VSS Signal assistance?
 

Is anyone in a position to capture the vss signal on a reed switch car (metro, starion)? My scope is on the fritz.

Problem: The vss is too noisy on some cars, need to know what it looks like in order to filter it properly. Plain old debouncing won't work because it seems the vss pulse peak to peak duration is less than your typical debounce period (was seeing 8342 microseconds @53mph, typical debounce is like 20000 microseconds). I added a tiny bit of debounce (3ms) and was able to put a more normal vssPulsesPerMile figure in, but the instant reading is jumpy.

It isn't a problem for hall effect cars because that is not a noisy physical switch.

I wonder if there is noise from the electromechanical parts of the speedometer/odometer? I don't know if I can defeat this problem with software alone. But would appreciate any insights into how to fix this.

Anyone seen a metro ECU schematic? Maybe someone has an old one I can dissect?

Edit, ok I think I have a "debounce" strategy, for dealing with this. I have to change it so it looks for X number of milliseconds in a given state before deciding to increment the vss pulse counter. Only tricky bit is the timing of the pulses should be done on the leading edge, and not whenever the pulse settles down. I think it is doable with the event scheduler though, instead of just disabling the pin interrupt for a given amount of time I will schedule an "increment the counter" event that will get overridden with each junk pulse.

Disregard, I forgot there was an existing laptop .wav file recording of a metro vss, wanna see? It is kinda fugly :)
http://opengauge.org/diympggauge/vsswaveform.GIF

But it looks like the "wait for it to settle down" strategy should work. It looks like just waiting till there is 2 milliseconds of clean signal would do it. But it will start flaking out again above 75mph. 1 millisecond might be enough, but I can't remember the last time I went 75 :) Maybe I will leave changing it to 1 millisecond as an exercise for the user ;)

What about this option on some cars, a add on hall effect. Put 1-2 magnets on halfshaft close to tranny and mount a hall effect like this one. Only like $3 and would put out consistent readings. Hall Effect Switch | Low Sensitivity | Manufactured by Melexis

And all you would need to know at that point is tire size to know pulse rate.
155/80-12 927 rev/mile
1 magnet = 927/mile
8 magnets = 7416/mile
You could attach magnets to large hose style clamp and strap to inner cv joint case.

I have a 'scope - but my extension cord isn't long enough to do meaningful on-the-road testing :). I have an inverter - maybe I can make it all work that way and get some pictures for us. But before I do that, I'm going to put a small capacitor in parallel with the zener and see if I can filter the bounces...

I bet dcb's already thinking along those lines for his motorbike. (He's already got the magnet/pickup installed, in the form of a bicycle computer.)

EDIT: FYI the Audiovox cruise control I added to my car came with a magnet kit for just this purpose - for older cars that may not have a VSS signal.

Ok, I might be overthinknig the vss reed switch bit (LOL). Probably some more code changes before I'm done

But took a dive today with a SG also and mpguino vesion 0.70 and heres where it landed:

odometer: 42.5 miles
mpguino: 42.33 miles
SG: 41.7 miles

So I am liking the distance tracking pretty well, rough indications have it at 0.4% off of the odometer even with the reed switch. Still want to research it some more and retest the saturn to make sure that is still accurate. But if the trip is accurate then I have to look elsewhere for the jumpy instant mph reading.

Looks like the Mpguino is pretty accurate :thumbup:, what car was this on the Saturn?

This was on the metro, the one with the bouncy vss reed switch that has been giving a bit o'trouble (had to double the vsspulsespermile initially). I was very happy with the saturn readings, both distance and fuel consumption at last check, but I've messed with the distance code a bit since then.

Looks like the v.70 fixed the Starion VSS bounce! Pulses per mile = 8204! GREAT JOB and THANKS!