Inertial Measurement Units (IMU) Information

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Selecting inertial measurement units (IMUs)

Inertial measurement units (IMU) are self-contained systems that measure linear and angular motion of an object or vehicle. Measurements are summed over a time period to determine the instantaneous position, velocity, orientation, and direction of movement.

Operation

Degrees of freedom

IMUs measure six degrees of freedom. This includes the measurement of linear motion over three perpendicular axes (surge, heave, and sway), as well as rotational movement about three perpendicular axes (roll, pitch, and yaw). This yields six independent measurements that together define the movement of an object or vehicle.

Sensor types

The IMU is comprised of at least two dedicated sensors, one or more linear accelerometers and one or more gyroscopes or angular accelerometers. An optional magnetometer may be integrated into the unit to calibrate against orientation drift.

Accelerometers detect the direction and magnitude of change in velocity. Simple accelerometers measure linear motion while biaxial and triaxial accelerometers detect a change in velocity over a plane or three-dimensional space, respectively. The IMU possesses a triaxial (sometimes referred to as a triad) accelerometer, or otherwise uses multiple accelerometers that are aligned across perpendicular axes. To learn more about accelerometers, please visit GlobalSpecs' accelerometers specification guide.
Gyroscopes detect the angular rate or orientation about a given directional vector. The angular rate is relative to a reference surface. The IMU uses multi-axis gyros to provide measurements in three orthogonal directions. These angular movements must be aligned with those of the accelerometer. To learn more about gyroscopes please visit GlobalSpecs' gyroscope selection guide.

Working principle

Selecting IMUs inertia measure IMUs operate by use of reference data, bias values from an initial starting point, and calculate changes to these values using its integrated sensors. A central processing unit calculates directional information; position, speed, orientation, and direction of movement, at a given time in space using the IMU. The sensors suffer from orientation drift as they calculate these variables using a process known as dead-reckoning and are subject to accumulative errors.

Dead reckoning

Dead reckoning is the calculation of current position by use of a previously determined location and the advancement of that position by a known or estimated directional speed over an elapsed time. The process was first used in marine navigation and relied upon manual measurements. IMUs calculate accurate directional information using integrated sensors and operate using these same principles.

Orientation drift is the propagation of orientation errors. Small measurement errors of acceleration and angular velocity produce larger errors in velocity that are compounded into even greater errors in position. Orientation drift, the difference between the actual position and orientation from the measured values, increases with respect to time as measurement errors are compounded. IMUs typically incorporate some amount of calibration in order to compensate for orientation drift.

Calibration

Calibration parameters can be stored in the memory of an IMU and are automatically reflected in the measurement data. In-situ calibrations may also be accomplished by use of a magnetometer or GPS unit to correct for orientation drift and improve the accuracy of the directional information.