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Subject: Re: Car speed conversion error
Date: Thu, 21 Nov 2002 22:28:52 +0000
References: <3DDB30C2.D6EC77EB@autometer.de> <firstname.lastname@example.org>
NNTP-Posting-Date: Thu, 21 Nov 2002 23:00:12 +0000 (UTC)
User-Agent: tin/1.4.2-20000205 ("Possession") (UNIX) (Linux/2.4.19 (i686))
In sci.electronics.design Amy wrote:
> 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?
Have a careful look at what is available on the CAN bus feeding the
dashboard, you may get lucky and have the odo right there!
Another possibility is that there MAY be a PID available via the OBDII
connector giving ODO reading... I no longer have my copy of KW2000 and
friends. Remember that KM/Miles is not necassarily the same on the CAN
bus as your local convention.....
I would not be at all surprised if in at least some cases the rounding
in whatever CAN node does the calculation takes the form of
speed = scale_from_frequency(...);
can_speed_value = speed;
Which always rounds down! Also there are some vehicles which report
speed over CAN as the absolute value of velocity, so going backwards
gives a positive speed. Note that you cannot even use the gear
information from the transmission to establish actual travel direction
as they are not always related in the obvious way!
Yes, I got bitten by this at a previous employer, put car into reverse
while creeping foward and get instant 100% torque demand (going foward!).
We noticed it while writing the module test. The saving grace was the the
fuel cell on that vehicle took a few seconds to ramp up so 'full torque'
from idle was limited by battery power. Still would have been a mildly
unpleasent surprise while trying to get out of a tight parking space!
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