Switch Contact Design Help 1

[BrainFailure]

Howdy:

Not sure if this is the right forum.

I am working on a product with a motor that has an encoder and a gearhead. The encoder is an incremental type so I need some kind of sensor to signal every time the output shaft reaches a specific angle. Unfortunatly this sensor must fit around a .125 inch shaft inside a .195 inch hole.

My thought is to design some kind of wiper that makes and breaks contact with the shaft. From what I have read the biggest concern in wiper design is the effect of corresion on the contact resistance. I am also concerned about wear because the wiper needs to last 3 million shaft rotations.

Fortunately I only need maybe 1 mA of current and also it only needs to average about 60 rpms (1Hz).

For corrosion resistance I have heard Gold plating recomended for switch wipers. However I don't think Gold will have enough wear resistance. Right now I am thinking maybe electroless nickel against hard bronze.

Any suggestions for wiper contact materials? Are there any other factors that I am not considering?

 

[btrueblood]

Why not a hall effect IC and a small magnet?

 

[BrainFailure]

The problem with the hall effect sensor is that it would have to fit in an envelope of about .025 X .070 inch. Thats smaller than the typical SMD resistor.

 

[EdStainless]

An optical switch? You really need to stay away from contact.
I still like the Hall effect idea. I have seen HE sensors on integrated circuits. they can make them small.

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Plymouth Tube

 

[BrainFailure]

Wow. The Allegro A1172 is a hall effect sensor that is .015 X .078 X .078 inch. That is incy winchy! I think I can make this work. I will have to put it on a flex pcb.

 

[BrainFailure]

Just kidding. I found three hall effect sensor manufacturers with chip scale packages, and every single sensor in a chip scale package sleeps for 50ms in between readings. My the pulse train I am trying to catch has a pulse width of 350 us so this will not work at all.

How frustrating!

 

[BobM3]

Have you considered a slip ring assembly and brushes?