Ocean Engineering 69 (2013) 1–8

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Ocean Engineering

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Deflection and inclination measuring system for floating dock based onwireless networks

Guangxiang Yang a,b,n, Hua Liang a, Chao Wu c

a Chongqing Engineering Laboratory for Detection, Control and Integrated System, Chongqing Technology and Business University, Chongqing 400063, Chinab China Silian Instrument Group Co. Ltd., Chongqing 400700, Chinac Department of Civil Engineering, Monash University, Clayton, VIC 3800, Australia

a r t i c l e i n f o

Article history:Received 9 August 2012Accepted 13 May 2013Available online 6 June 2013

Keywords:Floating dockDeflectionInclinationMeasurementWireless networks

18/$ - see front matter & 2013 Elsevier Ltd. Ax.doi.org/10.1016/j.oceaneng.2013.05.014

esponding author at: Chongqing Engineerinand Integrated System, Chongqing Technoloing 400063, China. Tel.: +861 363 791 66 02.ail address: ygxmonkey@126.com (G. Yang).

a b s t r a c t

When going in or out of a dock, the self weight of the ship may cause the deflection of the pontoon deckand/or inclination of a floating dock. This paper investigates the measurements of the deflection andinclination of a floating dock within a wireless network. A deflection measurement model is constructedon the basis of the multi-points liquid levels in the connected pipes. The water levels at different pointsare measured by the pressure transmitters connected to the connection pipes along the longitudinaldirection of the floating dock. The inclination is measured by 4 pneumercators installed on 4 corners ofthe floating dock. The measured data are transmitted to a server with a wireless sensor network. Withthe proposed model, the deflection and inclination calculation methods are analyzed and presented. Theexperimental results show that the accuracy of the deflection (hog or sag) can reach 95% and the error forthe inclination measurement (heel or trim angle) is less than 8%. It is verified that the measuring systemin the wireless sensor network supports the proposed deflection model and data collision avoidingalgorithm. The measuring system could provide the required information during the operation of thefloating dock.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

A floating dock is a ship-repairing platform (Shan et al., 2009),on which the damaged ship and its steel structure below thewaterline can be conveniently repaired. Therefore it is widely usedin the port or shore areas.

The architecture of a floating dock is illustrated in Fig. 1, whichconsists of a pontoon deck, two wing walls on both sides of thepontoon deck, and multi ballast tanks under the pontoon deck.The pontoon deck supports the self weight of the ship. The twowing walls accommodate comprehensive facilities such as opera-tion channels, central controlling room, workers' rest rooms,electrical devices etc. The ballast tank is a water containerresponsible for controlling the rising or sinking of the floatingdock. When the ballast tanks are filled with water, the dock willsink, and vice versa. Fig. 2 shows a practical industry floating dock.

The floating dock faces safety issues when the ship goes in or out ofthe dock. Normally, the self-weight of the ship is transferred throughdocking blocks to the pontoon deck, resulting in the deflection andinclination of the dock. The deflection and inclination can be adjusted

ll rights reserved.

g Laboratory for Detection,gy and Business University,

by controlling the water in the blast tanks. The deflection over thelength of the floating dock and the inclination angles (both trim andheel) should be limited within a valid range to ensure the safetyoperation of a floating dock. In this way, the bending stress (DetNorske Veritas (DNV), 2012) associated with the deflection of thefloating dock can be limited. Therefore, the deflection and inclinationof the floating dock must be measured and displayed according toChinese “Rules for Classification of Floating Docks (2009)” (ChinaClassification Society (CCS), 2009).

The deflection and inclination (draft) monitoring of a floating dockhas been studied by some researchers. A kind of arithmetic for theadjustment of the floating dock was presented in Xie et al. (2006) andHan et al. (2009). Accordingly, a real-time control and simulationsystem was developed based on the platform of LabVIEW by Smithet al. (2009). Zeyi et al. (2011) investigated the management system ofthe floating dock. The design scheme and the practical operation effectof an assembly-type floating dock were systematically presented byZhi (2010). Chen and Wu (2007) presented another deflectionmonitoring system, and an intellectual control scheme of shipimmersion. These achievements have greatly improved the safeoperation of the floating dock. However, some issues still remainunsolved in this area, e.g. the calculation models are not accurateenough or it could not satisfy the stringent requirements in theindustry environment when the wired data transmission is used. Thedeflection model is not constructed on the basis of connection pipes.

