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measuring pulling force — Parallax Forums

measuring pulling force

mikeweezermikeweezer Posts: 27
edited 2005-10-18 00:21 in General Discussion
Hello all,

I need to be able to measure the force being pulled on a rope. That information can can go either into a computer via Serial or USB - or into the Basic Stamp. I've been recommended to Load Cells by Omega Engineering, its just that their prices are a bit high. Any ideas?

Thanks,
Mike

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2005-10-13 08:14
    You might try a general hardware store spring of suitable strength [noparse][[/noparse]and size] and a linear potentiometer. This would be something like a 'spring scale' that is used to measure either weight or pull, but a linear electronic input added to it.

    The position would be indicated by the resistance.

    If you need free movement of the rope, use a pulley to attach to the device.

    The only real problem I forsee is that the linear potentiometers tend to get dirty due to having an open slip.
    If you can position it 'face down', you will get better duribility. Also, you might put the whole device in a zip lock baggy as a weather protection.

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  • stamptrolstamptrol Posts: 1,731
    edited 2005-10-14 13:09
    Mike,

    If you're a scrounger, the usual benchtop weighing scales used at the grocery store or candy store have one or more small load cells in them. Many times they are not economical to repair so the scale companies junk them. Look under Commercial Scales in your yellow pages and give them a call, looking for scrapped load cells.

    Then, with a fairly simple op-ampp circuit and perhaps some mechanical mounting work you'll have something to play with.

    Tom
  • Alan BradfordAlan Bradford Posts: 172
    edited 2005-10-18 00:21
    Hi,
    You could scrounge an old encoder and use a spring to make a poor mans load cell.
    Mount a pulley on the encoder shaft and mount it so it cannot move under max load.
    Attach the spring to a fixed wall or structure. Attach a cable to the other end of the spring, wrap it 2-3 times around the encoder pulley·and then attach the other end to the load. You may need an Idler pulley to take the load off the encoder pulley if the load is greater than the encoder shaft can handle.
    As you pull against the load, the spring will streach and cause the encoder shaft to rotate proportional to the load.
    You will have to calibrate it with a known load, X number of counts per Y load.
    This will make a simple load cell.

    Good Luck
    Alan Bradford
    Plasma Technologies
    www.plasmatechnologies.com
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