MIT
Department of Ocean Engineering

13.016 Introduction to Geometric Modeling and Computation

Spring 1997


Project Description

An AUV Design and Simulation System
Issued: 20 March 1997
Project final demos and presentation: 8 May 1997
Project final report due: 15 May 1997



Introduction

Throughout the semester, we have been studying methods to mathematically represent curves and surfaces on the computer. We will now use this knowledge to design and analyze an AUV. The project will involve developing a database to maintain the geometry of an AUV, design tools to adjust its shape, analysis tools to predict the vehicle's performance, a simulation module to graphically illustrate the operation of the AUV, and tools to realistically render the AUV through ray-tracing. (top)


Division of project tasks

This project can be divided into the following five major areas:

  1. Functional design of the fairing of the AUV.
  2. Geometric design of appendages.
  3. Calculation of the AUV's performance characteristics.
  4. Simulation.
  5. Visualization.

Each person will be responsible for developing the software in one of the areas listed above and interfacing with the remaining students to allow integration of the various modules. Each task will be divided up into smaller weekly assignments which will come together in the end of the semester to form a complete AUV design, analysis, visualization, and simulation package. (top)


Functional design

The person responsible for the functional design of the AUV will be creating the graphical user interface environment with which the user will design the fairing for the AUV. The tasks to be performed will include parameterizing the AUV, defining the underlying data base, and creating the interface through which designers can use the program.

We will assume that the general shape for the AUV's main body is rotationally symmetric (ie. main body is a surface of revolution). There will be a curved nose, parallel mid-body, and a tapered tail. Under these assumptions, the shape of the AUV can be described in terms of relatively few parameters such as the angle of the front, maximum diameter, length of nose, length of mid-body, length of tail, etc. With a suitable selection of parameters, this person will define the underlying data base to model the AUV, drawing from the geometric representations we have discussed in class. A good solution may involve representation of the nose and tail profiles with cubic Bézier curves.

An additional responsibility will be to provide an interface through which a designer may interact with the program. Tools which may need to be developed should provide the capability to interactively adjust the parameters of the AUV. To complement the interactive modification, a basic automatic optimization scheme should also be developed by this person. Optimization will involve searching for the set of parameters which product the least drag subject to a set of user defined constraints, such as required displacement and center of displacement, cruising velocity, angles at ends, etc.

Work in this area will involve collaborative effort with the students working on the performance calculations (to optimize the design) and the visualization routines (to provide feedback to the designer). (top)


Design of appendages

The AUV will need thrusters and fins to provide the driving force, control, and stability. The design of these parts will be the responsibility of this person. Design tools might allow defining the form of a strut, a thruster duct, and positioning the two relative to each other and on the AUV, and the placement of a faired small motor casing and propeller inside the duct using support fins.

The information about the appendages will need to be communicated to the people in all other groups.(top)


Performance calculations

In order to evaluate a particular design, certain performance characteristics of the AUV must be evaluated. These include computing power curves, drag, buoyancy, center of buoyancy, added masses, added inertias, etc. To perform these calculations, the person developing this part of the project will need to evaluate integral properties of the AUV fairing such as the total surface area, displacement, and frontal projected area. Basic hydrodynamic principles will be used to compute the expected performance of the AUV (as in Problem Set 6). In addition, information about the buoyancy and location of the center of buoyancy are needed for the designer to decide how much equipment can be carried and how it should be distributed within the fairing.

Work in this area will involve collaborative effort with the person working on the functional design capabilities of the package. Information about a design's performance will need to be communicated to the optimization algorithm. (top)


Simulation

In order to provide graphical feedback to the designer about the AUV, simulation tools will need to be developed. These will include rendering the wireframe and shaded surface representations of the AUV fairing, appendages, and bathymetric map of the sea floor, and simulation of an AUV's mission. The user should be provided with the necessary tools to inspect the AUV interactively, permitting the AUV to be viewed from all sides and at different scale factors.

The motion of the AUV could follow a predefined mission path expressed as a uniform cubic (3D space) B-spline curve. The vehicle should follow this path and be aligned with its tangent.

Methods to interrogate the sea floor will also need to be developed. This will allow ``idealized sonar readings'' from the AUV to be displayed, showing the vertical distance to the sea bottom or the distance to an obstacle along some direction aligned with the longitudinal axis of the AUV.

The person performing this part of the project will need to communicate with the person defining the functional design capabilities (to provide a visual feedback as parameters are changed), and the person developing the appendage design module. (top)


Visualization

The final visualization capability which should be provided in the design package is the rendering of a ray-traced image of the AUV. This capability will realistically render the AUV fairing and appendages, allowing designers to create fancy images of the AUV to show to their customers.(top)


A sample AUV and its environment


Bathymetry

 
Figure 1: Sample bathymetry with AUV.

For this project, will use a bathemtric map of the Charles river.

Charles River basin bathymetry

The bathymetric data of the Charles River basin is arranged in a regular rectangular grid of 120 x 52 measurements, at a nominal 20 meter spacing. The points are stored as (X,Y,D) triples, with D (depth) given as a negative value, i.e. relative to (nominal) sea level.

The points are stored in row-major order in the file charles.GRID.

  N M
  X_1,1 Y_1,1 D_1,1
  X_2,1 Y_2,1 D_2,1
   ...
  X_N,1 Y_N,1 D_N,1
  X_1,2 Y_1,2 D_1,2
   ...
  X_N,M Y_N,M D_N,M

Note that a portion of the surrounding shore area (D = 0, shown in gray in the image below) is included in the data set.

(top)


Hermes fairing and appendages

  For an initial design, we will begin with Hermes' geometry. The overall dimensions of this AUV are (length overall) by (maximum width).

 
Figure 2: Scale plan view of Hermes.

We can parameterize the characteristics of Hermes as follows:

This list is by no means exhaustive, but provides a starting point for defining the geometry of the AUV. One might also wish to parameterize the location of the thruster for example.

 
Figure 3: 3 dimensional rendering of Hermes.
(top)


A sample of desired changes of the AUV

We would like to modify the design of the AUV presented in Section 3.2 to perform a new mission which requires greater endurance. To improve the endurance of the vehicle, we need to add more batteries. The addition of space to carry more batteries, however, changes the vehicle's weight, displacement, center of gravity, and center of volume. Ideally, we would like our functional design system to find the optimal shape of the AUV fairing in terms of our parameterization. The optimal design would be the one which:

has the center of gravity under the center of buoyancy ().
(top)