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Can I use the hall effect sensor without a bias magnet mounted to it? — Parallax Forums

Can I use the hall effect sensor without a bias magnet mounted to it?

winchmanwinchman Posts: 22
edited 2005-11-23 17:41 in BASIC Stamp
I've got the Melexis 90217 Hall-Effect Sensor and some bias magnets. Will the sensor work without the bias magnet mounted directly to the sensor? Will it pick up the change in flux as the magnets pass close by?

I wanted to mount the magnets on the rotating part (which is made of aluminum) and put the sensor on a bracket nearby. The radius to the magnets would be about 2.5", and I was planning on having them 180 degrees apart. Will this work reliably?

Roger

Comments

  • Ryan ClarkeRyan Clarke Posts: 738
    edited 2005-11-23 15:20
    Roger,

    The way a Hall-Effect sensor works is by a magnet passing by it, causing 0-1-0 or 1-0-1 transitions. I'm not sure I understand your correctly- The bias magnet is not attached directly to the sensor, if it were, you would see no transitions...


    Ryan

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    Ryan Clarke
    Parallax Tech Support

    RClarke@Parallax.com
  • Ken GraceyKen Gracey Posts: 7,400
    edited 2005-11-23 15:43
    Roger,

    I concur with Ryan. But I think you are asking how close the magnet needs to be to the sensor. In my experience I put the two within 1/8" of eachother.

    What you describe is similar to a project Jon and I put together:

    http://www.parallax.com/html_pages/robotics/machining/RPM_display.asp

    Ken Gracey
    Parallax, Inc.
  • winchmanwinchman Posts: 22
    edited 2005-11-23 16:31
    On page 5 of the application guide for the sensor, they show the south pole of the bias magnet bonded to the unmarked side of the sensor. A rotating gear is small distance away on the marked side of the sensor. Apparently, the change in flux is caused by the concentration of the lines of force as the gear tooth goes by.

    The application guide is here: http://www.melexis.com/prodfiles/MLX90217_rev9.pdf

    I looked at the RPM display on the milling machine, and I'm not sure how that works. If the magnet is centered on the end of the shaft with the hall effect sensor directly above it, how will it see a change in flux as the shaft rotates? Is the pole of the magnet on the edge instead of on the flat side?

    I've used hall effect sensors before on electronic ignition systems for small engines. They used a small magnet on the propellor hub with the sensor near by. I've never seen an installation like the one shown in the application guide, and I just wanted to make sure what I had in mind would work.

    I've attached a sketch to make it clearer (I hope).
    Roger
    000_1130 (2).JPG
    964 x 424 - 44K
  • Larry~Larry~ Posts: 242
    edited 2005-11-23 16:38
    There are gear sensors that have the Magnets and the hall effect sensor in one unit, and when a chunk of metal passes by
    it senses, such as a gear tooth. It may?? be possable to build this yourself.
  • stamptrolstamptrol Posts: 1,731
    edited 2005-11-23 17:41
    Winchman,

    The short answer is that both ways will work.

    The bias magnet setup sometimes gives the opportunity to make the sensor look like a normally closed switch instead of normally open. Or, sometimes the sensor has to be 'pushed' a little bit towards one state or other to compensate for the relative strength of the metal or magnet passing by it.

    Regards
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