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Components That Work with the Attitude & Heading Reference System

Components That Work with the Attitude & Heading Reference System
The Siliconreview
24 November, 2022

What is an Attitude and heading reference system? 

An attitude and heading reference system involves a system that utilizes an inertial measurement unit (IMU). This unit usually comprises microelectromechanical system (MEMS) inertial sensors that measure the acceleration, Earth’s magnetic field, and angular rate. These measurements are then used to estimate the attitude of a specific object.

Components of the Attitude and Heading Reference System

Typically, AHRS includes a 3-axis accelerometer, a 3-axis magnetometer, and a 3-axis gyroscope. These components are used to estimate a particular system's orientation. Each of these sensors exhibits unique limitations and contributes various measurements to the linked system.

Accelerometer

While an accelerometer is assumed to be measuring gravity only, it offers an AHRS with a measure of the system’s acceleration. Following this assumption, the accelerometer can calculate the roll and pitch angles from the direction of the gravity vector.

Nevertheless, any errors or biases subjected to the accelerometer measurements can cause inaccuracy when calculating the roll and pitch angles. Additionally, since an accelerometer is assumed to measure gravity only, additional dynamic motion can cause errors when calculating the system’s roll and pitch.

Gyroscope

A gyroscope allows the angular rate measurement of an AHRS system. The angular rate measurements are then combined to estimate a system's attitude. But, a system's first attitude measurements must be known to determine the current attitude.

However, this calculated attitude will drift unboundedly from the real system’s attitude. This happens due to bias properties and inherent noise in the gyroscope.

Magnetometer

While the accelerometer can only provide roll and pitch measurements, a magnetometer facilitates an AHRS with yaw measurements. It compares the magnetic field measurement surrounding the system to the Earth's magnetic field. It works like a traditional magnetic compass.

It is best to note that in most AHRS units, the measurements given by a magnetometer do not influence the roll and pitch angle estimates.

Unfortunately, using a magnetometer to estimate the heading accurately can be challenging. Besides, the Earth's magnetic field is weak; thus, magnetic disturbances like high power cables and huge metal structures can interfere with the Earth's magnetic field. This interference could cause errors in the estimated heading angle.

The interference caused by objects having fixed AHRS-like vehicles can be compensated using the soft & hard iron calibration. However, this can only happen when these disturbances are not time-varying. Nevertheless, advanced filtering techniques can be used to influence the impact of external disruptors in the environment. But, the effectiveness of these filtering techniques varies by application and manufacturer.

Challenges Experience in AHRS

While most limitations of accelerometers, magnetometers, or gyroscopes can be worked out by linking them, there are several challenges that come with AHRS. These challenges can lead to errors or inaccuracies when calculating the attitude estimates. They include sustained dynamic accelerations, external and internal magnetic disruptions, and AC and transient disturbances on the magnetometer and accelerometer.

You can check out Advanced Navigation to discover high-performance and accurate attitude and heading reference systems.