Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

Design of an Underwater Glider Platform for Shallow-Water

Applications

Nur Afande Ali Hussain1, Ting Ming Chung

2, Mohd Rizal Arshad

1,

Rosmiwati Mohd-Mokhtar1, Mohd Zulkifly Abdullah

2

1Underwater Robotics Research Group, School of Electrical and Electronic Engineering

Universiti Sains Malaysia, Engineering Campus

14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia

2School of Mechanical Engineering Universiti Sains Malaysia, Engineering Campus

14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia

Abstract

Underwater gliders are type of autonomous underwater vehicle that glide by controlling their buoyancy and attitude

using internal actuators. By changing the vehicle’s buoyancy intermittently, vertical motion can be achieved.

Characteristic of glider motions include upward and downward in a saw tooth pattern, turning and gliding in a

vertical spiral motion glides without using thrusters or propellers. This paper presents the development of the USM

underwater glider as the first prototype for shallow water applications. The prototype development involves vehicle

concept design using Solidworks™, vehicle simulations by Computational Fluid Dynamics (CFD) and MATLAB

Simulink™ as stage of the design process. Once the prototype fabrication and system integration are completed, it

will be tested for vehicle’s modeling and controller development using the system identification approach and will

be compared with the proven glider’s control model.

Keywords

Underwater Glider; Computational Fluid Dynamics;

1.0 Introduction

An underwater glider glides by controlling their buoyancy using internal tanks and pumps. Existing gliders have

fixed external wings and tails and control their attitude by moving internal masses and using external control

surfaces such as rudder. Gliders travel from place to place by concatenating a series of upward and downward

glides. Gliding flight is buoyancy driven, and does not use thrusters system. Thus, glider must change depth to glide.

They glide downwards and upwards in the ocean by controlling their buoyancy to make themselves negatively and

positively buoyant. Gliders may also hold their position by gliding against current. This will make them neutrally

buoyant and drift with the current or rest on the bottom. Through their use of buoyancy propulsion system and low

power design, gliders are capable of long-range and high-endurance deployments. The driving force of the glider

depends on hydrodynamic forces such as drag and lift, which is a function of the advance speed and the angle of

attack. The gliding nature of about one half knot while travelling makes it relatively slow in comparison with other

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

Autonomous Underwater Vehicles (AUV) that used the propellers. However, the gliders (without the propellers)

have longer ranges, reduce the underwater acoustics signals and are more difficult to detect.

Gliders can operate both in littoral (coastal) and very deep ocean environments. Depends on the control system,

the operations could involve gliders to maneuvering in the interest area for long periods of time. Using the suitable

control algorithm, it can be programmed to do transiting undetected between areas or waiting on the bottom for

several periods of time before beginning a new mission command. The emerging technology of gliders has

overcome many difficulties that lie on the conventional method of ocean sampling using the ships and drifters. For

instance, ships can be too expensive to be used and limited in number and availability while drifter cannot choose

their path through ocean. Satellite technology in navigation and communication can be utilized on gliders

application and changing the way we see the ocean environment. These platforms can provide a different view of

interior ocean with much higher spatial from the command center situated from far away.

Underwater glider design and development approach can be different from one another due to user

requirements, amount of funding and man power involved. It also relies on whether some user needs to utilize high

sophisticated sensory system and amount of prototype endurance in term of duration of mission and maximum

gliding depth. Some of the design examples can be seen as in (Arima, Ichihashi and Ikebuchi, 2008; Alvarez et.al,

2009, Graver, 2005; Saringer, 2009, Ross, 2006). Early development of prototype design using simulation can be

useful to investigate the vehicle behavior. CFD analysis can be useful for hydrodynamic forces approximation and to

study the prototype performance in advanced (Seo, Gyungnum and Choi, 2008; Ishak, 2006). We can study the

prototype shape to find the suitable design and formalized the internal component needed as a payload. CFD

simulation result can be verified based on the towing tank experiment using the force sensor. It measures the real

hydrodynamic forces and compared with the simulation results and shows high in correlation between them (Arima,

Ichihashi and Ikebuchi, 2008). The simulations result can be used as the tool for design and development in cost

effective way for underwater glider application.

