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Unread 16-01-2012, 21:17
iambujo iambujo is offline
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Hall Effect Latch as an encoder/counter?

Hello -
I have been looking at other methods to count rotational speed of a wheel, in this case of a wheel could be spinning at 50+ Hz. Those US Digital KOP sensors have their place, but can be hard to work with if they are not going on a COTS transmission/gearbox with the correct size shaft already available. I am not concerned about direction.

We first looked at the Rotary magnetic encoder in the FIRST Choice parts this year, but that isn't going to work based on the data sheet. It provides angle based on the orientation of the magnet's poles, and I don't think it would be realistic to sample it at a high enough frequency to catch a desired analog value or PWM width.

I had some Melexis US1881 Hall-Effect latches sitting in my bin of goodies and figured they might work as long as their switching speeds were fast enough and they were sensitive enough. We bench tested today with a CIM and two magnets (one north oriented, the other south 180 degrees seperate on the hub). We spun up the motor and got a perfect square wave on the logic analyzer over the whole range of RPMs. The thought is to just pass this output into a digital input, run the wpilib counter and timer functions on it and calculate frequency.

So this brings me to my question - has anyone done this before successfully on a competition system? It seems to work great as a bench test proof of concept. The biggest concern I have is ensuring we mount the magnets safely on the wheel. I looked through the robot rules manual and didn't see anything excluding the use of magnets (in general or in a manner such as this).

Any thoughts (good or bad)?
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