Background - Smartphone-Based Human Activity Recognition - page 14

2.3.2.2 Gyroscope
A gyroscope is a sensing device for measuring orientation (Woodman 2007 ). It has
been used in many applications such as inertial navigation systems, aerial vehicles
for stability augmentation (e.g. in quadcopters) and recently, it has been introduced
in electronic devices (e.g. smartphones, game consoles) for enhancing user inter-
faces and gaming experience. For HAR, this sensor has been employed in various
applications such as for the detection of activities (e.g. walking, stair climbing) and
transitions between postures (e.g. from standing to sitting) (Coley et al. 2005 ; Najafi
et al. 2002 ).
Gyroscopes have also been produced withMEMS technologies. However, sensors
of this type can only measure orientation indirectly. They estimate angular speed
instead which can then be integrated in time in order to obtain orientation. However,
it is required first to have a reference initial angular position to achieve this. These
sensors are also highly prone to noise which can cause measurement drift from
integration.
Several MEMS gyroscopes obey the tuning fork principle in which a couple of
test masses are driven to resonance (Yazdi et al. 1998 ). Then, their displacement with
respect to their plane of oscillation can bemeasured into a signal related to the angular
speed of the sensor. Similarly to the accelerometer, it is possible to measure angular
velocity (magnitude and direction) using arrangements of single-axis gyroscopes.
2.4 Smartphones: The Evolution of the Traditional
Mobile Phone
Since the launch of the first mobile phones to the market by the end of the 1970s
(Oliphant 1999 ), a remarkable growth in the production of these devices has been
observed over the years. Statistics have shown that by 2011, this technology reached
nearly 80% of the world population (Ekholm and Fabre 2011 ). This trend is clearly
showing that mobile devices will be accessible to virtually everyone in a matter
of years.
Conventional mobile phones offer the basic telephony service which allows peo-
ple to communicate wirelessly from any location, provided it is within the available
cellular network coverage. These devices also include other essential communica-
tion services such as Short Message Service (SMS), Multimedia Messaging Service
(MMS) and email.
The user need for new services such as the instant access to information and more
sophisticated communication alternatives has stimulated the emergence of smart-
phones. These devices, which are mainly a new generation of mobile phones, are
provided with more computing capabilities, integrated hardware, services and wire-
less communications than the traditional mobile phones. Web access, multimedia,
gaming, location-based services, image capture and video recording, are just a few