Ocean Engineering 235 (2021) 109355
3
C. Cheng et al.
Fig. 2.
Autonomous Underwater Vehicle.
Source:
Reproduced from
Vibhute
(
2018
).
on AUV. This makes it challenging to implement path planning methods
on AUV and different from other robot platforms on land or in the air.
The models are based on Fosson’s comprehensive work (
Fossen
,
2011
),
including general two dimensional and three dimensional kinematics
transformation and in-depth derivation of dynamics motion equations.
2.1. Kinematics
Kinematics is based on the geometric method to study the motion
of vehicles, without considering the influence of force and mass. To
describe the kinematic characteristics of AUV, we will apply the inertial
North-East-Down (NED) reference frame and the Body frame.
Fig. 2
describes AUV as a six degree of freedom (DOF) vehicle with two
rotational and translational velocity components in each dimension
(
Fossen
,
2011
). As shown in
Fig. 2
, the Body frame is attached to
the vehicle, and has its axes forward (surge), to starboard (sway) and
towards the keel (heave) (
Wiig
,
2019
). The translation velocities along
the
𝑋
,
𝑌
and
𝑍
directions are expressed as
𝑢
,
𝑣
and
𝑤
respectively,
and the rotation velocities are expressed as
𝑝
,
𝑞
and
𝑟
respectively. The
rotation angles of each axis are expressed as
𝜙
,
𝜃
and
𝜓
respectively,
and
𝐾
,
𝑀
and
𝑁
represent the moment in
𝑋
,
𝑌
and
𝑍
direction
respectively (
Vibhute
,
2018
).
The rotation matrix of 6-DOF AUV is composed of the basic rotation
matrix of
𝑋
,
𝑌
and
𝑍
axes. In the geometric analysis, the basic rotation
matrix of
𝑋
,
𝑌
and
𝑍
axes are:
𝑅
𝑥
(
𝜙
) =
⎡
⎢
⎢
⎣
1
0
0
0
𝑐𝑜𝑠
(
𝜙
)
−
𝑠𝑖𝑛
(
𝜙
)
0
𝑠𝑖𝑛
(
𝜙
)
𝑐𝑜𝑠
(
𝜙
)
⎤
⎥
⎥
⎦
(1)
𝑅
𝑦
(
𝜃
) =
⎡
⎢
⎢
⎣
𝑐𝑜𝑠
(
𝜃
)
0
𝑠𝑖𝑛
(
𝜃
)
0
1
0
−
𝑠𝑖𝑛
(
𝜃
)
0
𝑐𝑜𝑠
(
𝜃
)
⎤
⎥
⎥
⎦
(2)
𝑅
𝑧
(
𝜓
) =
⎡
⎢
⎢
⎣
𝑐𝑜𝑠
(
𝜓
)
−
𝑠𝑖𝑛
(
𝜓
)
0
𝑠𝑖𝑛
(
𝜓
)
𝑐𝑜𝑠
(
𝜓
)
0
0
0
1
⎤
⎥
⎥
⎦
(3)
Combining these principal rotations gives the rotation matrix
𝑅
as:
𝑅
=
𝑅
𝑧
(
𝜓
)
𝑅
𝑦
(
𝜃
)
𝑅
𝑥
(
𝜙
)
(4)
The relationship between the orientation and angular velocity is:
𝐽
=
⎡
⎢
⎢
⎣
1
𝑠𝑖𝑛
(
𝜙
)
𝑡𝑎𝑛
(
𝜃
)
𝑐𝑜𝑠
(
𝜙
)
𝑡𝑎𝑛
(
𝜃
)
0
𝑐𝑜𝑠
(
𝜙
)
−
𝑠𝑖𝑛
(
𝜙
)
0
𝑠𝑖𝑛
(
𝜙
)∕
𝑐𝑜𝑠
(
𝜃
)
𝑐𝑜𝑠
(
𝜙
)∕
𝑐𝑜𝑠
(
𝜃
)
⎤
⎥
⎥
⎦
(5)
Hence, the AUV kinematics in 6-DOF are:
̇𝜂
=
[
𝑅
0
3×3
ℎ𝑒𝑙𝑙𝑜𝑐ℎ𝑒𝑐𝑘𝑡ℎ𝑒𝑐𝑜𝑚𝑚𝑎𝑛𝑑
3×3
𝐽
]
𝜈
(6)
