What is AHRS?
An Attitude and Heading Reference System (AHRS) is a high-tech sensor system that is widely used in many fields, like the aviation, navigation fields, also including aerial vehicles, robotics and self-driving cars. It receives data in parallel in a number of sensors and then determines roll, pitch and yaw angles and heading of the device in three-dimensional space and allows this device to thereby enable itself very precisely.
Flight control systems require AHRS systems that supply an important function of aircraft whether manned or unmanned. They use two or more sensors to determine the orientation even on the dynamic maneuvers when individual sensors may give inaccurate information.
How AHRS Works
Advanced AHRS technology is taking advantages of an ingenious integration of sensors and algorithms to compute the orientation. The principle here is the sensor fusion, which is an integration of data in various sources to overcome such limitations of the single sensors.
Sensor Fusion
AHRS receives data of several sensors:
- Gyroscopes – Measure angular rate
- Accelerometers – Measure linear acceleration
- Magnetometers – Measure Earth’s magnetic field
Combining the data provided by these capture devices, AHRS may deal with the constraint of single sensors. To cite an example, gyroscopes contain a drift, whereas accelerometers are prone to vibration. Such input is fused by means of algorithms known as sensor fusion algorithms, such as the Kalman filter, to give stable and accurate orientation information.
Algorithms & Processing
Sophisticated algorithms work with data coming out of the sensor:
- Kalman Filtering – Optimal system state estimation
- Sensor Calibration – this is a correction of biasses and misalignment.
- Data Fusion – Brings together information of more than one sensor
- Vibration Compensation – Filters out high-frequency noise
The algorithms are executed in dedicated microprocessors and include sensors that have a processing rate of up to 1000Hz, and hence they can be responsive in real-time even in rough maneuvers.
These sensors (Gyroscopes, Accelerometers and Magnetometers) give their data which are fed to a filtering algorithm to remove the noise and drift of the sensors so as to arrive at correct attitude information. Popular algorithms in use are the following:
- Complementary filter: Integrates low frequency of accelerometer and high frequency channel of gyroscope, which is appropriate to low-cost systems.
- Kalman Filter: improves pose estimation, recursive estimation and prediction at the expense of increased computational complexity.
- Extended Kalman Filter ( EKF ): It is applicable when the models are nonlinear and can handle nonlinear behavior of sensor.
- Adaptive Kalman Filter: It is capable of varying parameters of the filter with regard to the status of the systems itself, in a bid to increase robustness.
Key Components of AHRS
| Component | Function | Technology |
|---|---|---|
| Inertial Sensors | Measure angular rates and accelerations | MEMS (Micro-Electro-Mechanical Systems) |
| Magnetometer | Detects Earth’s magnetic field for heading reference | 3-axis Fluxgate or Hall-effect sensors |
| Processing Unit | Runs sensor fusion algorithms | ARM Cortex-M or similar microcontrollers |
| Temperature Sensors | Compensate for IMU temperature compensations on sensors | Integrated thermistors |
| Communication Interfaces | Output data to other systems | RS-232, ARINC 429, CAN bus, Ethernet |
MEMS Technology Revolution
In the modern AHRS systems MEMS (Micro-Electro-Mechanical Systems) technology has transformed inertial sensing. MEMS sensors are:
- Significantly smaller than traditional sensors
- More power efficient
- Highly resistant to shock and vibration
- Cost-effective for mass production
- Capable of high update rates (up to 1kHz)
- Suitable for miniaturized systems
AHRS Applications
The AHRS technology has opened up to many industries in spite of aviation:
Aviation
The primary fly displays, the autopilot system and the flight control computers on commercial and military aircraft.
Unmanned Aerial Vehicles
Key to both stabilization and navigation of drones and autonomous flight activities.
Marine Navigation
Ship and submarine stabilization (particularly during heavy sea).
Automotive Systems
Advanced driver assistance systems (ADAS), rollover recognition and self-driving cars.
Robotics
Orientation of industrial arm and humanoid robot balance control.
Virtual Reality
VR Headsets and motion controller head tracking.What’s the difference between AHRS and IMU?
The major distinction between AHRS and IMU:
Raw sensor data (acceleration and angular velocity) will be provided by the IMU and it has to be calculated and processed by other, external devices.
As AHRS has an on-board processing unit, it can perform computation of attitude and heading information directly, hence better to use in real time control and navigation.
AHRS development and trend
As sensor technology and the algorithms keep improving, AHRS systems will tend towards being more accurate, more reliable, and cheaper. As an example, AHRS sensors based on MEMS are of small size and weight thus fit in different devices. Moreover, AHRS can be combined with GPS/Inertial Navigation system (INS) in expanding the precision and resilience of the navigation system.