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12th

IFToMM World Congress, Besançon (France), June18-21, 2007 CK-xxx

Modelling and Analysis of an Autonomous Underwater Vehicle

Via Multibody System Dynamics

Hongwei Zhang* Shuxin Wang

†

Tianjin University Tianjin University

Tianjin, China Tianjin, China

Abstract—Autonomous underwater vehicles (AUVs) have

become an interesting research area because of their emerging

applications in oceanographic survey. The dynamic modelling

and simulation of AUVs are important in the process of design

and analysis of t

he

maneuverability of AUVs. AUVs with

complex attached bodies can be considered as multibody systems

for more accuracy in calculation of dynamic behaviour and

maneuverability. In this paper, dynamic modelling and analysis

of VBS-AUV, which was developed in our lab for landing and

taking off on the sea floor, is performed based on multibody

system dynamics. The forces acted on each body of VBS-AUV

are considered and computed respectively. Hydrodynamic

coefficients used in the dynamic model are estimated or

evaluated with the empirical method, experimental method and

CFD. The comparison for calculating hydrodynamic coefficients

are discussed and simulated in the dynamic analysis of VBS-

AUV. Simulating and experimental results show that multibody

system dynamics will be an efficient tool to describe detailed and

accurate models of AUVs with complex attached bodies.

Keywords: autonomous underwater vehicle (AUV),

multibody system dynamics, hydrodynamic coefficient,

computational fluid dynamics (CFD)

I. Introduction

Autonomous underwater vehicles (AUVs) have become

an intense area of oceanic research because of their

emerging applications in oceanographic survey. In the

process of design and analysis of maneuverability of

AUVs, simulating the movement of AUVs with dynamic

modelling is an indispensable procedure [1], [2]. The

dynamic behaviour of AUVs have been the subject of

considerable interest for many years [3], [4], [5], [6].

However, with the expanding of AUV’s function, the

geometric shape and mechanical construction of AUVs

are becoming more and more complicated. Some of the

AUVs are designed with complex attached bodies for

more complicated oceanographic tasks. Simplified model

considering an AUV with complex attached bodies as one

rigid body will cause inaccuracy in the calculation of

dynamic behaviour and maneuverability. Therefore,

efficient tools are required to describe detailed and

accurate models of AUVs with complex attached bodies.

Multibody system modelling has matured to the point that

*

E-mail : zhwtrue@

†

E-mail : shuxinw@

it is suitable for such applications [7], [8]. An AUV can be

viewed as a multibody system consisting of several rigid

bodies, such as a base body, attached bodies, rudders,

elevators and propeller. Accordingly, the hydrodynamic

coefficients of the whole AUV including both the base

body and attached bodies should be estimated or evaluated

for using in dynamic models. Generally, the

hydrodynamic coefficients can be derived by three

methods, i.e. empirical method, captive model

experimental method and CFD method. The object of this

paper is to describe the dynamic behaviour of an AUV

developed in our lab (named as VBS-AUV) using

multibody system dynamics, and to demonstrate how the

various hydrodynamic coefficients estimated or evaluated

with three methods affect the dynamic behaviour of the

VBS-AUV respectively.

The remainder of this paper is organized as follows. In

Section 2, VBS-AUV is introduced. The dynamic model

of multibody systems for VBS-AUV is presented in

Section 3. The hydrodynamic coefficients evaluation and

simulation are outlined in Section 4. Section 5 concludes

the paper.

II. Description of VBS-AUV

VBS-AUV (Autonomous Underwater Vehicle with

Variant Buoyancy System) is a modular low-cost

autonomous underwater vehicle which can land on the sea

floor and take off. To accomplish the task for

oceanographic survey, six primary subsystems are

required: (1) variant buoyancy system (VBS), (2)

propulsion, (3) power, (4) antenna, (5) navigation sensors,

(6) and oceanographic sensors. Structurally VBS-AUV

can be described as six sections: nose, main cabin, battery

cabin, ballast tanks, ADCP (Acoustic Doppler Current

Profiler) and the tail, as shown in figure 1. VBS-AUV

weights 195 kg in air. And it is 3.2m long with the base

body diameter of 0.3m. The principal parameters are

shown in table 1.

Maximum working depth 120 m

Voyage speed 2m/s (4 knot)

Cruising range 50 km

Navigation system GPS/DVL

Source of power Lithium-ion

TABLE I. Principal charateristics of VBS-AUV

1

12th

IFToMM World Congress, Besançon (France), June18-21, 2007 CK-xxx

For an extended measuring period, two ballast tanks are

designed to be used as landing and bottom-sitting

structures. The capacity of soft landing and bottom-sitting

of AUV will lead to the minimum power consumption by

putting the propulsion system into a sleep mode during

bottom-sitting state. And measuring operations close to

the seafloor will not be influenced by the propulsion

system. Additionally, the ballast tanks will support the

other sections of the AUV. After sitting on the sea bottom,

the inflate valve of VBS-AUV will be active to take in

water to land on the sea floor and get higher weight for

stability. The ballast tanks can be also used as the

unloaded objects of the AUV in an emergency.

III. Multibody Modelling of VBS-AUV

VBS-AUV is structurally composed of several sections,

as shown in figure 1. Considering the functions of the

different sections and the contributions to the dynamic

behavior of the system, VBS-AUV can be described as a

multibody system with six bodies. These bodies are

denoted as B

k

(k=1 to 6) respectively, that is, base body

(B

1

), rudder(B

2

), elevator(B

3

), propeller(B

4

), left and right

ballast tanks(B

5

and B

6

), as shown in figure 2.

E−XYZ

is

the inertial frame and the

O−xyz

is the base body frame.

The lower-numbered body array [8]

of VBS-AUV is

shown in equation (1).

⎧

K

=(1,2,3,4,5,6)

(1)

⎨

L

(

K

)(0,1,1,1,1,1)

=

⎩

main cabin

nose

battery

cabin

tail

ballast tank

ADCP

Fig. 1. General layout of VBS-AUV

G

6

M

2

M

3

Y

S

2

(S

3

)

O

6

T

W

Z

E

c

4

F

T

O

2

(O)

3

S

1

X

F

1

c

2

(c

3

)

F

2

F

3

G

2

(

G

3

)

G

4

M

1

F

S

6

6

c

R

O

1

G1

M

6

G

1

c

y

O

6

R

G6

S

5

F

5

z

B

2

O

5

O

B

4

B

1

R

G 5

M

5

B

5

B

3

x

B

6

G

5

Fig. 2. The multibody modelling of AUV

2