The application of an expert system for simulation investigations

in the aided design of ship power systems automation

Ryszard Arendt*

Faculty of Electrical and Control Engineering, Technical University of Gdansk, ul. Narutowicza 11/12, 80-952 Gdansk, Poland

Abstract

A structure and function of information system for aided design of ship power subsystem automation is presented. More detailed a function

of evaluation of design solutions based on simulation investigations is described.

Data describing a designed power subsystem is introduced in interactive mode to an expert system, then production rules evaluate

correctness to the substance and form of the design. In the case when there are no mistakes, the expert system creates simulation model of

designed subsystem and calls program MATLAB-SIMULINK. After a simulation session the program environment returns to the expert system.

In the paper a knowledge representation of designed ship power subsystem, and a choice of component model structures of subsystems,

which enable an application of production rules for simulation models creation and design correctness evaluation are shown. Also an

algorithm of simulation models creation of designed subsystems and an exemplary session of simulation investigations of ship propeller

subsystem are presented.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Expert system; Simulation; Design; Ship power system; Dynamic

1. Introduction

The design of ship systems automation includes a range of

important systems among which we can distinguish: power,

freezing, air-conditioning, auxiliary installation, navigation,

rolling stabilizer, steam production, and loading systems

(Bertram, 1999; Kowalski, Arendt, Meler-Kapcia, &

Zielinnski, 2001; Lee & Lee, 1999; Wu, Guo, Chen, &

Chen, 1999). A great complexity of the systems, a diversity

of the applied technical solutions as well as the strict

requirements of classification societies slow down the design

process and the evaluation of the proposed solutions. A

complex multi-functional information system has been

developed that aids ship systems automation design (Arendt,

2002; Kowalski, Arendt, Meler-Kapcia, & Zielinski, 2001).

For this purpose a series of research studies was carried out

that concerned mainly: investigation and description of the

design process of power systems automation, development

of databases and knowledge bases connected with the

process. The specificity of ship systems automation design

was defined as well as the designers’ tasks during particular

stages of the design process (preliminary design, commission

project, technical project) with the documentation specified

by classification societies, ship owners and shipyards. The

equipment database was developed together with component

elements and ship systems automation and the requirements

of classification societies. The database, created in MS

Access contains information on automation objects and

systems of existing ships as well as catalogue information

concerning control elements. The aided design of ship

systems automation is largely based on the use of information

on automation design of already built ships. Serving this

purpose, the algorithm was developed which determines

similarity of a ship being designed to particular ships on the

basis of the information included in the database. To create

the knowledge system for aided design of ship systems

automation, a shell expert system Exsys Developer was used.

The program is characterized by rule-oriented knowledge

representation various methods of inference, the possible use

of fuzzy logic and co-operation with databases and other

programs. A stimulation investigation subsystem was

created that co-operates with the database (Arendt, 2002;

Kowalski, Arendt, Meler-Kapcia, & Zielinski, 2001).

An use of mathematical models of elements, units and

ship systems may constitute a source of deep knowledge for

an expert system. The application of simulation investi-

gations during the design stage enables: evaluation of

efficiency and correctness of the assumed design solutions,

0957-4174/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.eswa.2004.05.011

Expert Systems with Applications 27 (2004) 493–499

www.elsevier.com/locate/eswa

* Tel.: þ48-583-472-157; fax: þ48-583-472-487.

E-mail address: rarendt@ely.pg.gda.pl

the choice of parameters for control and adjustment

systems, the evaluation of static and dynamic properties of

the designed systems, as well as the development of the

methods of system diagnosis.

The application developed in the expert system Exsys

Developer V.7.0, Multilogic Inc. aids the design process of

control devices of ship power systems and performs two key

functions:

† enables collection of the data describing the structure and

elements of the designed subsystem and evaluation of the

correctness to the substance and form of the design,

† creates M-file of SIMULINK and enables a series of

simulation investigations in order to evaluate the static

and dynamic properties of the designed subsystem.

The implementation of simulation investigations in the

expert system required systematization and a series of issues

had to be resolved, which included:

† choice of model structures of system component

elements that would ensure close typology of a

simulation model in regard to project diagram, which

facilitates the development of the knowledge rules

creating simulation models of the system (Arendt, 1998);

† defining criteria and developing methods of model

parameters selection that would ensure reliability of

modeling of certain types of real objects (Arendt, 2001);

† developing mathematical model libraries of system

component elements (Arendt, 1998, 2001, 2003);

† normalization of system evaluation criteria in classi-

fication societies’ regulations.

In this paper the following issues are included: knowl-

edge representation for the designed power subsystem, rules

evaluating correctness of the completed design as well as

the algorithms of simulation models creation of the designed

subsystems.

