Biomedical Engineering Reference
In-Depth Information
Fig. 16.2
The Wiimote sensor bar has two groups of IR LEDs at fixed widths
away, the controller's camera loses contact with it. The third example has the same
configuration as the second example, but with the sensor bar on the ground directly
below the Wiimote, at the user's feet. When the user repeats that forward motion,
the sensor bar reports the Wiimote moving up in the sensor bar's z-axis, whereas
the Earth and Wiimote data are the same as in the previous example. These exam-
ples show that each FOR captures important, different and incomplete information.
The following sections explain in more detail what this incomplete data looks like,
but it is important to keep in mind how the frame of reference may alter what the
data really means.
16.2.1.2 The Sensor Bar Connection
The sensor bar connection (SBC) is one of theWiimote's two primary spatial sensors.
It occurs when the Wiimote's infrared optical camera points at a sensor bar and sees
the infrared (IR) light emitted by its LEDs. A sensor bar has LEDs on each side
(see Fig.
16.2
), with a known width between them. This produces IR blobs that the
Wiimote tracks and reports in x and y coordinates, along with the blob's width in
pixels. To improve tracking distance (the Wiimote can sense blobs up to 16 ft away),
the sensor bar LEDs are spread in a slight arc, with the outer LEDs angled out and
the inner LEDs angled in. Interestingly, any IR source will work such as custom IR
emitters, candles or multiple sensor bars, provided you have themeans to differentiate
between the sensor bars. More details on extracting distance information between
Wiimote and Sensor Bar can be found in [
29
].
16.2.1.3 3-Axis Accelerometer
The second Wiimote input is the device's 3-axis accelerometer. The accelerometer
reports acceleration data in the device's x, y and z directions, expressed conveniently
in g's (approximately 9
s
2
), which is a common unit of many devices employ-
ing 3-axis accelerometers such as cell phones, laptops and camcorders. With this
information, the Wiimote is able to sense motion, reporting values that are a blend of
accelerations exerted by the user and by gravity. As the gravity vector is constantly
oriented towards the Earth ((0,0,1) in Earth's FOR), the gravity vector can be used
.
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