PSI - Issue 64

Lukasz Scislo et al. / Procedia Structural Integrity 64 (2024) 2246–2253 Lukasz Scislo et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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is usually the most appropriate. Excitation mechanisms fall into four categories: shaker, impactor, step relaxation and various non-contact excitation methods like sound or air pressure. 1.2.2. Sensing Technology Four main features must be considered when selecting vibration measurement instruments and vibration analysers: accuracy, sampling frequency, and ambient conditions. After deciding on the excitation system that forces the structure into motion, the sensor for motion measurement needs to be selected. According to the application, the choice between contact and non-contact devices must be made. However, different applications call for different ways of measuring vibration. The most common method of measuring vibration is with an accelerometer mounted to the test structure (Fig.1a). But there are three primary reasons why contact accelerometers (or strain gauges) may not work for an application: • The mass of the accelerometer would influence the vibrating structure, or it is not allowed to place the sensor on the object due to its value or historical significance (art, sculpture, historical objects and sites) (Fig.1b) ; • Rotating equipment makes wiring virtually impossible; • Measurements need to be performed in extraordinary environmental conditions, such as extremely hot/cold surfaces, radiation areas, and strong magnetic fields, or in the case of historical objects and sites, access to the area is restricted. Measuring vibration without directly mounting to the test structure is possible due to the fact that some vibration sensors do not require direct contact with the source of the vibration. The three typical cases of non-contact sensors are: • A microphone or Acoustic Pressure Sensor; • Laser Displacement Sensor (Vibrometer); • Eddy Current or Capacitive Displacement Sensors. Fig.1a presents the classical approach to dynamic testing, using typical equipment such as electrodynamic shakers and modal hammers for energy input and accelerometers as sensing technology. However, in the case of low-mass structures and historical projects, the approach should be fully none-contact (Fig.1b). Currently, the most significant advantages are associated with the use of Laser Doppler Vibrometer, which can also be used as an Internet of Things (IoT) sensor that streams the data to the cloud for future specialised analysis.

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Fig. 1. Measurement equipment essential components a) classical contact approach; b) non-contact approach with 3D LDV as an IoT sensor and cloud analysis.