What if your inner ear, which acts like a gyroscope to keep you balanced, was also equipped with a compass? Then you would be close to the human equivalent of an attitude and heading reference system (AHRS), an electromechanical instrument used to determine where an object is in space and where it’s headed.
An AHRS uses gyroscopes, accelerometers, and magnetometers (compasses), mounted on three axes, to provide real-time 3D orientation or, in the language of navigation, roll, pitch, and yaw. An AHRS can also provide the raw data from its sensors, such as 3D acceleration, angular velocity (rate of turn), and magnetic field. Being able to provide all this information makes AHRSs very valuable when it comes to creating gaming devices, robots, aerial drones, underwater vehicles, and other systems that involve physical movement.
The MEMS Advantage
AHRS systems have been around for some time, but until only fairly recently, they’ve been heavy, large, and expensive. Now, thanks to microfabrication, it’s possible to manufacture tiny versions of conventional electromechanical devices. Known as MEMS (microelectromechanical systems), these miniature structures implement things like pressure sensors and microphones, as well as the gyroscopes, accelerometers, and magnetometers used in AHRSs. MEMS AHRSs are less costly to manufacture than their larger counterparts and, in a growing number of cases, perform better, with a greater degree of accuracy.
Because MEMS AHRSs present such an attractive combination of features – they’re small, they don’t weigh much, they’re quite accurate, and they’re relatively inexpensive – they’re going where no other AHRS has gone before. They’re finding their way into places that would be too heavy, too large, and too expensive to support an AHRS built using traditional optical or mechanical methods.
So just how good is MEMS-based motion tracking and navigation? Most MEMS gyroscopes – and by extension, the MEMS AHRSs they’re used in – are considered “industrial grade.” The graph below gives that term some context.
Gyroscopes for industrial and consumer applications are increasingly MEMS-based
Industrial-grade gyroscopes are less precise (and less expensive) than the “tactical-” and “strategic-grade” devices used in high-end applications like naval submarines, guided missiles, and some spacecraft, but they’re more precise than “consumer grade” devices, which are typically used in automotive subsystems like anti-lock brakes, airbags, and active suspension systems. Consumer-grade MEMS gyros have also started appearing in gaming controllers and entertainment systems. You might have already used one – they’re in the Nintendo® Wii™, Sony® Move, and the latest iPhone®, and more are on the way.
At the high end of the industrial grade, new MEMS products can, to a certain degree, rival the performance of very expensive fiberoptic gyroscopes (FOGs). In 2010, for example, MEMS inertial measurement units (IMUs) comprised almost 40% of the commercial market, compared to just 13% for FOGs (Yole Developpment). As MEMS technology continues to evolve, this trend is expected to continue.
Breaking it Down
In general, industrial-grade MEMS AHRSs can be found in three major market segments. They’re in entertainment, used by game developers and Hollywood filmmakers to support full-body motion capture for special effects. They’re in movement science, where athletic coaches, physical therapists, and doctors use them for motion capture in sports, rehabilitation and clinical applications. And, most widely, they’re in the industrial sector, which can be subdivided into three application categories: control and stabilization, measurement and correction, or navigation.
Control and Stabilization
MEMS AHRSs are used to control and stabilize things like the aerial cameras in video drones and antenna systems on ships. If the platform is, for example, mounted on a rocking ship or a moving survey aircraft, the AHRS helps keep the platform stable. This kind of work requires quick reactions, so the low latency of the typical MEMS AHRS (less than 2 ms) is an especially important feature in this category.
A GPS-enabled MEMS AHRS controls and stabilizes an unmanned aerial vehicle (UAV) from the Northern Research Institute (NORUT) in Norway
Measurement and Correction
MEMS AHRSs are also used for measurement and correction, often in imaging systems such as underwater acoustics (sonar, echo sounders), and portable laser scanners. The AHRS corrects the sensor data captured by the sonar or laser scanner, or it determines which direction the imaging device is pointing. When used in conjunction with a synchronized GPS module, the AHRS has enough information available to automatically determine the position of the image.
A MEMS AHRS gives the humanoid robot “Flame,” from the Delft University of Technology, a sense of balance
In navigation, a MEMS AHRS and a MEMS IMU can be used in combination as a backup for a GPS module. That way, if there are GPS outages, the system can still deliver the necessary readings. Having this kind of insurance policy makes navigation more robust under high dynamics and in challenging GPS conditions. Also, adding MEMS components to a GPS system can improve its altitude readings by as much as an order of magnitude or more.
A MEMS AHRS helps navigate SAROV, an unmanned vehicle from Saab Underwater in Sweden
How it Works
In many ways, the gyroscope triad is the most important part in the AHRS, since it has such a strong influence on accuracy. The gyro triad measures the rate of return (the angular velocity) and as a result can provide the change in orientation. To prevent drift, the gyro triad is referenced by the complementary sensors, which provide compensation. The accelerometers compensate for attitude (roll/pitch), while the magnetometers compensate for heading (yaw). A digital signal processor (DSP), supported by a microcontroller (MCU), fuses all these signals together in a Kalman filter. The resulting output is a stable and robust 3D orientation.
To further improve the accuracy of the orientation estimates, some MEMS AHRS systems use an algorithm for sensor fusion to estimate sensor component calibration parameters during operation. Advanced AHRS systems also can include additional sensors, such as GPS receivers and barometric pressure sensors, to improve motion tracking even more.
MEMS AHRSs are already at the heart of a broad range of intriguing applications, and some of the most exciting developments today are in the MEMS gyroscope market. Improvements in the technology are pushing consumer-grade gyroscopes to a higher performance class, and that means a broader range of consumer products will begin to use relatively advanced motion-tracking functions.
A major trend for the future is further physical integration of all the required components for an AHRS. That would mean a single package that includes all the MEMS sensors plus the DSP and the MCU. Also, advances in the software used with MEMS AHRSs promise to bring MEMS-based products to even more new places, including body-area networked IMUs and AHRSs.
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