Fig. 1. The architecture of a floating dock.

Fig. 2. A floating dock in operation.

Fig. 3. The architecture of the monitoring system.

G. Yang et al. / Ocean Engineering 69 (2013) 1–82

More seriously, the measuring results are generally transmitted to acentral room by a manually reading or a wired system.

Wireless Sensor Networks (WSN) (Kovács et al., 2010) is beingincreasingly used in many fields, such as industrial monitoring,structural health monitoring (Quek et al., 2009, 2011), agriculturestorage, environment and education (Dikovic et al., 2011). Thewireless sensors are integrated into damage detection systems byQuek et al. (2009). An algorithm to patch the lost data is proposed.The design and evaluation of a wireless sensor network system forstructural data acquisition is presented by Xu et al. (2004). A suiteof algorithms for self-organization of wireless sensor networks inwhich there is a large number of mainly static nodes with highlyconstrained energy resources is presented by Sohrabi et al. (2000).The design of the sensor nodes is important, since they are basicunits of these networks. The design of a WSN depends signifi-cantly on the application, and it must consider factors such as theenvironment, the application's design objectives, cost, hardware,and system constraints (Yick et al., 2008).In this paper, thedeflection and inclination measurement model of a floating dockis proposed by implementation of WSN, with which deflection andinclination related parameters can be easily measured and trans-mitted. This model is constructed on the basis of the liquid level inconnection pipes. All sensors in the same line can test their ownliquid levels which are connected by pipes. When the linedeflected, the maximum deflection can be calculated by the liquidlevels measured at each sensor point. Traditionally the deflectionand inclination are measured with a wired system or by manuallyreading. All data packages of sensor are planned to access to thereceiver with a BEB algorithm, data collisions are lowered.Furthermore, the Silicon Laboratories Si4432 ISM Band transceiver(Silicon Laboratories, 2010) is introduced in the measuring system,so that the cost of the WSN can be greatly reduced and the existingprotection and monitoring systems can be extended.

2. System architecture

2.1. System topology

The topology of the proposed deflection and inclination mea-suring system is presented in Fig. 3. Each RF (Radio Frequency)

Module node is connected to a sensor, which is a transmitter usedfor detecting the water level in the connection pipes. RF modulesends the detected data to the receiver via wireless network. Thereceiver is responsible for collecting the results of all RF modulesand transporting the data to the monitoring server through RS485communication link.

2.2. Structure of the measurement instrumentation

According to Chinese “Rules for Classification of Floating Docks(2009)”, independent systems for measuring the deflection of thedock over its length shall be installed if the full length exceeds90 m. The deflection should be readable from the control room ofthe dock. In order to measure the deflection of the floating dock,the connection pipes principle (Yang and Notodirdjo, 2011) isapplied. Connection pipes are used in many fields for detecting theparameters like level, pressure, deformation etc. Liquid leveltransmitters are set up according to the locations of the connec-tion pipes, so that the liquid level at various testing points can bedetected and thus the deflection can be calculated. The connectionpipe and the liquid level transmitters are illustrated in Fig. 4.

A deflection and inclination measurement scheme is illustratedin Fig. 5. The deflection can be measured by the sensors connect-ing to the pipes which are installed on each of the two wing walls.In addition, four pneumercators are installed at the four corners ofthe floating dock for inclination (draft) measurement.

3. Wireless networks solution

3.1. Introduction of the RF transceiver

The Si4432 (Chen et al., 2011) is a highly integrated, single chipwireless ISM (Industrial Scientific Medical) band transceiver. Thelow receiving sensitivity (–121 dBm) coupled with +20 dBm out-put power ensures the extended range and the improved linkperformance. Built-in antenna diversity and its support for fre-quency hopping can further extend range and enhance theperformance. The Si4432 offers advanced radio features including

Fig. 4. The architecture of the connection pipes.

Fig. 5. The architecture of the connection pipes equipment.

