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34 Focus | Inertial measurement units packages. While they provide more protection against vibration, their use is not as straightforward as with plastic encapsulated devices, as designers need to consider the thermal effects. Ceramic will not expand as much as the printed circuit board underneath, which can cause the connections between the package and the board to break if there is a thermal mismatch. Fibre The advent of the semiconductor laser has dramatically reduced the cost, size and power consumption of the FOG. The laser uses two opposing beams in the same fibre optic cable, which can be up to 5 km long, and changes in the rotation of the device can be detected through an interferometer. These have been used in larger unmanned aircraft but have often been too heavy and costly for smaller systems. Modern semiconductor processes allow the fibre to be easily connected to the laser cavity (self-aligned), eliminating a costly step in the manufacturing. A key advantage of a FOG is that it has no moving parts, so the reliability is higher than with MEMS devices. In one development, a FOG has been combined with low-noise MEMS accelerometers for an IMU with a diameter of 88.9 x 73.7 mm (3.5 x 2.9 in), a weight of 700 g and a typical power consumption of 5 W. While this level of technology is used for weapons systems, the same FOG is used in several prototype driverless vehicles and unmanned underwater systems. These provide precise azimuth measurements that an autonomous car’s logic processing unit and control systems need in order to determine motion through a curve. One IMU includes a FOG and accelerometers in a compact, lightweight package to provide six degrees of freedom and acceleration data to precisely track the position and orientation of the car, even when GPS is unavailable, helping the car stay on course. A FOG IMU was also an integral part in a competition designed to showcase robots capable of intervening for and even replacing humans in high-risk situations such as fires, earthquakes and other natural disasters. This highlights that FOG gyros continue to have a major role to play in many unmanned system designs. Fibre ring laser gyros are also still used for IMU systems, although more so in maritime applications as these can be large and power-hungry. They use multiple lasers in a cavity to detect changes in rotation. Quartz A quartz-based accelerometer can provide higher reliability for navigation applications as it also has no moving parts, relying on the response of an electrically stimulated crystal. This can deliver a mean time between failures of 500,000 hours, or 17 years, compared with five years for a FOG. Bias stability (the measure of the bias performance) is one of the key factors in the choice of an accelerometer. This is the offset for the sensor, and should be as linear and repeatable as possible, as hysteresis in the bias leads to errors that cannot be resolved by compensation. The accelerometer, while larger than a MEMS sensor, also provides a bias of less than 4 mg and less than 160 µg for repeatability, which is a big difference from a MEMS accelerometer. A crystal accelerometer is also more resistant to shock and vibration than a MEMS sensor. December/January 2017 | Unmanned Systems Technology A cylindrical gyroscope uses a quartz crystal with no moving parts to achieve higher reliability (Courtesy of Innalabs) Researchers have been able to build quantum sensors that cool a cloud of ions so that the quantum states are entangled