IMU

Magnetometers

Magnetometers are able to measure and determine relative changes in magnetic fields. They usually record measurements in degrees or radians. The most common use of magnetometers in robotics is to act as a compass. Knowing which way North is can help you orient yourself more accurately when solving navigation problems. This sensor can still accurately read while an object is static, or not moving.

Accelerometers

Accelerometers are used to precisely measure acceleration. Acceleration is measured in meters per second squared (m/s^2). Many of the accelerometers you will use in robotics measure in three-axes and will measure lateral and vertical acceleration. You can use this information to infer velocity if you do not have something that can directly measure velocity. It is important to note that this sensor will not be providing relevant measurements when a robot is in a steady state (i.e. not moving or moving at a constant rate).

Gyroscopes

Gyroscopes are used for measuring rotational motion and recording movement in degrees per second or revolutions per second. Many of these sensors are based around a technology called MEMS or microelectromechanical systems. They use very tiny moving parts to actually measure the movement of an object. A gyroscope reads motion and therefore it will not be able to provide any information for a robot that is static.