AHRS | Attitude and Heading Reference System

AHRS | Attitude and Heading Reference System

Attitude and Heading Reference System (AHRS) : Description, How AHRS works & Applications


Sensors on three axes make up an attitude and heading reference system (AHRS), which provides roll, pitch, and yaw information for aircraft. These include solid-state or Microelectromechanical systems (MEMS) gyroscopes, accelerometers, and magnetometers and are sometimes referred to as MARG (Magnetic, Angular Rate, and Gravity) sensors. The old-fashioned mechanical gyroscopic flight instruments are to be replaced by them.

The on-board processing system in an AHRS, which gives attitude and heading information, is the primary distinction between an AHRS and an Inertial measuring unit (IMU). In contrast, an IMU transmits sensor data to a separate unit that determines attitude and heading. With sensor fusion, reference vectors such as gravity and the Earth's magnetic field are used to offset drift from the gyroscopes' integration. An AHRS is a more cost-effective alternative to traditional high-grade IMUs that simply include gyroscopes and rely on the gyroscopes' high bias stability since it produces a drift-free orientation. An AHRS can be used for attitude determination as well as inertial navigation..

To compute the solution from these several inputs, a non-linear estimation technique called an Extended Kalman filter is usually used.

In both business and commercial aircraft, AHRS is widely used and dependable. The primary flight display is often created by integrating AHRS with electronic flight instrument systems (EFIS), which are the heart of glass cockpits. An Air data, attitude and heading reference system (ADAHRS), which provides extra information including airspeed, altitude, and outside air temperature, can be created by combining AHRS with air data computers.

Basic Design of AHRS

Three pieces of equipment—a magnetometer/flux valve, a 3-axis gyroscope, and three accelerometers—combine to form the AHRS. Each of them performs a clear function.

The horizontal elements of our magnetic field are studied using a magnetometer or flux valve.

The device's linear motion can be measured with the help of the gyroscope (roll, yaw, and pitch axis).

The longitudinal, lateral, and vertical axes of the aircraft are studied by the accelerometers in order to determine the linear motion of the craft.

The engineers were able to replace the six LRUs (Line Replaceable Units) with a single device thanks to AHRS's tiny design. The weight, size, power requirements, and wiring of the vehicle are all significantly reduced by the AHRS's small design.

In addition to its compressed construction, AHRS differs from a traditional vertical gyro in a number of ways.

The automatic vertical erection would fade out in the conventional equipment when the aircraft's roll angle exceeded a predetermined threshold of 5 to 10 degrees. It will begin to wander freely and make a vertical mistake.

In contrast, AHRS employs a Schuler-tuned, velocity damped vertical erection loop. The advanced military-grade equipment system known as the velocity damped Schuler-tuned vertical erection loop does not require a modest vertical cutoff angle. In its actual local vertical, it can sustain stable continuity.

Occasionally, conventional gyros can experience gimbal lock. The most well-known problem with traditional gyros is gimbal lock, which is caused by the absence of acceleration and rate feedback terms.

As compared to AHRS, which measures the earth-based aircraft's body axis and provides the information to the automatic flight control system in distinct coordinate reference frames, there is no such aggravation with AHRS.

AHRS | Attitude and Heading Reference System
AHRS Layout 

AHRS box

Essential Components of The AHRS Box

The AHRU is the main element of AHRS (attitude heading reference unit). Gyros, accelerometers, power sources, and other instruments are all included in the AHRU box, which is used to measure the acceleration forces, rate of change, attitude of the aircraft, and magnetic heading.

AHRUs come in both analogue and digital varieties. Depending on the need, both are employed in various industries.

Sometimes an air data sensor is added to the AHRS to increase efficiency in order to reduce weight, wiring, and space requirements.

What distinguishes the AHRS from the IMU? The primary distinction between an AHRS and an IMU is that the latter has an on-board processing system created expressly to track attitude and heading information. Because of its superior construction, AHRS eliminates the intermediary that IMU depends on. IMU only transmits sensor data to a separate device that computes attitude and heading, in contrast to AHRS which has a dedicated onboard system for attitude and heading data. 

AHRS Uses 

AHRS used on some of the top technology include:

  • Aviation 
  • Manufacturing Robotics
  • Agricultural Surveying
  • Aquatic Navigation
  • Drone Localization and many more.

All industries make use of AHRS in different ways. The following are the top three most widely used applications of AHRS:

  • Utilizing it to determine the platform's orientation as a function of time as a recording device (consider flight data).
  • By directly connecting the angular rate outputs with the control loop, it may be used to establish and maintain platform stabilisation.
  • As a strategy for controlling attitude: within a few years, the cost of AHRS has significantly decreased due to technological improvement. Prior to this, AHRS has only been applied to military and commercial aircraft. AHRS can be used to build a closed-loop attitude control system for robotic vehicles as well as helicopters, aeroplanes, blimps, and Quadrotors.

The manufacturer and intended use of the AHRS will determine its size. These days, it's possible to find AHRS that fits on a penny. Our AHRS at Inertial Sense combines the MEMs gyros, accelerometers, magnetometers, and barometric pressure while also including all of the IMU features to deliver the best estimation.

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