The Twin-Burger is a fully autonomous vehicle designed as a versatile test bed for software development. To cover a variety of intelligent behaviors, the vehicle carries a powerful computer system and multiple sensors to get information on its surroundings. It should be noted that the Twin-Burger is not a cruising type vehicle but expected to perform various stationary tasks in experiments. To design the vehicle in small size for convenience of handling, its depth rating and energy duration were determined to be enough for the experiments in a testing tank or shallow water areas.
The Twin-Burger is an open-frame-structured vehicle which measures 1.54 m long and weighs approximately 120 kgf in air. Dimensions and specifications of the vehicle are shown in Table1. General arrangement of the vehicle is illustrated in Fig.2. Most of the onboard instruments including the vehicle computer system, sensors, and other electronic circuits are mounted in twin FRP pressure hulls. A battery cylinder and FRP hulls are attached to the frame getting high separation between the center of gravity and that of buoyancy, which causes good static stability in rolling and pitching
Actuators and Sensors
The vehicle is propelled by four 40 W thrusters in four-degrees-of-freedom control modes. Two main thrusters which generate surging force and yawing moment are located on the side of the body parallel with the longitudinal axis. The others are a vertical thruster and a side thruster located in the middle of the body. They generate heaving and swaying movements, respectively.
Position of the vehicle is calculated by dead reckoning navigation based on the data measured by propeller-type speed sensors, a depth sensor, and a small sensor unit called AHRS (Attitude and Heading Reference System). The vehicle carries multiple sensors for information acquisition from the surroundings, such as eight channels of ultrasonic range finders and a CCD camera with a tilt-pan mechanism on the front cap of the cylinder. With the range finders, the vehicle is able to get relative position to the surroundings and to find obstacles which should be avoided.
The CCD camera allows the vehicle to perform vision based behaviors, such as observation of underwater structures, navigation with landmarks, etc.
For multi-vehicle or diver-vehicle operations, the vehicle has an ultrasonic command link system and a vision-based communication system using EL (Electro-luminescent) panels. Fig.3 shows the concept of the vision-based communication system which is used when the vehicle and a diver are located in a short range. A set of five EL panels displays a command pattern which consists of 4 bits of coded signals and 1 bit of status signal. The command patterns are captured by the CCD camera or human eyes and decoded by computer vision techniques or human recognition. It is expected that the system reduces psychological pressure of a diver because of the visibility of the vehicle's intentions.
In order that the Twin-Burger behaves according to commanded missions, appropriate swimming capability should be given to the vehicle by introducing a motion control methodology. It is difficult to get precise equations of motion of the Twin-Burger from hydrodynamic model tests, such as PMM (Planar Motion Mechanism) tests, because the configuration of the vehicle is relatively complex and expected to be modified frequently, and the nonlinearity should be considered when the vehicle cruises at low speed.
Here, a sliding control method, which is a robust control strategy to model uncertainty, nonlinear dynamics and unknown disturbances, establish for trajectory control of underwater vehicles by Yoerger and Slotine is applied to the Twin-Burger vehicle. Since the nonlinear cross coupling terms which appear in the Twin-Burger's equations of motion can be taken as parameter uncertainties in this method, an independent sliding controllers can be designed for each mode of four-degrees-of-freedom motion of the vehicle, that is, surging, swaying, heaving and yawing.