So, I spent some time staring at the front wheel yesterday, and came up with an idea to rig the HES off the fork, and fit some small rare earth magnets in the bolts that hold the brake rotor onto the wheel. Gave the wheel a spin, and it worked... but not very well. Not well enough. I was using a bipolar latching HES, and I have to say, that is NOT the way to go for this application. When I placed my HES order, I didn't know there were different types available, much less what the differences were. The latching bipolar needs to see both North and South poles to work properly when you're reading off the face (as intended). I realized I could read off the edge of the sensor, and it would be happy with one pole or the other, and would 'see' it one way (North, for example) as it approached the front, and see it the other (South, for example) as it swept past and the sensor was picking it up from the back side. It seems to really reduce the size of the usable air gap when you do this though, and I didn't manage to get a reliable pulse train, and didn't have confidence I ever would get a reliable pulse train with that approach.
So, I've got three ideas competing for my next attempt:
- Rig a unipolar HES instead, which I suspect will give me more reliable pulses
- Rig a back-biased HES (aka gear tooth sensor, what's used in ABS speed sensors) to just pick up the rotor bolts directly (no magnets to attach, yay!)
- Try to vampire off my ABS sensor, either directly or with a divide-by-12 circuit.
Does anyone know what the upper limit of the MPGuino's VSS pulse-per-mile capability is? I see lots of chatter up in the 16,000/mile range, but I may be getting in the ballpark of 85,000/mile if I go straight off the ABS sensor (I think - I haven't observed the pulse train on a scope, since I don't have one, but the sensor ring has about 100 teeth on it, and I'm guessing the wheel circumference is around 72 inches). I'm not even sure if there's a clean way to pick up that signal, and I'm still deciding if I even feel good about messing with the ABS system in this way... If the speed of that pulse train is too much for the MPGuino, then the /12 circuit should get it in a reasonable range.
But the gear-tooth speed sensor approach feels good from the simplicity and not messing with ABS system angles.
Any of these options are cheap enough to pursue, IMO.
Any suggestions/experiences?
Jason