Biomedical Engineering Reference
In-Depth Information
to discover part of the Wiimote's orientation in terms of earth's frame of reference
using:
tan
−
1
a
z
a
y
pitch
=
tan
−
1
a
z
a
x
roll
=
.
Unfortunately, determining yaw in the Earth's FOR isn't possible because the Earth's
gravity vector aligns with its z-axis. Another unfortunate issue is that determining the
actual acceleration of the Wiimote is problematic owing to the confounding gravity
vector. To determine the actual acceleration, one of the following must take place:
•
the Wiimote must be under no acceleration other than gravity so that you can
accurately measure the gravity vector (in which case, you already know the actual
acceleration is zero);
•
you must make assumptions about the Wiimote's orientation, thus allowing room
for errors;
•
you must determine the orientation by other means, such as by the SBC or a
gyroscope (we discuss this in more detail later).
The implications for orientation tracking by the accelerometers are that theWiimote's
orientation is only certain when it is under no acceleration. For this reason, many Wii
games require that users either hold the Wiimote steady for a short period of time
before using it in a game trial or have it point at the screen, oriented to the SBC.
16.2.1.4 Wii MotionPlus
This attachment uses two gyroscopes to report angular velocity along all three axes
(one dual-axis gyro for x and y and a single-axis gyro for z). Mechanical gyroscopes
would typically be too large and expensive for a Wiimote so it uses MEMS (micro-
electromechanical system) gyroscopes, which operate using a vibrating structure,
are inexpensive, use little power and are fairly accurate. A MotionPlus-augmented
Wiimote provides information on changes to the Wiimote's orientation, alleviating
many of the device's initial data limitations.
While the MotionPlus isn't yet fully reverse engineered, we know it reports orien-
tation changes in two granularities, fast and slow, with fast being roughly four times
the rate per bit. The gyroscope manufacturer reports that the two gyroscopes have
a linear gain but that the different gyroscopes report values in two different scales,
so there is no single scaling factor. Additionally, temperature and pressure changes
can impact this scale factor and change the value associated with zero orientation
change.
Merging the acceleration and gyroscopic data isn't simple as both sensors have
accuracy and drift errors that, albeit small, amount to large errors over short time peri-
ods. When using the infrared sensor bar, you can compensate for these accumulating