Challenge in control system for underwater glider is the underwater environment, in which it can be classified

as unstructured and unknown. Suitable control methodology and strategies can be chosen to overcome these

unknown parameters. One of the control strategies is to divide the control architecture into three specific layers such

as mission planning layer, task planning layer, and the behavior layer (Yu, et al., 2006). This type of controller

integrates the human’s intelligence into the control system and can assure the preciseness in planning level. There is

also some dedicated controller design for sharp turning strategies using Linear Quadratic Regulator (LQR) for

underwater glider during reflection near the sea bottom and collision prevention (Wang, Zhang and Wang, 2009).

Robust PID controller worked effectively during diving control phase in autonomous underwater vehicle combining

glider capabilities (Alvarez et al., 2009). Apart from this, there are also research undertaken on model-based

feedback control for gliders as for example in Leonard and Graver (Leonard and Graver, 2001) and the study on

stability and control of gliding vehicle as being discussed by Bhatta (Bhatta, 2006).

With careful design, buoyancy-driven gliders are quiet and used little power (Graver, 2005; Leonard and

Graver, 2001). Underwater gliders can be utilized in remote sensing for physical, chemical and biological

oceanography. Other possible applications include used as communications gateways or navigation aids and military

application such as tactical oceanography and maritime reconnaissance. Attractive glider characteristics include of

autonomous operation, high endurance and range, and low cost. Gliders are well suited to extended missions such as

collecting sensor data while move along a chosen path or maintaining position. Gliders can operate autonomously

either as individual task or in a group and may adaptively adjust their missions according to remote instruction or

according to sensor information.

Based on the remarkable issues that glider can overcome, together with the positive advantages and wider

application area that can be utilized, therefore, developing another new platform for glider system will become the

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

interest of this paper. In summary, this paper goes as follow. Section 2 describes the system design of the USM

underwater glider prototype. In section 3, system implementation of the prototype design, CFD simulation analysis

and simulation equation of motions are elaborated. Section 4 explains the discussion of the findings. Finally,

conclusions are given in section 5.

2.0 System Design of the Underwater Glider

The USM underwater glider is under development in order to be the first prototype platform to investigate the

buoyancy driven mechanism and vehicle dynamic motion. The platform is designed to be operated on shallow sea

water not more than 30 meters depth (Ali Hussain, Arshad and Mohd-Mokhtar, 2009). To complete the design,

fundamental knowledge on flight dynamic is adopted as it shares some common features in terms of the system

dynamics (Etkin, 1959). The careful and deep understandings on fluid mechanics are also important as to make sure

the developed platform is robust and reliable with the real practical situation (Douglas, Gasiorek and Swaffield,

2001; Yunus and Cimbala, 2006). Upon completion of the prototype, it will be tested for it workability in giving the

forward motion. The shape of the underwater glider will be in circular cylindrical form based on the proven design

concept. This is due to underwater hydrodynamic pressure effect and the capability of this shape to resist this effect

efficiently. Another advantage is a good hotel load for the hardware such as electronic components. Extra payload

can be achieved by designing the shape hull longer or in modular shape. This will give better accessibility on

different subsystem too. As for the same volume, a circular cylinder provides with better hydrodynamic as compared

to spherical form. For the docking purposes, circular cylindrical shape is easy to implement than others. Figure 1 and

Figure 2 display the hull shape design and the internal frame design respectively.

The overall length of the vehicle is 1.3 meters with a diameter of 0.17 meter. In this underwater glider design,

National Advisory Committee for Aeronautics (NACA) 0012 airfoils as a wing geometrical construction is selected.

The first two digits indicate the symmetrical of the airfoil and the last two digits indicates the maximum thickness

percentage compare to it chord size. NACA airfoil series are the guidelines being used to construct the wing airfoil

coordinates. Although these concepts of constructions are restricted for airplanes design, it can be adopted to the

underwater environment due to gliding concept as propulsion mechanism for the vehicle. These wings design are

crucial to give the vehicle sufficient Lift-Drag ratio as a conversion from vertical velocity to forward motion. The

total wing span for this vehicle is 1 meter without sweep design. Table 1 gives the general specification for the USM

underwater glider.

