Depth Control Methods of Variable Buoyancy AUV
Andrei V. Medvedev, Vladimir V. Kostenko, Anton Yu. Tolstonogov
Institute of Marine Technology Problems FEB RAS, Vladivostok, 690950, Russia
(Tel : +7-950-282-5156; E-mail: tolstonogov.anton@gmail.com)
Abstract—Autonomous Underwater Vehicles (AUVs) are
widespread and effective tool for oceanographic research. The
main applications of AUVs are long-term missions for survey
of wide area and patrolling. However the key issue for long
term cruising range is energy storage. One of approaches to
reduce energy consumption is using of the variable buoyancy
system (VBS) on the AUV. This approach allows to compensate
AUV’s buoyancy and to reduce energy on hovering due to it.
Two combined depth control methods based on cooperative work
of propulsion system and variable buoyancy system has been
presented in this paper. Estimation of the methods based on
calculation of energy consumption and transient response time
has been made. Comparison of ones with methods based on
standalone work of propulsion system and VBS has been made.
Provided methods was proved on simulation and water pool
tests on variable buoyancy AUV “CH-2” designed at Institute
of Marine Technology Problems FEB RAS year ago.
Keywords—AUV, depth control, variable buoyancy system,
propulsion system, hovering.
1. I NTRODUCTION
Autonomous Underwater Vehicles (AUVs) are most efficient tools for echolocation and photo imaging of the seafloor
on purpose to wrecks finding, as well as geological survey
and mapping. Also AUVs play an important role in ecological
environment monitoring [1]. And the key issue for AUV long
range is energy storage. Thus reduce power consumption of
vehicle motion is expedient.
The AUV’s buoyancy has a significant influence on motion
of an AUV and its power consumption. Buoyancy deviation
of the estimated values is caused by pressure variation from
one hand and variation of water density, depending on its
temperature and salinity from other hand [2], and also from
depth of submergence.
An effective way to reduce energy consumption while
moving is equipping an AUV with variable buoyancy system
(VBS). This system is widespread in gliders [4], [5] and
autonomous buoys [3].
The main type of AUV motion during echo graphic and
photo imaging tasks and ecological observation is depth or
altitude control. Meanwhile a scenario of power effective and
silent depth control have special interest. Impact of AUV’s
buoyancy change on its moving and depth control by VBS
are described in detail in the book [6] and papers [7], [8], [9].
Presented papers described in detail depth control by VBS of
AUV without propulsion system.
The object of study of this paper is analyzing of transition
process with different methods of depth control for AUV
equipped with both VBS and propulsion system.
Fig. 1. AUV “CH-2” equipped by variable buoyancy system.
The remainder of this paper is organized as follows: section
2 outlines the system specification and design of VBS. Section
3 considers description of used methods for depth control and
description of computational MATLAB model. Finally, section
4 shows result of trial run in water pool with different methods
for depth control.
The proposed algorithms were implemented and tested
on the variable buoyancy AUV “CH-2”. This vehicle was
designed and implemented at Institute of Marine Technology
Problems FEB RAS (Russia, Vladivostok city) and equipped
with variable buoyancy system. This system is composed of
two identical modules with the oil bladder in bow and stern
of the vehicle. Volume of each oil bladder can be changed
independently. This provides variable of buoyancy of the
vehicle and variable static pitch in same way. Also “CH-2”
is equipped by propulsion system, which one consists of two
horizontal and two vertical thrusters. Such type of propulsion
system provides control of 5 DOF.
2. V EHICLE D ESCRIPTION
AUV “CH-2” (fig. 1) has beed designed at Institute of
Far Eastern Branch of the Russian Academy of Sciences last
year and tested during previous summer season. The vehicle
equipped with built-in closed circuit VBS. Table 1 presents
the main technical specifications of the vehicle.
VBS consists of two identical modules in a bow and stern
side of the vehicle (fig. 2). Each module contains a solid oil
tank with hydraulic fluid, an oil bladder and a hydraulic pump
(3). The displacement volume of the bladder can be changed
by pumping oil between the oil tank and the bladder with
the hydraulic pump, so buoyancy of the vehicle can changed
without changing of its weight. Control of variable buoyancy
carried out from the electronic unit and based on rotation
count to buoyancy calibration. An independent work of the
two modules allows an opportunity of control buoyancy of
TABLE 1
T HE MAIN TECHNICAL SPECIFICATIONS OF AUV “CH-2”.
Dry weight, [kg]
87.5
Buoyancy range, [kg]
-2.5. . . 2.5
Length [m]
2.33
Diameter [m]
0.59
Max depth, [m]
150
Autonomy, [hour]
32
Max speed, [m/s]
2.5
Fig. 4. The control loop of the method propulsion.