Fig. 6. Hardware block diagram of RF Module.

Fig. 7. Architecture of the receiver module.

G. Yang et al. / Ocean Engineering 69 (2013) 1–8 3

continuous frequency coverage from 240–960 MHz in 156 Hz or312 Hz steps allowing precise tuning control. Its direct digitaltransmit modulation and automatic PA (Power Amplifier) powerramping promise the precise transmit modulation and reducedspectral spreading. Therefore, the compliance with global regula-tions including FCC (Federal Communications Commission), ETSI(European Telecommunications Standards Institute), ARIB (Asso-ciation of Radio Industries and Businesses), and 802.15.4 d regula-tions can be ensured.

3.2. RF module design

The RF Module is the sensor node in WSN devised with si4432transceiver. The si4432 is designed to work with a microcontroller,crystal, and a few passives to create a low cost system. Voltageregulator is integrated on-chip which allows a wide range ofoperating supply voltage conditions from +1.8 to +3.6 V.A standard 4-pin SPI bus is used to communicate with themicrocontroller. Three configurable general purpose I/Os (Input/Output) are available to satisfy the needs of the system.

The RF Module is responsible for acquiring output signal fromthe sensors and transmitting the data to the receiver. A STM8series microcontroller is chosen as the MCU (Micro Control Unit)of the module, and output signal of 4–20 mA current is connectedto AD (Analog to Digital) transfer module. When MCU gets the ADresult, it sends the data of water level to RF transceiver. Theinternal architecture is shown in Fig. 6.

3.3. Receiver module design

The receiver is the key device of wireless network for commu-nication with RF modules. Si4432 transceiver module is integrated

in the receiver for radio frequency communication. STM32103 chipis the MCU of receiver.

The receiver module gets all the data from RF modules andtransmits them to the server via RS485 communication interface.The server is located in the control room of the floating dock. Withthe collected data from all of the sensor nodes, the monitoringsystem on the server can work out the maximum deflection andinclination. The result is then transmitted to the receiver forpossible display and alarm. The internal hardware block diagramof the receiver is shown in Fig. 7.

4. Parameters measurement methodology

According to the measurement scheme described in Section2.2, the deflection and inclination of the floating dock can be easilymeasured.

4.1. Deflection measurement model

The sensor used for detecting the deflection is Rosemount 1151pressure transmitter (Rosemount 1151 pressure transmitterreference manual, 2008) which offers a variety of configurationsfor differential, gage, absolute and liquid-level measurementsincluding integrated solutions for pressure, level, and flow.

Considering the cost and complexity, five testing points on eachwing walls are designed in this paper. The deflection measurementmodel on the wing wall is presented in Fig. 8.

Theoretically, the water level of the connection pipes at allpoints is the same and is referred to as the initial value. The waterlevel sensors are equally distributed along the pontoon deck asshown in Fig. 8. At the beginning, each point has the same waterlevel, linit, which is

linit ¼ l1 ¼ l2 ¼ :::¼ lnðn¼ 5Þ ð1Þ

Fig. 10. Deflection model—hog—no inclination.

Fig. 11. (a) Deflection model—hog—inclination (b) connection pipes show of (a).

G. Yang et al. / Ocean Engineering 69 (2013) 1–84

In which, n is the total number of sensors. In this case, it is 5. Andthe deflection Δhi of each point is ideally equal to zero

Δhi ¼ li−linit ¼ 0; i¼ 1� n ð2Þ

4.1.1. Hog type deflection measurementFirstly, it is assumed that the deck has an upward deflection-

hog as shown in Fig. 9. This hog type deflection can be generatedwhen the middle ballast tanks are filled with less water. Themeasurement of such deflection is then discussed under twoconditions: when there is no dock inclination and when there isa dock inclination.

Deflection measurement when there is no inclination. Underthis condition, the deflection pattern of the pontoon deck ispresented in Fig. 10, in which li is the initial liquid level of theith testing point on the pontoon deck; li0 is the new liquid levelvalue due to the pontoon deck deflection. xi (i¼1–5) representsthe length of each point.

