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Gyros, Accelerometers & Inertial Measurement Units (IMU)

A gyroscope is made up of a wheel that is able to spin on an axle, with the whole construction mounted in a frame. When the wheel spins, the gyroscope resists attempts to change its position, due to angular momentum. The gyroscope thus can maintain a particular orientation in space, and so when mounted on an unmanned vehicle such as a UAV, ROV or AUV, it can be used to detect any changes in relative position between the gyroscope and the vehicle.

UAVs can be subjected to wind and other forces in many different directions, and similarly an unmanned underwater vehicle may be affected by tides and currents. The information from a gyroscope can therefore be used to stabilize the vehicle in turbulent conditions.

Three-axis gyroscopes measure rotation rate around the roll, pitch and yaw axes. A six-axis gyroscope adds a three-dimensional accelerometer, thus providing both static and dynamic acceleration measurements.

Accelerometers use acceleration measurements to figure out orientation relative to the surface of the earth. They are constructed from MEMS (microelectromechanical systems) components such as electromechanical capacitors, which vary their capacitance with acceleration.

The combination of gyroscopes and accelerometers allows an unmanned system to respond quicker to unexpected forces such as strong gusts of wind and unexpected sharp turns. Since accelerometers are noisy over the short term and gyroscopes drift over time, the combination of the two allows for greater position accuracy.

An IMU (inertial measurement unit) encompasses gyroscopes, accelerometers and other sensors such as magnetometers and can also include a GPS or other GNSS unit. The IMU fuses together information from all these sources to provide measurements that can be used to calculate orientation and velocity. Sophisticated algorithms such as Kalman filters are used in order to account for noise and drift.