Unmanned Systems Technology 023 I Milrem Multiscope I Wireless charging I Logistics insight I InterGeo, CUAV London & USA show reports I VideoRay Defender I OS Engines GR400U-FI I Ultrabeam Hydrographic Ultra-2 I IMUs

82 Focus | IMUs, gyros and accelerometers As closed-loop technologies become cheaper and smaller, however, manufacturers are increasingly likely to move towards offering only closed- loop accelerometers, because of their longer-term stability of performance and greater lifespan. This performance can be rated using a number of key metrics and matched to the requirements of the vehicle, with unmanned systems in sensitive operations (such as military, industrial or urban environments) meriting IMUs with higher bandwidths and measurement ranges. Bandwidth indicates the rate at which the accelerometer outputs data to update its readings. For high-end unmanned vehicles, an accelerometer typically exhibits a bandwidth between 300 and 500 Hz, or close to 1 kHz for a critical application where position estimates need the fastest possible updating. An accelerometer’s range specifies the maximum acceleration the system can measure, and is typically given as upper and lower bands measured in m/s 2 or units of g . For professional unmanned air and ground vehicles, this figure can reach ±15 g (or ±8 g for marine vehicles), or up to ±40 g for UAVs undertaking missions in dynamic or high-risk environments, such as for military or special forces. As well as noting the robustness of an accelerometer’s performance capabilities, engineers and integrators should pay close attention to how far its errors have been minimised during the calibration process. For example, ‘random walk’ or ‘noise density’ is a vital measure of the accelerometer’s random fluctuations over time. While a few different types of this can be measured, of particular importance is how velocity readings generated from the accelerometer can drift as a result of random walk. The degree of this is given by the ‘velocity random walk’, measured in m/s per hour (written as m/s/ √ h), m/s per second per Hz (written as m/s 2 / √ Hz), or milli- g of gravity per Hz (written as m g / √ Hz). Another key but undervalued error metric is vibration rectification error (VRE), which is an anomalous shift in the offset of the accelerometer caused during the rectification of AC vibrations to DC. VRE is particularly important for incline measurements that depend on measuring DC output, but it can vary widely between suppliers and is not often specified by most to begin with, although it is worth noting that higher bandwidths can lead to the introduction of higher frequency in-band vibrations, increasing the VRE. And as accurate data across variations in temperature is key, examining the operating temperature range can be important. As an industry standard, this tends to range from around -40 to +80 C. MEMS gyroscopes MEMS gyroscopes operate using the Coriolis effect to detect roll, pitch and yaw. The Coriolis effect typically refers to the pattern of deflection taken by objects moving within a rotating frame of reference. MEMS gyros use a proof mass locked within a single plane as their central component, which can be fabricated by bulk deposition or etching. This mass is normally a vibrating structure, most often shaped like a tuning fork or ring. As with accelerometers, they are often made from silicon or quartz. Gyroscopes using these kinds of structures are typically referred to as vibrating structure gyroscopes (VSGs) or coriolis vibratory gyroscopes (CVGs). In a ring-like silicon gyroscope, for example, the VSG should be supported and suspended in free space by an appropriate support, such as a set of springs or spokes around the ring. The design should take care to make the ring as geometrically perfect as possible, to optimise for balance, thermal stability and bias across extremes of vibration, shock and temperature. The planar vibration is controlled by transducers or actuators, and the angular rate response is also measured by them. The transducers will have typically been fabricated from lead zirconate titanate, particularly in the form of a piezoelectric thin film (PZT). PZT remains common among less costly gyros, but suffers from significant drift over time and temperature compared to other transducers. While temperature- induced errors can be compensated for by integrating independent temperature sensors into the gyroscope’s PCB (which most systems do), age-induced errors are less manageable. PZT-type gyros might therefore be best used either in lower-grade unmanned systems or in GNSS-aided navigation systems in which stability is only required over a small timescale. Gyroscopes increasingly use capacitive transducers, which results in VSGs that sense angular velocity December/January 2019 | Unmanned Systems Technology Open-loop accelerometers are cheaper than closed-loop ones, but are less stable and need more regular calibration (Courtesy of Physical Logic)