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Melexis 90217 Hall-Effect sensor for gear tooth counting — Parallax Forums

Melexis 90217 Hall-Effect sensor for gear tooth counting

garyggaryg Posts: 420
edited 2014-03-23 20:03 in Accessories
Hi
Has anyone use this sensor " Melexis 90217 Hall-Effect Sensor" for purpose of detecting machine screw heads
on a robotic platform?
I just today noticed that Parallax sells this sensor.
In one of my projects, I've been trying to do wheel measurements, but am trying to decide on which sensor to use.
It's my 1st differential drive platform.
I'm thinking that I would be satisfied with +- 4" or so accuracy per revolution at this time.

It would be very convenient to place machine screws around my wheel mounts and be able to count distance traveled.
Data sheet says that gluing a magnet to this sensor would enable it to detect the magnetic field changes due to disturbance caused
by metal object placed near the sensor if a magnet is glued to the back of the sensor.

I'm currently using a BS1 to drive two HB25 motor controllers.
I will be using a Propeller Quickstart board in the very near future on this project to replace the BS1.

I would appreciate any comments concerning using this Melexis 90217 Hall-Effect Sensor in an application such as this.

Am I considering going down a troublesome path?

Thanks
Garyg

Comments

  • TtailspinTtailspin Posts: 1,326
    edited 2014-03-08 07:46
    It works great!
    You need to take care to set the screws as evenly as possible, the hall effect sensor is very sensitive and accurate.
    Also, you don't need to use something as big as a screw head, in fact if it's to big, the sensor will detect the edge of the screw,
    not a problem if your wheels only go in one direction, but you will lose wheel position accuracy(about the width of the screw head) when you switch direction.

    The thing is designed to detect the tip of a gear tooth, so it takes a surprisingly small piece of metal for detection. something the size of a finish nail with the head cut off..




    -Tommy
  • garyggaryg Posts: 420
    edited 2014-03-09 14:05
    Ttailspin
    Do you use the 3/8" diameter magnets glued to the back of the 90217 Hall-Effect sensor that Parallax sells?
    I think I'll be getting a couple of these to try out.
    Thanks
  • TtailspinTtailspin Posts: 1,326
    edited 2014-03-09 19:56
    Yes I did glue the magnet to the chip... err, it's actually the other way round, glue the tiny chip onto the magnet..It's pretty small. :)
    Check this thread out for some action pictures...

    I     put a small drop of 'Crazy Glue' onto the magnet face, then carefully applied the Melexis chip to the magnet, and held it for the obligatory 30 seconds..



    -Tommy
  • garyggaryg Posts: 420
    edited 2014-03-10 16:44
    Hi
    One more question before placing an order for sensors and magnets.
    I promise, really.

    How can one tell the north pole from the south pole of the magnet?

    Thanks for your help.
    I believe this will be a good adventure.
    Gary
  • TtailspinTtailspin Posts: 1,326
    edited 2014-03-10 20:59
    The magnet is wide enough to stand on edge, I just put a mark on the face that kept pointing north..

    Find the widest area away from any metal, try to stand the magnet on edge, let go, and one face should face north.
    The magnets are very powerful, so keep any spare magnets far away while performing this stunt..:)

    Your adventure awaits!


    -Tommy
  • Alex.StanfieldAlex.Stanfield Posts: 198
    edited 2014-03-13 19:23
    Gary, you may find my experience in this thread http://forums.parallax.com/showthread.php/149621-MLX90217-reliability-problems

    See the last post for a summary

    As for telling the right orientation I just hold the magnet against the sensor and then swipe some metal infront looking for changes on the output.

    Hope it helps
    Alex
  • garyggaryg Posts: 420
    edited 2014-03-13 22:35
    Thanks
    I tried a couple of simple tests with a magnet that I had sticking around my shop.
    Standing the magnet on edge was 1st attempt. That didn't work out well. Too much friction from my table.
    Then I remembered something from the distant past.
    I made a small boat 2"x2" and stood the magnet up in a bucket of water.
    I can determine magnet north side now.

    Alex
    Good info. I'll be getting a few of these and do some experimenting.

    Thanks again
    Gary
  • bnrgbnrg Posts: 14
    edited 2014-03-21 16:00
    Hi Ttailspin,
    I have tried attaching the 90217 to the magnet with Scotch super glue but it will not maintain a good bond-any light handling makes it fall off. I cleaned the parts with alcohol, tried gentle abrading with sand paper, but still no luck. What kind of glue did you use and does anyone have other suggestions? Rubber cement didn't work either. It is rated to 150 C, so would a piece of heat shrink work without damaging the sensor?
    Thanks,
    Bob
  • TtailspinTtailspin Posts: 1,326
    edited 2014-03-21 22:43
    The exact brand, Gorilla, super glue, impact-tough formula.
    Super Glue is not so good if it has aged and thickened, only good for about 3 months after opening, so buy more little bottles, instead of one big bottle.

    Make sure you can see lots of small scratches on the magnet face, 100grit should do the trick.
    Crazy Glue not working, that's just crazy...:)


    You could wrap the sensor and magnet with heat shrink tube, just make sure you will have clearance between the tube and gear teeth, or whatever you are measuring.
    The sensor has to be 'danger close' to get accurate and reliable readings, 1/4" is to far away...



    -Tommy
  • bnrgbnrg Posts: 14
    edited 2014-03-23 20:03
    Tried the gorilla brand and it worked perfectly! Thanks Tommy. The scotch brand material that I used was brand new so there must be a formulation difference-guess they are not all the same.
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