Table 1- Specification of USM Underwater Glider

Dimension 0.17 m (Diameter)

1.3 m (Length)

1.0 m (Wing Span)

Operation Depth 30 meters maximum

Operation Time More than 2 hours

Main Power Lithium-Ion

Sensors Eco-Sounder Transducer

IMU 5 Degrees of Freedom Gyrocompass

Depth Sensor

Distance Sensor

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

Figure 1- Hull shape Design

Figure 2 – Internal Frame Design

The pressure hull is made of high pressure PVC for easy manufacturing process. Since there is no propellers

system in this platform, the buoyancy system and movable mass mechanism behavior will be monitored by the data

logger system for workability of the mechanism during underwater testing stage. The maximum weight of USM

underwater glider will not exceed 30kg including all payloads, future sensors and instrumentation equipment. The

design also includes a future expansion of multi sensor such as two acoustic altimeters and Conductivity,

Temperature and Depth (CTD) module for auto-heading, auto-depth and auto-altitude capability, as well as for

survey mission. Mission control system such as diving control and surface navigation will be developed in order to

do the heading, depth and altitude mode. It shall be portable to handle by one or two personnel for ease of

transportation and due to Launch and Recovery (LAR) aspect. With this portability system, multiple gliders can be

launched in a mission without needed any special mechanical equipment. By having this, the payload of all

equipments can be reduced significantly (Jun et al., 2009).

3.0 System Implementation of the Underwater Glider

3.1 Electrical and Sensors System

PIC programmable chip will be used as the main controller circuit as it consumes less energy for endurance of the

vehicle. For heading and orientations of the vehicle, Inertial Measurement Unit (IMU) of 5 Degree of Freedoms

(DOF) provides the sensing of surge, heave, sway, rolling and pitching. This will give some important decision

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

making for control design and system identification processes. By using a CruzPro™ active acoustic transducer (see

Figure 3) and KELLER™ depth sensor, the vehicle can be programmed to execute the auto-depth and auto-altitude

routine. Due to the slow horizontal motion of the glider, these capabilities will allow the vehicle to perform

underwater profiling over a quite large number of areas. For each sensor and actuator, PC-104 will be fitted to the

vehicle for future data logging system. The main power to the vehicle is 9 AH Lithium Ion battery that able to

support up to 3 hours of operation.

Figure 3- CruzPro™ Acoustic Transducer

3.2 Buoyancy engine

The buoyancy engine system of the underwater glider is based on the ballasting tank which has a piston to push out

and pull in the water inside the vehicle. The piston tank is driven by 12V DC brush motor and capable of injecting

maximum of 500ml of water in 30 meter depth. The motor can be changed to high torque DC motor in order to

improve vehicle capabilities to go deeper and to overcome the underwater pressure. The ballasting mechanism is

monitored by using micro switch to estimate the ejecting volume of water. The micro switch is able to estimate the

internal piston location for detail dynamic analysis as this system provides the desired depth and altitude control.

This DC motor is driven by the 10A driver circuit, being interfaced with the micro processor chip, which act as a

controller for the system. Figure 4 and Figure 5 show the enhanced 10A motor driver and the piston tank

respectively.

Figure 4- Enhanced 10A Motor Driver

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Figure 5- Piston Tank

3.3 Movable Mass System

The movable mass platform consists of stepper motor control mechanism for precise controlling position of the

platform. The platform will be integrated with the Lithium-Ion battery for movable mass to save the payload space.

Stepper motor is integrated with the infrared distance sensor in order to know the exact position of the movable

platform. Again the data can be analyzed to gain the dynamic behavior of the vehicle.

3.4 Vehicle Design

Based on the calculation of sea water density of 1027kg/m3, the minimum vehicle weight is around 23kg in order to

make it slightly in negative buoyancy. It is of important also to consider all the volume of the vehicle inclusive of

the internal component mass and the vehicle size in order to estimate overall shape of the vehicle. The center of

gravity (CG) will give some estimation on proper place to install the movable mass system into the underwater

glider. Calculations are done by using Datcom™ software. CG will be estimated according to the current

components arrangement. In future, all the add-on components will be arranged in reference to this CG. Table 2

shows the calculation of CG based on the location of sensors and hardware components. In this case, Y axis is the

length of the vehicle.

