{"id":34,"date":"2012-05-18T04:50:49","date_gmt":"2012-05-18T04:50:49","guid":{"rendered":"http:\/\/bestperformancegroup.wordpress.com\/?page_id=34"},"modified":"2014-09-25T19:11:12","modified_gmt":"2014-09-26T02:11:12","slug":"mems","status":"publish","type":"page","link":"http:\/\/bestperformancegroup.com\/?page_id=34","title":{"rendered":"Inertial Sensor Motion Tracking"},"content":{"rendered":"

Inertial Measurement Units (IMU) are the latest advancement in motion tracking having been adapted from aerospace and military industries. \u00a0An IMU is a Micro-Electro-Mechanical System (MEMS) electronics module<\/a> and is typically comprised\u00a0of 3 accelerometers, 3 gyroscopes, and optionally 3 magnetometers. \u00a0IMUs with 3 axis accelerometers and 3 axis gyroscopes (either as tri-axial sensors or 3 independent axial sensors) are commonly referred to as 6 degrees of freedom (DOF) IMU sensors. \u00a0The inclusion of a 3 axis magnetometer is some times referred to as 9 DOF IMU sensors; however, technically these should still be referred to as 6 DOF IMU sensors with associated 3 DOF digital compass (magnetomers) as technically the magnetometer sensors are not inertial sensors. \u00a0\u00a0\"Fig_5\"<\/a>In most modern applications, the accelerometers, gyroscopes, and magnetometers used in 6 DOF and 9 DOF IMUs are usually each triad sensors, which insures axis orthogonality. \u00a0Axis orthogonality is very important for orientation matrix calculations. \u00a0In an accelerometer triad, the sensor generates three analog signals describing the accelerations along each of its axes produced by and acting on the object to which it is attached. \u00a0The most significant of these sensed accelerations is the acceleration caused by gravity. \u00a0There are two types of gyroscopes: an integrating rate gyro measures changes in orientation whereas a rate gyro measures changes in angular velocity. \u00a0A tri-axial rate gyro sensor outputs three analog signals describing the object’s angular velocity changes about each of the three sensor axes. \u00a0Even though the IMU is not located at the object’s center of mass, the angular rate measurements are not effected by linear or angular accelerations. \u00a0Most modern day IMUs also include a magnetometer triad which serves basically as a digital compass in the Earth’s significant magnetic field and is used to determine absolute orientation of the IMU in the Earth’s North-East-South-West (NESW) global reference frame. Magnetometers are used to help define an initial orientation and further positions are derived by integrating the gyroscope data. \u00a0\"ime\"<\/a><\/p>\n

Independently these sensors excel at different things, but the combination of these 3 sensor types in a 9 DOF IMU offers a very powerful motion tracking technology. Magnetometers offer very poor orientation accuracy for fast movements, but exhibit almost zero drift over time. \u00a0An integrating scheme using angular rate gyros reacts quickly and produces accurate rotations to quick movements, but errors are accumulated over time resulting in drift. \u00a0The integration routine requires starting from a known orientation. \u00a0Combining the inputs from all 3 sensors allows for quick and accurate position and orientation determination with a small amount of drift over time if an advanced Kalman filtering routine is implemented.
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