# Preliminary Study of Hovercraft Control System

1

Preliminary Study of Hovercraft Control System

Cordova

Supervisor: Dr. Ir. Mohammad Rohmanuddin, MT.

Study Program of Engineering Physics, Faculty of Industrial Technology,

Bandung Institute of Technology

Labtek VI Building, Jl. Ganesha 10, Bandung, Indonesia, 40132

e-mail:

Abstract

In designing a hovercraft, there are many factor should be considered, such as skirt

design or plenum. We have to consider also the materials, structure, thrust, drag,

dynamic effect, and stability. Due the hovercraft using rudder mechanism to steer, the

fan must produce the sufficient air speed for the rudder to be effective as a control

surface. As a result, a hovercraft cannot generate a pure torque and is thus unable to turn

in place. The rudder is also mechanically limited and cannot produce force directed

along the lateral direction of the hovercraft. For these reasons, a hovercraft with is an

under actuated system. In this project, control system for steering a hovercraft and

provide lift for the hovercraft will be studied. Engineering software, MATLAB and

Simulink, will be used in designing the control system. This project is the first step to

design and build a hovercraft control system.

Keywords: Stability, rudder, lateral direction, under actuated.

I. Introduction

Indonesia is an archipelago country

which 70% of its area consist of water.

Indonesia has many locations pass by

the river flow path. Beside that,

Indonesia has also locations where risk

from natural disaster, such as

earthquake, flood, and even tsunami.

Due to that geographic condition,

Indonesia needs a vehicle which can

move fast and reliable in many

conditions, to use in transportation,

emergency condition, coast guard, and

search and rescue. Since the hovercraft

can move in almost all terrain

conditions, this vehicle is one of the

solutions which can satisfy those

requirements.

A Hovercraft, or Air-Cushion Vehicle,

is an amphibious vehicle designed to

travel over any sufficiently smooth

surface - land or water - supported by a

cushion of slowly moving, low-pressure

air, ejected downwards against the

surface close below it. To produce the

air cushion, the propeller is used to

provide lift by keeping a low-pressure

air cavity under the craft full of air.

Figure I.1: Main parts of the hovercraft; 2

1. Thrust fan

2. Air flow

3. Lift fan

4. Plenum/flexible skirt

II. Objective

The objective is to study a control

system for the hovercraft. The control

system is consists of motor control

system, and surface movement control

system.

III. Methodology

1. Derivation of the dynamic

system model.

2. Control system model

representation.

3. Simulate, and analyze control

system using MATLAB and

Simulink.

IV. Dynamics of Hovercraft

Dynamics of hovercraft motion is has 6

degrees of freedom, they are 3 degrees

in translation dynamics (surge, sway,

heave), and 3 degrees in rotation

dynamics (roll, pitch, yaw). A few

assumptions are made, such as

neglected roll, pitch, and heave, because

these movements have no considerable

influence. Since the hovercraft only

have lateral motion. So that, in this

project, the hovercraft assumed only has

3 degrees of freedom, which is surge(u),

sway(v), and yaw(r).

Two coordinate frames are chosen,

which is a body-fixed frame X0Y0Z0 on

the hovercraft and an earth-fixed frame

XYZ.

Body-fixed

Earth-fixed

Y

Z

X

X0

Y0

Z0

v

w

u

p

r

q

ß

Figure IV.1: Coordinate frames

State-space method is used to

representing the dynamics of hovercraft.

Since this method makes the dynamics

of hovercraft is easier to understand.

A state equation is a first order, vector

differential equation. It is represent the

equation of hovercraft motion. The

state-space general expression is:

x Ax Bu

y Cx Du

+ =

+ =

n

∈ x R is the state vector

m

∈ u R is the control vector

A is the state coefficient

B is the driving matrix

Dynamics of hovercraft:

1

r R

R

r R R

Y Y Y

r

V V V

r r

β

β

β β δ

β δ

−

+ + =

+ + =

Obtained the state-space representation

of the dynamics of hovercraft:

[ ]

*

' '

'

1

0 1

R

R

v

R

r

Y Y

r r

y

r

δ

β δ

β β

δ

β

−

= +

=

1

tan

v v

u u

β

−

= = side-slip angle

= yaw moments 3

δ

R = rudder deflection

2 2

V u v = + = speed of the hovercraft

V. Dynamics of Electric Motor

Electric motor is used to control the

speed of the propellers which provide

lift force for the hovercraft. Lift force is

cause by the air pressure ejected

downwards against the surface.

Air Inlet

Air Outlet (low-pressure) Air Outlet (low-pressure)

Lift Propeller

Motor

Figure V.1: Hovercraft’s plenum principle

Lift Force = Pressure x Surface Area

The electric motor input is dc current,

and the output is the angular speed of

the motor which depends on the torque.

t e t

in

k k k

T V

R R

T J

ω

ω

− =

& =

Obtained the dynamics of the electric

motor:

e t t

in

k k k

I

JR J

ω ω

+ − =

T = Motor torque

J = Inertia of the load

ke = Motor’s electric constant

kt = Torque constant

R = Electrical resistance

Iin = Current input

ω = Angular velocity

VI. Control System Design and

Simulation Progress

Simple block diagram of the hovercraft

automatic steering control system

C HD

S

A

Input Output

Amp

C = Controller

Amp = Signal amplifier

A = Actuator (rudder)

HD = Hovercraft dynamics

S = Sensor (rate gyro)

For manual steering system, sensor is

not needed and controller only has an

amplification function, like an

amplifier, for the input signal.

Simple block diagram of the automatic

motor speed control system

Cm

Sm

Am

Input Output

MD

Cm = Motor speed controller

Am = Signal amplifier

MD = Motor dynamics

Sm = Speed sensor

The control system simulation using

MATLAB and Simulink has not done

yet and is still in progress.

VII. References

[1] Ogata, Katsuhiko. 1997. Modern

Control Engineering.New Jersey:

Prentice Hall International, Inc

[2] Franklin, Gene F, J David Powell,

Abbas Emami.1994.Feedback

Control of Dynamic System.USA:

Wesley Publishing Company 4

[3] McLean, Donald.1990.Automatic

Flight Control System.UK:

Prentice Hall International, Inc

[4] Boldean, Ion and SA Nasar. 1999.

Electric Drives. Florida: CRC

Press

[5] Friedland, Bernard. 1987. Control

System Design an Introduction to

State Space Methods. Singapore:

McGraw-Hill Book Co.