2. The structure of the expert system

Basing on a detailed analysis of the scope of activities

performed in particular stages of the ship power systems

design, the expert system was assumed to carry out two

main functions (Kowalski, Arendt, Meler-Kapcia, &

Zielinski, 2001):

† preliminary design encompassing bidding project and

commission project,

† main design encompassing technical project.

In each of these tasks we can distinguish some basic

functions, such as:

† Gathering input information on the designed ship and the

equipment selected for installation in the engine room.

The information should be stored in the database of

multi-access mode. The system can check their

completeness.

† Searching databases for a completed design in order to

find identical or similar solutions of control devices.

† Choice of solution on the basis of up-dated system base

and automation components base. The system shows

either the existing solutions of the completed designs or

the lack of solutions.

† Turning to a user (a designer) for a decision in case of the

lack of ready-made solutions or in case of possible

alternative solutions (ambiguous solutions).

† Displaying on a screen (as schemata, description or

index) the achieved fragmentary solutions (for example

solutions concerning particular objects or ship power

subsystems) for a user to accept.

† Taking into account the requirements of a given

classification society and international conventions

during the inference process (searching for solutions).

† Generating the outcome design documentation in a form

of a technical description of automation, lists and other

text files, schemata, graphic projects as well as data

(databases) to be used by other systems.

† Data preparation and conducting simulation investi-

gations of the designed control devices.

The realization of the functions listed above required a

complex structure of the expert system, whose elements are

presented as a block diagram in Fig. 1.

3. Knowledge representation of a designed ship

power subsystem

Decision-making on the design aided by an expert

system consists of choosing (assigning) the values for (to) a

chosen set of variables. A set of variables with possible

variants of values describes a knowledge of the designed

system necessary for creating models and conducting

simulation investigations in the program environment of

MATLAB/SIMULINK.

Fig. 1. The general structure of system with the database.

R. Arendt / Expert Systems with Applications 27 (2004) 493–499494

For each of the designed subsystems, the following data

are defined:

(a) component elements:

† general types of elements such as engine, a

coupling, a generator;

† particular types of elements e.g. Diesel engine ZV

40/48 Zgoda-Sulzer;

† the number of elements for a given general type.

(b) structures:

† indication of an initial element

† connections of an initial element with an inter-

mediate element;

† indication of subsequent intermediate elements;

† further connections of intermediate elements;

† indication of final elements and their connections.

In the recently developed application of the expert

system the ship power subsystems designer can enter data in

the dialog mode. The program asks for the following data:

† the number of general type component elements of

subsystem;

† the proper names of the used elements;

† the sequence of elements transforming energy, starting

with the initial element until the final one.

A finite set of component elements of the designed ship

power subsystems was assumed. One can use in maximum:

† 4 diesel engines (DIESEL1–DIESEL4),

† 4 shafts (SHAFT1–SHAFT4),

† 4 couplings (COUPLING1–COUPLING4),

† 4 control pitch propeller screws (CONTRPROP1–

CONTRPROP4),

† 4 constant pitch propeller screws (CONSTPROP1–

CONSTPROP4),

† 3 summing torque gears (SUMMGEAR1–SUMM-

GEAR3),

† 2 distributing torque gears (DISTRGEAR1 – DIS-

TRGEAR2),

† 2 shaft generators (SHAFTGENER1–SHAFTGENER2).

The collected data are set up in a frame (file structur.frm).

In Table 1 there is an example of the data that describe the

propeller subsystem consisting of two middle-speed Diesel

engines, working through couplings, a summing torque

gear, a shaft for a constant pitch propeller screw.

The data collected in the frame are displayed on screen in

order to be accepted by the designer as correct.

4. The choice of component element model structures

of a ship power subsystem

The task of ship power subsystems is to generate and

convert mechanical energy—propeller subsystems as well

as to generate and convert electric energy—electric power

subsystems. The system should be analyzed as a coherent

unit because the power link chain may include, for example:

– Diesel engine—mechanical energy source,

– synchronous generator—mechanical/electrical energy

converter,

– induction motor—electrical/mechanical energy con-

verter,

– propeller screw—the receiver of mechanical energy.

The analysis of ship power systems is complicated by a

great variety of the employed devices: the ones generating,

transmitting and receiving mechanical energy, mechanical

energy converters, electric energy receivers as well as

electric energy converters. The applied technical solutions

allow for various configurations of the employed elements,

which involves the amount and variety of the employed

elements together with the possible topology (Arendt,

1998).

