How to measure the speed of car by phone with accelerometer and gyroscope?

Question

I want to know current speed of car and make a passed path. I have an Android phone with accelerometer and gyroscope which sent me data. This is the data in phone system of coordinate that probably wouldn't the same as coordiante system of car.

How I can transform this accelerations and rotations to car system of coordinate?

Answer 1

The generic answer for your generic question is no. The acceleration measures the changes in the speed, so the best you could get from acceleration, is the speed variation.

To get the absolute speed you would have to have the initial speed and add it to the speed change:

v(t) = v0 + a*t

So, if you would have a car moving along a straight line, and your device was fixed to the car, you could get easly the speed changes (although measurements errors will add up and quickly lead to discrepancies)

In practice you will face many issues trying to implement it, namely:

• You need the initial speed to be determinate based on the same referential as the acceleration. This would require some measurements and a lot of trignometry, as you would get both values from different sensores at different rates.
• The car will not move in a straight line, so your acceleration referential will be constantly moving (a lot more of trignometry and calculus).
• If the device is in the user hand, the device movements in relation to the car will increase even more the calculations (and accumulated errors).

Regards.