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From: firstname.lastname@example.org (Amy)
Subject: Re: Car speed conversion error
Date: 20 Nov 2002 07:09:56 -0800
References: <3DDB30C2.D6EC77EB@autometer.de> <email@example.com>
NNTP-Posting-Date: 20 Nov 2002 15:09:56 GMT
Thanks for all the answers and information! They are all very useful
> 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?
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