Inertial Measurement Unit (IMU)
- Anthony Luo
How it works
An Inertial Measurement Unit (IMU) measures inertial values of the body. Common values include:
Acceleration
Rate of rotation
Compass heading (less common)
Note that these are all measured as rates - an external AHRS system must provide values such as speed, angle, orientation, etc. Sometimes these AHRS systems are built into the “Absolute Orientation Device” (which is very similar to an IMU).
See more about how accelerometers and gyroscopes work - they may be implemented differently depending on your sensor.
IMU’s are very sensitive to vibration and tend to “drift” a lot. This, combined with the fact that you often need to integrate or derive results from the inertial measurements means that they are unsuitable for prolonged use unless extensive software or hardware investment is made to improve the quality of the system.
Regular IMU’s like the BMX160 or MPU6050 will deviate by up to a degree every few seconds (maybe a degree a minute with high quality AHRS software), while high quality AHRS systems like the VectorNav VN-300 will deviate by no more than 2 degrees per hour.
What do we use it for?
Warg typically uses IMU’s with an AHRS system to determine orientation of an object or aircraft! We commonly use IMU’s that are built into Flight Controllers, or the following external devices:
Name | Link | DOF |
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BMX160 | < deprecated link > (replaced by BMI160 https://www.bosch-sensortec.com/products/motion-sensors/imus/bmi160/ ) | 9 |
MPU6050 | https://invensense.tdk.com/products/motion-tracking/6-axis/mpu-6050/ | 6 |
VN-300 | (Not strictly an IMU - just contains one) | |
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Constraints & Mounting Requirements
Please refer to manufacturer documentation for sensor-specific orientation. Certain flight controllers may also have IMU’s built in with vibration dampening. Be very careful that you are not worsening or obliterating the performance of the system by changing the physical mount.
General mounting requirements are listed below:
X, Y, Z correct | Refer to the airframe requirements or mounting requirements along with sensor datasheet to determine correct X,Y,Z mounting orientation. Mounting incorrectly will result in catastrophic failures (usually) |
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Vibration isolation | Depending on the requirements of the system, it may be necessary to isolate IMU’s from high frequency noise to help improve accuracy. |
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