Therefore, the deflection Δhi (i¼1–5) at each point can becomputed by the following equation

Δhi ¼ li−li0 ð3ÞNote that there is no deflection for the two end points x1 and

x5 on the line, we have

Δh1 ¼Δh5 ¼ l1−l10 ¼ l5−l50 ¼ 0 ð4ÞWhen the deflection of each sensor point is calculated by Eq.

(4), the deflection at any point on the line can be obtained with aLAGRANGE interpolation algorithm. LAGRANGE interpolation is awell known, classic technique for interpolation, using an approx-imating or interpolating function to fit N+1 known discrete pointswith an Nth degree polynomial. Thus, the deflection at any pointalong the full length of pontoon deck can be computed as

ΔhnðxÞ ¼ ∑n

k ¼ 0ΔhklkðxÞ ð5Þ

where

lkðxÞ ¼ ∏n

j¼ 0j≠k

x−xjxk−xj

ð6Þ

Fig. 8. Deflection model.

Fig. 9. Deflection model—hog.

Deflection measurement when there is an inclination. Underthis condition, the deflection is coupled with inclination measure-ment as shown in Fig. 11.

In Fig. 11(b), point P is one of the points equipped with sensorson the deck. Point O and W are two end points on the pontoondeck. Before the deflection, point P is overlapping with point B,line OBW is a ramp line. The inclination angle of α between lineOBW and the horizontal line (X axis) can be measured by foursensors as will be discussed in Section 4.2. When deflection isgenerated, OBW becomes an arc OPW. The projection of point P onthe X axis is C. The cross point of line PC and line OBW is A. Theprojection of point P on line OAW is B. As the angle α is very small,OC can be regarded as approximately equal to OB. The straight lineMDN refers to the water level in the connection pipes. Accordingto the Torricelli's Experiment, regardless of the inclination of theconnection pipes, the water level height DC is a constant. We have

DC ¼ linit (linit is the initial liquid level of each point whenpontoon deck have no deflection.)

OC ¼ xi ði¼ 1; 2; … 5Þ. xi is the length of ith point. In this case,i¼3.

DP ¼ li0 (li0 is the new liquid level of each point when pontoondeck is deflected. In this case, i¼2.)

AC ¼Δli ¼ xi tan ðαÞ Δli is the liquid level difference of eachpoint when deck is deflected with inclination.

Accordingly, the PA is given that

PA¼DC−DP−AC ¼ linit−li0−xi tan ðαÞ ð7Þ

From Fig. 11(b), the deflection at point P is PB (it is Δhi; i¼ 2),we have

Δhi ¼ PB¼ PA cos ðαÞ ¼ ðlinit−li0−xi tan ðαÞÞ cos ðαÞ ð8ÞSpecifically, as point O and point W are end points, there are no

deflections on these two points. Generally, if there is no inclina-tion, angle α is 0, the deflection Δhi at each point is given as the

G. Yang et al. / Ocean Engineering 69 (2013) 1–8 5

following equation

Δhi ¼ PB¼ PA cos ðαÞ ¼ ðlinit−li0−xi tan ðαÞÞ cos ðαÞ ¼ linit−li0 ð9Þ

This equals to the values under the condition of “There is noinclination”.

It should be noted that, the above derivations are based on theassumption that the inclination angle α is less than 901, that ispoint “W” is higher than point “O”. Similar results can also bederived when the inclination angle α is greater than 901, that ispoint “W” is lower than point “O”. The general equation when thedeflection pattern is a hog is

Δhi ¼ðlinit−li0−xi tan ðαÞÞ cos ðαÞ; αo π

2

ðlinit−li0−xi tan ðπ−αÞÞ cos ðπ−αÞ; α4 π2

(ð10Þ

Similarly, on the basis of LAGRANGE interpolation, the deflec-tion at any point along the full length of pontoon deck can beinterpolated as

ΔhnðxÞ ¼ ∑n

k ¼ 0ΔhklkðxÞ ð11Þ

4.1.2. Sag type deflection measurementThe floating dock may also experience sag type deflection when

loaded with a ship as briefly shown in Fig. 12.Similar to hog type deflection measurement when the inclina-

tion angle α is considered, the deflection at each sensor point isgiven by

Δhi ¼ PB¼ PA cos ðαÞ ¼ ðxi tan ðαÞ−ðlinit−li0ÞÞ cos ðαÞ¼ −ðlinit−li0−xi tan ðαÞÞ cos ðαÞ ð12Þ

