Driveshafts - Part II

Use multiple sensors, spaced 90 deg, and two magnets, spaced so as to maintain the shaft's balance?

Cheers, James Arthur

Generally one magnet won't affect balance, but if you are worried, use two.

Mount two sensors, offsetted from each other by a few degrees. The phase between the two sensor outputs will give you operating direction. Simple application, I bet there are chips to do it already on the market...

that one's for sensing gear teeth, not magnets

maybe this:

here's one that I have used in the past:

but it might not be suitable for installing in the undercarriage of a vehicle.

I had a very small neodymium magnet positioned about a quarter inch or

5/16" away from the sensor IIRC. I'm sure you could have the magnet farther away than that, just don't know exactly how far. You might be able to do some research and figure it out from the gauss figures on the data sheet.

I am no expert on this but have done similar things in industrial control. A very long time ago. One important aspect is to deal with all the crud that can accumulate down there over time. Either it must not matter or the rotation of the shaft has to move enough of it out of the way, all the time. What we used were sensors that had an oscillator and receiver coil, arranged in a U-shape. IOW there was a large slit in this potted device. The output was digital, a sharp drop in current when interrupted because that stalls the internal oscillator. These devices would keep working even when badly covered up with oil sludge.

This device was bolted to some flange on the frame and a partial disk would interrupt that on every rotation. Basically a piece of metal fastened on the shaft. In your case you would need two such sensors next to each other and the partial disk would have to cover both but not much more. Then you can detect which one is interrupted first so you also have the direction. You'll need some (simple) logic to cover the case where the shaft reverses direction while being sensed.

Anyhow, if this is for heavy duty industrial use and reliability is paramount I suggest to talk to these guys and AFAIR they have a US subsidiary:

--
Regards, Joerg

http://www.analogconsultants.com

Why don't you just tap off of the speed sensor in the transmission?

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Or one of the ABS speed sensors...

two sensors, or cheat and tap the reverse-light switch.

Bye. Jasen

This does seem to be the right approach. I was thinking multiple magnets, but multiple sensors helps with the bigger problem of shaft balancing! And, this approach should be fairly "easy" to ruggedize.

It also solves the problem of trying to find a single sensor with 2 pickups and a reasonable sensing distance.

Believe it or not, a lot of off-road construction type vehicles don't have backup lamps, so that's no help. Also, as there are several model trucks involved with this project, we want to stay away from tapping into the vehicle's speedo - which may not function in reverse anyway(?). In fact, some of the vehicle's don't even have speedos, so that's out too.

Originally, I wanted to use an accelerometer. This way, we don't have to "touch" the vehicle at all, which saves installation labor. However, the software development time to use such a sensor and get it working right looks like it will exceed our window for deliverables..., unless there were an off the shelf solution which so far, we have not located. Plus, it looks like those sensors require substantial calibration post-installation, which is not very appealing.

Thanks for the help. I will check the links out and give 'em a try.

-mpm

Many vehicles for heavy duty use don't have any of this, especially older ones.

--
Regards, Joerg

http://www.analogconsultants.com