Continuous Body Monitoring - Ambient Intelligence for Scientific Discovery - page 207

3 The SenseWear Armband - Specifications

The SenseWear armband is comfortable to wear, can be worn continuously, and
will store up to 14 days of physiological data before the data needs to be retrieved.
The armband is also equipped with wireless capability for retrieving the stored
data and also for providing real-time feedback. The specifications in Table 1 are
for SenseWear II.

3.1 Sensor Detail

The following provides more detail about the sensors in the Bodymedia armband.
Specifically the 2004 model SenseWear Pro 2 . After the physical sensors are char-
acterized we will consider why the important thing is not what is being sensed,
but what can be detected. For example, one can not purchase a pedometer sen-
sor, a calorie-burn sensor, nor a sleeping sensor. Yet these are what SenseWear
ultimately monitors and reports. From the physical sensors are derived synthetic
sensors, contextual information, and other measures [2].

Accelerometer. The accelerometer is a 2-axis MEMS device that measures mo-
tion. The motion can be mapped to forces exerted on the body and the gravity
information can provide valuable context information for predictive algorithms.
The accelerometer has a resolution of approximately 0.005g and is presently cal-
ibrated to match a reference gravity of one g to within 0.05g on the longitudinal
axis, 0.06g on the transverse axis. The axes are relative to an upright armband,
longitudinal being from ground to sky and transverse parallel to the ground.
Worn on the back of the upper arm, transverse is through the chest.

Heat Flux. The proprietary heat flux sensor is a robust and reliable device
that measures the amount of heat being dissipated by the body at the location
of the armband. The sensor uses materials with very low thermal resistance
and extremely sensitive thermisters. It is placed in a thermally conductive path
between the skin and the side of the armband exposed to the environment (see
Fig. 5). A high gain internal amplifier is used to increase the resolution of the
measurements to a resolution of m 2 . The accuracy of the heat flux

sensor is tested in a wide range of temperatures, with a low inter armband
variability about room temperature. At two regions of particular interest, the
heat flux sensor has a resolution of 0.267 W

< 0.3 W /

m 2 in thermal equilibrium at 22 ◦ C
(room temperature); and a resolution of 0.291 W

/

m 2 heat flux at
32 ◦ C (skin temperature). The high gain amplification results in a very sensitive
instrument, highly suited to higher rate sampling.

m 2 / at 50 W /

Galvanic Skin Response. Galvanic skin response (GSR) represents electrical
conductivity between two points on the wearer's arm. The GSR sensor in the
armband includes two hypoallergenic stainless steel electrodes integrated into the
underside of the armband connected to a circuit that measures the skin's con-
ductivity between these two electrodes (see Fig. 5). Skin conductivity is affected
by the sweat from physical activity and by emotional stimuli. GSR is used as an