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An autonomous underwater vehicle (AUV) is a robot which travels underwater without requiring input from an operator. AUVs constitute part of a larger group of undersea systems known as unmanned underwater vehicles, a classification that includes nonautonomous remotely operated underwater vehicles (ROVs) –controlled and powered from the surface by an operator/pilot via an umbilical or using remote control. In military applications AUVs more often referred to simply as unmanned undersea vehicles (UUVs).
laser Doppler vibrometer
A laser Doppler vibrometer (LDV) is a scientific instrument that is used to make non-contact vibration measurements of a surface. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface. The output of an LDV is generally a continuous analog voltage that is directly proportional to the target velocity component along the direction of the laser beam.
Heterodyne detection is a method of detecting radiation by non-linear mixing with radiation of a reference frequency. It is commonly used in telecommunications and astronomy for detecting and analysing signals. The radiation in question is most commonly either radio waves (see superheterodyne receiver) or light (see Optical heterodyne detection or interferometry). The reference radiation is known as the local oscillator. The signal and the local oscillator are superimposed at a mixer. The mixer, which is commonly a (photo-)diode, has a non-linear response to the amplitude, that is, at least part of the output is proportional to the square of the input.
block diagram of the LDV
Figure 1 shows the principle block diagram of the LDV we developed. The LDV is composed of transceiver unit and signal processing unit. A single frequency fiber laser with the linewidth of 6 kHz at wavelength of 1550 nm is used as the transmitter. The output light is divided into two beams by a fiber splitter. Transmitted light is frequency-shifted of 55 MHz by an acousto-optical modulation (AOM), and the residual light is taken as the reference laser beam (LO). The modulated beam is transmitted to glass perpendicularly through an optical circulator and a telescope. Then, the reflected beam with Doppler shift due to the vibration of glass is received by the same telescope, and it is mixed with LO by a fiber coupler.
Finally, the beat signal is detected by a fiber coupled InGaAs PIN detector. Demodulation circuits are important modules of the signal processing unit. In the demodulator circuit module I, the high-pass filtered output voltage signal of the PIN detector is the intermediate frequency (IF) input of the quadrature demodulator RF2713 and the double frequency of the heterodyne frequency shift of 55 MHz which drives the AOM is the local oscillator input of RF2713.
Using RF2713 for I/Q quadrature demodulating and passing the low-pass filters, the in-phase (I) and the quadrature (Q) output signals can be obtained. Moreover, the demodulator circuit module II is necessary for the purpose of real-time display. The I/Q signals are differentiated and then multiplied cross with each other as illustrated Fig. 1. In the end, the audio signal is constructed by the sum of the two signals, and can be played with the sound card and speaker. Hence, the output analog audio signal corresponding to the reliable and real-time speech signals Uout(t) can be described as
Principles of operation
A vibrometer is generally a two beam laser interferometer that measures the frequency (or phase) difference between an internal reference beam and a test beam. The most common type of laser in an LDV is the helium-neon laser, although laser diodes, fiber lasers, and Nd:YAG lasers are also used. The test beam is directed to the target, and scattered light from the target is collected and interfered with the reference beam on a photodetector, typically a photodiode. Most commercial vibrometers work in a heterodyne regime by adding a known frequency shift (typically 30–40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg cell, or acousto-optic modulator. A schematic of a typical laser vibrometer is shown below. The beam from the laser, which has a frequency fo, is divided into a reference beam and a test beam with a beamsplitter. The test beam then passes through the Bragg cell, which adds a frequency shift fb. This frequency shifted beam then is directed to the target. The motion of the target adds a Doppler shift to the beam given by fd = 2*v(t)*cos(α)/λ, where v(t) is the velocity of the target as a function of time, α is the angle between the laser beam and the velocity vector, and λ is the wavelength of the light.
Basic components of a laser Doppler vibrometer Light scatters from the target in all directions, but some portion of the light is collected by the LDV and reflected by the beamsplitter to the photodetector. This light has a frequency equal to fo + fb + fd. This scattered light is combined with the reference beam at the photo-detector. The initial frequency of the laser is very high (> 1014 Hz), which is higher than the response of the detector. The detector does respond, however, to the beat frequency between the two beams, which is at fb + fd (typically in the tens of MHz range).
The output of the photodetector is a standard frequency modulated (FM) signal, with the Bragg cell frequency as the carrier frequency, and the Doppler shift as the modulation frequency. This signal can be demodulated to derive the velocity vs. time of the vibrating target.
advantages of an LDV over similar measurement devices such as an accelerometer are that the LDV can be directed at targets that are difficult to access, or that may be too small or too hot to attach a physical transducer.
Also, the LDV makes the vibration measurement without mass-loading the target, which is especially important for MEMS devices.
LDVs are used in a wide variety of scientific, industrial, and medical applications. Some examples are provided below:
Aerospace – LDVs are being used as tools in non-destructive inspection of aircraft components.
Acoustic – LDVs are standard tools for speaker design, and have also been used to diagnose the performance of musical instruments.
Architectural – LDVs are being used for bridge and structure vibration tests.
Automotive – LDVs have been used extensively in many automotive applications, such as structural dynamics, brake diagnostics, and quantification of Noise, vibration, and harshness (NVH).
Biological – LDVs have been used for diverse applications such as eardrum diagnostics and insect communication.
Calibration – Since LDVs measure motion that can be calibrated directly to the wavelength of light, they are frequently used to calibrate other types of transducers.
Landmine detection – LDVs have shown great promise in the detection of buried landmines. The technique uses an audio source such as a loudspeaker to excite the ground, causing the ground to vibrate a very small amount with the LDV used to measure the amplitude of the ground vibrations. Areas above a buried mine show an enhanced ground velocity at the resonance frequency of the mine-soil system.
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