Inertial Measurement Unit
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An Inertial Measurement Unit is a closed system that is used to detect altitude, location, and motion. Typically installed on aircraft or UAVs, it normally uses a combination of accelerometers and angular rate sensors (gyroscopes) to track how the craft is moving and where it is.
Typically, an IMU detects the current acceleration and rate of change in attitude (i.e. pitch, roll and yaw rates) and then sums them to find the total change from the initial position.
An IMU stands in contrast to the GPS system, which uses external satellites to detect the current position.
IMUs typically suffer from accumulated error. Because an IMU is continually adding detected changes to the current position, any error in the measurement is accumulated. This leads to 'drift', or an ever increasing error between what the IMU thinks the position is, and the actual position.
IMUs are normally one component of a navigation system. Other systems such as GPS (used to correct for long term drift in position), a barometric system (for altitude correction), or a magnetic compass (for attitude correction) compensate for the limitations of an IMU. Note that most other systems have their own shortcomings which are mutually compensated for.
The term IMU is widely used to refer to a box, containing 3 accelerometers and 3 gyroscopes. Accelerometers are placed such that their measuring axis are orthogonal to each other. They measure the so-called "specific forces" (inertial acceleration - gravity).
Three gyros are placed such that their measuring axis are orthogonal to each other, measuring the rotation rates.
One or more temperature sensors are included, maybe already incorporated in each accelerometer or gyroscope, or as an additional sensor used to calibrate the raw data from the other sensors.
To achieve superior accuracy, the box may be designed such that the temperature is controlled and kept constant. The walls of the box are made of materials that minimize electromagnetic interference. If the output signals are analog, electrical noise must be reduced in the cables and analog to digital converter. If the output data is already in digital format, time delays become the major concern.
The data provided by an IMU box is all that's needed to perform dead reckoning. The first use of such a box was in a ship, and today almost every ship has one. Satellites have one also. Almost anything that must somehow use some electronics to know its acceleration, velocity and/or attitude has an IMU.
Nowadays one can buy an off-the-shelf MEMS based IMU for approximately 2000 euros. This is perfectly adequate for many purposes. By modifying the kinematic equations and running the data with a Kalman filter, the data from an IMU can be transformed into an AHRS system (Attitude and Heading Reference Systems). This measures roll and pitch.
The dead-reckoning integration of the rate gyros will cause error by drift, but the observation of the gravity vector by the accelerometers serves as an external observation of the local vertical. This corrects most drift errors.
In dynamic environments such as a jet fighter, the gravity will be masked by the aircraft's body accelerations. In these cases the IMU is normally coupled with the GPS or other sensors. This brings us a bit closer to a Navigation and Guidance System and a bit away from the IMU. This is why a lot of engineers don't really make a distintion between an IMU and an inertial guidance system.
See also inertial guidance system
IMU's can, besides navigational purposes, serve as orientation sensors in the human motion field. They are frequently used for revalidation (quantification of progress in revalidation), sports technology (technique training) and animation. It is a good substitute for optical and magnetic tracking systems (see also Motion capture).
