If you watched any of the outdoor events broadcast from the Sochi Winter Games, then you may have noticed the small, spider-like video drones hovering above the snowboard and ski competitions. These unmanned helicopters, used for aerial filming, are remarkably maneuverable. They can get closer to the action, and follow it more smoothly, than a camera mounted on a crane or in a conventional helicopter.
What makes video drones so nimble? A sophisticated system for motion control and stabilization, called an attitude and heading reference system (AHRS), that uses miniature electromechanical gyroscopes, accelerometers, and magnetometers (compasses), mounted on all three axes. These miniature components, known as MEMS (microelectromechanical systems), enable small, lightweight motion-tracking systems that are more accurate, more reliable, and less expensive to manufacture than ever before.
Unmanned Aerial Vehicles (UAVs)
MEMS AHRSs make it possible to equip a small helicopter, with a maximum payload of about 5 to 10 kg, with the motion-tracking technology necessary to maneuver the vehicle in the air and keep it on course. In the past, combining all the necessary requirements for control and stabilization meant using a highly accurate (and thus bulky and heavy) inertial measurement units (IMUs) or an expensive (and also bulky) fiber-optic gyroscope (FOG). Either way, the mechanism would be too much for a small helicopter to handle. MEMS AHRS products are light enough to be carried easily, and accurate enough to perform the necessary measurements. Also, real-time operation requires quick responses, and the latest MEMS AHRS products, which offer latencies of less than 2 ms, are fast enough to keep up.
Video drones like this one are made possible by MEMS AHRSs
When airborne, the drone experiences the near-constant presence of vibrations and long-lasting accelerations that can lead to temporary navigational errors and, over time, make the drone drift off-course. The latest class of vibration-rejecting MEMS gyroscopes, featuring high bandwidths, combine with powerful signal-processing algorithms to reject vibrations with frequencies up to 200 Hz, and this improves short-term accuracy.
Long-lasting accelerations cause issues reading the accelerometer next to gravity. If these accelerations aren’t properly compensated for, the roll and pitch values will be incorrect and the vehicle will begin to drift. New sensor-fusion algorithms, which manage the readings of multiple sensors, help MEMS AHRSs detect these accelerations and adapt according to the measured dynamics. Sensor fusion also helps make MEMS devices more resistant to the magnetic distortions caused by steel, permanent magnets, and electric currents. High-performance sensor-fusion algorithms make it possible to compensate for these kinds of magnetic distortions and adapt the heading estimation accordingly.
Mobile 3D Mapping
Beyond video drones and other UAVs, an emerging application for MEMS technology is mobile 3D mapping, which involves using imaging devices, mounted on moving platforms, to create a three-dimensional map. By using MEMS products to build an internal navigation system (INS), the same kind of mapping techniques used by the custom-built vehicle for Google Street View more widely affordable. For example, a low-cost laser scanner or thermal camera, a MEMS-based INS that outputs position and orientation, and a laptop with control software can all be combined in a cargo box that fits in any ordinary car. Such a box might be used by municipalities to monitor public works. The laser-scanning solution is light enough to be carried by a drone, so it could also be used for aerial mapping. Fire brigades in national parks and other forested areas could use the mapping-equipped drone to look for hot spots, without having to hire a special aircraft, and a pilot, for the job.
The ability to cope with vibrations and magnetic directions is opening up opportunities for low-cost MEMS AHRSs in the shipping industry. MEMS AHRSs make it possible to monitor the motion, acceleration, and velocity of cargo loaded anywhere on the ship. That information, combined with weather data, wave measurements, and other readings, lets the MEMS AHRSs chart the best course and velocity to minimize roll, optimize fuel consumption, and protect the cargo. Without the accuracy and robustness of current-day MEMS-based motion trackers, it would be next to impossible to develop an application like this cost-effectively.
When used as part of a cargo ship’s navigation system, a MEMS AHRS can help chart a better, more efficient course
Another application of MEMS AHRSs is on container ships that use large kites for propulsion. The kite is attached to the forward deck and the MEMS AHRS is used to steer the kite. The kite has to move at just the right moment to ensure maximum efficiency, and the MEMS AHRS determines this moment. As with an unmanned helicopter, such a large-scale kite is subject to high accelerations that can impact the accuracy of the controller. Also, any weight or size added to the kite makes it less efficient. Using a small yet precise MEMS AHRS delivers the right degree of accuracy without adding drag. MEMS AHRSs can also be used in a similar way to control kites attached to turbines in offshore wind farms.
A MEMS AHRS is used to control this kite-propelled cargo ship
Higher accuracy, better algorithms for sensor fusion, the miniaturization of components, and relatively lower manufacturing costs make MEMS-based products an attractive alternative to traditional optical and mechanical navigational devices. Video drones are perhaps the best known application for these kinds of MEMS-based solutions, but there is rapid growth in other applications, too. Mobile 3D mapping, navigational systems for cargo ships, and wind-propelled ships equipped with kites are just a few examples. The unique combination of features, and the low overall cost, make these pint-sized MEMS-based systems the most likely leaders in the next generation of motion-tracking applications.
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