The ship power systems design requires qualitative and

quantitative analysis in static and dynamic state of a system

Table 1

Frame with collected data describing the designed propeller subsystem of

a ship

Element Name Connections

DIESEL1 PC3V COUPLING1

DIESEL2 PC3V COUPLING2

DIESEL3

DIESEL4

SHAFT1 W1 CONSTPROP1

SHAFT2

SHAFT3

SHAFT4

COUPLING1 SP1 SUMMGEAR1

COUPLING2 SP1 SUMMGEAR1

COUPLING3

COUPLING4

CONTRPROP1

CONTRPROP2

CONTRPROP3

CONTRPROP4

CONSTPROP1 S2

CONSTPROP2

CONSTPROP3

CONSTPROP4

SUMMGEAR1 PRZ1 SHAFT1

SUMMGEAR2

SUMMGEAR3

DISTRGEAR1A

DISTRGEAR1B

DISTRGEAR2A

DISTRGEAR2B

SHAFTGENER1

SHAFTGENER2

R. Arendt / Expert Systems with Applications 27 (2004) 493–499 495

and, in this case, simulation investigations are found to be

very useful. The goal is to organize the component elements

model structures is such a way that they could be directly

connected to each other according to the assumed

configuration of the ship power system.

The automation of creation of ship power system

simulation models based on schemata and descriptions of

a design requires maintenance of topological consistence

between a designing scheme and the developed structure of

simulation model, which is not always attainable since some

essential state variables can be confounding in nature.

The creation of a power system simulation model must

take into account a cause and effect relation between the

subsequent component elements of a system. Among the

elements models the state variables are ’exchanged’ that

ensure a correct cause and effect description. The choice of

structures of component element models mainly consists in

defining the set of state variables conveyed to the other

models.

Defining as state variable the torque and angle paths in a

model of unit propeller seems to be very convenient. During

simulation investigation of propeller units it is simple to

define torsional stress of shafts of the component elements

and the conditions exceeding the accepted values. The speed

and angle acceleration values result from the angle path

changes in the time and are easy to obtain with the use of

differential blocks.

In Figs. 2 and 3 a diagram of an exemplary propeller unit

is shown as well as the structure of its simulation model.

The choice of state variables: voltage amplitude, current

amplitude, frequency, and phase shift in power subsystems

models enables the monitoring of all important parameters

in electric circuits.

In Figs. 4 and 5 an exemplary structure of a tubular rudder

and the structure of its simulation model are presented.

The given analyses show that it is possible to match

the structures of a simulation model of propeller unit

component elements that ensure the topological closeness

between simulation models and system design schemata.

The only difference appears in the number of the conveyed

data (signals), which results from the different level of

abstraction in the designer’s model—the design schema and

the simulation model.

The structures of a model of a ship electrical power

system show smaller topological consistence; however,

simple production rules were provided to create serial and

parallel circuit models of energy receivers.

The obtained structures of simulation models enable

the algorithmization of simulation model production

procedures on the basis of the design schemata of a ship

power system. In order to describe a design schema in the

form of a list of component elements and a list of defined

connections, one can generate simulation program files,

describing simulation models of power systems.

5. Algorithms of formal and substantial correctness

evaluation of collected data

In order to evaluate the correctness of the collected data,

the knowledge in the form of production rules is used. The

rules evaluate the correctness of the employed elements and

the assumed structure of the designed power subsystems.

Some of the principles are presented:

† it is unacceptable to connect directly two Diesel engines

(10/10);

† between a Diesel engine and a propeller screw an

intermediate element—a shaft should be used (5/10),

† if Diesel engines work parallely through an summing

gear, then between the Diesel engine and the gear a

coupling should be used (6/10);

Fig. 2. An exemplary structure of a propeller unit, the following symbols

are accepted: 1, middle speed Diesel engine; 2, a flexible coupling; 3, a

friction clutch; 4, a gear; 5, a constant pitch propeller screw.

Fig. 3. The structure of a model of the propeller unit, shown in Fig. 2.

Fig. 4. An exemplary structure of a tubular rudder; the following symbols

are accepted: 1, middle speed Diesel engine; 5, a control pitch propeller

screw; 6, generator; 7, induction motor.

Fig. 5. The structure of a model of the tubular rudder, shown in Fig. 4.

