How is the acceleration data formatted in Empatica Connect?

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Empatica devices have an onboard MEMS type 3-axis accelerometer that measures continuous gravitational force (g) applied to each of the three spacial dimensions (x, y, and z). The scale is limited to +-2g (by default) on the E3 and E4 and can be extended to +-8g with custom firmware.

Figure 1 is an example of Accelerometer data from Connect, the subject moves into position, walks, runs, and then simulates the turning of a car's steering wheel. The dimensional axes are depicted in red, green and blue.

The format is described in the info.txt file that is included in each session archive.

The accelerometer is configured to measure acceleration in the range [-2g, 2g]. Therefore the unit in this file is 1/64g.
Data from x, y, and z axis are respectively in first, second, and third column.


For analytic purposes you can interpret the raw data as follows:

  • xg = x * 2/128
  • a value of x = 64 is in practice 1g

 

Along with the raw three-axis acceleration data Empatica provides a moving average of ACC

 

In figure 2, the same data is visualized using the moving average . from Connect, the subject moves into position, walks, runs, and then simulates the turning of a car's steering wheel. The dimensional axes are depicted in red, green and blue.


Each second (32 samples) the acceleration data is summarized using he following method:

  • sum+= max3(abs(buffX[i] - prevX), abs(buffY[i] - prevY), abs(buffZ[i] - prevZ));

The output is then filtered:

  • avg=avg*0.9+(sum/32)*0.1;

 

To change your Connect visualization preferences:

click on your account name in the upper-right corner of the Connect Application, select the "Preferences" tab, and then chose your preferred visualization.

 

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