In the formula, the generalized velocity vector and position vector are
respectively represented by the following matrix:
𝜂
= [
𝑋, 𝑌 , 𝑍, 𝜙, 𝜃, 𝜓
]
T
(7)
2.2. Dynamics
Referring to the
Fossen
(
2011
) maneuvering model, the nonlinear
equation of the dynamic motion for AUV has the following general
form:
𝑀
(
̇𝜈
) +
𝐶
(
𝜈
)
𝜈
+
𝐷𝜈
+
g
(
𝜂
) =
𝐵𝑓 ,
(8)
where
𝑀
denotes the inertia matrix,
𝐶
contains Coriolis and centripetal
terms, and
𝐷
indicates the hydrodynamic damping matrix. Vector
𝑔
denotes a combination of gravity and buoyancy, vector
𝑓
is the control
input variable of AUV, which is transformed into the force and moment
of control motion by matrix B.
The control input vector is:
𝑓
= [
𝑇
𝑢
, 𝑇
𝑞
, 𝑇
𝑟
]
T
,
(9)
where,
𝑇
𝑢
is the thrust force produced by rotating the propeller,
𝑇
𝑞
is the
vertical motion control force generated by the left and right rudders,
𝑇
𝑟
is the horizontal motion control force generated by the upper and
lower rudders.
Assuming that the center of gravity (
𝑥
𝑔
, 𝑦
𝑔
, 𝑧
𝑔
) and buoyancy of AUV
are rigid bodies at one point, the 6-DOF translational and rotational
motions will be expressed as follows:
𝑚
[
̇𝑢
−
𝑣𝑟
+
𝑤𝑞
−
𝑥
𝑔
(
𝑞
2
+
𝑟
2
) +
𝑦
𝑔
(
𝑝𝑞
−
̇𝑟
) +
𝑧
𝑔
(
𝑝𝑟
+
̇𝑞
)] =
𝑋
𝑡𝑜𝑡𝑎𝑙
𝑚
[
̇𝑣
−
𝑤𝑝
+
𝑢𝑟
−
𝑦
𝑔
(
𝑟
2
+
𝑝
2
) +
𝑧
𝑔
(
𝑞𝑟
+
̇𝑝
) +
𝑥
𝑔
(
𝑞𝑝
+
̇𝑟
)] =
𝑌
𝑡𝑜𝑡𝑎𝑙
𝑚
[
̇
𝑤
−
𝑢𝑞
+
𝑣𝑝
−
𝑧
𝑔
(
𝑝
2
+
𝑞
2
) +
𝑥
𝑔
(
𝑟𝑞
−
̇𝑞
) +
𝑦
𝑔
(
𝑟𝑝
+
̇𝑝
)] =
𝑍
𝑡𝑜𝑡𝑎𝑙
𝐼
𝑥
̇𝑝
+ (
𝐼
𝑧
−
𝐼
𝑦
)
𝑞𝑟
− (
̇𝑟
+
𝑝𝑞
)
𝐼
𝑥𝑧
+ (
𝑟
2
−
𝑞
2
)
𝐼
𝑦𝑧
+ (
𝑝𝑟
−
̇𝑞
)
𝐼
𝑥𝑦
+
𝑚
[
𝑦
𝑔
(
̇
𝑤
−
𝑢𝑞
+
𝑣𝑝
) −
𝑧
𝑔
(
̇𝑣
−
𝑤𝑝
+
𝑢𝑟
)] =
𝐾
𝑡𝑜𝑡𝑎𝑙
𝐼
𝑦
̇𝑞
+ (
𝐼
𝑥
−
𝐼
𝑧
)
𝑟𝑝
− (
̇𝑝
+
𝑞𝑟
)
𝐼
𝑥𝑦
+ (
𝑝
2
−
𝑟
2
)
𝐼
𝑧𝑥
+ (
𝑞𝑝
−
̇𝑟
)
𝐼
𝑦𝑧
+
𝑚
[
𝑧
𝑔
(
̇𝑢
−
𝑣𝑟
+
𝑤𝑞
) −
𝑥
𝑔
(
̇
𝑤
−
𝑢𝑞
+
𝑣𝑝
)] =
𝑀
𝑡𝑜𝑡𝑎𝑙
𝐼
𝑧
̇𝑟
+ (
𝐼
𝑦
−
𝐼
𝑥
)
𝑝𝑞
− (
̇𝑞
+
𝑟𝑝
)
𝐼
𝑦𝑧
+ (
𝑞
2
−
𝑝
2
)
𝐼
𝑥𝑦
+ (
𝑟𝑝
−
̇𝑝
)
𝐼
𝑧𝑥
+
𝑚
[
𝑥
𝑔
(
̇𝑣
−
𝑤𝑝
+
𝑢𝑟
) −
𝑦
𝑔
(
̇𝑢
−
𝑣𝑟
+
𝑤𝑞
)] =
𝑁
𝑡𝑜𝑡𝑎𝑙
,
where
𝑚
is the mass of the vehicle,
𝐼
is the inertia in the specific axial
or transverse direction, and (
𝑋
𝑡𝑜𝑡𝑎𝑙
, 𝑌
𝑡𝑜𝑡𝑎𝑙
, 𝑍
𝑡𝑜𝑡𝑎𝑙
) and (
𝐾
𝑡𝑜𝑡𝑎𝑙
, 𝑀
𝑡𝑜𝑡𝑎𝑙
, 𝑁
𝑡𝑜𝑡𝑎𝑙
)
are the total external forces on each degree of freedom.
The above modeling of AUV put some unique constraints in its
motion control with path planning methods which makes it different
from other mobile robots. On the one hand, as a highly nonlinear
system, AUV is affected by the change of mass center, buoyancy center
