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36 Focus | Inertial measurement units MEMS-based accelerometers and gyro sensors can use similar structures, although the gyro sensors are made to resonate in order to pick up changes in orientation, so advances in the design of the MEMS structures benefits both types of sensor. The automotive industry has driven the development of MEMS sensors, particularly accelerometers, which are used to trigger airbags in a crash, and for stability control. While this has delivered high-volume production though, the packaging is optimised for the automotive environment and may not be suitable for the higher performance requirements of an IMU. MEMS-based IMUs can now compare with FOG-based systems in terms of accuracy, and do not necessarily have the export restrictions that can apply to FOG sensors. This advance in MEMS-based sensor performance has been achieved through a combination of packaging and the software algorithms. By moving to packaging that allows the sensor to be in a hard vacuum, the sensor’s quality factor (also called the Q) can be boosted from 500 (in standard plastic packaging) to 30,000. This high Q gives a bias of 0.1°/hour, comparable to FOG-based systems, and compares to a bias (or drift) of degrees per second for commercial devices. Developing that packaging technology for mass production was a major step forward in its own right. There are many different types of structure for MEMS sensors, although all MEMS gyroscopes make use of the Coriolis effect, which can be detected more easily on small structures than large ones. By resonating a micromachined structure so that its mass is vibrating towards and away from the centre of rotation, any tangential movement can be measured to provide the angular velocity. Some MEMS sensors use a large mass that responds to the change in direction, some have rings that are excited by permanent magnets, while others use ceramic piezoelectric materials to provide the resonance. These all have different responses, from linearity to dynamic range. Using semiconductor manufacturing techniques allows several devices to be built on one piece of silicon with the associated control-loop signal processing. While that may be suitable for developers who control the sensor’s design and manufacturing to produce an IMU though, it may not suit a third-party IMU integrator who uses different types of sensor on different axes of a unit. Controlling the entire process allows developers to optimise each element of the IMU, however, from the design of the sensor to the feedback loop and signal conditioning for the sensor data, as well as the packaging of the sensor and the IMU. It also allows more knowledge of the performance of the sensor over time, which is a key element in building a calibration model of the IMU. Many MEMS sensors have an open- loop output, which means they just provide the raw data from the sensor. Sensor makers though are moving to a closed-loop output to improve the accuracy, adding in signal processing alongside the sensor to compensate for known effects. Operating in a closed-loop mode gives better scale-factor performance for navigation calculations by minimising errors, and this is currently a major focus for IMU designers. The closed-loop performance of MEMS IMUs is approaching that of open-loop FOGs but still not challenging closed- loop FOG units. Size The advantage of MEMS sensors is that they allow a smaller IMU design. They are down to 5 cm 3 for an 82 cm 3 module, and even under 2 cm 3 with a housing of 29 cm 3 . There is a trade- off however between IMU size and performance, as larger sensors provide higher performance. That means there is pressure on the supporting electronics to be made even smaller to reduce the overall size of the housing. One sensor and IMU maker has been working with a US research agency to get the sensor design under 1 cm 3 to meet the target of an IMU that is under 100 mm 3 (that is, less than 5 mm on a side). This can be achieved with chip- scale MEMS sensors where the silicon sensor is not packaged and mounted directly on the board, with packaging around the whole IMU. The issue with that though is the manufacturing yield of the IMU, because the construction is more like a chip module than a printed circuit board. A lower manufacturing yield drives up the cost of course, which is already higher for such a small system. December/January 2017 | Unmanned Systems Technology Compensation of the MEMS-based accelerometers and gyroscopes in a tactical-grade IMU can be handled by a dedicated digital signal processor (Courtesy of Analog Devices)