Inertial Measurement Unit
The Inertial Measurement Unit (IMU) is typically a combination of three sensors: accelerometer, gyroscope, and magnetometer. IMUs can directly produce measurements of acceleration, angular velocity, and heading. Sensor fusion techniques can be used to estimate orientation in 3D space.
Measurement: Linear acceleration (accelerometer), angular velocity (gyroscope), heading (magnetometer/compass). 3D orientation (roll/pitch/yaw) when values are combined using sensor fusion techniques.
Ideal operating conditions: Can be used in nearly any conditions. Magnetometer needs to avoid EM noise and ferrous materials.
- Can be used in nearly any conditions – indoor/outdoor, day/night, uneven surfaces. Requires few assumptions to use. This versatility makes the IMU a useful addition to any sensor setup.
- Good for mitigating discrete jumps in position and orientation measurement when fused with other sensors.
- Useful in both 2D and 3D applications.
- Produces a reasonable 3D orientation estimate.
- Acceleration measurements can be integrated to estimate velocity and position. However, these estimates are noisy and will accumulate error over time. Fusing the acceleration data with other sensors is a better approach.
- Very inexpensive. Decent IMUs are available for less than $20.
- Requires initial calibration
- It is difficult to work directly with raw IMU data due mainly to noise. Filtering algorithms are needed to extract useful information. Higher end IMUs can filter the data before outputting it.
- Gyroscopes are susceptible to drift.
- Magnetometer/compass output is unreliable. Must be corrected based on where it is being used in the world. Even then the compass heading is inaccurate (+-20 degrees or worse). Also vulnerable to external magnetic sources.
An accelerometer is a sensor that measures proper acceleration. Proper acceleration is not necessarily coordinate acceleration (∆v/∆t). Instead, an accelerometer measures the forces acting on the test mass relative to free fall. This means that at rest, an accelerometer will read an acceleration upward of 9.81m/s^2. There are multiple methods of measuring acceleration. Typically, it involves some sort of test mass that either deforms or causes another part of the circuit to deform. This deformation can either cause a change in voltage, capacitance or resistance. This change is then measured and the sensor's direction of travel is calculated.
The gyroscope is a device for measuring angular momentum. A mechanical gyroscope consists of a spinning disk suspended within two rings with the ability to move freely in any direction. Modern MEMS gyroscope devices that you will find in modern electronics are based on a vibrating structure. Combining a gyroscope and an accelerometer with sensor fusion provides greater precision in determining the orientation and motion of a device.
A magnetometer is an instrument that measures both the strength and direction of the Earth’s magnetic field to provide compass readings for the IMU. Combining a gyroscope, accelerometer, and magnetometer provides orientation and motion of the device relative to the Earth’s magnetic poles. These devices are susceptible to magnetic noise produced by DC motors and other electronics. Therefore most manufacturers offer an offset variable to calibrate to maintain precision.