I'm interested in using a microcontroller (PIC probably) to interface to a car/truck odometer to track usage (read only of course ;-).
Haven't had much luck w/Google/DejaNews finding relevant info, so I was wondering if anyone here had any experience they'd be kind enough to share, particularly what type of interface circuit to use, best place/way to get the signal off the vehicle, and what types of signals are available.
thanks
On 27 Jun 2003 11:48:52 -0700, snipped-for-privacy@mindspring.com (Larry) wrote:
It depends. Different vehicles have different sensors and signals. Modern vehicles usually have a digital Vehicle Speed Sensor (VSS) signal that can be found various places in the vehicle, including the engine control module harness, an instrument cluster harness, and somewhere near the sensor (usually near the transmission). The characteristics of this signal vary. The first step to understanding a particular vehicle is to get the service manual and understand the relevant part of the schematics, and try to figure out what kind of sensor and signal it uses. I have seen optical, variable reluctance, and hall-effect sensors. After that, I put the target vehicle on stands in the garage and put it in gear and observe the signal at various speeds with a scope. I have seen open-drain, open-source & active signals of various voltages, DC and AC. The number of pulses per rotation is also variable, and can be quite high for vehicles with ABS. On some cars, this signal counts rotations of the transmission output shaft, on others it is usually one of the undriven wheels. I then start experimenting with the interface circuit to get a useable digital signal without interfering with the vehicle computers. Depending on the signal characteristics, you may need to pull up, pull down, isolate with a diode, and/or add a buffer amp. If you use this signal, you need to make sure that you are not interfering with it because it can confuse the engine management and/or ABS systems. If the car has cruise control, this can be an easy way to test the signal for interference.
My experience comes primarly from working with rally computers, which are essentially precision odometers. Most off-the-shelf rally computers will only work with a particular type of signal, so we often leave the original vehicle sensor alone and add one or two sensors (known as "probes") just for the rally computer. This allows us to avoid interfacing with and interfering with the engine management system, to use a sensor that is directly compatible with the particular computer and to have redundant sensors for instant switch-over in case of failure. Depending on your application, you may find it easier to simply add your own sensor and connect that to your computer. I would recommend googling on "rally computer probe". Here are some companies that I have dealt with that sell odometer sensors to the rally market:
Good luck.
-- Chuck Cox SynchroSystems Motorsport Computers Hopped/Up Racing Team
or you could use a different approach and get the data from a gps in the vehicle
share,
the
Aren't the U.S. Interstate mile markers reasonably accurate?
The distance-to-go signs in California only have a casual relationship to reality. They seem to include a +/- 5 mi. randomization factor for amusment of CalTrans personnel.
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