% correct.

    Buut..if it where me, i would use your variable resistor as a voltage divider.

    Then build one of the many voltage meter circuits out there to display distance in terms relative to voltage.

    -luvv-
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    sully,

    Since you posted this on another forum, and got many questions there, you may want to post all the asked for info here, as well as the photo. ;) The more info you give, the easier it is to help you.

    Ken
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I would use a drum of about 10 cm circumfrence, note that the circum. lies part way into wire; use a measuring tapes spring return spring to rewind wire on drum. Attach a slotted disc, 10 slots, on one end of drum Use 2 optical slot detectors, 90 deg out of phase. I believe there is an explination in AACs Volume 4.
    Or, can use drum & 10, or greator.turn pot, with a stronger return spring. Use an OP Amp, to adj. output V to cover 0 to 5, or as desired.
    Vol IV- Ch 11- synchronous counters- last page
     
    Last edited: Feb 23, 2012
  5. KMoffett

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    Dec 19, 2007
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    Actually, the OP only needs one opto-detector, since they are only measuring in one direction (that, from an earlier post). The "tape measure" spring is called a "constant force" spring in the industry.

    Ken
     
  6. #12

    Expert

    Nov 30, 2010
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    It could be as simple as this.
     
  7. paulktreg

    Distinguished Member

    Jun 2, 2008
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    How accurately does it need to measure the position?
     
  8. Bernard

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    Aug 7, 2008
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    " as simple as this " ; if it is a 10 turn pot, but believe a little under & over shoot would be prudent.
     
  9. #12

    Expert

    Nov 30, 2010
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    It's a ten turn pot. That's why it is labeled, "10 turns". The unlabeled pot is for overshoot compensation. If it needs to be compensated for undershoot (below zero volts), that's a bit complicated for a concept drawing with no accuracy requirements specified by the OP.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Don't you think you would need a level wind mechanism to keep the cable/wire/string on the surface of the drum (think bait casting reel)? Also, the fact that the cable is wound in a helix has to be considered when designing the drum, in addition to the diameter of the cable.
    These things are expensive for a reason.
     
  11. Bernard

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    Aug 7, 2008
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    Ron, point well taken & considered. Wire, or cable. that I am looking at is the type used on drafting board sliding arm & is .o3 in, ss. Ten turns only takes .3 in, so with constant tension should lay down in one layer. If required, a simple level wind could be added; two gears. threaded rod [ screw], & traveling nut.
     
  12. Ron H

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    Apr 14, 2005
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    Our OP hasn't come back. I'm wondering if 1cm resolution would be sufficient.
    I have always found that "simple" mechanisms like this are not so simple, unless you have access to a machine shop. Where do you get a ≈10cm circumference drum that will couple to the shaft on the pot? Where do you get the slotted disk? I suppose it could be printed on clear mylar. Attaching the spring might even be a challenge.
     
  13. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    There are free programs out there to create and print any encoder pattern on Mylar with a laser printer.
    You don't actually have to attach the constant force spring to the cylinder. If you maintain >1.5 wraps minimum it will hold tight.

    (S)He's also posted over on the Electro-Tech Forum. Someone there suggested an accelerometer. Sounds like a good, inexpensive, high-tech solution.

    Ken
     
  14. Bernard

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    Aug 7, 2008
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    Re Ron H: Where do you get a drum; If the max equipment is a drill press, then a 1.5 in hole saw makes a nice 1.25 in drum, which can be filed down or built up with thin metal, ie beer can. Looks like " Gym equipment project" & this one are kaput.
     
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