Thread: VSS problem
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Old 06-13-2016, 04:26 AM   #5 (permalink)
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Originally Posted by t vago View Post
Glad to help. :-)

If this is so, then it would appear that your Alfa is set up to deliver around 18000 pulses per mile, or about 11200 pulses per kilometer. I have not performed a definitive analysis of the upper limit of what the MPGuino can handle, but it seems that you may have found its upper VSS limit, given its parameter settings.

This has a lot to do with digital signal theory. Specifically, your Alfa might have made a VSS signal for the MPGuino, that is higher than what is known as the Nyquist frequency. That is defined as the frequency at which something can be digitally sampled without introducing errors. That is typically 1/2 the sampling frequency.

From what you have described, it appears that the MPGuino, as programmed with default parameters, cannot sample VSS signals that are more than about 187 Hz. That would be the Nyquist frequency for the MPGuino, which means that the MPGuino sampling frequency is about 374 Hz.

What you're seeing is this:

Once your Alfa's VSS goes above 187 Hz, the MPGuino will effectively see a frequency of (374 - VSS) Hz, and that will cause your indicated speed to fall at speeds above 60 kph. At around 120 kph, you'll probably see a speed of 0.

With most cars, they will never see this problem as their VSS signals would not go much above 10000 pulses per mile. However, I am thinking that your Alfa has a VSS that is... 18000 pulses per mile? Is that right?

I think you will just need to adjust a parameter in the MPGuino itself.

There is something called "VSS Debounce" and it is set to a program default of 2. This is for purely mechanical vehicle speed sensors that use a reed switch to generate a VSS signal.

As your Alfa uses an electronically generated VSS signal, you can safely set "VSS Debounce" to zero. Do this, and tell me if your problem goes away.
I don't know if it is 18000 pulses per mile (mine also in km i think) .. hmm.. can't read the scope so well, and car technical manual doesn't state.
Someone has made a fine diagram of the signal amplifier if that would be a help to help.
EDIT did you mean what i use in the settings?, got it set at 26280 VSS, it is where it seems ok correct for low speeds.
MPGuino indicates zero speed at exactly 55km/h~ 34mp/h

Is "VSS Debounce" the same as VSS delay?, cause i don't recall seeing "VSS Debounce" in settings. Have tried both setting that to 2..1.. and Zero, and still the same.

Last edited by mirkwood; 06-13-2016 at 07:40 AM..
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