Submersibles

Software for Autonomous Underwater Vehicles (AUVs) is usually complex. It involves several
complicated tasks such as controller, path planner, map builder, and sensor processor.
Distributed processing is unavoidable for such a complex software system with real-time response
requirements. Once these processes are distributed over several computers, it is essential
that they have some mechanism to communicate to each other to share information.
Each process might communicate with several of the other processes. Interprocess communication
becomes an important issue. This thesis discusses the design and implementation
of a generic distributed toolkit that facilitates the development of distributed software for
AUVs. This toolkit makes the details ofinterprocess communication transparent to the programmers
involved in the AUV software development. The toolkit provides efficient direct
communication between processes and does not impose any constraints on the architecture
of the distributed software. Conventional techniques for monitoring/debugging of sequential programs are not applicable
for distributed processes. This is because the correctness of a real-time distributed program
is determined not only by its "logical" correctness but also its "timing" correctness.
Monitoring mainly comprises of observation of the system during its operation. Debugging
is mostly a graphical "post-mortem" analyses of the dumps generated by the distributed program.
This thesis also presents the design of a window based generic graphical monitoring/
debugging tool.

An experimental investigation of an oscillating hydrofoil propulsion system working behind a model of a small submersible was conducted. Tests were carried out both for a single foil and for systems of two foils. The tests with two foils considered various possible interactions between the foils. In order to understand the influence of trailing vortices of the foils on efficiency of the propulsion system, the two foils were tested for three different combinations of their relative wing spans. All of the above tests were carried out for two types of foil motion, one in which the pitch distribution was prescribed and the other in which the pitch depended on the motion characteristics.

This thesis presents the design and implementation of an underwater network communication protocol. The goal is to enable several autonomous underwater vehicles (AUVs) to form a communication network and to exchange information during at-sea missions. The focus of this work is on the upper layers of the protocol: Network and Transport layers. Routing is a critical issue since all the nodes forming the network are moving. A study and comparison of existing routing algorithms is presented. Two routing algorithms have been chosen and implemented in the network layer of the protocol: Flooding and Destination Sequence Distance Vector Routing. The protocol has been tested on several types of simulated missions. An analysis of the results is proposed for each mission.

Case-based reasoning (CBR) is a powerful reasoning paradigm for many application domains like planning, diagnosis, classification, and decision making. Recognizing solutions of past instances which are similar to the problem in hand is the central concept of CBR. Accordingly, the main research issues in CBR are efficient indexing, retrieval, and evaluation of cases. Generalization of indices has been a major concern as it directly influences the size of casebases and the ability to recognize the right candidate cases. This dissertation work presents a novel indexing scheme--using fuzzy sets to represent case indices and fuzzy aggregation operators to evaluate case matches. The proposed scheme, REFIC (REasoning from Fuzzy Indexed Cases), provides a flexible and transparent scheme to generalize case indices leading to smaller casebases. A hierarchical aggregation of different index matches is suggested for case evaluation. Also, for continuous variable domains, it is proposed to combine the solutions of a small subset of best matching cases as opposed to the conventional approach of selecting and modifying a single best one. These schemes are demonstrated by implementing a case-based navigation planner for autonomous underwater vehicles (AUVs). This navigation planner comprises of an annotated map database, a case-based path planner, and a hybrid fuzzy-CBR based reactive navigation module. The annotated map database provides a general framework for modeling the navigational environment. Annotations attached to objects and geometrical query handling are two main features of this database. Using this system as a spatial casebase, an off-line path planning system for AUV missions is designed. The obstacle avoidance module employs CBR to dynamically select promising directions of movement and to activate a subset of navigational behaviors. This reactive navigation scheme has been found to be very robust under noisy sensor data and complex obstacle distribution patterns.

In this dissertation, the digital signal processing techniques required for a 3-D sonar imaging system are examined. The achievable performance of the generated images is investigated by using a combination of theoretical analysis, computer simulation and field experiments. The system consists of a forward looking sonar, with separate projector and receiver. The projector is a line source with an 80 degrees by 1.2 degree beam pattern, which is electronically scanned within a 150 degree sector. The receiver is a multi element line array, where each transducer element has a directivity pattern that covers the full sector of view, that is 150 degrees by 80 degrees. The purpose of this sonar system is to produce three dimensional (3-D) images which display the underwater topography within the sector of view up to a range of 200 meters. The principle of operation of the proposed 3-D imaging system differs from other commonly used systems in that it is not based on the intensity of backscatter. The geometries of the targets are obtained from the delay and direction information that can be extracted from the signal backscatter. The acquired data is further processed using an approach based on sequential Fourier transforms to build the 3-D images. With careful selection of the system parameters, the generated images have sufficient quality to be used for AUV tasks such as obstacle avoidance, navigation and object classification. An approach based on a sophisticated two dimensional (2-D) autoregressive (AR) model is explored to further improve the resolution and generate images with higher quality. The real time processing requirements for image generation are evaluated, with the use of dedicated Digital Signal Processing (DSP) chips. A pipeline processing model is analyzed and developed on a selected system.