Intelligent Wearable Interfaces

CHAPTER 7

Ubiquitous 3D Digital Writing Instrument

7.1 INTRODUCTION

A MAG-μIMU, which is based on Micro ElectroMechanical Systems (MEMS) gyroscopes, accelerometers, and magnetometers, is developed for real-time estimation of human hand motions. Appropriate filtering, transformation, and sensor fusion techniques are combined in the Ubiquitous Digital Writing Instrument to record handwriting on any surface. An extended Kalman filter (EKF) based on a MAG-μIMU (micro inertial measurement unit with magnetometers) is designed for real-time attitude tracking and is implemented to record handwriting.

In classic IMU applications, the Kalman filter uses the gyroscope propagation for transient updates and correction by reference field sensors, such as gravity sensors, magnetometers, or star trackers. A process model is derived to separate sensor bias and to minimize wide-band noise. The attitude calculation is based on quaternion, which when compared with Euler angles, has no singularity problem. According to this filter framework, a Complementary Attitude EKF is designed by integrating the measurement updates from accelerometers and magnetometers alternatively, in order to compensate the attitude observation error caused by sensor limitations, such as inertial accelerations, magnetic field distortion, and attitude ambiguity along each reference field. Testing with synthetic data and actual sensor data proved the filter will rapidly converge and accurately track the rigid-body attitude. The ...