Comparing Eqs. (8) and (12), the deflection of the dock deck,regardless of hog or sag type deflection, can be given by a generalequation as follows

Δhi ¼ðlinit−li0−xi tan ðαÞÞ cos ðαÞ; αo π

2

ðlinit−li0−xi tan ðπ−αÞÞ cos ðπ−αÞ; α4 π2

(ð13Þ

When Δhi40, it is a hog (Fig. 11), Δhio0, it is a sag (Fig. 12).

Fig. 12. (a) Deflection model—sag—inclination (b) connection pipes show ofdeflection model—sag (no inclination).

4.2. Inclination measurement model

Four transducers are arranged on the four bottom corners ofpontoon deck as shown in Fig. 13. Transducers No. 1 and No. 2 aredistributed on the port of the dock, and lie on the forward and aftrespectively. Similarly, transducers No. 3 and No. 4 are installed onthe starboard of the dock, and lie on the forward and aftrespectively. These transducers are pressure based and water-proof measuring devices which are named pneumercators. Theyare easy to install and suitable for applications in slurries and dirtyliquids. Therefore these transducers are reliable for inclination(draft) measurement of the floating dock.

Two inclination angles are to be measured: trim angle in thelength direction of the dock and heel angle in the transversedirection. Given that the length of the dock is LD, the width of dockis WD. The trim angle in Fig. 14 can be obtained by the measuringthe liquid levels at the forward and the aft. Assuming that thereadings of the transducers are diði¼ 1� 4Þ. When there is noinclination, the draft readings on the four corners are equal to theinitial value. If the floating dock inclines, we have

Trim angle in the portside

tan ðβÞ ¼ d1−d2LD

ð14Þ

Because the trim angle is very small and the value of d1−d2 isvery small compared to the LD. Therefore, sin(beta) is approxi-mately equal to the tan(beta). And trim angle in the starboard

tan ðβÞ ¼ d3−d4LD

ð15Þ

For fairness, considering the inclination in the port and star-board of the floating dock, the average difference should be usedfor relative trim angle computation, as given by the followingequation

tan ðβÞ ¼ ððd1−d2Þ þ ðd3−d4ÞÞ=2LD

ð16Þ

Thus, the relative trim angle is given by

β¼ arctanððd1−d2Þ þ ðd3−d4ÞÞ=2

LD

� �ð17Þ

The relative heel inclination angle model is presented in Fig. 15.The heel angle γ is calculated by the measuring results of liquidlevels at the port and the starboard. According to the discussionabove, we have

Heel angle at the forward

tan ðγÞ ¼ d1−d3WD

ð18Þ

Fig. 13. Transducers distribution of the inclination measurement.

Fig. 14. Trim measurement model.

Fig. 15. Heel measurement model.

Fig. 16. “Yuexiu Mountain” floating dock.

G. Yang et al. / Ocean Engineering 69 (2013) 1–86

Or heel angle at the aft

tan ðγÞ ¼ d2−d4WD

ð19Þ

Similarly, the average measuring value on the port and star-board of the floating dock is used for relative heel angle calcula-tion, which is given by

γ ¼ arctanððd1−d3Þ þ ðd2−d4ÞÞ=2

WD

� �ð20Þ

Fig. 17. The sensors' location in “Flying Dragon Mountain” floating dock.

5. System verification experiments

The proposed floating dock deflection and inclination measur-ing scheme is validate by the experiments. The experiments weresuccessful conducted on two floating docks, “Flying DragonMountain” and “Yuexiu Mountain” (shown in Fig. 16) in Guangz-hou, China. The experimental data on the “Flying Dragon Moun-tain” are chosen for analysis in this paper. The parameters of“Flying Dragon Mountain” needed for the measurement are listedas follows

