Measure vehicle speed using Odometer

[mostafanfs]

How can I measure vehicle speed using its odometer sensor?
I think its output pulse has a low frequency around 200Hz in 100Km/h speed.
Do I have to count its pulses in a defined time or should I measure the pulse width ?
In either ways what is the necessary calculations or formula?

 

[Daniel_Arg]

I once made an rpm meter. I remember counting the pulses within a given time period, like half a second was much easier and yield higher resolution than measuring the time between pulses. On the other hand counting the pulses in a given period gives you a much smaller refresh rate than measuring the time between pulses. Those are my two cents.

 

[c_mitra]

The derivative of the odometer count with respect to time is the vehicle speed. In a car, I believe, that there is only one cable connected to the wheel to measure the speed and distance. The derivative is taken by a rotating magnet and eddy current shield controlled by a spring.

You can either count the number of pulses in unit time or time between two pulses.

I do not understand the last question.

 

[mostafanfs]

Thank you
Well there should be some sort of coefficient or something, right?

 

[Daniel_Arg]

Yes, but it's probably specific to a manufacturer and/or model. You measure the rotational speed of the output axis of the gearbox. How those rpm translate into a given speed depends on the car wheel size.

Depending on the car you could also get the speed by reading the ECU through the OBDII port.

 

[Warpspeed]

Frequency of 200 Hz at 100 Kmh is borderline for the counting pulses over time method but it would work.

Just count pulses over a half second period and you get 100 pulses at 100 Kmh.
That will work, but you will notice two things.
The reading updates twice every second, which is o/k, but the last digit will change randomly.

If you sit on 100 Kmh the reading might flicker between 99 and 100, or 100 and 101 twice per second which could become really annoying.
There are a couple of ways to fix that, but it will slow down the update rate.

The other method of measuring the time between pulses means the speedometer will update every pulse, or in 5mS at 100 Kmh. You can average that and get a pretty smooth display without the last digit jumping, and it will update really fast, except at very low road speeds.

Once you have measured the time interval using input capture, you divide a fixed constant by the time interval.

In this example 100 Kmh = 200 Hz = 5.000mS
If you make the constant 500.
500 divided by 5.0000mS = 100 Kmh display

42 Kmh = 84Hz = 11.905mS
500 divided by 11.905 = 41.999 Kmh display