{mosgoogle}Generally experiments in a test tank are carried out in advance to get the dynamic characteristic of underwater robots, and various motion control laws are evaluated based on obtained dynamics of the robot. In this study, we made a small and lightweight AUV as a testbed for control performance evaluation. The AUV has two modes, “cruising mode” in which the motion is controlled by lift force of the main wings, and “hovering mode” controls the heave motion by rotating both thrusters to vertical direction. Mounted instruments are limited in necessary minimum quantity aiming at the smaller AUV. In the AUV, two servo motors and two thrusters are mounted for motion control. The servo motors control the angles of thrusters and main wings, where the two thrusters are attached.{mosgoogle}

 
 

I. INTRODUCTION
Underwater operations are very difficult activities for human beings. Particularly, works in the extreme environment such as hydrothermal vent, underwater volcano, active fault on seafloor, maintenance of underwater structures and biota below ice bergs in the polar zone, are very dangerous operations. Underwater robots are expected as new tools to work in the extreme environment. For the operations in the sea, unpredictable environment, it is important to plan the robots mission closely in advance by considering the maneuverability of the target robot, robot dynamics, control laws, sensing ability and so on.
In this paper, a glider-tyep small AUV named, "SeaBird", is developed as a testbed vehicle for evaluation of glider-type AUV and also to join an underwater robot competition "Underwater Robot Festival 2006" held in Kobe University. The concept of the robot is (i) small enough to handle the robot with one person, (ii) whose operation environment is limited to pools or test tanks, (iii) two operation modes, cruising and hovering can be controlled using two thrusters and two wings. In the following sections, the details of the robot and experimental results are described.{mosgoogle}

 

II. DEVELOPMENT OF “SeaBird”
A. Overview of “SeaBird”
“SeaBird” is designed to be small and lightweight enough to be capable of carring out experiments with one person. In order to develop the robot with inexpensive cost, two side thrusters are made using a maxson DC geared-motor, an acrylic pipe, ABS resins, O-rings for water proof. The each wing with a thruster can be rotated using a DC servo motor for hobby use. SeaBird can control  four motions, heaving, surging, yawing and rolling changing two thrusters power and wing angles. One feature of the underwater robot is that the direction of thrusters can be changed together with wings. The advantage of this structure is that the robot has two motion mode, cruising mode and hovering mode, and the disadvantage is
complexity of control.

 

The robot is designed with a 3D CAD software “Autodesk Inventor”. The specifications of SeaBird are shown in Table 1. The overall length is 450 [mm], the width is 710 [mm] and the weight is 5.0 [kg]. The control circuits are made based on PIC and dsPIC from Microchip to fit the inside of the acylic pipe body. As the sensory system, a pressure sensor to measure the depth, a magnet compass sensor for heading angle and a CMOS camera are installed. The maximum design depth is 10 [m]. Figure 1
shows the overview of SeaBird.

 

B. Design of “SeaBird”
1) Calculation of maximum depth of cylinder housings The maximum depth of “SeaBird” is examined based on following equations [1].

 

2) Design of hull
{mosgoogle}Electric equipments such as micro computers, sensors, circuit and battery, etc. are packed inside of the hull. The configuration of hull is shown in Fig.2. The sensors and electric control board is attached to the ABS mounter. One wall side has 6 waterproof connectors for electrical communication with the outside as shown in Fig. 3. The circuit board, the battery, and sensors are fixed to this side of lid part. Therefore, all contents can be taken out easily from the hull by opening the wall of connector side.
The waterproof is realized by putting the O-ring in the lid parts and the joint parts of the cylinder.

 

Fig. 1 Overview of developed robot “SeaBird”

 

(3) Design of thruster
Figure 4 shows the developed thruster. The thruster is mounted on a cylinder that contains a 6.5[W] DC motor. In order to realize water proof structure, two O-rings are attached arount the shaft, which is connected outside propeller directly. The structure of the thurster is shown in Fig. 5. The propeller to get the impellent is 4-bladed made by brass and its size is  60[mm].
The DOF of robot depends on the number of thruster. “SeaBird” has only two thrusters because variable thruster mechanism changes the direction of thrusters. “SeaBird” has two types of propellers whose blades consist of forward and backward phase pitch angle (Fig. 6). One propeller has left rotation blade, and another has right rotation. If the propeller with the same phase blade in the thruster is used for surge motion, the robot is forced rolling because of the propeller rotates in the same direction.

 

Therefore, the robot does not advance straight even if the propeller is rotated same speed. Left and right propellers cancel the moment by pitch angle in opposite phase to prevent this problem.

 

(4) Design of the variable thruster mechanism

Figure 7 shows the variable thruster mechanism. The variable thruster mechanism consists of a DC servomotor, a case made from the ABS resin, and a shaft made from ABS resin. The servomotor is covered with the upper and lower two cases as shown in Fig. 7, and the cases are attached to the robot. The shaft to manipulate the wing is fixed to the servomotor. The thruster is installed to the mechanism.

