Inertial measurement unit


An inertial measurement unit 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. IMUs are typically used to maneuver aircraft, including unmanned aerial vehicles, 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. A wireless IMU is known as a WIMU.

Operational principles

An inertial measurement unit works by detecting linear acceleration using one or more accelerometers and rotational rate using one or more gyroscopes. Some also include a magnetometer which is commonly used as a heading reference. Typical configurations contain one accelerometer, gyro, and magnetometer per axis for each of the three principal axes: pitch, roll and yaw.

Uses

IMUs are often incorporated into Inertial Navigation Systems which utilize the raw IMU measurements to calculate attitude, angular rates, linear velocity and position relative to a global reference frame. The IMU equipped INS forms the backbone for the navigation and control of many commercial and military vehicles such as manned aircraft, missiles, ships, submarines, and satellites. IMUs are also essential components in the guidance and control of unmanned systems such as UAVs, UGVs, and UUVs. Simpler versions of INSs termed Attitude and Heading Reference Systems utilize IMUs to calculate vehicle attitude with heading relative to magnetic north. The data collected from the IMU's sensors allows a computer to track a craft's position, using a method known as dead reckoning.
In land vehicles, an IMU can be integrated into GPS based automotive navigation systems or vehicle tracking systems, giving the system a dead reckoning capability and the ability to gather as much accurate data as possible about the vehicle's current speed, turn rate, heading, inclination and acceleration, in combination with the vehicle's wheel speed sensor output and, if available, reverse gear signal, for purposes such as better traffic collision analysis.
Besides navigational purposes, IMUs serve as orientation sensors in many consumer products. Almost all smartphones and tablets contain IMUs as orientation sensors. Fitness trackers and other wearables may also include IMUs to measure motion, such as running. IMUs also have the ability to determine developmental levels of individuals when in motion by identifying specificity and sensitivity of specific parameters associated with running. Some gaming systems such as the remote controls for the Nintendo Wii use IMUs to measure motion. Low-cost IMUs have enabled the proliferation of the consumer drone industry. They are also frequently used for sports technology, and animation applications. They are a competing technology for use in motion capture technology. An IMU is at the heart of the balancing technology used in the Segway Personal Transporter.

In navigation

In a navigation system, the data reported by the IMU is fed into a processor which calculates attitude, velocity and position. A typical implementation referred to as a Strap Down Inertial System integrates angular rate from the gyroscope to calculate angular position. This is fused with the gravity vector measured by the accelerometers in a Kalman filter to estimate attitude. The attitude estimate is used to transform acceleration measurements into an inertial reference frame where they are integrated once to get linear velocity, and twice to get linear position.
For example, if an IMU installed in an aeroplane moving along a certain direction vector were to measure a plane's acceleration as 5 m/s2 for 1 second, then after that 1 second the guidance computer would deduce that the plane must be traveling at 5 m/s and must be 2.5 m from its initial position. If combined with a mechanical paper map or a digital map archive, the guidance system could use this method to show a pilot where the plane is located geographically in a certain moment, as with a GPS navigation system — but without the need to communicate with or receive communication from any outside components, such as satellites or land radio transponders, though external sources are still used in order to correct drift errors, and since the position update frequency allowed by inertial navigation systems can be higher the vehicle motion on the map display can be perceived as smoother. This method of navigation is called dead reckoning.
One of the earliest units was designed and built by Ford Instrument Company for the USAF to help aircraft navigate in flight without any input from outside the aircraft. Called the Ground-Position Indicator, once the pilot entered in the aircraft longitude and latitude at takeoff, the unit would show the pilot the longitude and latitude of the aircraft in relation to the ground.
Positional tracking systems like GPS can be used to continually correct drift errors.

Disadvantages

A major disadvantage of using IMUs for navigation is that they typically suffer from accumulated error. Because the guidance system is continually integrating acceleration with respect to time to calculate velocity and position , any measurement errors, however small, are accumulated over time. This leads to 'drift': an ever-increasing difference between where the system thinks it is located and the actual location. Due to integration a constant error in acceleration results in a linear error in velocity and a quadratic error growth in position. A constant error in attitude rate results in a quadratic error in velocity and a cubic error growth in position.
Positional tracking systems like GPS can be used to continually correct drift errors.

Performance

A very wide variety of IMUs exists, depending on application types, with performance ranging:
To get a rough idea, this means that, for a single, uncorrected accelerometer, the cheapest loses its ability to give 50-meter accuracy after around 10 seconds, while the best accelerometer loses its 50-meter accuracy after around 17 minutes.
The accuracy of the inertial sensors inside a modern inertial measurement system have a more complex impact on the performance of an inertial navigation systems and can be found in .

Sensor errors

Gyroscope and accelerometer sensors behavior is often represented via a model based on the following errors, assuming they have the proper measurement range and bandwidth:
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 then very important to consider the needs regarding stability, repeatability, and environment sensitivity, on both short and long terms.
Targeted performance for applications is most of the time better than sensors absolute performance. However, sensor performance is repeatable over the 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 factory through a dedicated calibration sequence using multi-axes turntable and climatic chamber. They can either be computed for each individual product or generic for the whole production. Calibration will typically improve sensors raw performance by at least two decades.

Assembly

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 side, if suspension is likely to enable IMU performance increase, it has a side effect on size and mass.