Optical tachometers / wheel speed sensors

[Edmund_SURE]

Has anyone had any success with optical wheel speed sensors?

Have you had issues with sunllight and shadows causing missed teeth or mistriggers between teeth?

Measuring wheel speeds has always been a "we'll get it working if we have time at the end" job, so naturally we've never had working wheel speed data (at least not since 2009). This year the team is much more organised and actually getting all the data we want is being sorted.

However, have the problem that no-one on the team has experience with hall effect sensors, and the electrically-proficient team members have never seen optical sensors used outdoors. So making something that is going to work, not look rushed, be light, and without breaking the bank is prooving tricky.

We're hoping to work in a very confined area of the car, so we'd prefer to use a reflective ring rather than a blocking ring so both halves of the sensor are on the same side of the ring, but any help would be apriciated.

Diagram of examples of what I'm talking about, prefer right version over left. (http://abrobotics.tripod.com/Ebot/EbotImages/encdisks.gif)

(New here, not entirely sure where such discussions are supposed to go.)

[Swiftus]

Hall / VR both pretty easy to implement. If you use a hall effect sensor which was designed as a wheel speed sensor for a car, you should be pretty confident it will work for you. Hall is easier to think about since it is a digital signal rather than an analog waveform. There are some cars which use optical, but magnetic of some sort is usually more reliable

http://www.diyautotune.com/catalog/trigger-wheels-c-48.html?osCsid=322af7e70af60fe279571a5d6440a2ad

http://trigger-wheels.com/store/contents/en-uk/d5.html

[Pete Marsh]

UWA used an optical wheel speed pickup based around a PC mouse sensor for many years. The trigger wheel is inside the upright, between the wheel bearings, so well protected from the environment.

It was very successful and uniquely able to do a high pulse/rev count easily.

Pete

[mech5496]

We do hall/magnetic as well, and never looked back.

[Edmund_SURE]

Seems the answer is "no, unless you count fancy optical mouse stuff". Alright thanks, I'll drop the idea and we'll carry on get hall sensors working.

Edit: By the way, on the offchance, has anyone had any success buying bike ABS rings and matching sensors and getting that work for the benefite of using ring-sensor sets that you know work together?

[Tim.Wright]

I'm sure people have used off the shelf sensor + trigger wheels but in my opinion it will be more work to adapt that to your spindle instead of just designing trigger wheel yourself. If you make sure there is the possibility to adjust the sensor to trigger wheel gap ( i.e. via shims or washers on the hall sensor bracket) then its almost impossible to stuff up a hell sensor arrangement.

The only possible gotcha is figuring out if you need to use a pull up resistor or not.

For what its worth, the auto industry are moving to magnetic encoder technology for wheel speed sensors. This replaces the trigger wheels with a smooth annular ring which has a magnetic trigger pattern burnt into it. They give much higher resolution outputs than conventional hall triggers (which I doubt you would need) but they are more resistant to contamination than an optical sensor. They tend to be nicely small and light as well because racecar:
http://pmmonline.co.uk/files/professionalmotormechanic/ASB%20FACE%20NTN%20APRIL%202012.jpg

Though I think they might be more sensitive to misalignment than a hall sensor.

[Goost]

Maybe you could look at these:
http://www.skf.com/group/products/bearings-units-housings/engineered-products/sensor-bearing-units/motor-encoder-units/index.html

probably pricey, but would package nicely.

We used hall effect sensors on a trigger wheel inside the upright (between bearings). Protects the parts nicely and looks clean, not much harder to implement (maybe easier as you don't have to make a tab/bracket to hold the sensor.

[Kevin Hayward]

Tim,

The optical sensors that Pete mentioned were news to me. When I was at UWA the rings inside the upright housing housed about 50 small magnets. Alternating polarity for successive pairs. Very neat. Must have replaced them later on. Sensor unit was built in-house. Back then the hall effect sensors were a bit pricier than now, and still packaged just as badly. Was a neat implementation, and awesome to watch. The sensor unit had a led for on and a green led that flashed everytime a signal was sent.

Custom stuff like that was a bit common in the early days of UWA. We made our own hall effect sensors for converting an F4 into fuel injected, and over a few years developed in-house voice coil actuators for electronic shifting. Less inertia than similar powered solenoids.

So many good off-the-shelf solutions today that it seems a bit odd that you would have bothered with any of it back then, but it was a good experience to be involved with.

The only thing I could say to the youngsters building cars today with their drexler diffs, fuel injected base engines, and militera steering racks is "get off my lawn"

Kev

[Swiftus]

The only thing I could say to the youngsters building cars today with their drexler diffs, fuel injected base engines, and militera steering racks is "get off my lawn"

Kev

^^^ Perfect! :D ^^^