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Hall sensor and magnet as precision switch? — Parallax Forums

Hall sensor and magnet as precision switch?

T ChapT Chap Posts: 4,223
in General Discussion
I got in some magnets and Melexis hall sensor today to play with. I want to find some options for various situations where a physical snap switch is not a good option. In experimenting with the hall sensor, the switching seems very precise and consistent. The only thing is that the point of switching ON versus OFF is not exactly the same point which is no big deal. Anyone ever use hall sensors for precise switching? I just need to be within a millimeter accurate and repeatable, which this is seeming to be good at.

Comments

  • tonyp12tonyp12 Posts: 1,951
    The smaller the magnet and the more sensitive the hall sensor will give you better result as you're trying to avoid large fringe area.
    It's like looking at the sun and stepping 10feet to the right, its still look as it at the same spot.
    Do it with small flashlight, it's completely out of field if you even just move 2feet.

  • Heater.Heater. Posts: 21,230
    T Chap,
    The only thing is that the point of switching ON versus OFF is not exactly the same point...
    Interesting. That sounds like a Hall Effect switch has hysteresis which I did not know they had. Not that I ever played with one.

    If so you probably cannot eliminate it but you should be able to reduce it as said above.
  • T ChapT Chap Posts: 4,223
    http://www.digikey.com/product-search/en?keywords=US5881EUA-AAA-000-BU-ND

    This is what I got to test.

    It says 30mT Trip, 30mT Release but testing by hand seems to want to release the switch at a slightly greater distance than the trip. Could be an illusion as they seem specific about the trip levels. I will mount the magnet on a micrometer and get some specifics. These magnets are tripping at just under .5" which is good for my projects.

    Magnets:

    http://www.digikey.com/product-search/en?keywords=469-1004-ND
    http://www.digikey.com/product-search/en?keywords=469-1003-ND


  • tonyp12tonyp12 Posts: 1,951
    edited 2016-04-22 16:58
    I recommend this one, push-pull output (saves power instead of 47K-100K pullup being engaged for extended times)
    north or south magnet, gable end sensing, hysteresis, sleeps with 0.05% duty latching the state at 20/sec equal 5uA.

    http://www.mouser.com/ProductDetail/Murata/MRMS211H/?qs=/ha2pyFadujKXmrGA1Dnrn7Z1kYM9CINEntbnTL5sdc=
    http://www.murata.com/~/media/webrenewal/products/sensor/amr/position-rotation/amr_spec_mrms211h.ashx?la=ja-jp
  • ElectronsRfunElectronsRfun Posts: 5
    Hi TChap,

    I have a lot of experience using magnets and a ratiometric Hall Sensor, along with using magnets as a switch. I have found, a magnetic switch does always seems to release, noticeably, further than the trip distance. I never could get that early L.I.S.A. alarm project to work the way I wanted (which is how I got to the Propeller). That being said, I have many projects that I use successfully everyday that use a magnetic switch and especially the Hall sensor. In fact my wife, who has advance ALS, relies on the Hall Sensor project every second of everyday, too call for help when she needs it, she has less than 1/4" willful movement in her entire body, but uses the Hall sensor with and extremely high success rate. The Hall sensor is connected into a 12 bit ADC then to the propeller for a resolution of about 0.001 units. (I think that's correct).

    Anyway, the projects are at Zappbots electronics page and use to be on Chris Savage's Savage Circuits website.

    Have fun and good luck.

    Jason
  • T ChapT Chap Posts: 4,223
    Testing with a micrometer with magnet taped to it, shows the release is slightly greater distance about .02" , but for my cases that is fine. Nice addition to list of sensors to use in the future.
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  • JasonDorieJasonDorie Posts: 1,930
    I know a number of people who use hall effect switches as homing and limit switches on CNC machines, because the precision is significantly better (and more repeatable) than mechanical switches.
  • jmgjmg Posts: 15,173
    T Chap wrote: »
    I got in some magnets and Melexis hall sensor today to play with. I want to find some options for various situations where a physical snap switch is not a good option. In experimenting with the hall sensor, the switching seems very precise and consistent. The only thing is that the point of switching ON versus OFF is not exactly the same point which is no big deal. Anyone ever use hall sensors for precise switching? I just need to be within a millimeter accurate and repeatable, which this is seeming to be good at.

    At a fixed temperature, and with user-preset, that should be fine.
    Note in volume production you will get variations in magnets, magnet orientation, and sensor trip all of which will add to your errors.
    You may also want to plot Distance vs temperature on a dozen units, to check that effect.

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