Minimal inductive proximity sensor?

Reflection sensor? Farnell has about half a page of "reflective optical sesnsors", most of them costing a bit under $10 each in small quantities.

If the cast iron wheel had any significant residual magnetism, you might be able to do something with a linear Hall effect sensor and comparator, but it would be more expensive,

------------- Bill Sloman, Nijmegen

Gritty, dirty, oily environment...

What would be the basic setup for this approach? An oscillator on the remote board that the PIC would monitor the frequency of?

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Capacitive proximity sensor. Basically, just a plate that senses
when another plate (the spoke) goes by with air as the dielectric.

Right - that wipes out optical sensors, unless you can get the system to squirt filtered oil at your sensor all the time.

----------- Bill Sloman, Nijmegen

I considered that. The wheel speed is slow, but it can be at 100RPM for long periods so I thought the resulting reliability would be questionable.

Sounds like you need a nonmagnetized variable reluctance pickup, like the ones used for aircraft fuel flow sensors. It looks like a spark plug, sort of; just a ferrous metal tube with a center post barely peeking out, and a coil inside. Inductance increases as a gear tooth/spoke/turbine blade comes close.

You can signal condition this with an oscillator that uses the coil in its tank (observe frequency shift) or you can put it in an AC bridge.

Google "magnetic proximity sensor." You can buy these with the necessary electronics inside if you don't want to do it yourself. You'd have to make a lot of these to justify homebrew.

John

[...]

The low cost way to do it is to make the oscillator at the PIC side of the capacitor and have the object provide a ground that increases the capacitance.

The PIC is wired to the plate at two points. One is an output from the PIC that drives the plate through a resistor and the other is an input. The PIC asserts a high for a long time and then a low. It then measures how long it takes for the input to go low too. It compares this value to a value from longish ago to remove drifts. If the time gets longer, the object has arrived.

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kensmith@rahul.net   forging knowledge

Maybe the old fashioned approach would work for you - microswitch with wand protruding into wheel arch - reliable, dust and water etc proof units available - very reliable - use the PIC to count and de bounce etc

David

Dave wrote:

I can't imagine this working when the probable capacitance change would only be a few dozen picofarads on the sensor board. I'm more inclined to believe a seperate monitored oscillator might work.

Thanks, that looks like a pretty reasonable approach. I'll do some experimenting...

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No, I think that method would be too "drifty", and the frequency too
high because of the size of the plate.

What I\'d do is something like this:


          +HV        +V     +V            +V
           |          |      |             |
           |          |      |            [R7]
          [R1]       [R2]    |             |
     ||    |          |      |    +--[R6]--+
     || |  |          |      |    |        |
     || |--+---[C2]---+------|----+--|+\\   |
     || |             |      |       |  >--+
     || C1           [R3]  [POT[

Ok, now I understand what you're suggesting. Somehow I was thinking you meant a split plate.

[...] 10pF times a 1Meg resistor gives a 10uS change in the time constant so whats the problem?

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kensmith@rahul.net   forging knowledge