User's Manual

C-2 Actiwatch Hardware
Actiwatch
Details
Accelerometer
The shape of the accelerometer makes it most sensitive to motion changes
in certain orientations. However, it is an omnidirectional sensor, resulting
in sensitivity to motion changes in all directions. The Actiwatch stores an
integration of movement (including directional aspects) as activity “counts”.
Thesamplingfrequencyis32Hz.
Accelerometer orientation
Sensor Calibration and Reproducibility
EachActiwatchhasbeenprogrammedwithacalibrationcoefficienttonormalize
data between watches. During calibration at the factory, all Actiwatch devices are
normalizedtoremovemost,ifnotallvariationbetweendevices.Regardlessof
thisfeature,weemphasizethatrelativelevelsofactivitywithinadataset,oras
measured by the same device, should be used for drawing inference rather than
absolute values between units. This is due to the great variation in mounting the
device.
Note: To obtain the most repeatable results, it is important to develop a
standardized mounting and positioning protocol.
Time Keeping
The computer clock sets the Actiwatch start time. Within the device a crystal
oscillator keeps elapsed time. When a sampling epoch is chosen, Actiwatch waits
a specific number of oscillations before storing the number of activity counts.
This also applies to delayed starting times. The time-keeping circuitry does not
initiate until the device has been removed from the reader.