Son-Cheol Yu

Time-Delay Controller Design for Position Control of Autonomous Underwater Vehicle Under Disturbances

This paper presents an enhanced time-delay controller (TDC) for the position control of an autonomous underwater vehicle (AUV) under disturbances. A conventional TDC performs well when the involved data acquisition rate is fast. However, in AUV control applications that use a Doppler velocity log (DVL) navigation system, we cannot keep the data acquisition rate sufficiently fast because a DVL sensor generally supplies data at a slow acquisition rate, which degrades the performance of the TDC. To overcome this problem, we propose an integral sliding-mode controller to be supplemented to the conventional TDC to improve the control precision even if the DVL navigation system is in operation. The proposed controller is computationally simple and robust to unmodeled dynamics and disturbances. We performed computer simulations and experiments with the Cyclops AUV to demonstrate the validity of the proposed controller.

Integral sliding mode controller for precise manoeuvring of autonomous underwater vehicle in the presence of unknown environmental disturbances

We propose an integral sliding mode controller (ISMC) to stabilse an autonomous underwater vehicle (AUV) which is subject to modelling errors and often suffers from unknown environmental disturbances. The ISMC is effective in compensating for the uncertainties in the hydrodynamic and hydrostatic parameters of the vehicle and rejecting the unpredictable disturbance effects due to ocean waves, tides and currents. The ISMC is comprised of an equivalent controller and a switching controller to suppress the parameter uncertainties and external disturbances, and its closed-loop system is exponentially stable. Numerical simulations were performed to validate the proposed control approach, and experimental tests using Cyclops AUV were carried out to demonstrate its practical feasibility.

Development of mooring-less robotic buoy system using wave powered renewable energy

For long-term operation, a wave-energy harvesting robotic buoy system that holds position automatically without anchoring line is proposed. We made a prototypes which consists of the propulsion system and power generation system. We adopted the Wave glider platform to hold the buoy position without mooring, and developed a flip-type self-rectifying wave turbine system (WTS). We developed modeling equations to describe the proposed system and simulated them using numerical software, then tested their station-keeping ability and energy-harvesting power in wave tank. Developed prototype patrolled within a 3-m diameter area, and the WTS generated 10.1 W of power. The station-keeping area was smaller than that of the existing mooring buoy, which means that the robotic buoy system can obtain precise and accurate observation data in the ocean without additional costs for mooring systems, and its wave-harvested energy can provide sufficient electric power to maintain the whole system.

A study of magnetic fluid seals for underwater robotic vehicles

Underwater robotic vehicles (URVs) normally use mechanical seals with o-rings, rubber boots, PTFE or Grafoil wedges, or V-rings. Because mechanical seals can cause an unexpected malfunction, we try to adapt magnetic fluid seal (MFS) for underwater robotic vehicles. For reliable design of the MFS, we use numerical simulation of the flow in the MFS using finite element method. The results obtained from experimental investigations of the operation of the MFS contacting with pressurized water will be compared to numerical simulation results

The development of multi-functional autonomous amphibious vehicle

This paper addresses multi-purpose autonomous amphibious vehicle. It could operate 3 terrains; land-sea-air, a 3-in 1 amphibious vehicle. Usually the vehicle runs as AUV in underwater. It has tracks to run on-land and seabed and works as AGV, ACV. On the pressure hull, there is a waterproof cargo with self-sealing hatch. This enables agent vehicles launch such as UAV, on seabed. This study focuses on the feasibility the proposed vehicle. Experiments were carried out to demonstrate the feasibility.

A robust loop-closure method for visual SLAM in unstructured seafloor environments

This paper addresses the problem of visual simultaneous localization and mapping (SLAM) in an unstructured seabed environment that can be applied to an unmanned underwater vehicle equipped with a single monocular camera as the main measurement sensor. Monocular vision is regarded as an efficient sensing option in the context of SLAM, however it poses a variety of challenges when the relative motion is determined by matching a pair of images in the case of in-water visual SLAM. Among the various challenges, this research focuses on the problem of loop-closure which is one of the most important issues in SLAM. This study proposes a robust loop-closure algorithm in order to improve operational performance in terms of both navigation and mapping by efficiently reconstructing image matching constraints. To demonstrate and evaluate the effectiveness of the proposed loop-closure method, experimental datasets obtained in underwater environments are used, and the validity of the algorithm is confirmed by a series of comparative results.

Development of passive acoustic landmark using imaging sonar for AUV's localization

We proposed and tested a passive acoustic landmark that can provide accurate navigation information for an autonomous underwater vehicle (AUV). The AUV used imaging sonar mounted on the vehicle to recognize shadows of the columntype landmark, and can measure vehicles own pose and relative position from the landmark which was set in a known position. The results of the proposed method had acceptable error (<;1 %) in the calculated position of the AUV. The proposed localization method will be useful to correct drift errors of inertial navigation systems for the AUV or to find the same position repeatedly.

Development of energy-harvesting unit and propulsion unit for a robotic buoy system

In this paper, we described about development of a robotic buoy for Internet of maritime thins (IoMT) technology. For IoMT, stand-alone data conllector is a key technology. The energy harvesting robotic buoy is proposed in the context. The proposed system is composed of three components: propulsion system, energy harvesting system, and buoyancy control system. The proposed system can hold its position within an uniform bound without mooring, and harvests wave energy. As a preliminary study, we developed hydrodynamic model for the whole system, and conducted simulation. Through the simulation, we implemented station keeping algorithm, and investigated optimal design parameters. To verify the simulation results we constructed prototype and carried out a series of experiments: station-keeping test, and power generation test. As a result, the whole system was proved that the propulsion system has sufficient manoeuvrability for station keeping within 3 m, and peak power generation was 8 W.

Imaging sonar based real-time underwater object detection utilizing AdaBoost method

We propose a real-time underwater object detection algorithm using forward-looking imaging sonar. Considering the characteristics of sonar image, the Haar-like feature is used to construct each weak classifier. We construct a strong classifier by combining several weak classifiers. An adaptive Boosting (AdaBoost) algorithm is utilized to determine coefficients of each weak classifier and weights of training dataset. Moreover, we improve the efficiency of calculation using a cascade structure. To verify our method, we use the field data obtained by hovering-type AUV “Cyclops”. From this data, we create a training dataset and conduct the learning process of detector. The experiment results show the accuracy and tolerance of the object detector made by the proposed approach.

Robust Sonar-Based Underwater Object Recognition Against Angle-of-View Variation

Underwater object recognition based on deployed mobile nodes (underwater vehicles) is difficult, because the shape of an object in a sonar image is significantly changed depending on the direction from which the object is approached. The approaching directions of deployed mobile nodes cannot be predicted in advance. To solve the problem, the conventional underwater recognition algorithms use realistic template images generated by sonar image simulators, and compare the template images to the actual sonar image. However, the realistic sonar image simulation is computationally complex, and the comparison requires some preprocessing methods, such as image segmentation. To solve the problem, this paper proposes a sonar image simulator-based underwater object recognition algorithm. The proposed algorithms that is motivated by the generation mechanism of a sonar image can directly compare the actual sonar image and the simulation image generated by a simple sonar image simulator.