Sea-Moon Kim

An integrated navigation system for autonomous underwater vehicles with two range sonars, inertial sensors and Doppler velocity log

This work presents an integrated navigation system for autonomous underwater vehicles with inertial and acoustic sensors. The integrated navigation system is based on a strap-down inertial measurement unit (IMU), where auxiliary navigation sensors are two range sonars, a Doppler velocity log (DVL), a magnetic compass and a depth sensor. The main idea of the paper is to improve the performance of the IMU-DVL navigation system by introducing additional range information. The range sonars can measure the distance and the incident angle from a reference transponder on an underwater station. The range sonars are modeled to improve the navigation algorithm, which could provide valuable information for the IMU-DVL navigation to reduce estimation error, especially when the DVL cannot detect the bottom reflection. Simulation was conducted with the 6-d.o.f. nonlinear numerical model of an AUV in lawn-mowing survey mode at tidal flow, where the bottom reflected DVL information is unavailable. Simulations illustrate the effectiveness of the integrated navigation system assisted by the additional range measurements.

Visual servoing for underwater docking of an autonomous underwater vehicle with one camera

Docking systems are required to increase the capability of autonomous underwater vehicles (AUVs) to recharge the batteries and to transmit data in real time in underwater. This paper presents a docking system for an AUV to dock into an underwater station with one camera installed at the nose center of the AUV. To make a visual servo controller, this paper presents an optimal control algorithm based on a state equation composed of the AUV dynamics and the optical flow equation. The control inputs of the AUV are generated with the projected target position on the CCD plane of the camera and with the AUVs motion. This paper introduces a test bed AUV for underwater docking developed in Korea Research Institute of Ships and Ocean Engineering (KRISO), KORDI of Korea. The AUV is propelled with one thruster and controlled with two horizontal planes and two vertical planes. Experiments will be performed in Ocean Engineering Basin of KRISO to demonstrate the effectiveness of the modeling and control law of the visual servoing AUV. The AUV identifies target position with processing the captured image of the lights mounted around the entrance of the dock. This paper also presents a strategy to identifier the target with lights arrays.

Structural-acoustic coupling in a partially opened plate-cavity system: Experimental observation by using nearfield acoustic holography

In order to understand the cause and effect relation between a structure and a fluid, many studies on structural-acoustic coupling have been done. However, the studies were restricted to the interaction between only a structure and a fluid located on one or the other side of the structure. It is our aim to understand the coupling mechanism of a generally coupled system that has direct interaction between a finite interior fluid and a semi-infinite exterior one. We believe that this configuration allows the structure to interact with the fluid of the finite volume and that of the infinite one, thus providing a more general structure-fluid coupling (or structural-acoustic coupling) mechanism. For this purpose, we selected a partially opened plate-cavity system which has two different modally reacting boundary conditions: a plate and a hole. In order to understand the physical coupling phenomena of the selected system, visualization of the sound fields was performed experimentally. We used near field acoustic holography to estimate sound field variables, such as pressures and intensities. Examining the acoustic variables, we found that there are two types of coupling mechanisms depending on frequency and associated wavelength. One is where the plate and the cavity are so strongly coupled that the plate can be considered as a source. In this case, the system radiates acoustic energy effectively through the plate. The other is where the coupling interaction behavior decreases the radiation efficiency. The frequencies that determine whether the plate is a good or bad radiator are found to be around the natural frequencies of the plate.

Sparse Underwater Acoustic Channel Parameter Estimation Using a Wideband Receiver Array

This paper considers a channel parameter estimation problem using a wideband multichannel receiver array. We estimate the sparse underwater acoustic communication channel parameters, such as time-delay, incidence angle, Doppler frequency, and complex amplitude of impinging multipath components. Multichannel signals from a receiver array are modeled to obtain additional sparsity in the incidence angle and the angle-delay-Doppler-spread function is defined to parameterize the channel. To estimate the principal entries of the angle-delay-Doppler-spread function, we propose a modified version of the orthogonal matching pursuit (OMP) algorithm to utilize a redundant dictionary. We introduce a space-alternating scheme which divides the entire parameter search space into smaller subsets to avoid handling large parameter search space. The performance of the proposed method is evaluated using two experimental data: one from large-scale water tank with the capability of generating surface gravity wave and the other from shallow sea water which shows sparse channel structure. Our results demonstrate that the proposed method accurately estimates the time-varying multipath channel parameters with lower residual error than the OMP.

Performance Analysis of Channel Compensation and Channel Coding Techniques based on Measured Maritime Wireless Channel in VHF-band Ship Ad-hoc Network

In this paper, the parameters of the RTT (Radio Transmission Techniques) for SANET (Ship Ad-hoc NETwork), which is considered for the next generation maritime communication systems, are set up. A channel model has been analyzed based on the practical measured maritime wireless channel in VHF (Very-High Frequency) for SANET system. Also, by considering the frame structure including preamble, guard time and pilots for both single and multi-carrier systems, the BER (Bit Error Rate) performances are evaluated and analyzed in the aspects of channel compensation and channel coding techniques. Based on the simulation results, optimal modulation & coding schemes are suggested for SANET. That is, in single-carrier system by using differential modulation schemes, channel compensation is not necessary. However, channel coding is helpful to achieve additional gain. On the other hand, when 16-QAM modulation is employed in multi-carrier system, the implementation of both channel compensation and channel coding techniques show huge performance gain for various of K values, which are related to different maritime environments, and the rolling effects of wave.

