PSI - Issue 44

Mauro Mazzei et al. / Procedia Structural Integrity 44 (2023) 1212–1219 Mauro Mazzei et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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3.5. Optical MEMS accelerometer A new coarse-to-fine optical MEMS accelerometer based on the Fabry-Pérot (FP) interferometer has also been proposed. The structure consists of a test mass suspended by machined nano springs. The deflection of the test mass due to the applied acceleration is detected using FP cavities that constitute the optical system of the device. 3.6. Capacitive accelerometer Capacitive accelerometers are based on a change in electrical capacitance in response to acceleration. Accelerometers use the properties of an opposing capacitor plate for which the distance between the plates varies in proportion to applied acceleration, thus altering the capacitance. This variable is used in a circuit to eventually provide a voltage signal proportional to acceleration. Capacitive accelerometers are capable of measuring both constant and slow transient and periodic acceleration. AC capacitive acceleration sensors basically contain at least two components; the primary is a 'stationary' plate and the secondary plate is attached to the inertial mass, which is free to move within the case. Fig. 1. Capacitive accelerometer sensing concept. 4. Design requirements Our project involves the design and development d a monitoring system capable of integrating a wired or wireless network of sensors, mounted on the structure under consideration, to measure significant physical quantities of structural response, actions, and environmental conditions. After well addressing the single-direction sensing module, we propose a three-axis structured model that becomes the node to be integrated into a networked sensor array. In particular, the primary task is to continuously measure infrastructure load sources: environmental (wind, seismic action) and man-made (traffic). 4.1. Selected configuration Bridges can be designed to support large amounts of weight, but the vibrations associated with them must be controlled. Although bridges appear to be solid structures fixed in place, they are exposed to the effects of vibration. If the force is applied to the bridge is at a frequency that matches the natural frequency of the bridge, the vibration within the bridge will be amplified in a phenomenon called mechanical resonance. In situations where the mechanical resonance is strong enough, the resulting vibrations can cause a bridge to collapse due to movement. Typically, the longer the span, the lower the resonance frequency of the bridge. Lower frequencies are also associated with vibrations of large displacement amplitude. Some famous cases demonstrate the importance of understanding the resonant frequency of the bridge, knowing what might excite the frequency and how it can be handled. The worst of these is the Tacoma Narrows Bridge, at the time the third largest suspension bridge in the world, which collapsed when the wind-induced vortex drop coincided with the bridge's natural frequency. Without sufficient damping, the resonances grew until when the bridge collapsed. The Millennium Bridge over the River Thames in London provides another example. The thousands of people who crossed the bridge on the opening day caused the bridge to vibrate, as a result of which pedestrians were then inadvertently fell in step with these vibrations, amplifying them and causing oscillations resulting in the bridge swaying from side to side.