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81 surface micro-machining, which involves depositing and etching material for the MEMS structures onto a substrate. An alternative is bulk micro-machining, in which the substrate wafer is etched-into and the MEMS device is created out of the substrate. Silicon remains the most popular material for making MEMS IMUs, as is typical for other ICs. Reasons include its low electrical resistivity and its ability to bend without being damaged, as well as its wide availability and applicability in electronic circuit printing. Accelerometers In IMUs for unmanned systems, MEMS accelerometers determine acceleration by detecting and measuring changes in capacitance, or the degree to which energy in the form of an electric charge can be collected and stored in a circuit or component thereof. Since a key factor of capacitance is the geometry of and distance between two charged but electrically separated components, accelerometers are designed as flexible mechanical systems so that measurable changes in capacitance can result. This is most often manifested as a MEMS capacitor that has two key parts. One of them is a silicon mass that has been either loaded on springs or suspended by strings, and featuring a number of silicon electrodes in the shape of long, thin ‘teeth’ or ‘fingers’ that are combed along the length of the mass. The other key part is a fixed silicon structure, typically housing the spring- loaded mass and laminated onto it with glass bonds or similar, hermetically enclosing or ‘sandwiching’ it into a single axis of freedom. This has similar electrodes extending from it, running parallel between those of the first structure, with some fluid (typically air) filling the spaces between both structures. Any acceleration causes the spring- loaded mass to move along its axis, altering the distance between the electrodes and therefore the capacitance. This change can be sensed by various methods – using oscillators or AC voltmeters, for example – to convert the changes into corresponding rates of acceleration. Accelerometers can also be configured to sense and measure inclines, to compensate for gyroscopic drift in incline sensing. Such ‘inclinometers’ detect changes in the effect of gravity on an accelerometer’s axes, but these changes can be masked when a vehicle accelerates at a constant rate, or rotates in a way that induces centripetal acceleration on the accelerometer. And as the displacement of the proof mass is proportional to the rate of acceleration, accelerometers with larger and more expensive MEMS structures (with greater ranges of displacement) can measure capacitance and therefore acceleration more precisely. Smaller systems will be cheaper and lighter but less accurate, particularly as more powerful signal processors will also drive up the cost and SWaP requirements of a higher-grade IMU. Another key distinction between cheaper accelerometers and more expensive ones is whether they are open or closed loop. This refers to whether there is a feedback source built into the MEMS architecture to generate a rebalancing force, to drive the spring- loaded mass back towards its null (initial) position while still maintaining its range of motion. While designing and integrating an actuator or other source of feedback can be expensive, such closed-loop configurations require less long-run calibration owing to their electro- mechanical self-correction for ‘noise’ (such as extremes of temperature or vibration). Open-loop accelerometers can thus suffer from a relative lack of stability and linearity in their readings, but they remain far more affordable, and can give sufficient data quality if regularly calibrated and enclosed against temperature, vibration, shock and other hazards. High-quality enclosures are generally made from aluminium or titanium, with mechanical stops to protect and stabilise the fixed part of the capacitor and keep its laminated plates holding the proof structure in place. IMUs, gyros and accelerometers | Focus Unmanned Systems Technology | December/January 2019 Unmanned vehicle accelerometers use capacitive sensing to determine changes in acceleration (Courtesy of SBG Systems)