Inertial measurement unit
An inertial measurement unit (IMU) is an electronic device that measures and reports a body's specific force, angular rate, and sometimes the orientation of the body, using a combination of accelerometers, gyroscopes, and sometimes magnetometers. When the magnetometer is included, IMUs are referred to as IMMUs.[1]
For broader coverage of this topic, see Inertial navigation system.IMUs are typically used to maneuver modern vehicles including motorcycles, missiles, aircraft (an attitude and heading reference system), including uncrewed aerial vehicles (UAVs), among many others, and spacecraft, including satellites and landers. Recent developments allow for the production of IMU-enabled GPS devices. An IMU allows a GPS receiver to work when GPS-signals are unavailable, such as in tunnels, inside buildings, or when electronic interference is present.[2]
A very wide variety of IMUs exists,[13] depending on application types, with performance ranging:
To get a rough idea, this means that, for a single, uncorrected accelerometer, the cheapest (at 100 mg) loses its ability to give 50-meter accuracy after around 10 seconds, while the best accelerometer (at 10 μg) loses its 50-meter accuracy after around 17 minutes.[14]
The accuracy of the inertial sensors inside a modern inertial measurement unit (IMU) has a more complex impact on the performance of an inertial navigation system (INS).
Gyroscope and accelerometer sensor behavior is often represented by a model based on the following errors, assuming they have the proper measurement range and bandwidth:[15]
All these errors depend on various physical phenomena specific to each sensor technology. Depending on the targeted applications and to be able to make the proper sensor choice, it is very important to consider the needs regarding stability, repeatability, and environment sensitivity (mainly thermal and mechanical environments), on both short and long terms.
Targeted performance for applications is most of the time better than a sensor's absolute performance. However, sensor performance is repeatable over time, with more or less accuracy, and therefore can be assessed and compensated to enhance its performance.
This real-time performance enhancement is based on both sensors and IMU models. Complexity for these models will then be chosen according to the needed performance and the type of application considered. Ability to define this model is part of sensors and IMU manufacturers know-how.
Sensors and IMU models are computed in factories through a dedicated calibration sequence using multi-axis turntables and climatic chambers. They can either be computed for each individual product or generic for the whole production. Calibration will typically improve a sensor's raw performance by at least two decades.
High performance IMUs, or IMUs designed to operate under harsh conditions, are very often suspended by shock absorbers. These shock absorbers are required to master three effects:
Suspended IMUs can offer very high performance, even when submitted to harsh environments. However, to reach such performance, it is necessary to compensate for three main resulting behaviors:
Decreasing these errors tends to push IMU designers to increase processing frequencies, which becomes easier using recent digital technologies. However, developing algorithms able to cancel these errors requires deep inertial knowledge and strong intimacy with sensors/IMU design.
On the other hand, if suspension is likely to enable IMU performance increase, it has a side effect on size and mass.
A wireless IMU is known as a WIMU.[16][17][18][19]