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www.sea-technology.com December 2015 / st 21 I nertial sensors have been used for motion sens- ing at and below the sea for more than 100 years. One of the frst underwater vehicle naviga- tion applications was a gyrocompass used in the British Royal Navy's submarine HMS E1 in 1913. Interestingly, this pioneering instrument was pro- duced by the Sperry Gyroscope Co., a company still alive today under the name Atlantic Inertial Systems and its joint venture business, Silicon Sensing Systems, and it's not without reason that the companies are co-located in Plymouth, Eng- land, a maritime city steeped in naval technology and history. Since these early days, gyroscopes and accel- erometers have been applied to a multitude of tasks at sea, such as vehicle and payload stability control, guidance, navigation, attitude, heading and positioning. For more than half a century, these large precision instruments were complex mechanical assemblies with bothersome bear- ings and springs, all hand-built by experienced craftsmen with clock-making skills, each one be- ing unique. Today, Silicon Sensing can achieve almost the same measurement precision using silicon microelectrome- chanical systems (MEMS) inertial sensors, and they just keep getting better and smaller. The question remains as to where the ultimate boundaries of performance, size and cost of this technology lie, and what are the implications for under- water vehicle systems today and in the future. Inertial MEMS MEMS gyroscopes and accelerometers are produced us- ing wafer-scale processes derived from the semiconductor industry to fabricate hundreds or thousands of tiny identi- cal micromechanical structures in silicon using photo- lithographic and deep reactive ion etch (DRIE) techniques. While still at wafer level, microns-thick metal or piezoelec- tric transducers are deposited by sputtering particles onto the silicon, which provide the drive and pick-off signals to and from the sensor. Once suitably packaged, together with their associated electronic control circuitry, you have either a single-axis angular rate sensor (gyro) or a linear acceler- ometer. In the case of the gyro, the silicon structure, which is in fact a tiny silicon ring, is vibrated at its resonant frequency such that, when rotated, the Coriolis force effect generated causes the vibration mode to move around the structure, from which a signal is generated, the magnitude being pro- portional to the applied angular velocity. The accelerometer is much simpler to comprehend: Like a spring, the beam-like structure defects under the g-force of any applied linear acceleration along its sensitive axis resulting in a change of the capacitance between adjacent elements, thus generating a signal proportional to linear ac- celeration usually referred to in units of gravity, or g. High-Performance Inertial Microelectromechanical System MEMS IMU for Next-Generation Underwater Guidance, Navigation By Eric Whitley • Stephen Clarke Typical Allan Variance plot for the gyro in the DMU30 IMU.