The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: Car speed conversion error
References: <3DDB30C2.D6EC77EB@autometer.de> <email@example.com> <firstname.lastname@example.org>
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?
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup