actigraphcorp.com	  	  April 28, 2015

ActiGraph White Paper
What is the Utility of Inertial Motion Unit (IMU) Data?
I. Introduction
ActiGraph has recently released the ActiGraph GT9X Link, which contains an Inertial
Measurement Unit (IMU). The IMU sensors and data output units of measure are listed inTable 1 below.
Sensor Type
3 axis accelerometer (+/- 16g)
3 axis gyroscope
3 axis magnetometer
Temperature sensor

Units of Measure
G’s
Degrees/second
microTeslas (µT)
Celsius - 0C	  Table	  1:	  IMU	  sensors	  and	  units	  of	  measure	  IMU data are sampled and stored at a 100 Hertz rate. The CSV file in Table 2 below
provides an example of how these data are displayed.

1	  	  	  	  	  	  	  Table	  2:	  IMU	  data	  sample,	  each	  sensor	  has	  a	  3-­‐axis	  output	  (X,	  Y,	  Z)	  in	  orde	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	   	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900	  	  1	  	  actigraphcorp.com	  	  II. Magnetometer
The magnetometer sensor can be used to determine direction via the Earth’s magnetic field.1
During an initial evaluation of the magnetometer, the ActiGraph Link was laid flat on a table
top with its top pointing north (see Figure 1 below). The device was then rotated 360 degrees
clockwise. Figure 2 shows the graphed output data.

North	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Figure	  1.	  Link	  tabletop	  test	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Figure	  2.	  Graphed	  magnetometer	  data
1. For	  best	  results	  magnetometer	  data	  should	  be	  coupled	  with	  acceleration	  data	  in	  order	  to	  eliminate	  effects	  from	  non-­‐zero	  elevation	  and	  bank	  angle	  values.	  	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900	  	  2	  	  actigraphcorp.com	  	  In Figure 2, the X-axis data (shown in blue) has a minimum level of approximately -6 µT(micro Tesla), which corresponds with north. As it is rotated, the blue line peaks atapproximately 34 µT, pointing south.
As expected, the Y-axis demonstrated a similar peak to peak deflection of 40 µT (-15 µT to25 µT), but lagging by 90 degrees (starting orientation was east). The Z-axis data did notchange significantly during the rotation, indicating that this axis was oriented perpendicular
to the tabletop (elevation and bank angles ~ 0o). Repeating the measurement with ahandheld Link device with non-zero elevation and bank angles resulted in variations in theZ-axis output.
These data were used to set up the compass table in Figure 3. Because the X-axis goesfrom low to high as it rotated from north to south, the Y-axis data is necessary to determine
if it is facing east or west while it is between its max and min. Therefore, in this simple
experiment, if the Y-axis is above 4 µT, the ActiGraph Link is facing east, and if it falls
below this value, it would be facing west.
An initial review produced the table below.	  
Figure	  3.	  Magnetometer	  compass	  table	  	  	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900	  	  3	  	  actigraphcorp.com	  	  A second test examined the ActiGraph Link in the vertical position (simulating a hipworn device), with the top up and the face northward. The ActiGraph Link was thenrotated twice (see Figure 4 below).	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Figure	  4.	  Graphed	  IMU	  data	  with	  Link	  in	  vertical	  position

Note that the axis orientation has changed with the Z-axis data serving as the north/south
indicator axis and the X-axis as the east/west indicator axis. Table 3 displays the ActiGraph
Link directional orientation.

Table	  3.	  Direction	  orientation	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900	  	  4	  	  actigraphcorp.com	  	  As noted earlier, when the ActiGraph Link was not horizontally oriented, the Z-axis valuechanged. When the device is worn at the hip, this becomes the Y-axis. With further study
and data conditioning, these data may help determine if the individual is going up or down
hill while ambulating.
*A Note About the Earth’s Magnetic Field:
The two apparent poles are referred to as dipolar, with a positive peak generally lying
somewhat south of the true locus of the anomaly and a depression (or negative) in themagnetic field occurring to the north at mid-northern latitudes.
Magnetometry: Nature’s Gift to Archaeology
Kenneth L. Kvamme chapter 9, pg 209

III. Gyroscope
The gyroscope measures rotational velocity in units of degrees per second (deg/s). Using thesame data set from the initial compass test with the ActiGraph Link laid flat on the tabletop,
the gyroscope data were plotted as shown in Figure 5.
100	  0	  -­‐50	  1	  179	  357	  535	  713	  891	  1069	  1247	  1425	  1603	  1781	  1959	  2137	  2315	  2493	  2671	  2849	  3027	  50	  -­‐100	  Gyroscope	  X	  Gyroscope	  Y	  Gyroscope	  Z	  -­‐150	  -­‐200	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Figure	  5.	  Plotted	  gyroscope	  data	  with	  Link	  flat	  on	  tabletop	  The graph in Figure 5 shows that only the Z-axis responded, indicating that it was rotated ina single axis plane. The signal is negative, which corresponds to a clockwise rotation.
Because the gyroscope output is in degrees per second, a column was added to the data
output to display the summed degrees per epoch. These data are stored at a rate of 100	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900	  	  5	  	  actigraphcorp.com	  	  samples per second; therefore the degree displayed must be divided by 100 to match theepoch period. The summed degree column was plotted in Figure 6 below.

Degrees	  0	  -­‐50	  1	  169	  337	  505	  673	  841	  1009	  1177	  1345	  1513	  1681	  1849	  2017	  2185	  2353	  2521	  2689	  2857	  3025	  50	  -­‐100	  -­‐150	  -­‐200	  Degrees	  -­‐250	  -­‐300	  -­‐350	  -­‐400	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  	  Figure	  6.	  Summed	  degrees	  added	  from	  the	  gyroscope	  Z-­‐axis

Figure 6 shows that the ActiGraph Link was rotated approximately a negative 360 (o) degrees(measurement was 359.01o). By looking at the number of epochs to rotate the full 360
degrees, the average rate of rotation can be determined. The rotation started at approximately
1000 and ended at approximately 3000, and since the sample rate is 100 per second, the 2000
epochs would be divided by 100 to equal approximately 20 seconds. This indicates therotation took about 20 seconds and 18.0o in one second (average).

IV. Conclusion
It has been shown that by monitoring the magnetometer data, directional information can beharvested. As mentioned, the inclusion of accelerometer data is needed in order to further refine
and improve accuracy of this sensor endpoint. Additionally, this testing demonstrates that
determining the rate of rotation and the amount of rotation is possible using the gyroscope data.
Each axis would need to be evaluated independently and some type of vector magnitude
algorithm would need to be applied in order to evaluate the three dimensional rotation. Withfurther research and study there are many possible applications for these data.	  	  	  	  49	  E	  Chase	  Street	  	  	  	  |	  	  	  	  Pensacola,	  FL	  32502	  	  	  	  |	  	  	  	  ph:	  850.332.7900	  	  	  	  |	  	  	  	  f:	  850.332.7900	  	  6