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Recommended Flexiforce sensor mounting? — Parallax Forums

Recommended Flexiforce sensor mounting?

OrionOrion Posts: 236
edited 2005-07-22 23:34 in General Discussion
I'm about to use the Flexiforce sensor to measure the amount of liquid in a barrel.· I'm only looking for a rough gauge of how much is left, let’s say 10 divisions. IE >90%, 89-80%, 79-70% etc.· The barrel will weigh in at about 160lbs when full.· My current idea is to build a platform with four legs and under one of these put the flexiforce sensor.·
·
This post a while back shows the weight around the active portion of the sensor. http://forums.parallax.com/forums/default.aspx?f=10&m=79076&g=79400#m79400
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My thinking was to put the weight on the active portion.· What would be the correct way to use this thing??

Comments

  • SPENCESPENCE Posts: 204
    edited 2005-07-17 16:52
    Use 3 legs. 4 legs can cause problems. 3 will guaruntee
    pressur on one leg.

    73
    spence
  • OrionOrion Posts: 236
    edited 2005-07-17 17:31
    Good idea! Thank you. Still do I have the right idea for mounting the sensor? Putting the weight on the active portion with some spilling over? Let’s say 2" square legs with the sensor sandwiched in the middle of a leg.


    square leg
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    |·······| XX |
    |······ | XX | Flexiforce
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    |
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  • KenMKenM Posts: 657
    edited 2005-07-17 17:46
    The post you refer to was mine.

    The main point I wanted to make with that post is for the readings from the sensor to be reasonabably repeatable,· some attention to applying the force in a repeatable manner is needed.

    The picture may not show well, but I had a clear plastic ruler on top of the sensor, then the weights on the ruler. This was done in an attempt to get a relatively uniform distribution of weight on the sensor.

    Than as you saw, there was a different reading when the same amount of weight is used, but a little off center.

    I·think the three legged platform idea should work.

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    Ken
  • SPENCESPENCE Posts: 204
    edited 2005-07-17 17:59
    3 legs in a triangle give a proportional output. No teter-toter effect is possible.

    In the case of 4 legs, it is possible to spend many hours and shims getting just the correct height on each leg for the leg on the sensor to give a reasonable output.

    73
    spence
  • OrionOrion Posts: 236
    edited 2005-07-17 18:48
    "The main point I wanted to make with that post is for the readings from the sensor to be reasonabably repeatable, some attention to applying the force in a repeatable manner is needed."

    Thanks for the help, that clears it up. I thinking that I will have blocks around the barrel so that it's centered on the platform each time. Mabey even a level on the platform to verify that things are ok.
  • KenMKenM Posts: 657
    edited 2005-07-17 20:49
    The sensor is still perfectly usable with a proportioanal distribution on three legs.
    Orion is not concerned with actual weight, just a proportion of full, so the sensor output can easily be correlated to 90~99%, 80 to 89% and so on.
    SPENCE said...
    3 LEGS IN A TRIANGLE GIVE A PROPORTIONAL OUTPUT. NO TETER-TOTER EFFECT IS POSSIBLE.

    IN THE CASE OF 4 LEGS, IT IS POSSIBLE TO SPEND MANY HOURS AND SHIMS GETTING JUST THE CORRECT HEIGHT ON EACH LEG FOR THE LEG ON THE SENSOR TO GIVE A REASONABLE OUTPUT.

    73
    SPENCE
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    Ken
  • Tracy AllenTracy Allen Posts: 6,666
    edited 2005-07-18 17:24
    The output of the flexiforce sensors is roughly Resistance = K / Force. Or, Force = K / resistance. If you put one flexiforce under each leg of the tripod and connect them in parallel, the output will be Force = K( 1/R1 + 1/R2 + 1/R3). That is, the total force upward is the sum of the three contact points, which may vary if the load shifts. This is a non issue if the load willl not shift, in which case one sensor under one single leg would give a proportional output. The F=1/R dependency is rough but it does allow rough force summing by connecting the devices in parallel. We used that once for detecting animals presence on feeding platforms connected as a tripod using rubber feed centered on 3 sensor elements.

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    Tracy Allen
    www.emesystems.com
  • OrionOrion Posts: 236
    edited 2005-07-22 23:34
    Thanks to all that replied.

    This may be of some interest to any one that has the sensor.

    http://www.tekscan.com/pdfs/FlexiforceUserManual.pdf
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