CG in X direction,

CG in Z direction,

In these calculation, the CG are located at the (821.6mm, 0, -0.22mm) coordinate point.

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

Table 2- Example of CG Calculation

COMPONENT WEIGHT, M

(KG)

Distance, X

(mm)

M x X Distance, Z

(mm)

M x Z

PISTON TANK 0.770

VOLUME 1 0.71 377.59 268.09 0 0

VOLUME 2 0.04 495.59 19.82 -8.04 -0.3216

VOLUME 3 0.02 488.09 9.76 -7.9 -0.158

BATTERY 1 0.5 665.25 332.625 57.806 28.903

BATTERY 2 0.5 EACH 889.08 889.08 25.306 12.653

BATTERY 3 -32.194 -16.097

PC 104 1.5 OR

0.5 EACH

1052.125 1578.19 27.806 13.903

-17.194 -8.597

-62.194 -31.097

3.5 CFD Analysis

In performance analysis of underwater glider, estimation of hydrodynamic forces is important. As underwater glider

is forced to move through fluid, drag and lift forces that generated will have significant effect on the motion and

performance of the glider itself. The drag and lift forces depend on the density of fluid, velocity of fluid, size, shape

and orientation of vehicles body. Drag force coefficient, CD and lift force coefficient, CL are dimensionless number

which represent drag and lift characteristics of the vehicle.

Drag force coefficient, CD=FD/ (

(1)

Lift force coefficient, CL= FL/ (

(2)

Here, A refers to the platform area of main wings. For small angle of attack (positive and negative), Fluent 6.3.26 is

used to run the simulation. The resultant drag and lift force is transformed into drag and lift coefficient according to

the equations (1) and (2) mentioned above. For glider modeling, it cuts into half at the symmetry plane. This is used

to shorten the simulation time and reduced memory usage. Figure 6 and Figure 7 demonstrate some results of lift

and drag coefficient for different angle of attack. For underwater glider at zero angle of attack, it shows that the total

pressure distribution is quite symmetrical especially at the body of the vehicle. Waves occurred at the back region of

the glider as fluid pass through glider body. Figure 8 shows some contour on total pressure for underwater glider at

zero angle of attack.

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

Figure 6- Lift And Drag Coefficient Vs Angle Of Attack

Figure 7- Lift/Drag Ratio Vs Angle Of Attack Of Glider

Total Pressure On Symmetry Plane,Zero Angle Of Attack Contour Of Total Pressure , Zero Angle Of Attack

Figure 8- CFD Analysis

-4

-2

0

2

4

-20 -10 0 10 20

LIFT

/DR

AG

RA

TIO

ANGLE OF ATTACK

LIFT/DRAG RATIO VS ANGLE OF ATTACK

LIFT/DRAG RATIO

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

Figure 9.1- Bottom View Of Total Pressure

Distribution, Upward 2 Degrees

Figure 9.2- Bottom View Of Total Pressure

Distribution, Upward 10 Degrees

Figure 9.3- Top View Of Total Pressure

Distribution, Upward 2 Degrees

Figure 9.4- Top View Of Total Pressure

Distribution, Upward 10 Degrees

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

Figure 9.5- Bottom View Of Total Pressure

Distribution, Downward 2 Degrees

Figure 9.6- Bottom View Of Total Pressure

Distribution, Downward 10 Degrees

Figure 9.7- Top View Of Total Pressure

Distribution, Downward 2 Degrees

Figure 9.8- Top View Of Total Pressure

Distribution, Downward 10 Degrees

Figures 9.1-9.8 show a total pressure distribution of underwater glider for upward and downward Angle of

Attack (AOA). As angle of attack increases positively from two to ten degree, total pressure on bottom surface

increases to nearly factor of two especially at the location of fixed wing and tails. Nose of glider and leading edge of

fixed wings has shown the highest of total pressure distribution. For upper surface of glider, changes of total

pressure are less. However, region with low pressure has increased (blue region on main wings area) accordingly

with angle of attack. This is the area where main wings joint with glider body. For diving condition, pressure force

acting on upper surface is higher as lift force is acting downward. As angle of attack increases negatively, total

pressure acting on upper surface is also increased.

