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From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: Car speed conversion error
References: <3DDB30C2.D6EC77EB@autometer.de> <firstname.lastname@example.org> <email@example.com>
Date: Fri, 22 Nov 2002 09:21:18 -0500
NNTP-Posting-Date: Fri, 22 Nov 2002 09:21:12 EST
Organization: Bell Sympatico
> 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
> float speed;
> int can_speed_value;
> 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!
> Regards, Dan.
Is there any standardization of the data format on automotive
implementations of CAN? If so, which standards apply? If not, is there a
good source for this info?
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