Improvement on an inertial-Doppler navigation system of underwater vehicles using a complementary range sonar

This work presents an improvement of an inertial-Doppler underwater navigation system for unmanned underwater vehicles (UUV) using a complementary range sonar. The inertial-Doppler navigation system is generally based on an inertial measurement unit (IMU) and a Doppler velocity log (DVL) accompanying a magnetic compass and a depth sensor. Although conventional navigation systems update bias errors of the inertial sensors, scale effects of the DVL, the estimated position slowly drifts as time passes. This paper proposes a measurement model with additional range sonar to improve the performance of the IMU-DVL navigation system for underwater vehicles. The proposed navigation model based on inertial navigation systems includes the scale effects, the bias errors of the DVL, and the error model of the range sonar, where the order of the system states is 21. An extended Kalman filter was adopted to propagate the error covariance, updated the measurement errors and correct the state equation when the external measurements are available. Simulation conducted with the 6-d.o.f. equations of motion of an AUV in lawn-mowing survey mode illustrates the effectiveness of the IMU-DVL navigation system assisted by the additional range measurement.

A model estimation and multi-variable control of an unmanned surface vehicle with two fixed thrusters

This paper presents a dynamic model estimation using system identification (SI) technique and a multivariable controller design for an unmanned surface vehicle (USV) with two fixed thrusters. The catamaran shaped USV has been developed for marine research and surveying exploration in costal area. To validate the automatic control performance of USV, which is designed by classical PID controller, we carried out experiments to keep the USVs position at a fixed point and to track predefined positions. As a result, we have found that it needs time-consuming efforts to tuning the weight between heading and speed controller since the yawing and surge motions are tightly coupled to the two thrusters. In order to solve the problem, it is necessary to introduce the multivariable controller design method. And a numerical dynamic model is required for the model based design. This paper addresses the estimation of a dynamic model of the USV based on the experimental results and the design of Linear-Quadratic (LQ) controller based on a multivariable control method. To verify the efficiency of the designed controller using the estimated dynamic model, numerical simulations were carried out.

Time-varying Underwater Acoustic Channel Modeling for Moving Platform

This paper is concerned with modeling the time-varying underwater acoustic channel especially when its time variability is induced by transmitter and/or receiver motion. The relative motion between transmitter and receiver creates a Doppler shift of the received signal and introduces a time-scale variation to the channel which is characterized by Doppler spread and channel coherence time. In order to simulate the physically probable time-varying channel, we suggest a channel model which is based on the ray theory. It exploits the eigen-ray information of ray tracing to determine the discrete signal transmission paths and adds them to random diffusive multipaths whose amplitude and phase are modeled by Rayleigh and uniform distribution, respectively. If the carrier frequency is quite large, it satisfies the uncorrelated scattering assumption and gives the scattering function which shows how the signal is distributed over range and frequency. We apply the suggested model to a case of an underwater platform moving in the shallow water and present its result.

Navigation and Control System of a Deep-sea Unmanned Underwater Vehicle 'HEMIRE'

This paper presents a hybrid underwater navigation and control system for positioning, guidance and control of a deep-sea unmanned underwater vehicle (UUV), HEMIRE. For precise navigation of the UUV, the hybrid navigation system is designed based on strap-down IMU (inertial measurement unit) accompanying with USBL (ultra-short base line), DVL (Doppler velocity log), range sonar, depth and heading sensors. Initial localization and position reference of the UUV are performed with the USBL when the vehicles are in stationary condition. This paper also presents the characteristics of the UUV and the system constitution of the surface control unit. HEMIRE is equipped with two hydraulic manipulators, ORION, which are remotely controlled at the surface vessel via fiber optic communication. An operator can control the manipulators with a workspace-controlled master arm as well as a parallel-type master arm. This paper describes the task-oriented control of the tele-operated robotic arms mounted on HEMIRE and its application to task-oriented joint configurations.

Preliminary study on the inertial-Doppler localization of a deep-sea launcher hanging on a cable

This paper presents a localization method of a deep-sea launcher with a hybrid underwater navigation system. The under water navigation system mainly composed of an inertial measurement unit (IMU) and a Doppler velocity log (DVL). The implementation of DVL can improve the navigational performance of the IMU when an underwater vehicle works near sea bottom. A dynamic error model of a DVL-aided inertial navigation system is designed to implement an indirect feedback Kalman filter, and a measurement model is also designed. This paper demonstrates the enhanced performance of the DVL-aided hybrid navigation by conducting rotating arm test in ocean engineering basin at KRISO. The IMU and the DVL are embedded in a small fish attached at the rotating arm. Additional depth sensor and a magnetic compass are introduced in the measurement model. In addition, this paper also conducts a dead-reckoning navigation using the DVL and the magnetic compass. The dead-reckoning navigation system also shows better performance than the IMU-only navigation system.