and hydrodynamic coefficient (
Xiang et al.
,
2014
). On the other hand,
AUV has underactuated characteristics, and parts of its speed cannot
be directly controlled, but can only be generated when the propeller
and rudder angle are manipulated. For example, when AUV rotates,
part of the forward speed will be converted into swing speed, and the
heave motion is controlled in a similar way. In addition, the collected
sensory data will affect the performance of path planning methods for
AUV. General mobile robots use the electromagnetic wave generated
by laser sensor to navigate and avoid obstacles
Ye and Borenstein
(
2002
), and some of them use the camera (
Biswas and Veloso
,
2012
)
to obtain images directly to judge the position of obstacles. However,
because AUV works in the marine environment, and it detects the
surrounding obstacles according to the ultrasonic wave generated by
Ocean Engineering 235 (2021) 109355
4
C. Cheng et al.
sonar sensor (
Kimball and Rock
,
2008
). Data collected with sonar
sensors will be interfered by underwater sound waves, which will result
in inaccurate information and low-quality data received and largely
affect the accuracy of obstacle avoidance for AUV. In addition, AUV
path planning also needs to consider the influence of ocean currents.
Moreover, underwater tasks performed by AUVs usually need to follow
some specific path patterns. For example, when AUV completes some
unknown search and detection missions, there is usually an algorithm
to generate a coverage path in the task editor, which requires AUV to
bypass unforeseen obstacles when executing this path mode (
Bagnitckii
et al.
,
2017
). In some situation, multi-AUV also needs to follow a certain
formation mode to improve the efficiency of the mission (
Ding et al.
,
2014
).
Although these make it more challenging to implement general path
planning methods for AUV compared to other mobile robots, many re-
searchers have proposed effective path planning methods to overcome
these challenges. In Sections
3
and
4
, we will review most popular path
planning methods for AUV in terms of global path planning with known
static obstacles and local path planning with unknown and dynamic
obstacles.
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