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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1.3. Introduction to Laser Doppler Vibrometers

Laser displacement sensors use triangulation with a transmitting and receiving lens. A laser beam is emitted toward the target through a transmitting lens. The light then reflects back towards the sensor and is directed by a receiver lens to a receiving element as the target moves closer and farther away, the angle of the reflected light changes (it is focused on a different position on the receiving element). Displacement sensors are preferable in applications that prevent using accelerometers, such as rotating components, or when the accelerometer's mass would have too much influence on the system's motion or cannot be attached due to process restrictions. A great example is a modal analysis of ultra-lightweight carbon structures for the detectors (Fig.2a) or space technologies (Guinchard et al., 2018) and implementing such a technique for quality assurance purposes on the production lines (Scislo, 2021).

PSV-500 Laser Doppler Vibrometer

RSV-150 Laser Doppler Vibrometer

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b)

Fig. 2. Example of contact-less measurement of: a) ultra-light objects (example from CERN- Mechanical Measurement Lab); b) Field measurement of force in bridge hangers and cables.

It must be pointed out that the 3D Laser Doppler Vibrometer is an advanced optical instrument. Thus, it can perform reliable measurements in different environments, both for small (Fig.2a) and large objects (Fig.2b); however, the main problem is the signal quality's dependency on surface quality (Schewe and Rembe, 2021). As a result, this study's motivation is to show techniques and possible improvements of test setups to overcome some of the practical problems of field measurements, especially in the case of fragile objects, e.g., light materials, but also in an application for historical, heritage objects that are often protected from instrumentation installations. The paper investigates the problem of choosing a non-contact excitation source and how the light is backscattered from the different surfaces to enhance the signal strength or give access to invisible surfaces. Finally, some concerns will be

presented regarding the choice of equipment, mounts and signal processing parameters. 2. Selected methods for measurement enhancements with the use of 3D LDV 2.1. Contactless measurements with the excitation using a sound pressure source

Due to the fragility of some objects, their low mass, or the lack of possibility of attaching both sensor/s and exciter, the problem of finding alternatives is evident in the case of many objects, including historical ones. In the case of sensor technologies, it is still possible to use a microphone or, e.g., a LDV. However, the problem exists with replacing the source of object excitation. In most applications, the contact exciter can be substituted for the source of sound pressure (loudspeaker) if the object is excited strong enough for the sensor to measure the response.