Freematics Forum

Hi,

I have several questions about acceleration.

1. It seems like the data values have biases, specifically xbias = 30, ybias = -3, zbias = -83. Please, see the attached image of the histograms of the three accelerations _after_ the biases have been subtracted.

2. The positive direction of the x acceleration axis points backwards, i.e. when acceleration (after subtracting the bias) is positive the car is slowing down, and vise versa.

3. The last question is about the units. At first I assumed that, after subtracting the biases, the units are m/s^2. However, now I found the topic (viewtopic.php?f=2&t=1982&p=3936&hilit=acceleration#p3936) that states that acceleration should be computed using the following formula:
actual acceleration at X axis (in m/s^2) = x_accelerometer_sensor_data *160(to correct approximation done in original code) * 9.81(value of g)/ 16384
with some mysterious conversion factors of 160 and 16384.
What are these coefficients? And more importantly, back to my first question, is there a bias in the data?

Thank you

RE: #2

I suspect this has to do with the orientation of the OBDII port in your vehicle and whether the Freematics dongle is 'phased' properly.

Some code will need to be written to calibrate the x, y, z axis with respect to the port.

For instance in one of my vehicles the port allows the Freematics dongle to be facing with the label upward, and in another car,
the label is down.