Inertial measurement unit

An inertial measurement unit, or IMU, is the main component of inertial guidance systems used in air-, space-, and watercraft, including guided missiles. An IMU works by sensing motion — including the type, rate, and direction of that motion — using a combination of accelerometers and gyroscopes. The data collected from these sensors allows a computer to track a craft's position, using a method known as dead reckoning.

How they work

An IMU works by detecting the current rate of acceleration, as well as changes in rotational attributes, including pitch, roll and yaw. This data is then fed into a computer, which calculates the current position.

For example, if an IMU installed in an airplane were to detect that the craft accelerated westward, resulting in a calculated speed of 500 miles per hour, and detected no other accelerations for 1 hour, then the guidance computer would deduce that the plane must be 500 miles west of its original position. If combined with a computerized system of maps, the guidance system could use this method to show a pilot where the plane is located geographically, similar to a GPS navigation system — but without the need to communicate with any outside components, such as satellites.

A major disadvantage of IMUs is that they typically suffer from accumulated error. Because the guidance system is continually adding detected changes to its previously-calculated positions "(see dead reckoning)", any errors in measurement, however small, are accumulated from point to point. This leads to 'drift', or an ever-increasing difference between where the system thinks it is located, and the actual location.

For example, if an individual were blindfolded, moved in a series of directions, and then asked where they think they are, they would only be able to estimate their final position. The more a person were moved while blindfolded, the more inaccurate their guess of where they have ended up. IMUs work in a manner similar to that which human beings use to detect motion, and although they yield considerably more accurate motion sensing than a human being is able to perform, they are still not perfect, and their errors can accumulate in a similar way.

IMUs are normally only one component of a navigation system. Other systems are used to correct the inaccuracies that IMUs inevitably suffer, such as GPS, a barometric system for altitude correction, and a magnetic compass.

Construction

The term IMU is widely used to refer to a box containing three accelerometers and three gyroscopes. The accelerometers are placed such that their measuring axes are orthogonal to each other. They measure inertial acceleration, also known as G-forces.

Three gyroscopes are placed in a similar orthogonal pattern, measuring rotational rates (meaning how the craft is rotating in reference to the Earth's gravity).

Uses

IMUs are primarily used in vehicle-installed inertial guidance systems. Today almost every water-going vessel has one. Most aircraft are also equipped with IMUs.

IMUs can, besides navigational purposes, serve as orientation sensors in the human field of motion. They are frequently used for sports technology (technique training), and animation. They are a competing technology for use in motion capture technology.

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