For velocity flow profile, it shows a shock wave appears at the back of the glider as the fluid passing through.

For upward AOA, as angle of attack increases, the shock wave behind the glider expands as well, and it spreads

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

towards upper surface of the glider. For downward angle of attack, shock wave expands in different direction which

spreads to lower surface of the glider. Pressure force acting on upper surface of the glider is higher than lower

surface. Nose of glider also shows a high resistance against the fluid flow where pressure distribution is very much

higher as compared to other region.

Upward 2 Degrees Upward 10 Degrees

Downward 2 Degrees Downward 10 Degrees

Figure 10- Velocity Profile

3.6 Simulation of Equation of Motions

Simulation of equation of motions in the vertical plane was done using the MATLAB Simulink in order to

investigate the buoyancy driven mechanism in advanced (Graver, 2005; Kan et al., 2008; Gardner, 2001). The

performance of the vehicle is analyzed by simulation, and this, can be very useful for glider design and behavior

understanding. The simulation result will be used for validation purposes, while comparing the real experimental

data with the proven underwater glider model (Leonard and Graver, 2001; Bhatta, 2006).

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Figure 11- Forward Velocity (m/s)

Figure 12- Pitching Angle (radians)

Figure 13- Angle of Attack (radians)

Figure 14- Lift

Figure 15- Drag

Figure 16- Depth (meter)

In this result, we have selected the gliding downward motion with zero initial condition. The controlling

conditions are: Forward Velocity=0.3m/s, Pitch Angle=-0.5 radians and Net Buoyancy =25 grams. During the 500

Int. J. Intelligent Defence Support Systems, Vol. 3, No. 3, 2010

seconds of simulation, the vehicle is gliding downward towards the 40 meters depth. Based on these results, we are

able to compare with the CFD findings, and this will become the additional data information for this research. The

simulation results have been justified with similar test field conducted by (Graver, 2001). Figure 11 to Figure 16

show some of the glider dynamics based on the MATLAB Simulink design investigation.

4.0 Discussion

The USM underwater glider is the first glider prototype that has been developed at the Underwater Robotic Research

Group (URRG) USM. The vehicle can be occupied with sensors payload depending on the purpose and applications.

In this prototype, we obtained the model of the glider’s dynamics by using the system identification approach. Using

the low cost sensors it is sufficient to know the vehicle dynamics based on the manipulated control inputs. Eco-

sounder transducer is used as the alternative for low cost altitude data collector as compared to the proven expensive

altitude transducer. The simulations results which represent the vehicle dynamics and behaviors have provided with

important information for the understanding of the vehicle behavior in general. In CFD analysis, the findings are

based on the shape of the vehicle design whereas in MATLAB simulations, it is based on the control inputs

configuration. By combining all these results, we may get the complete picture of the system design overall. All the

gathered information will be very useful for future underwater prototype development.

5.0 Conclusion

Based on the results and findings, we have gathered enough information for developing our first prototype of glider

system. The CFD simulation results show the gliding motion in different AOA. It also demonstrates some possible

gliding angle as to investigate the vehicle movement during gliding. The total pressure distribution to the vehicle’s

body while gliding is also observed. This information will be used in manufacturing and fabrication process for

higher stresses area. The simulation of motion in the vertical plane shows some vehicle characteristic of the glider

motion based on the ballast rate and movable mass displacement. These inputs will manipulate the vehicle

characteristic in terms of depth, gliding angle, forward velocity etc. Based on the findings, the system identification

approach is used to obtain the vehicle’s control system model. Later, the outcome based on the system identification

approach will be compared to the simulation model to give information about the input and output parameters. The

control system hardware can be implemented to the prototype so it can be workable and reach its optimum criteria

according to the prototype design aspect.

Acknowledgement

The authors would like to thank the National Oceanographic Directorate (NOD) of the Ministry of Science and

Innovation (MOSTI), NOD-USM 6050124 and USM-RU-PRGS under Intelligence Control & Modeling of

Underwater Glider for the research grant awarded.

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