Total length of the pontoon deck LD¼184.32 m;Total width of the wing wall WD¼47.00 m:Lifting capacity¼16,000 t;Maximum permissible deflection Dmax¼LD�0.5‰¼92 mm;Maximum permissible trim βmax¼1.001 (Zhao, 2007);Maximum permissible heel γmax¼2.001 (Sun, 1995);The detailed position of each sensor is introduced in Fig. 17. It is

noted that the unit in this figure is meter (m). The five sensors areplaced on the bottom of pontoon with the same interval which is40 m in the figure. The sensors are installed just closed to the twowing walls. All of the RF modules are installed on the top of thetwo wing walls, which can avoid modules being immersed into seawater when the floating dock is sinking. The sensor and the RFmodule are connected with a pair of copper cables which cantransferred the output signal of sensor to the RF module. Theoutput signal of sensor is a DC (Direct-current) current of 4–20 mA.

The developed monitoring system software is shown in Fig. 18,in which the deflections of the port and starboard, and theinclination angles (trim and heel) can be displayed. If the deflec-tion and inclination exceed the maximum permissible values,alarm sounds and warning light is illuminated. The operator canget enough information from the software and adjusts the deflec-tion and inclination of the floating dock by pumping compensatingwater in the corresponding blast tanks.

5.1. Deflection measuring experiment

The calculated deflection results were compared with theresults from finite element analysis (Xu and Shen, 2011; Duanet al., 2007). The finite element analysis is a reliable and efficientmethod for structure calculation, validation and verification.Therefore the calculation results are used for the calibration of

the deflection. The relative error ration in Table 1 is the calculationresults divided by difference of calculation result and the mea-sured result. The experiments were carried out under differentship loading conditions: Light Loaded floating dock and Heavyloaded floating dock. The measured results and calibrated resultsare listed in Table 1.

According to Table 1, the maximum measured deflection doesnot exceed the maximum permissible deflection. The maximummeasuring error of the deflection in this system is about 5.0%,which indicates that the experimental results satisfy the designspecification.

The measuring results at the three different points when ship1 was lifted in the floating dock are shown in Table 2. The “maxproportion of LD” means the proportion of maximum measureddeflection to the full length of pontoon dock. In this study, LD isgiven at the beginning of this section, it is 184.32 m.

From Table 2, the maximum deflections Δhi measured atdifferent sampling time always occur at point 3, which meansthat the pontoon deck has a sag deflection and the maximumdeflection occurred in the middle of the pontoon deck. Thedeflection increases with time, indicating that the pontoon deckis gradually loaded more during the lifting process. In this case, themaximum deflection does not exceed 0.5‰ of LD.

5.2. Inclination measuring experiment

With the model described in Section 4.2 the inclination (trimand heel) angles can be calculated. The conventional method ofinclination measurement is manual observation of the rulercalibrations. This method is accurate but time consuming and lessefficient. In this paper, the manual observation method is used forcomparison purpose. There are 4 m calibrations equipped at thefour corners of the two wing walls. When the floating dock rises ordrops, workers at the four corners would read the real time metersand report the results to the central room. In this way, the

Fig. 18. Interface of the deflection and inclination measurement software.

Table 1Measured deflections under different loading conditions.

n Conditions Maximumdeflection (mm)

Point Proportion of LD Calculationresults (mm)

Relative errorratio (%)

1 No load 0 0 0 02 Ship 1 55.2 3 0.299‰ 56.6 2.53 Ship 2 75.6 3 0.410‰ 79.7 5.1

Table 2Measured deflections when lifting.

n Time (s) Point Δh2 Point Δh3 point Δh4 Max proportion of LD (‰)

1 0 2 0 3 0 4 0 02 120 2 −2.2 3 −8.8 4 0 0.0483 250 2 −8.3 3 −16.3 4 −7.6 0.0884 370 2 −11.4 3 −24.7 4 −11.5 0.1345 490 2 −18.6 3 −37.5 4 −17.8 0.2036 610 2 −20.1 3 −42.2 4 −19.4 0.2297 730 2 −26.6 3 −55.2 4 −24.3 0.299

Table 3Measured inclination angle on different loaded ships.

n Conditions Maximum trim β (1) Maximum heel γ (1) Manual observation trim β (1) Manual observation heel γ (1) Error ratio trim (%) Error ratio heel (%)

1 No load 0 0 0 02 Ship 1 0.66 −0.74 0.69 −0.77 4.3 3.93 Ship 2 0.98 −0.87 1.06 −0.93 7.5 6.5

G. Yang et al. / Ocean Engineering 69 (2013) 1–8 7

inclination angle can be calculated. The computer measuredresults are then compared with the manual observations. Theinclination experiment results are listed in Table 3.

