Forums: Automotive: alOBD Scanner

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VSS and Distance Measurement

Good Evening Alexandre,
This isn't a problem report but rather a question. I have purchased your ScanGenPro Android app and it does everything as advertised. I have a need to measure accurate distance travelled for an field application. Could you explain the estimated accuracy of distance measurement using ScanGenPro... do you determine distance using integration of VSS every second or so or is there another method you employ? Do you have any idea as to the accuracy of the distance measurement under constant speed and under a situation where speed varies according to traffic? I have seen a couple of online articles about using OBDii VSS data in a taxi billing application, but I didn't not examine their mathematical data to get a feeling for the accuracy of distance measurement in typical traffic. Would you think your method could get results in the 0.001 mile ballpark? Thanks for any answer and thanks for the great Android app.

WaltK
Tue, 04 Jan 2011 01:45:04 +0000

Dear WaltK,
Thank you for purchasing alOBD ScanGenPro for Android. I am glad that you're considering it for your field application.
As to your question, the application itself does not measure anything. All information in PIDs is coming from vehicle's OBDII computer. The car PCM(ECU) is measuring all distances and speed and my app is just reporting data provided by the computer.
As to the accuracy of the distance data, I suspect it should be as accurate as your vehicle's odometer. The odometer accuracy must stem from the corresponding federal regulations, which I do not know.

I apologize, I couldn't be more helpful.

Thank you again.

anonymous
Wed, 05 Jan 2011 16:58:34 +0000

OK, thanks. US Federal requirements only require that the VSS reported via OBDII are to nearest 1MPH (or 1KM/Hr) and that the parameter is updated every 100 milliseconds. Of course the original VSS analog signal is a square wave pulse train which, in essence, is super accurate since it is measuring revolutions of a shaft (generally) in the gearbox. Thus there is virtually no error there assuming you know the amount which the vehicle moves per pulse, and that is a fixed amount if we ignore expansion of the tires, etc. Of course when the OBDII signal is reported it is an approximation of the VSS signal frozen and reported 10 times per second, and then only to the nearest 1MPH (or 1KM/HR). So it has a variance when compared to the "count" of the original VSS pulses. It is a shame that the standard does not require reporting of the total pulses which occured on VSS SINCE THE LAST REPORTED VALUE. In that way a more accurate approximation of speed could be calculated as well as a very accurate estimate of distance travelled.
Anyway, thanks for the response.
WK

WaltK
Sat, 08 Jan 2011 14:34:44 +0000

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