What is IMU Misalignment?
When the sensitive axes of the IMU sensors (the axes of accelerometers and gyroscopes) are not orthogonal (perpendicular) to one another in the case of the inertial measurement unit (IMU), the term Misalignment(orthogonality) is used. Preferably, the three planes ( X, Y, Z ) of the IMU become orthogonal to each other and create so-called orthogonal coordinate system. Nevertheless, manufacturing mistakes, installation mistakes or mechanical structure flaws can make the actual sensor axes not perfectly orthogonal thus causing correlations in the measured data and also the accuracy of the navigation system.
Key Insight
Misalignment is when accelerometers and gyroscopes do not have a perfect orthogonal and perfect alignment with the reference frame of the device. The misalignment angles in the order of (1-2°) can result in a large error in position and orientation calculations over some time.
Key Concepts of Misalignment
Orthogonality Error
In the event that the x, y and z axis of the IMU sensors are not set to be perfectly perpendicular to one another. This lack of orthogonality means that there is cross-axis sensitivity meaning that motions in one axis can give rise to effects on others.
Axis Misalignment
The orientation of the IMU package may cause problems due to the physical orientation not matching the references frame of the IMU package. In such cases, this is normal where the IMU is attached at an angle to the PCB or general device chassis..
Sensor-to-Sensor Misalignment
Accelerometer and gyroscope in the same IMU module do not aligned on same axis. This is a problem that is especially problematic with respect to sensor fusion algorithms.
Thermal Effects
Fluctuations in temperature inducing physical expansion/contraction of the IMU package, giving rise to a misalignment that varies as time goes on. Consequently, compensation must be carried out dynamically.
Misalignment Impact on Navigation
Misalignment errors accumulate with the passage of time, most especially, in dead reckoning navigation systems. The various angles of misalignments influence the accuracy of positioning and this is as follows:
| Misalignment Angle | Position Error after 1 min | Position Error after 5 min | Critical Applications Affected |
|---|---|---|---|
| 0.5° | 0.5 meters | 12 meters | Consumer drones, AR/VR |
| 1.0° | 1.0 meters | 25 meters | Autonomous robots, Industrial AGVs |
| 2.0° | 2.1 meters | 52 meters | Precision agriculture, Surveying |
| 5.0° | 5.2 meters | 130 meters | Tactical-grade navigation |
Why Misalignment Matters
With a speed of an aircraft at 50km/h, a 1° misalignment can result in the estimated 8 meters deviation in position in 1 minute of flight. The error increases quadratically with time and hence needs correction of the misalignment in case of any application that necessitates accuracy of navigation.
Correcting IMU Misalignment
There are a number of methods that are used to quantify misalignment errors and correct them. The hardware calibration together with software algorithms is the best method:
Laboratory Calibration
The precision turntables and rate tables have been used in order to characterize misalignment in controlled circumstances. This provides the best calibration parameters, and special equipment is also needed.
Multi-position Calibration
Finding a number of orientations of the device that it is known to be and doing some IMU measurement at each of the orientations. The technique may be done in the field and no special equipment is required.
Sensor Fusion Compensation
Estimating the misalignment parameters of the operation of Kalman filters or complementary ones by means of fusing the data obtained by IMU to the information obtained by GPS, magnetometer, or visual odometer.
Temperature Compensation
Storing the values of misalignment parameters at various temperatures and compensating the same in respect to real time measurements using onboard sensors.
Applications Where Misalignment Matters
Aerial Drones & UAVs
Clear navigation can only occur when the right attitude can be gauged. As a consequence of misalignment, estimates of position drifts degrade further and in case of GPS-denied operation, the results become poorer.
Autonomous Mobile Robots
The AGVs and Warehouse robots are using dead reckoning within the measurement of landmark. Misalignment will create a problem to develop position error.
Vehicle Navigation
Automotive systems use the IMUs in dead reckoning when in tunnels and also urban canyons. The issue of misalignment degrades the level of accuracy in positioning.
AR/VR Systems
Tracking is a very sensitive thing and the orientation information must be precise. Jitter and drift in virtual environment are the consequences of misalignment.