Simulation and Visualisation of Docking
This report describes the work performed on subtask 5.2 of the
EURODOCKER project. The objective of this task was to develop a
simulation tool capable of simulating docking operations involving
Maridan's underwater vehicle, Martin, and the docking station
designed for this project by S3I and built by IW. To achieve an
accurate imitation of Martin's behaviour an advanced hydrodynamic
model was obtained from Maridan and incorporated into the simulator.
Similarly, to achieve a truthful appearance of the vehicle and the
docking station, accurate CAD drawings were adapted to the simulator.
Throughout the EURODOCKER project, and not least in this subtask,
a high degree of modularity is pursued. The simulator is designed
in this spirit, allowing easy incorporation of new visual or
dynamical models of the vehicle, the docking station and the
environment. By using dedicated and widespread tools for both
simulation and visualisation, modelling of the simulated entities
is greatly facilitated. The result is a generic and modular
simulation and visualisation environment that can easily be
adapted to new scenarios involving different entities and
surroundings.
The dynamical modelling is performed in Simulink for Matlab, which is
probably the most common tool for modelling and simulating physical
systems. This tool allows models to be expressed exactly as the
programmer desires, whether this is discrete, continuous, in the
time domain or in the frequency domain. The modular structure of
Matlab further makes it easy to separate the physical model from
the simulation management and thus to easily modify or replace the
core of the model. As it is possible to communicate over a network
(or the local computer) from Simulink using sockets, the model can
be run in parallel and communicate with other programs on any other
platform supporting sockets.
The selected visualisation tool, VRML, provides a similar modularity
and generality. Firstly, tools exist to convert almost any CAD format
to VRML, enabling geometric models to be easily imported from whatever
format they already exist in without having to redraw these. And, equally
important, VRML 3D browsers can be installed as free plug-ins within
common free 2D browsers, such as Netscape or Internet Explorer.
Furthermore, as VRML can exchange messages with external programs written
in Java, scenes can be enhanced with Java's powerful computational and
network facilities. For instance, positional data can be received via
sockets either from the web or from other programs on the local computer,
interpreted and treated in Java and then finally routed to the VRML scene.
All communication between different blocks in the simulation is performed
according to the IEEE standard for Distributed Interactive Simulation, DIS.
This standard is written for military simulations involving large number of
entities distributed over a network. DIS defines standardised protocols to
communicate simulation parameters and entity positions with a minimum of
overhead. By complying with this standard, compatibility with other
researchers in the fields of computer simulation and autonomous underwater
vehicles is achieved.