What Is an Inertial Measurement Unit?

Andy Hill

An inertial measurement unit (IMU) is a key component of navigational equipment used in airplanes, spacecraft and water-borne vehicles. They utilize accelerometers, electrical reporting mechanisms, and gyroscopes to analyze and measure acceleration rates, rotational change, and orientation. IMUs are also utilized in guided missile navigation systems. The inertial measurement unit is used to measure gravitational forces, commonly referred to as g-forces.

An inertial measurement unit is an integral part of an aircraft's navigational equipment.
An inertial measurement unit is an integral part of an aircraft's navigational equipment.

Working in a similar manner to inertial navigation units (INU) via a series of gyroscopes and accelerometers, the inertial measurement unit measures gravitational forces and is able to produce time-line records of the measurements. The records provided by an inertial measurement unit are passed to a computer, which can then calculate the vehicle position based on the reported velocity, directional records, and elapsed time. This positional record can also be transposed directly onto an electronic mapping system to provide the operator or pilot with a geographical location. This method of navigational control is similar in nature to global positioning systems (GPS) but negates the need for external contact with satellite positioning installations, which harkens to the well-known term, dead reckoning.

Miniature inertial measurement units are used on the space shuttles.
Miniature inertial measurement units are used on the space shuttles.

One problem inertial measurement units have been known to suffer is positional drift, whereby incremental errors in recording are carried over to the next set of measurements. This positional drift — referred to as accumulated error — can lead to some inaccuracies in the reported location, particularly over longer periods of time. Additional components of navigation systems are designed to correct these incremental errors, including magnetic compasses, externally mounted velocity sensors, and GPS control.

Commonly housed in boxes of varying metals, inertial measurement units usually share space with three gyroscopes and three accelerometers. There are several manufacturers who produce the measurement units in different styles and with different levels of accuracy. Some inertial measurement units are constructed for specific applications, such as the miniature inertial measurement unit (MIMU). The MIMU was developed specifically to be used during space travel to provide stability control to the spacecraft. MIMUs are used in satellite control, lunar modules, and on the space shuttles themselves.

An essential item of navigational equipment, the inertial measurement unit is can be used to control both manned and unmanned craft. The units are utilized in the majority of floating craft as well as a large number of airplane navigation systems. The technology is also used in more abstract applications, such as for motion-capture technology and as an integral part of the Segway personal transportation device.

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One major omission: an IMU measures inertial data which is gravity and the earth rotation rate. There are many items that are labeled "IMU" but do not measure the earth rate, these are not IMU's but "sensor packs". To actually navigate autonomously, it must measure the earth's rotation rate.

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