According to Table 3, the maximum measured trim or heelangles are less than the maximum permissible inclination angle.The maximum error of measured inclination angle in this systemis about 7%. Therefore, the measurement result is accurate enoughfor the floating dock application.

The measured trim and heel angles are compared with themanual observations in Figs. 19 and 20, respectively. The solid linerepresents the manual observation result and the dashed line

represents the measured one. The sampling time interval is 10 s.According to the figure, the measurement curves generally com-pares well with the manually observed results.

In these two subsections, deflection and inclination experi-ments are implemented by the deflection and inclination modelwith the monitoring system. The proposed monitoring systemcollects the data from sensors to calculate the required para-meters. The calculation methods come from the proposed liquidlevel measuring algorithm. With the equipped connection pipes,the deflection and inclination angle are measured in the proposedmonitoring system.

Fig. 19. Measured and manually observed trim angle.

Fig. 20. Measured and manually observed heel angle.

Table 4Data transmission experiment.

n Distance(m)

Data lossrate (%)

Data rate(bps)

Data packagelength (bits)

Error datarate (%)

Collectiontime (s)

1 10–125 0.7 1200 64 0 0.962 10–125 1.2 2400 64 0 0.883 10–125 1.8 4800 64 0 0.85

G. Yang et al. / Ocean Engineering 69 (2013) 1–88

5.3. Wireless transmission experiment

Silicon Laboratories Si4432 ISM Band transceiver is designed towork on 433.9 MHz frequency band. Since it is a very slow adjustingprocess of the floating dock, the data transmission rate of 1200 bps(bits per second) is adequate for the system application. The datapackage is totally 8 bytes length. As mentioned above, there are 10sensor nodes in the WSN. To avoid and reduce data collision in thenetwork link, a Binary Exponential Back off (BEB) algorithm (Yanget al., 2012) is introduced in the wireless network. Theoretically thereceiver collects all the data packages from the 10 sensor nodes within0.53 s with a 1200 bps baud rate. Considering the package loss causedby link collision,1.07 s is the maximum upper time limit. The samplingperiod is 2 s. Table 4 presents the experimental results. In this table,the data loss rate is the ratio of actually received bytes by receiver tothe totally transmitted bytes by each RF modules connected to thesensor nodes. The error data rate is the ratio of received correct bytesby receiver to the original bytes transmitted by each RF modules.

According to Table 4, the maximum data loss rate is less than 0.7%with the 1200 bps data rate which consumes an average collectiontime of 0.96 s. When the data rate grows, the data loss rate increases.When the data rate is less than 4800 bps, the data loss rate and errordata rate are still accurate enough for the slow adjustment process offloating dock. The error data rate is 0% shows that the WSN is reliableand there is no error bytes received. But a very small amount of datawill be lost when the data rate increasing. Experimental resultsvalidate the reliability of the proposed wireless sensors network.

6. Conclusions

A deflection and inclination measurement system in a WSN of thefloating dock is proposed in this paper. The deflection measuringmodel of a floating dock is presented. This model is constructed basedon the liquid level in the connection pipes. The data transmittedwithin a WSN are planned to access to the receiver with a BEB

algorithm, data collisions are avoided. The experimental resultsindicate that the measurement accuracies of deflection and inclinationmeasurement models are about 95% and 93%, respectively. Thewireless data transmitting is reliable with a lower data loss ratio of0.7%. The traditional parameters monitoring system of floating dock isimplemented by a wired one. As the poor work environment in thefloating dock, a wireless sensor networks system can eliminate theinconvenience and lower the cost of a wired measuring system.Therefore, it is cost effective and more convenient than a wiredsystem.

Acknowledgments

This work is funded in part by Chongqing Education CommitteeCooperation Foundation, China under Grant KJZH11213, ChongqingScience and Technology Commission, China (CSTC2012jjA40037).

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