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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Car speed conversion error
References: <3DDB30C2.D6EC77EB@autometer.de> <email@example.com>
Date: Wed, 20 Nov 2002 15:27:56 GMT
NNTP-Posting-Date: Wed, 20 Nov 2002 07:27:56 PST
> Thanks for all the answers and information! They are all very useful
> to me!
> > Your question is unclear to me. The VSS output I am familiar with
> > is a digital pulse train, the particular car I worked with had a
> > VSS that output 2000 pulses per mile. To calculate mph you could
> > either count pulses received in a fixed time period (bad- not very
> > good resolution at normal speeds for a reasonable update interval)
> > or measure the amount of time it took to receive a certain number
> > of pulses. The tolerance of the conversion was controlled by the
> > timer/counter resolution, not by any sort of analog -> digital
> > signal level conversion.
> I do not know too much about car design (although I drive one =P);
> forgive me if I didn't use the right term.
> Let me clarify my question. We can calculate the car speed based on
> the digital pulse train from the VSS. But nowadays, most of the cars
> have some kind of networking inside them; it can be using J1850
> protocol or CAN protocol. For example, if the ABS unit or cruise
> control unit requests for the speed information, the speed information
> will be sent out in the format of "0" or "1" (2 bytes unsigned number
> for J1850) rather than the original pulse train from the VSS. So my
> question is what is the conversion error when the original pulse train
> from VSS gets converted to 2-byte data? And if there is any regulation
> the tolerance for this conversion?
> From what you guys pointed out, the error in speed can come from tire
> size/pressure variation, chipset/oscillator tolerance, timer
> resolution and sampling time.
> The reason I want to know about these things is I want to read the
> data bus in a car regularly (say 10 times per second) to get the speed
> information, and integrate it with time to get the distance travelled.
> In your opinions, what would be the worse case percent error in
> distance, taking into account all kinds of errors?
The error can be quite large if there are modifications. For example, I
have a vehicle that came with 7.50X15 highway tires from the factory.
The vehicle is now equiped with 31/10.50X15 off road tires. The odometer
error is known to be about 20% for that vehicle. That is, if the
odometer says it has gone 100 miles, the correct distance is 120 miles
more or less. Where I live, there are some ethnic persons who will not
be named who do violent modifications the other way. I saw a pickup
truck with spring shackles that cleared the ground by a whopping 0.5
inches. The odometer error for that is greater than 30% on that brand of
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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