using sensor fusion can produce the following measurements: ■ Gravity – specifically the earth’s gravity and excludes the acceleration caused by the motion being experienced by the device. An accelerometer measures the gravity vector when the IMU is stationary. When the IMU is in motion, the gravity measurement requires fusing data from an accelerometer and a gyroscope and subtracting out the acceleration caused by motion. Applications that require orientation detection with respect to the earth can use gravity measurements. ■ Linear acceleration – equivalent to the acceleration of the device as measured by the accelerometer, but with the gravity vector subtracted. IMU linear acceleration can be used to measure movement in three-dimensional space. The accuracy of this value depends on the tracking accuracy of the gravity vector. ■ Orientation (attitude) – the set of Euler angles including yaw (azimuth), pitch, and roll, as measured in units of degrees. ■ Rotation vector – derived from a combination of data from accelerometer, gyroscope, and magnetometer sensors. The rotation vector represents a rotation angle around a specified axis.
Figure 1: DFRobot’s SEN0140 10 DOF MEMS IMU sensor board integrates an accelerometer, a magnetometer, a gyroscope, and a barometric pressure sensor. (Image source: DFRobot)
IMUs can be used for a variety of applications including consumer (mobile phones), medical (imaging), industrial (robotics), and military (head tracking). The required IMU accuracy depends on the application requirements.
possible movement of a rigid body in space can be expressed as a combination of the six basic DOF. However, in the world of IMUs, there are numerous references to 9 DOF and even 10 DOF sensors. This nomenclature can be rather confusing given that there are only six total DOFs used to describe movement. The nine DOF number comes from adding up the DOF for each type of sensor contained inside the IMU. So, if an IMU has a 3 DOF accelerometer, a 3 DOF gyroscope, and a 3 DOF magnetometer, then it’s called a 9 DOF IMU. Adding an atmospheric pressure sensor to the mix for measuring altitude creates a 10 DOF IMU.
Six degrees of freedom
DOF refers to the possible movements of a rigid body within three-dimensional space. There are only six DOF in 3D space: three DOF for linear translation (forward/back, up/down, left/ right) and three DOF for rotations (pitch, yaw, and roll). No matter how complex the motion, any
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