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33 for developers to differentiate the design; it also has an impact on the design of the sensors. For example, driverless car IMUs will need two-axis accelerometers and gyros because the third axis, height, is less relevant. By contrast, accelerometers developed for safety applications where early notification of tipping (as in a change in height) is essential will need a three-axis sensor. This design approach can be extended to using different types of sensors on different axes. For example, a marine IMU may need more accurate height information from the motion of waves (called the ‘heave’) so the vertical axis would have a sensor with higher accuracy but a lower dynamic range (as the wave height doesn’t change very quickly). That is very different from a UAV where the height can change quickly, and here the height data can be supplemented with an altimeter or pressure sensor. Technologies All of this means there is room for many different types of sensor technology in an IMU’s design, depending on the performance and reliability requirements. IMU makers therefore regularly assess the various sensors on the market, often choosing technologies depending on the application and even the axis being measured. These technologies can range from the early laser-based ring gyros to more modern fibre optic gyros (FOGs) and oscillating piezoelectric gyros, as well as micromachined MEMS devices. Similarly, accelerometers can be designed in different ways, based around quartz crystals or different MEMS structures. While MEMS devices are smaller and lower power than other accelerometer and gyro technologies, they have struggled in the past to reach the performance levels required for IMU designs, although that is changing. At the other end, FOG devices – while highly accurate and reliable, having no moving parts – have been too large and costly for many mobile applications, but that too is changing. The performance of an IMU is a complex combination of sensor technology, the packaging of the sensors, the accuracy of the test and calibration data and the associated algorithms, as well as the algorithms and software implementation in the navigation system. Changes in sensor design, the type of packaging and the software all have an impact on the performance of the overall system. For example, specific design skills are needed when integrating sensors in more rugged, ceramic Inertial measurement units | Focus IMUs for marine systems sometimes need highly accurate measurement of the height of a craft above water level, requiring more complex signal processing algorithm design (Courtesy of SBG) Unmanned Systems Technology | December/January 2017 A small, high-precision fibre optic gyro (FOG) is key to a driverless car’s performance. The red spot represents light moving through the FOG’s optical circuit of coiled fibre; this circuit is the FOG’s sensing unit (Courtesy of KVH)