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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However, due to specific properties, this kind of solution works well paired with a 2D or 3D LDV, offering a fully contactless measurement procedure. An example is presented in Fig. 3.

Carbon composite sandwich

Signal strength indicator

Loudspeaker with the mount

a)

b)

Fig. 3. Example of using a loudspeaker as an exciter for carbon fibre composite sandwich structure (CERN- Mechanical Measurement Lab): a) Loudspeaker placement; b) 3D LDV sensor heads placement.

For the loudspeaker, the waveform generator can implement different functions. As the goal is to go through the whole frequency range, the typical exciter function choice is the sine sweep or chirp function. A sine sweep test is characterised by a single frequency input as the base excitation. Frequency itself, however, varies over time. When testing sine sweeps, low frequencies may be tested first, then high frequencies, and vice versa. One cycle is defined as going from low frequency to high frequency, then going from high frequency back to low frequency, as required by some specifications. A chirp occurs when a pulse's rising edge and its falling edge have slightly different frequencies. Using both functions is correct from the measurement methodology point of view. However, considering both functions' application with the loudspeaker is problematic due to the sound effect of both functions on the people in the vicinity of the test station. Thus, it is preferable to use a pseudorandom function. A pseudorandom signal has the amplitude to define the sinusoid used to stimulate the structure. The amplitude can be made to correspond to higher amplitudes, resulting in structural resonance. This puts more energy where it is needed most. Multiplying the phase of the pseudorandom signal can reduce the peak factor (peak to RMS ratio) of the output voltage for amplifier/speaker integration. The crucial practical consideration is the optimal placement of the sound source, which should be close to the object (ideally less than 5cm). It must be pointed out that measuring on a completely black surface increases the noise level and decreases the signal strength (Fig.3b). However, it is still possible to acquire reliable data, it is suggested to enhance the signal strength and repeat the test. This problem has been evaluated in the next chapter. 2.2. Enhancements of signal strength In the case of signal strength enhancement, typical solutions are using retro-reflective tape, white or black tapes, or/and, in some cases, mirror/s (Hasheminejad et al., 2018). According to some 3D LDV manufacturers, the difference in noise levels according to the surface is significant. In this case, mirrors allow for maximal optical signal strength. Measuring on a white surface increases the noise level by a factor of up to 50. Finally, measuring on a completely black surface increases the noise level by a factor of up to 300. As a 3D Laser Doppler Vibrometer can measure ultra-low vibration amplitudes under challenging environments, it is still often necessary to enhance signal strength. It is usually the case with some black surfaces or transparent ones. An example of such a problematic black surface has been introduced in the previous chapter and is visible in Fig. 3. The typical solution for this problem is to use retro-reflective tape (e.g. 3M ScotchliteTM remote sensing sheeting). The tape consists of microglass spheres (diameter ≈50 µm). The purpose of each is to scatter light back along the path of the incidence beam according to low reflection and similar to the use of a mirror. It must be