The thruster is rotated by the mechanism. The water proof of mechanism is realized with the O-rings. Two O-rings are used in the shaft part as well as the thruster.

(5) Design of circuit board
The mounted electronic circuit board is developed using a circuit design CAD tool, EAGLE. Figure 8 shows the developed circuit board. The circuit board has micro computers such as a PIC18F8720, a dsPIC30F5011, motor drivers, optical couplers, a DC/DC converter, three three-terminal regulator, sensors, a camera and so on.

 

The sensory system consists of a pressure sensor for depth measurement, a magnet compass sensor for heading angle and two encoders thruster rotation speed. The power switch is made using a magnetic reed switch for turning on power from the outside. Figure 9 shows the system configuration. The PIC18F8720 is the main CPU for control system and it sends the control signals to every elements. The PIC18F8720 communicate with a PC by RS232 serial
communication.

The dsPIC30F5011 is a sub CPU to control the camera (CMOS-EYE). The dsPIC30F5011 communicates with the PI18F8720 by I2C communication protocol. In order to remove the influence of the noise from the motor, the PIC18F8720 and the motor are insulated with optical couplers. SeaBird” has the Ni-MH battery that has 14.4[V], 3300[mAh] as the power supply. The proper voltage of each elements are the heading sensor, encoders, the PIC18F8720, dsPIC30F5011, optical couplers, motor drivers are 5V, the servomotors are 8V, and the pressure sensor is 12V. Therefore, the power supply voltage is lowered to appropriate voltage using the DC/DC converter.

The DC/DC converter is insulation type for insulating the PIC and each motor. As the servomotor needs the electrical current that exceeds the allowance of the DC/DC converter, the power supply is made by the parallel-connected three-terminal regulators.  The power supply for the DC motor of the thruster is supplied 14.4[V] from the battery.(6) Design of wing SeaBird” has two wings for the improvement of the stability of the robot, and control performance on both ends. The NACA0012, which is the symmetry wing, is adopted [5].

 

The airfoil is rotated by the servomotor, the wingspan is calculated so that the torque necessary for rotation must not exceed the maximum torque of the servomotor [6]. The moment of the wing is calculated with the total of the moment of rotating wing and the moment of wing in the uniform stream.

 

III EXPERIMENTs
A. The measurement of impellent
The measurement of the impellent about two developed thrusters is carried out to obtain the caliburation curve. The experiment is to measure the thrust force when the duty ratio of PWM signal is changed every 50 counts from 0 to 1024 counts. The thruster is attached to a 6 axis force sensor which is fixed to the experimental implement.

The impellent is measured 5 times, and the mean values are assumed to be observation values. Figures 11 and 12 show the experimental results. The horizontal axis of both figures is the input voltage changed from the PWM signal. The thruster where power in direction of retreat when the rotation of the motor installed in the experimental implement is positive is “Thruster 1”, and another is “Thruster 2”. Both thrusters show the characteristic that the force for forward direction is almost twice larger that the backward force. Each result of the measurement shows the rate coefficient of the approximate line of the forward on
the result of thruster 1 is 4.17E-2, the backward is 2.13E-2.

 

On the other hand, the result of the thruster 2 is the rate coefficient of the forward is 4.07E-2, the backward is 2.18E-2. The result shows thruster 1 and thruster 2 have the similar tendency mentioned above.

 

 

IV CONCLUSIONS
In this research, we developed a glider type small AUV as a testbed for characteristic evaluation. “SeaBird” has necessary equipment such as the battery, sensors, etc. At the present stage, the posture is controlled by the control of rotational speed of the thruster and the control of angle of the wing. Future tasks are to improve the autonomy of the robot. The robot obtains pressure and heading data to control depth and traveling direction. The PID controller will be implemented on the robot by these data are fed back.{mosgoogle}

 

And the mechanism that moves the center of gravity of the robot will be newly produced and mounted. The weight for the buoyancy adjustment is moved back and forth with the new mechanism. As a result, the center of gravity moves. This mechanism is mounted on "SeaBird", and control pitching of the robot.

REFERENCES
[1] T. Ura, S. Takagawa “The pandect of underwater robots” Seisando bibliopole, pp. 152-153, 1994
[2] Department of Oceanography at Tokai University “From space to the abyssal floor, Illustration oceanic outline” Kodan

     company scientific, 1998, pp.122-125
[3] S. Ohata “Development of mosaic processing system by the small AUV” KIT, Master's thesis, pp.30-32
[4] T. Fjii “Research on intellectual action of AUV” Tokyo University, 1993, pp.1-6
[5] http://www.pagendarm.de/trapp/programming/java/profiles/NACA4.html
[6] K. Kudo, T. Hasegawa, M. Sirakasi “Mechanics of fluids” CORONA company, 1994, p.203-207

 

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