International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 4

Number 2

June 2010

205

1 INTRODUCTION

During precise control of ship movements at small

velocities and when dynamic positioning task is per-

formed, disturbance has considerable impact at the

control accuracy. Particular influence to the ship

movements are during trials on isomorphic ship

models which are used for captains training and also

for control systems research in the Ship Handling

Research and Training Centre at Silm Lake near

Ilawa, Poland. Most of isomorphic ship models have

been built in the 1:24 linear scale. According to the

laws of mechanical similarity during trials on the

lake time run √24 ≈ 5 times faster than during opera-

tions on the real ship. The same scale 1:√24 ≈ 1:5

must be used for comparison of velocities in the real

world and on isomorphic ship models. Therefore

even small wind disturbance during tests affects ship

model like quite strong wind and wave disturbance

acts the real ship (Szalangiewicz 1996; Morawski

2007).

To take into consideration the external disturb-

ance in the control process the appropriate mathe-

matical model of wind and wave disturbance is re-

quired. The model enables a design of more precise

control systems and also more sophisticated com-

puter simulations can be done. This paper describes

design of the measurement system, which is used for

collecting wind and wave parameters on the Silm

Lake. The registered data will be base for develope-

ment of mathematical models of disturbance on the

lake.

2 MEASUREMENT SYSTEM DESIGN

The wind and waves measurement system diagram

is given in figure 1. It consists of measurement sen-

sors, which are installed on a dolphin fixed in the

lake bottom in some distance form a lakeshore. The

measurements are collected by a remote computer,

which can be installed on a shore or on the ship

model.

Microprocessor

system

Wave

measurer

RS485

pulses

anemometr

RS232

radio

modem

PC at a

shore

radio

modem

Wave

height

sensor

Fig. 1. Wind and waves measurement system.

Measurement System for Wind and Waves

Characteristics Registration on the Silm Lake

L. Morawski, J. Pomirski, P. Sikora & R. Sokol

Gdynia Maritime University, Gdynia, Poland

ABSTRACT: This paper describes the system for registration of waves and wind disturbance. Ultrasonic an-

emometer measures wind parameters. Capacitor sensor is used for measurement of wave height. The wave

sensor changes its capacitance according to the immersion of the sensor in water. The measurement system is

controlled by the microprocessor system, which collects data from the sensors and retransmit them to the

computer via radio modem. The system is used for design and simulation of control systems for isomorphic

ship models on the Silm Lake near Ilawa, Poland.

206

2.1 Anemometer

The ultrasonic anemometer measures the time taken

for an ultrasonic pulse to travel from one transducer

to the opposite transducer and then compares it with

the time taken for another pulse to travel in the op-

posite direction (Fig.2).

Fig. 2. Ultrasonic wind speed measurement

The speed c

of the pulse travelling in the same

direction as wind and opposite direction c

are giv-

en by:

−==

+==

(1)

where:

– is a wind speed component, which is paral-

lel to the 1-2 line,

c – is a speed of the sound in the air (it depends

on air temperature),

L – is a distance between both transducers.

If both times T

and T

are known, it is possible to

calculate wind speed V

and speed of the sound in

the air c:

)(

TTL

−

(2)

Gill Instruments' WindObserver II has been used

for the wind measurements. It has two pairs of ultra-

sonic transducers, which are used for calculation of

two perpendicular components of the wind speed.

Both components can be easy converted to the polar

co-ordinates i.e. wind speed and direction. Both co-

ordinates are calculated with 2% accuracy. The data

are transmitted 10 times per second in the NMEA

0183 format through RS422 interface to the micro-

processor system (WindObserverII User Manual).

2.2 Wave height measurer

Electronic part of the wave measurer (Fig.3) consists

of:

− a sensor, which capacitance changes due to im-

mersion of the sensor in a water

− a monostable oscillator, which forms pulses ac-

cording to the capacitance of the sensor,

− a transmission module which receives data from

anemometer and sends data to the PC on a shore,

− a microcontroller which controls an operation of

the measurer.

Fig. 3. Electronic circuit of wind and waves meter.

Capacitor sensor is used for measurement of

wave height. It is formed by two electrodes (Fig.4).

The flat bar is one of electrodes. The bar is also the

element of mechanical structure of the sensor. Se-

cond electrode is composed by the thin copper wire

with a teflon isolation coat. The wire is parallel to

the bar. The permittivity of the air and the water are

different, so the capacitance of the wave sensor de-

pends on the wave level as it causes what part of the

sensor is under the water.

The sensor immersion in the range 0-500mm cor-

responds to the capacitance variance from 40pF to

400pF. The capacitor is a part of the monostable os-

cillator composed by a popular LM555 chip (LM555

Data sheet). In this chip the time pulse width strictly

depends on the connected capacitance. Pulse genera-

tion is strobbed by the microcontroller. The micro-

controller also measures the pulse time width. The

32 partial measurements approximate final result.

Measurement is available 10 times per second. Rela-

tionship between the pulse time width and the sensor

immersion (the wave height) was determined by ex-

207

periment. The damping of the electrodes by water

mainly influences the measurement accuracy. Static

error of wave height measurement is less then 1mm,

but maximum dynamic error is bigger – it was esti-

mated to 5mm.

Fig. 4. Wave sensor – capacitor partially immersed in the wa-

ter.

Additional microcontroller tasks are collecting of

the data from the anemometer and then retransmit-

tion of both the wave and wind parameters to a

computer through radio modem. Transmission baud

rate of the radio modem is limited to the 4800B. Be-

cause the measurements are available every 100ms

(10Hz), and the NMEA format of data received from

anemometer causes long time of transmission

(70ms), and moreover 32 measurements of wave

height are performed in parallel, therefore the mi-

crocontroller program has been quite difficult to

write. Transmission of the data to the shore reduces

power requirements for the measurement system, so

the weight of the equipment installed on the lake is

lower, therefore even thin dolphins, far away from a

shore could be used for the measurer fixing.

3 CONCLUSION

Examples of the wind and waves parameters was

recorded on the lake and given in the figure 5.

The described measurement system enables regis-

tration of wind and waves parameters and finally it

will be possible to construct of appropriate mathe-

matical models of wind and waves disturbance

which appears on the Silm Lake during trials on

isomorphic ship models.

Data are transmitted by radio modem, so the sen-

sors are light enough to be mounted even on the thin

beacon fixed to the lakebed. Another advantage of

the measurer is small power consumption, therefore

battery supply can be used to register data for many

hours. The slow baudrate has been selected for

transmission, it caused some problems in software

design, but finally the cheap radio modem can be

used, moreover an influence of the disturbances is

reduced and the distance range covers nearly whole

lake.

100

150

200

250

300

350

Wind direction

[deg]

100

150

Wind speed

[m/s]

100

150

-10

-5

Wave height

[0.01m]

t [s]

Fig. 5. Direction and speed of wind and level of wave record-

ed on Silm lake.

REFERENCES

LM555 Timer. 2006. Data Sheet, National Semiconductor.

Morawski, L. 2007. Charakterystyki probabilistyczne i wid-

mowe wiatru w badaniach modelowych jednostek pływają-

cych na jeziorze Silm. IV Ogólnopolska konferencja "Pro-

blemy naukowo-techniczne w wyczynowym sporcie

żeglarskim". Proc. symp., Warszawa-Gdynia.

Szelangiewicz, T. 1996. A simulation model for a mooring po-

sitioning system of a vessel in presence of wind, current

and waves, Polish Maritime Research, Gdańsk.

WindObserver II Ultrasonic Anemometer, User Manual, Doc

No. 1390-PS-0004, Lymington UK,