An Attitude Heading and Reference System, better known as an AHRS, is a 3-axis sensor system that provides real-time 3D attitude position – pitch, roll, and heading. The primary function then of an AHRS is to provide orientation data. AHRS are designed to replace traditional gyro-based instruments and to provide superior reliability and accuracy.
Some of the many applications for AHRS include control and stabilization, measurement and correction, and navigation. An example of control and stabilization could be where a camera or antenna mounted on a system such as a plane or ship needs to be stable. Measurement and correction best applies to imaging systems where an AHRS is used to ensure the direction the imager is pointed. And in navigation, an AHRS can be used to provide orientation and direction.
AHRS consist of magnetometers, microelectromechanical systems (MEMS) accelerometers, and MEMS gyroscopes on all three axes. In other words, a MEMS-based AHRS includes sensors for 3-axis magnetic, 3-axis acceleration, and 3-axis gyro. These sensors, combined with a built-in processor, create an inertial sensor system fully cable of measuring the attitude of objects in 3D space.
The sensors in AHRS use algorithms to estimate this attitude in 3D space. Some AHRS units will use traditional Kalman filter algorithms that use magnetic and acceleration measurements to estimate the time-varying gyro bias in real-time. Other AHRS systems utilize modified non-Kalman filters that compute an estimation of orientation in real-time. A potential advantage of these modified algorithms is that they can outperform traditional Kalman filter-based sensors by providing real-time optimization of performance for varying magnetic or dynamic operating environments.
Magnetometers are used in AHRS to measure the direction of the magnetic field at a point in space. A more traditional magnetometer would be a fluxgate system. Though this technology provides good accuracy and reliability, it is not conducive to a MEMS-based AHRS due to its larger form size and greater power requirements. An alternative to fluxgate technology is a magneto-inductive (MI) sensing technology. Not only does this technology provide the desired smaller form factor and low power requirement, MI also provides very high resolution – higher than what competing technologies such as anisotropic magneto resistive (AMR) sensors can provide at similar cost.
Accelerometers measure proper acceleration – the rate at which the velocity of an object is changing. They measure the static (gravity) or dynamic (motion or vibration) acceleration forces of a given object. The ideal accelerometer in an AHRS provides long term stability, low vibration error, and reliability.
AHRS demand very precise gyroscopes as the quality of these devices greatly impacts the overall performance of the inertial sensor system. An example of a very high end gyroscope is a fiber optic gyroscope, commonly known as a FOG. FOGs provide extremely precise rotational rate information due to their lack of moving parts. However, FOGs have a great deal of inherent development and manufacture costs as well as a larger form factor and higher power demands. As technology improves, MEMS-based gyroscopes have closed the performance gap on some FOGs. When factoring in lower cost and power requirements, MEMS-based devices provide an excellent answer for the need of precision in a gyroscope.
MEMS-based Attitude and Heading Reference Systems (AHRS) continue to develop and improve in both technology and application. As the requirements of both military and commercial systems evolve, there is increasing demand for continuous improvement. Both existing systems and those in development must incorporate SWAP-C – Size, Weight, Power, and Cost – standards. Simply put, demand will increasingly require systems and their components to be smaller, lighter, use less power, and all at a lower cost. AHRS are no different in this initiative. Manufacturers must adhere to these principles, all while improving the performance of AHRS. Manufacturers who fail to adjust to these demands will find themselves left behind.
The Sparton AHRS-M2
The Sparton AHRS-M2 is a micro-sized, light weight, low power Attitude Heading Reference System with a revolutionary built-in adaptive-calibration mode. Sparton AdaptCal™ provides continuous 3D adaptive in-field calibration while Sparton’s AdaptNav™ sensor fusion algorithms allow the system to provide accurate attitude and heading outputs.