Debugging the Tachometer Signal

I spent this evening, trying to figure out why the tachometer was flat-lining under acceleration. After talking with some EMI suppression engineers at work, they suggested covering the sensor with a steel shield to prevent magnetic fields from interacting with the sensor.


So, I headed out to the grocery store and got a nice steel can of organic tomato soup. After dinner, I washed out the can and cut the bottom two inches off with some tin-snips. After covering the sensor with this and revving up the motor, I still had the same problem. Rats!


The next experiment was to purchase a piece of 24 gauge steel plate from Parkrose Hardware and cover the face of the motor behind the sensor to see if I could prevent magnetic fields from interfering. I drilled a 1" hole to fit over the spinning magnetic disk and drilled two holes to match the mounting holes.

Result: After installing this under the plastic spacer ring, the tachometer failed to work at all. Rats again!

However, this did provide a clue to what was going on. The steel plate (along with some wiggle room) added about 1/16" spacing to the sensor. After taking some more measurements, I found that the spinning disk was only just inside the bottom edge of the Zolox sensor.

As a third experiment, I added 1/4" of spacer washers under the spinning magnetic disk to push it deeper inside the sensor housing. This actually worked fine, at least for mild acceleration. The spacer was loosely added, so I'll make something more permanent tomorrow after I get some decent spacer hardware.

In short, make sure the spinning magnetic disk is deep inside the Zolox sensor housing to make it work.

Good Night,
Tim