Use IMUs for precise location data when GPS won’t suffice
consume very little power, and they’re available from a variety of sources with a wide range of resolution and accuracy. With these characteristics IMUs can be used to augment positioning information from GNSS receivers. (See “ Design Location Tracking Systems Quickly Using GNSS Modules.”)
Gyroscopes track relative movement independently from gravity, so errors from sensor bias or integration result in a position error called “drift.” ■ Accelerometer sensors measure linear acceleration,
intensity varies between 45 and 55 µT, at an angle between 50 and 80 degrees. By computing the angle of the detected earth’s magnetic field, and comparing that measured angle to gravity as measured by an accelerometer, it is possible to measure a device’s heading with respect to magnetic north with high accuracy. An adjustment based on the current latitude and longitude is needed to get a true north heading. ■ Pressure sensors measure differential or absolute pressure with units typically in hectopascal (hPa) or milliBar (mbar), which are equivalent. Standard atmospheric pressure
including acceleration components caused by device motion and
acceleration due to gravity. The acceleration is measured in Gs, which are multiples of the earth’s gravitational force (1 G = 9.8 meters/ second2). Accelerometers are available with one, two, or three axes, which define an X, Y, Z coordinate system. Accelerometer data can be used to measure static device orientation by computing the measured angle of the device and compensating for gravitational force. Periods of complex motion can complicate
Anatomy of an IMU
Motion sensors react to and detect physical motion, including parameters such as acceleration, movement rate, or distance. Inertial sensors are a special class of motion sensor. IMUs integrate a number of motion sensors into one device and can provide high accuracy positioning information. They react to the motion of the sensor itself. IMUs incorporate one or more of the following motion sensor types: ■ Gyroscope sensors measure angular position changes, usually expressed in degrees per second. Integrating angular rate over time results in a measured angle of travel which can be used to track changes in orientation. Gyroscope sensors are available with one, two, or three axes corresponding to pitch, roll, and yaw angles.
at sea level is defined as 1013.25 hPa. Changes in
altitude cause corresponding changes in detected ambient air pressure and can be used to track vertical motion. Motion tracking using IMUs employs sensor fusion to derive a single, high accuracy estimate of relative device orientation and position from a known starting point and orientation. Sensor fusion involves combining the IMU’s various motion sensor outputs using complex mathematical algorithms developed either by the IMU manufacturer or the application developer. Position calculations
the orientation calculation. ■ Magnetic sensors measure
magnetic field strength, typically in units of microTeslas (µT) or Gauss (100 µT = 1 Gauss).The most common magnetic sensor used for mobile electronics is a three-axis Hall-effect magnetometer. The magnitude of the Earth’s magnetic field varies between 25 and 65 µT, and in angle of inclination depending on geographic location. For the continental United States, the
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