Laser vibrometer

Equations (33) and (34) demonstrate that the motion quantities 5 (displacement) and v (velocity) are encoded in phase and frequency modulation of the detector output signal, purely referenced to the laser wavelength A. Tobeableto recover the time histories s(t) and v t) from the modulated detector signal, adequate phase and frequency demodulation techniques, or both, are utilized in the signal decoder blocks of a laser vibrometer. 

Lawton, R., "MEMS Characterization Using Scanning Laser Vibrometer, Reliability, Testing, and Characterization of MEMS/MOEMS II, R. Ramesham and D. M. Tanner, Eds., Proceedings ofSPIE, Vol. 4980,2003, pp. 51-62. 

Noise and vibration measurements are made with microphones and accelerometers. Sometimes, sophisticated instruments like laser vibrometers are used for NVH work. Common noise fixes include cutting slots and/or chamfers on the brake pads and linings, the application of constrained layer damping noise insulators to the back of the brake pad shoe plates, and detuning the resonant frequency of mechanical brake corner components. 

Sohn H, Dutta D, Yang JY, DeSimio M, Olson S, Swenson E. Automated detection of delamination and disbond from wavefield images obtained using a scanning laser vibrometer. Smart Mater Struct 2011 20(4) 045017. http //dx.doi.org/10.1088/0964-... 

Tissue Vibration. Noncontact methods are preferred for measuring the surface motion of tissues. Laser vibrometers are commercially available with sufficient bandwidth and resolution for most studies. A direct mass load on the skin, together with the skin s elasticity, forms a mechanical low-pass filter (see Simple Lumped Models in Sec. 10.3.1). If a device is to be mounted directly on the skin, it must be of low weight (e.g., <3 g) and possess a comparatively large attachment area (e.g., >S cm ), in order for vibration to be recorded without attenuation of the motion at 80 Hz. An upper frequency limit of 200 Hz is theoretically achievable (—3dB) with a transducer and skin mount weighing 3 g and an attachment area of 1.8 cm. ... 

In Fig. 3.11, velocity motion detected by the laser vibrometer is shown at the middle and that of Lamb s solution is shown at the bottom. Remaika-ble agreement is confirmed except the latter reflections in the detected wave. Because a concrete block was employed in the experiment, these reflected motions are observed. At the top, frequency spectrum of the detected wave is shown as a solid curve and is compared with that of Lamb s solution denoted by a broken curve. Reasonable agreement is again confirmed. This result suggests an application of the laser vibrometer for the absolute calibration instead of the capacitive transducer. 

Hatano and Moro [1976] suggested the use of a reciprocity method, where sensors are coupled onto a steel plate. This approach uses Rayleigh waves. Other methods described in the literature suggest using a laser-vibrometer to measure the displacement of the free surface of the sensor, or a network analyzer (Weiler and Grosse 1995 Grosse 1996). 

Fig. 5.5. (top) Example of frequency response functions of a resonant and a so-called broadband transducer. The frequency response function was measured using a laser-vibrometer (middle) and a face-to-face method (bottom). Amplitudes are on a linear scale. 

Vibrational Measurements. Laser vibrometers make use of the Doppler shift, which occurs when one laser beam experiences a frequency shift relative to the other because of the motion of the sample. These interferometers are used in many industries to measure vibration of moving parts, such as in airline or automotive parts, or parts under stress, such as those in bridges. 

The resonance spectrum, obtained by continuously exciting the object over a wide frequency range (sine-wave fi quency sweep) and measuring its response, provides an acoustic signature of the object. The measurements can be made with direct contact transducers, such as piezoelectric crystals, or with a non-contact setup using a speaker for excitation and a laser vibrometer for response measurement. Typicalfy, the frequency sweep range used for chemical munitions lies between 1 KHz and 30 KHz and the entire frequency sweep can be carried out in less than 60 seconds. 

If the velocity decoder on the vibrometer is used, the displacement information can stiU be obtained by integrating the velocity signal as a function of time. Fourier analysis can be performed to transform the displacement-ampHtude-versus-time signal into the frequency domain. Even though the laser vibrometer can achieve picometer resolution, it can t detect strain by a dc voltage alone. Only alternating compressive and expansive strain can be measured with this method. Fortunately, this includes combined ac and dc field excitations. 

Three US boosters (1 1, 1 1.5, and 1 2 gain) were used to provide the vibrational peak-to-peak amplitude range. The actual peak-to-peak amplitudes as measured with a laser vibrometer, were 50, 60, and 70 pm for the 1 1,1.5, and 1 2 boosters, respeetively. 

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