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FEATURE SENSORS


MEMS Inertial Sensors: High-Tech Motion Sensors Don’t Miss a Thing By Dr. Thomas Frasch, product manager at First Sensor


capacitance to be recorded and measured. These silicon sensors are usually manufactured as a key MEMS element in bulk or in a surface micro-machining process. But this is not the case for the new First Sensor generation, which are manufactured in highly complex HARMS and AIM processes. HARMS stands for “high-aspect-ratio microstructures” and enables microstructures with an exceptionally high aspect ratio and minimizes any cross-sensitivities. AIMS (“air-gap-insulated microstructures”) reduces parasitic capacitances because the components are – as the name suggests – insulated with an air gap.


T


here are lots of good reasons for being able to detect even the slightest of movements – for example, in buildings and bridges that have to withstand high winds. And vibrations from rail vehicles and heavy machinery can also push structures beyond their tolerance threshold. High-precision recording instruments are essential for giving engineers a detailed picture of a structure’s condition and allowing them to respond where necessary. New, high-accuracy, capacitive silicon inertial sensors with outstanding resolution and extremely low noise density precisely measure not only the tiniest of movements, but also vibrations and changes of inclination. This allows deviations of any kind or size to be identified with micrometer-precision and establish whether the stability of a structure is at risk. However, movement measurements on buildings is just one of many potential applications. These sensors are also invaluable tools in geo-engineering, navigation and robotics. They can also be used for the predictive maintenance of technical facilities. Potential sources of faults can be detected and rectified at an early stage to prevent major damage and downtime due to repair work further down the line. This not only saves time, materials and money, but also ensures that sensitive and critical installations can continue to operate without problems.


MONOCRYSTALLINE SILICON MICROSTRUCTURES The inertial sensors from First Sensor have monocrystalline silicon microstructures with a high aspect ratio.


26 JUNE 2020 | ELECTRONICS


These tiny sensors are constructed in the form of a micro electromechanical system (MEMS) and capable of registering accelerations in two spatial dimensions. This allows them to detect even the slightest changes in position or acceleration. The sensors achieve an excellent resolution of 10 µg or 0.0005° (2 arc seconds). As inclination sensors with measurement ranges of ±30°, the sensors achieve a noise density of less than 0.0004°/√Hz and resolutions of less than 0.0015° at a measurement frequency of 10 Hz. When used as acceleration sensors, they offer measurement ranges of ±3 g, ±8 g and ±15 g and achieve a noise density of less than 30 µg/√Hz and resolutions of less than 40 to 95 µg at a measurement frequency of 10 Hz.


KEY TECHNOLOGY FOR THE INTERNET OF THINGS


MEMS sensor technology is among the key technologies for the much- vaunted Internet of Things – and with good reason. An MEMS-based inclination sensor, for example, can register the deflection of a single human hair at a distance of 10 meters – the corresponding value is just 0.0005° and 2 arcsec or 10 µm/M. At the heart of the MEMS inertial sensors for the capacitive measurement of tilt, acceleration and vibration are tiny spring-mass systems with silicon webs just a few µm wide. During manufacture, the silicon sensor element is constructed in a patented micromechanical process. In response to acceleration, the masses suspended on springs are deflected, allowing the respective change in


HIGH-PERFORMANCE ASIC “COMMAND BRIDGE” The high-performance ASIC – an integrated circuit – is the control center and “brain” of the sensor. It reads the capacitive signals of the sensor element and transmits the measured value digitally. High-performance ASICs such as those developed by First Sensor offer extremely low-noise capacitive detection; provide optimal support for the nominal and differential capacitance range; are high-resolution with high a dynamic range; are equipped with a digital SPI interface for configuring the sensor ASIC system and reading out the sensor data; and feature a flexible signal filter. Since the sensors usually have to


function in harsh outdoor conditions such as in wind turbines, bridge pillars or deep in the earth, they naturally require special protection. The MEMS inertial sensors from First Sensor feature a hermetically sealed housing made from a ceramic substrate. It can be easily tailored to different conditions and requirement profiles depending on its application and is cost-effective to manufacture, even in small and medium quantities.


POWERFUL, COST-EFFECTIVE AND ENERGY-EFFICIENT Thanks to their miniature format, the MEMS inertial sensors from First Sensor are inexpensive to manufacture and consume comparatively little energy. This – along with their incredible capabilities and capacitances – makes them the ideal solution for current applications and an attractive alternative for new ones, especially in the fields of automation, robotics, predictive maintenance and geo-engineering.


FIrst Sensor www.first-sensor.com / ELECTRONICS


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