Magnetic Quadrature using Multipole Magnets

[wholder]

One odometry option I've been investigating (but have yet to try implementing) is the use of multipole magnetic rings in combination with specialized Hall sensors designed to work with them.  The AS5304 sensor from austriamicrosystems costs about $8.74 from DigiKey, see:

  http://www.digikey.com/product-detail/en/AS5304B-ATSU/AS5304B-ATSU-ND/2334777

and is designed to work with a ring magnet, like this one:

  http://www.ams.com/eng/Products/Position-Sensors/Magnets/AS5000-MR20-44

which can be purchased for less than $4, each.  The rind is a 44 pole, plastic bonded strontium ferrite magnet with an inner diameter of 24mm, an outer diameter of 32mm and a thickness of 1.5mm, so I think it might be able to fit easily into a RC wheel rim with room to clear the drive shaft.  But, as I say, this is still a research project, so I've yet to try and build anything.

The advantage of this approach is that it's light, dust and water immune and should give a extremely high accurracy since the AS5304 can generate 40 pules per pole period.  However, this accuracy might be an issue, as it would generate a lot more transitions per revolution than an optical approach.  But, with a hardware decoder, or a fast processor, it might be a good solution.  Just wanted to throw it out as an option in case anyone else is interested.

Wayne

[Jon Watte]

That should work great, as long as it's not too close to the actual motor.

Btw: the number of transitions is probably still much less than what you get from a 64 cpr encoder on the motor shaft (before gearing) from the Pololu gearmotors, and those are still not terribly fast -- 12,000 ticks per second or so, tops.

FWIW: The encoder on the Pololu motors is also magnetic, as is the pretty good 4096-position encoder used in the Dynamixel MX series of robot servos. So there's plenty of precedent for similar applications! (although not pushing them all the way to the wheel, like you suggest)

Sincerely,

jw

Sincerely,

Jon Watte


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[Russ Johnston]

AMS also makes this chip:
http://www.ams.com/eng/Products/Position-Sensors/Rotary-Magnetic-Position-Sensors/AS5040
that I have been playing with. I think the ring magnet version might be easier to attach though.

Russ

[Russ Johnston]

Now I remember why I didn't try the ring magnets before. While they are only $4 you have to pay $30 in shipping to get them.

Russ

On Tuesday, December 3, 2013 5:22:08 PM UTC-7, wholder wrote:

[Ted Meyers]

Sparkfun is now selling a magnetic wheel encoder kit for about $10: https://www.sparkfun.com/products/12629

Ted

On Tuesday, December 3, 2013 5:22:08 PM UTC-7, wholder wrote:

[spma...@163.com]

Hi , Wayne ,

The AS5000-MR20-44 ring magnet , You can get from China supplier Super Magnet Co.,Ltd  with lower price , product link as :  http://www.spmagnet.com/m_article/40-D32x24x14mm-44-poles.html