lb f z + M y
lr f x + M z


 ts =
, tl =
ls + lb
lr + ll
l
f
−
M
l
f

s
z
y
b
x − Mz

 tb =
, tr =
ls + lb
lr + ll
Fig. 2. Schematic view of the AUV “CH-2”.
TABLE 2
T HE MAIN SPECIFICATIONS OF VBS.
Volume per pump revolution, 10−6 m3
1.8
Max pump speed, 10−3 m3 /min
4.25
Max volume of bladder, 10−3 m3
2.5
Where M ∈ R4x4 is the thruster allocation matrix, f~ - the
generalized force vector, ~t - the thrust vector.
Where ts , tb respectively thrust of stern and bow vertical
thruster, ls , lb respectively theirs shoulder, tl , tr - respectively
thrust of left and right horizontal thruster, ll , lr respectively
theirs shoulder. We used a right-handed coordinate system
here, or, more specifically, BODY system [11]. fx force along
the longitudinal axis the vehicle, fz force along the normal
axis, My moment around the transversal axis, Mz moment
around the normal axis.
3. M ETHOD
3.1. Methods for Depth Control
Fig. 3. Principle of work of VBS module.
vehicle and its static pitch in came way [10]. Table 2 shows
the main specifications of VBS.
Also the vehicle is equipped with a propulsion system. It
consists of two horizontal thrusters and two vertical thrusters
(2). Each of these thrusters and are controlled using CAN bus
and have separate TCU. Same TCU are used for control of
the pumps. Four thrusters provide motion in heave and surge
axis and rotation around heave and sway. Vertical thrusters
provide total thrust in range from -84 N to 70 N. Horizontal
thrusters provide thrust in range from -86 N to 106 N. The
solution for a control allocation problem (1) for such thruster‘s
configuration can be found in the one way (2).
f~ = M ~t
New combined depth control method based on cooperative
work of propulsion system and buoyancy variable system has
been presented. Provided methods was proved on simulation
and water pool tests on variable buoyancy AUV “CH-2”
designed at Institute of Marine Technology Problems FEB
RAS year ago. New method was compared with standalone
propulsion and buoyancy variable system method based on
energy efficiency and transient response time.
There are two systems on the vehicle and each one of
them could controls depth of the vehicle: propulsion system
and VBS. It gave the opportunity to test several different
methods for depth control and to compare them correctly. Four
different methods in the article have been tested: propulsion,
buoyancy, combined1 , combined2 . The last two methods
have been presented in this article.
P ropulsion. This is a classic PID controller, output value of
which is used to control of propulsion system. Such approach
is largely universal: it does not require knowledge of current
buoyancy of a vehicle, fast and widespread. The main disadvantages of this method are continuous power consumption
and noisiness in water. The control loop of the method is
showed on figure 4.
Buoyancy. It is a PID controller in same way, but output
value of one is used to control of VBS. Due to low pump
speed this method is slow and energy-intensive. Advantages
Fig. 7. The control loop of the method combined2 .
Fig. 5. The control loop of the method buoyancy.
Fig. 8. The control loop of the method combined2 .
the method is showed on figure 8.
The following PID controller was used for presented methods:

t
P


 F (t) = Kp e(t) + Kd v(t) +
Iq
t=0
(1)

V


Iq = Ki T
aB
Fig. 6. Flowchart of transfer of buoyancy.
of this method are silent in water and energy-efficient after
transition process. The control loop of the method is showed
on figure 5.
Combined1 . This method is a combination of sequential
work of propulsion and buoyancy methods. First step of
the method is stabilizing of depth by propulsion method.
Second step is transferring the integral part of propulsion’s
PID controller to VBS system. The flowchart of buoyancy
transfer algorithm is presented on figure 6. General control
loop of the method is showed on figure 7.
Combined2 . This method is overlapping combination of
propulsion and buoyancy method. The aim of this method
is combination of advantages of each single method: fast
transition process of propulsion method and energy-efficient
of buoyancy method after transition process. Control loop of
Where are:
V = 0, |v(t)| ≥ b B = e(t), |e(t)| ≥ b
,
V = 1, |v(t)| < b
B = c,
|e(t)| < b
(2)
and Kp , Kd , Ki are coefficients for the proportional,
integral, and derivative terms, e(t) is error of value, v(t) is
speed of value, T is integration period, a, b and c are tuning
parameters.
3.2. Simulation System
Preliminary computational modeling of mentioned above
methods in Matlab Simulink have been made. Vertical motion
of vehicle has been assumed only:

(m + madd )v̇z




Fgrav
Fbuoy




F
gdr
= Fgrav + Fthrust − Fbuoy − Fgdr
= mg
= pgV
1
2/3
=
Cz Vz ρvz |vz |
2
(3)
TABLE 3
C OEFFICIENTS OF MODELING OF THE AUV CH-2.
Dry weight, kg
87.5
Drag coefficient along the axis of z
1.35
Addition mass, kg
87.5
Volume of the vehicle, m3 ,
0.0880
Fig. 11. The test result of the method combined1 .
Fig. 9. The test result of the method propulsion.
Fig. 12. The test result of the method combined2 .
TABLE 4
L IST OF WATER TEST RESULT.
Fig. 10. The test result of the method buoyancy.
Where: m is dry weight of vehicle, madd is addition mass
in water, vz is vertical speed, Cz is drag coefficient along the
axis of z, is water density and V is volume of the vehicle.
Coefficients of modeling listed in the table 3.
4. R ESULTS
The methods discussed above have been tested in water
pool. Stabilization of target depth 1.36 m was evaluated for
all discussed methods. Figure of transition process by different
depth control methods have been obtained (figure 9, 10, 11,
12). Evaluations of transition time and consumed energy are
listed in table 4.
5. D ISCUSSION
Four depth control methods were implemented. Two combined methods were presented and full scale testing of each
Name of method
Transition time, s
Consumption energy, 103 J
propulsion
12
0.22
buoyancy
37
4.76
combined1
80
2.58
combined2
30
2.47
discussed above methods was performed. The following conclusions can be drawn based on experimental results:
P ropulsion. This method is fast and low energy coast.
However further stabilization of target depth will be consume
constant nonzero power to compensate positive buoyancy of
the vehicle.
Buoyancy. It is much more slow and energy-intensive
(more in 22 times) than previous, but consumption power of
further stabilization of the vehicle is almost zero.
Combined1 . This method combines advantages of previous
method with good stability in the electrical power plant
network. But total transition time of this method including
buoyancy transfer algorithm is very long (almost in 8 times
compared with time of depth stabilization) and stability of this
method is highly dependent on precise of PID‘s integral part.
Combined2 . It is combines all advantages of combined1 ,
but this method more stabile as one does not require strict
requirements to precise of PID controller, but electrical power
plant network is pretty unstable.
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