Biomedical Engineering Reference
In-Depth Information
or angular velocity. Although this means the Wiimote's spatial data doesn't directly
map to a real-world positions, the device can be employed for this under constrained
use [
17
,
20
]. For these reasons, it makes a good study of how incomplete sensor data
can be fused together to make a useful controller.
16.2.1.1 Frames of Reference
The accelerometer and gyroscope sensors used in the Wiimote mean it has no single
Frame of Reference (FOR). Instead, there are three that need to be considered when
working with the device. Figure
16.1
shows the Wiimote's personal FOR where x,
y and z axes are labeled, along with rotations pitch, roll and yaw, respectively. The
Earth's gravity, detected by accelerometers, is a second FOR and a third FOR is the
Wiimote's relationship to the sensor bar.
The following three examples clarify why these FOR are important and how all
must be understood when working with the device. First, consider a user holding a
Wiimote naturally, when +z is up in both the Wiimote's and the Earth's FOR and the
Wiimote's front points away from the user and toward a sensor bar, which is usually
on top of a display. In this first example, the user moves the Wiimote toward the
sensor bar. This results in acceleration reported in the y-axis of both the Earth's and
Wiimote's FOR and the sensor bar reporting decreased distance between theWiimote
and it. In the second example, the user rotates the Wiimote down to point toward
the earth (a 90
◦
pitch). When the user again moves the Wiimote directly toward the
sensor bar, the controller reports that there is an acceleration in its z-axis. However,
the Earth's FOR has acceleration in its y-axis because the 90
◦
downward pitch didn't
change the Earth's FOR. The sensor bar has no FOR because as the Wiimote rotates
Fig. 16.1
The Wiimote, with labels indicating the Wiimote's coordinate system. Multiple coordi-
nate systems and partial spatial data make the Wiimote difficult to design for
