PSI - Issue 44

Federico Germano et al. / Procedia Structural Integrity 44 (2023) 902–909 F. Germano et al./ Structural Integrity Procedia 00 (2022) 000 – 000

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An application specific integrated circuit (ASIC) is used to interface with the MEMS sensor to detect these capacitance changes. The ASIC contains clocks, a demodulator, and other signal conditioning to convert the capacitance changes to an output voltage proportional to the applied acceleration. Changes in the MEMS geometry allow it to be scaled for various acceleration input ranges. A gas enclosed inside the structure provides damping. Open loop devices strictly measure applied acceleration by converting the change in capacitance seen by the sensing element to a voltage that is proportional to it. The proof mass is not constrained from movement inside the MEMS structure. Closed loop accelerometers utilize feedback in the form of a restoring electrostatic force that is applied to the proof mass to keep the capacitance equal on both sides thus maintaining the spring position centered within the structure. The amount of restoring force applied is then proportional to the applied acceleration. Closed loop systems have a key advantage over open loop systems in that the noise floor tends to be lower allowing the detection of lower levels of acceleration (higher signal to noise ratio) and they also have lower non-linearity as the spring mass maintains its centered position and damping changes are reduced. Closed loop accelerometers are sometimes referred to as force rebalancing accelerometers. Variable capacitances accelerometers differ from piezoelectric (or IEPE) type sensors in that they can sense acceleration down to 0 Hz, or static levels. This makes them useful for applications such as inertial measurement, tilt, and low frequency vibration measurement. Up to 2022, open loop MEMS are ameliorating their performance so much that they are currently blacklisting the closed loop MEMS from most of the applications, including monitoring of infrastructures.

1. Example of MEMS accelerometer

The MEMS technology exhibits huge variation between products in market range. What we are looking for as a requirement, to satisfy the target setting cited before, is following: • Low noise density. Proper noise density should be in the range of 0.007 mgRMS/√Hz for civil applications. Higher noise density will inevitably slow the warning identification of acceleration-based criteria for SHM. • Close-to-zero phase shift. To put a number, a 2 degrees maximum error in the 0-10 Hz range is considered low enough to extract properly modeshapes without occurring in mistakes due to the phase error shift. Of course any wifi commercial network cannot grant this capability, so accelerometer have still to be cabled in.

• Excellent thermal stability, in the order of typical 0.2 mg/°C.

• As cable will be necessary, the possibility to have a differential sensor output will help signal robustness. Differential output sensors will not show an increase in offset with increased cable length.