Beschreibung: This paper presents a robust cascaded control strategy to underwater robot thrust. The dynamics of surge motion, of propeller axial flow, of propeller shaft, and of electrically driven circuit in the motor constitute a cascaded system with respect to propeller thrust. Instead of the usual parameter perturbation, generalized modeling errors are considered in the plant, which may be parametric errors, ignored high-order modes, or some unmodeled dynamics in the underwater thrust system. External disturbances are also taken into account, which may be the random noises from mechanical or electrical equipment, or the environmental forces possibly induced by nonuniform currents, ocean internal wave, or cable tension. Combined with state feedback control, an online neural network (NN) compensator is introduced to identify the modeling errors, while L2-gain design is used to suppress the externally continuous or instantaneous disturbances. The Lyapunov's second method is applied to instruct the controller design, which guarantees the uniformly ultimately bounded (UUB) stability of the error system. By analyzing the tracking errors, it is recommended how to properly select the controller parameters. Good tracking performance and reasonable control inputs are illustrated by numerical simulations.

Beschreibung: A novel and fully automatic rigid fish cage system has recently been developed for deployment in the waters of Korea. The cage structure has 12 sides, incorporating a steel framework with a diameter and depth of 5.92 and 2.91 m, respectively. Attached to the steel framework is a housing for motor valves controlling variable ballast tanks, eight housings for two air compressors, a main control system, four batteries, a reserve air tank, four high air pressure tanks, 12 variable ballast tanks, and a seawater pump housing. The net of the fish cage is tightened across the frame to minimize volume reduction due to currents. The cage is outfitted with a control station located above the valve housing. With the control system, the buoyancy can be adjusted by utilizing compressed air stored in the four high air pressure tanks. The mechanical components of the ballast systems are operated by automated software that incorporates control and monitoring algorithms. The software initiates control of the ballasting components so the fish cage system can submerge if a preselected sea state occurs. The automatic control station incorporates a wind gauge, wireless communication printed circuit boards (PCBs), and a transmitting antenna. During operation, it monitors the wind speed, so the cage can be submerged before extreme sea states and then surfaced after the weather has passed and the conditions are considered safe. The control station also regulates the flow of air and seawater to and from the variable ballast tanks in response to the surface environmental conditions. Control can also be done remotely by a facility operator. In the development process, in situ tests were conducted to assess the performance of the submersion mechanism and the reliability of the automatic control system with a 1/4 size fish cage of similar construction. During the tests, the vertical position and inclination of the fish cage in the water column were measured. During the tests, the clo- e/open states of the motor valves that control the 12 variable ballast tanks were also assessed during descent and ascent operations. The successful performance of the 1/4 size fish cage during the tests showed promise that such a system could possibly be used on a much larger scale to avoid the rigors of the environment in support of commercial level offshore aquaculture.

Beschreibung: Plankton image classification is an important, yet challenging, problem in marine biology. This challenge can be attributed to: 1) large within-class variations; 2) large between-class similarity; and 3) large noise. To mitigate these problems, we propose a novel subspace classification framework, called pairwise nonparametric discriminant analysis for binary plankton image recognition. In this framework, first we decompose the multiclass recognition into a combination of pairwise binary classes, then train an appropriate classifier for each class pair using the nonparametric discriminant analysis technique (a newly developed subspace analysis technique) to effectively remove unwanted information (such as the within-class variations and the noise) and extract discriminant information (such as the boundary structural information), and, finally, combine all the pairwise classifiers using an efficient fusion rule for real-time classification. Extensive experiments are conducted on a large data set to show the improvement obtained by our new approach over the state-of-the-art ones.

Beschreibung: In this paper, sound scattering at a water/sediment periodic interface is modeled by an integral equation method to investigate the range of validity of the Rayleigh hypothesis. Due to the periodicity of surface, coupled integral equations are simplified by introducing the quasi-periodic Green's function, which can be evaluated with a relatively small number of terms of the spectral series. By comparing the scattering amplitudes obtained from the integral equation method and the method based on the Rayleigh hypothesis, the range of validity of the Rayleigh hypothesis is determined. For a given wavenumber $kappa$ of the periodic surface, the maximum surface amplitude $h_{max}$ beyond which the Rayleigh hypothesis is invalid decreases with frequency of incident waves. For the grazing angle $theta={hbox {90}}^{circ}$ , $h_{max}$ is larger than $h_{c}=0.448/kappa$ (the upper limit of surface amplitude determined by the criterion of the Rayleigh hypothesis) in the frequency range 2.5–40 kHz. However, for a small grazing angle, such as, $theta={hbox {35}}^{circ}$ , $h_{max}$ is observed to be smaller than $h_{c}$ for the large surface period $Lambda$ at frequency 40 kHz.

Beschreibung: In this paper, optical wireless has been addressed as a promising technology to provide high-bandwidth services for underwater communications. However, the significant attenuation degree due to high absorption and scattering of optical transmission in the water confines the achievable range of optical links to only few meters. One way to achieve transmission at long distances is to employ a dense network configuration where information can be transferred through a series of intermediate nodes acting as relays. In this study, we consider optical wireless network arrangements where nodes are floating at different depths into a service aquatic medium. We deploy an effective path loss model which incorporates the key factors that deteriorate the optical power, and we derive the achievable transmission range to satisfy connectivity criteria assuming intensity-modulation direct detection (IM/DD) with on–off keying (OOK). A set of numerical results is presented in order to reveal the interaction between various parameters such as error probability, wavelength, node density, transmitted power, data rate, etc., in order to achieve $k$ -connectivity. The proposed analysis could be the basis of deploying reliable underwater optical networks suitable to deliver broadband services at far distances.

Beschreibung: In this paper, a narrowband method based on the acoustical Doppler effect is proposed for estimating the full set of five flight parameters, along with the propeller blade rate, of a turboprop aircraft as it transits over a wide aperture hydrophone array in a straight line at constant speed and constant altitude. The five flight parameters describe fully the linear trajectory of the aircraft's transit. The basic principle of the proposed method is to measure the temporal variation of the instantaneous frequency (IF) of the acoustic signal received by each sensor of the array and then to minimize the sum of the squared deviations of the IF estimates from their predicted values over a sufficiently long period of time for all sensors. The minimization is performed numerically subject to a set of constraints on the source parameters to be estimated. The IF model for the received signal at each sensor of the array and the initial parameter estimates required for the numerical minimization are derived. The effectiveness of the proposed method is demonstrated using both simulated and real data.

Beschreibung: Real-time 3-D acoustical imaging technique is a key advance to broaden the scope and enhance the feasibility of underwater missions. In this paper, a real-time 3-D underwater acoustical imaging system to handle the actual tasks under a narrowband excitation is presented. The system consists of three parts: a transmitter, a receiving hydrophone array, and a signal processor. In this system, a distributed and parallel subarray (DPS) beamforming algorithm is proposed to process the acquired signals from a scene placed in the far field. The DPS beamforming algorithm is an approximate method for the sonar signal processing with an advantageous computational efficiency. The direct method (DM) and fast Fourier transform (FFT) beamforming are compared with DPS beamforming for the memory and computational requirements. Based on this algorithm, a prototype was developed, which has been extensively employed in the lake and sea trials. The trials demonstrate that the system can achieve the 3-D imaging of the scene and meet the real-time requirement of the underwater operations.