Orientation

The orientation sensor

The orientation of the Upoint remote is measured relative to the Earth's gravity and magnetic fields. Since the Earth's magnetic field changes from point to point on the Earth's surface, whereas the Gravity field is virtually constant, the magnetic sensors involved need to be able to measure the 3 components, but the acceleration ("gravity") sensors need only sense 2 components.

Notwithstanding this fact, the present prototype has some redundancy built into it and senses all 3 components of Magnetic as well as Gravity fields. That is, the 3 components of the Magnetic vector are sensed by a combination of the 1-axis and 2-axis magneto-resistive chips HMC1051Z and HMC1052L (manufactured by Honeywell), whereas the Gravitational vector is measured by the 3-axis MMA7260Q accelerometer (manufactured by FreeScale). Note that many other brands of accelerometers and magneto-resistive chips are out there, and prices continue to drop. Furthermore, other than magneto-resistive technologies (e.g., magneto-inductive) are also available, and essentially perform the same function.

1-axis and 2-axis magnetoresistive sensors: Honeywell's HMC1051Z and HMC1052L

3-axis accelerometer: FreeScale's MMA7260Q

Mathematical algorithms that derive Yaw, Pitch and Roll angles from these vectors are well-known, and are implemented in the PC based driver-software. Although the Roll angle is derived it is in fact not used by the algorithm, since it describes the motion of the remote around the pointing line, and hence doesn't affect the point-of-aim calculation.

Yaw, Pitch and Roll angles

Theoretically, Yaw and Pitch angles can be derived from gyroscope sensors, too, which measure angular velocity. Such angular velocities can be integrated over time to produce the same orientational information as produced by combinations of magnetometers and accelerometers. Gyros, however, experience a phenomenon called drift which, over time, causes their angular readings to become increasingly less accurate. Since the Upoint technology relies on being able to sense the remote's orientation in a fixed reference frame, gyroscopes alone are not suitable for determining Yaw and Pitch in the context of Upoint.

The present prototype, however, does employ the IDG300 2-axis gyro (manufactured by Invensense) as a means to stabilize the noisy Yaw and Pitch readings produced by the magnetic- and gravity sensors. It is anticipated that the use of adequate low-pass filters can replace this function of the gyro for the purposes of Upoint.

2-axis gyroscope: InvenSense's IDG300