VSS Vehicle Speed Sensor Output

[retepsnikrep]

Anyone have any accurate figures as to how the output of this relates to speed?

I saw Mikes earlier reply that it pulses at about 100hz at 90mph.

If anyone has any other readings that would be useful I need to feed the signal into my BMS and work out speed/wh mile etc.

Thanks

Peter

[EricZ]

Check out:

http://www.insightcentral.net/owners/mods/cruise2.htm

Starting at around the sixth paragraph, he talks about the VSS output.

[retepsnikrep]

Thanks just what I needed, missed that somehow

[Mike Dabrowski 2000]

Peter,
The MIMA code determines vehicle speed in the getspeed routine.
you may find info there to help.

;************************************************* *********************
;The VSS pulse rising edge generates an interupt(int2) timer1 is started in the ISR,
;on the next pulse ISR, the counter is stopped, the count transfered to VSSHI,VSSLO, the
;counter is cleared and restarted.
;clock = 625,000/sec
;clock/10/count/10= frequency of pulses in HZ
;the frequency of the VSS pulses in HZis nearly the same as the actual MPG.(close enough for now)
;Need to finish the MPH scale on the low end.
;************************************************* *********************

GETSPEED

[mikeross]

That's my problem right now.. must be a broken speed sensor

[archebald23]

on what signal was the output based? four-pulse or two-pulse? i was trying to get the same results on my abs speed sensor but the figures are way too distant.

[JimIsbell]

The OBDII output has speed as I am displaying it on my computer from there.

[retepsnikrep]

Guys I need some help with my VSS maths.

The VSS sensor gives 4550 pulses per mile. (Not sure where the 4550 came from?)
That's 75.83 pulses per second at 60mph.
My pic counter has a resolution of 0.000005 seconds or 5us
One VSS pulse at 60 mph is 2637 counts long.
One VSS pulse at 30 mph is 5274 counts long
One VSS pulse at 15 mph is 10548 counts long

How do I get from the Count figure to speed in mph?

I would like resolution to 0.1mph if possible.

[Right Lane Cruiser]

Peter, use the following:

3600sph / (count * 0.000005s * 4550p/mi)

In other words, using unit inspection...

Count (per pulse) multiplied by 0.000005s to get total duration of one pulse. Multiply by 4550 to get the total time in seconds at the current speed to travel one mile. That's in seconds per mile so the inverse is miles per second, then multiply by the conversion factor of 3600 seconds per hour.