R. Arendt / Expert Systems with Applications 27 (2004) 493–499496

† an output torque of a Diesel engine should not be

transferred to another one by a shaft (10/10);

† many shafts should not be used in one line transferring an

output torque (5/10);

† on the shaft output a coupling should not be used (2/10);

† on the shaft output an adding gear should not be used

(2/10);

† on the shaft output a distributing torque gear should not

be used (2/10);

† on the shaft output a shaft generator should not be used

(2/10);

† an output torque of an adding gear should not be

transferred to a diesel engine (10/10);

† between an summing gear and a propeller screw a shaft

should be used (3/10);

† an output torque of coupling should not be transferred to

Diesel (10/10);

† two couplings should not be connected in series (10/10);

an output moment of a distributing torque gear should not

be transferred to a diesel (10/10);

† between a distributing gear and a propeller screw an

intermediate element—a shaft should be used (5/10);

† between a coupling and a propeller screw an intermediate

element—a shaft should be used (5/10);

The rule concerning the last principle has the following

form:

Rule number: 94

IF:

[CONNCOUPLING2] ¼ ”CONTPROP1” OR

[CONNCOUPLING2] ¼ ”CONTPROP2” OR

[CONNCOUPLING2] ¼ ”CONTPROP3” OR

[CONNCOUPLING2] ¼ ”CONTPROP4”

THEN:

INCORRECT STRUCTURE OF THE SUB-

SYSTEM—THE COUPLING OUTPUT CON-

NECTED TO THE SCREW WITHOUT THE

INTERMEDIATE ELEMENT—A SHAFT—

CONFIDENCE ¼ 5/10

During the formulation of production rules evaluating the

formal correctness of the collected data the following

connections were taken into account:

The rules evaluating the correctness of the collected data

indicate the achievement of the adopted goal within the

accepted confidence (0/10–10/10). Depending on the goals

shown by the rules and its confidence, the rules determine the

further work of the program. The following cases are

possible:

1. the collected data are not complete, wrongly correlated or

the designed subsystem shows an unacceptable structure;

2. the collected data show an untypical structure of the

designed power subsystem;

3. the collected data are correct and complete.

In the case 1 a designer can finish the program or restart

the data collection describing the designed power sub-

system. In the case 2 a designer can restart the data

collection describing the designed subsystem or go on to a

simulation session. In the case 3 the program starts a

simulation session without the help of a designer.

6. Creation of simulation models of power subsystems

The data describing elements and the structure of the

designed subsystem collected in a frame (in the file

structur.frm) are the basis of simulation model

description.

The process of creating M-files of a simulation program

SIMULINK can be divided into two basic parts:

† the subsequent adds of M-files containing the description

of component element models to the M.-file of the

simulation model of the designed propeller subsystem;

† the creation of a graphical image of the simulation

model of the designed subsystem.

The created graphical image of the power subsystem

model is divided into discrete fields, to which row and

column indices ðm; nÞ are assigned. For an initial element

DIESEL1 a field (1,1) is assigned, a following elements

connected with DIESEL1 have fields ð1; 2Þ…ð1; nÞ; until

finite element is reached. A presence in the row of a

distributing torque gear (2 outputs) gives effect of assigning

fields with indices ð2; nÞ to elements connected with second

output. Subsequent Diesels are placed in fields ðm þ 1; 1Þ;

and the process of assigning field indices to subsequent

elements is repeated. The source of information concerning

the following connections of elements is a frame

structur.frm. The evaluated data are collected in a frame

placement.frm, which is used at the creation of M-file of

simulation program.

To define algorithms of creation of simulation program

M-file describing subsystem model, the formal analysis of

M-file description is done as well as processing of semantic

and linguistic data. One can say, that created M-file

describing the simulation model of the designed power

subsystem should have the following structure:

† the file name, declarations and formal initial descrip-

tions;

† the following base parts of file describing models of used

component elements, each finished with a line declaring

parameters of a macro-block placement on a graphical

image;

† the set of lines declaring macro-block connections,

results from an accepted structure of the designed

subsystem;

† the declarations and formal close of the file.

R. Arendt / Expert Systems with Applications 27 (2004) 493–499 497

According to what is stated the following files are edited:

† a file containing the name subsyst.m, declarations and

initial descriptions;

† files containing models of engines; files containing models

of shafts; files containing models of a flexible coupling

and friction clutches; files containing models of constant

pith propeller screw and controllable pith propeller

screws; files containing models of an adding moments

gears; files containing models of a distributing moments

gears; files containing models of shafts generators;

† a file with declarations and formal M-file close.

The procedure of the M-file creation with description of

the model of the designed power subsystem open the file

subsyst.m (clears an existing one). Next it adds to file

subsyst.m the name, declarations and formal initial

descriptions. The subsequent rows and columns of the

pleacement.frm are reviewed and for non-zero indexes of

rows and columns the subsequent files with component

element models are added. After each file is added the

procedure of calculating a placement of a macro-block in

the graphical image is started. The added file with a line

declaring placement parameters is complemented. After

review of the all pleacement.frm the procedure calculating a

placement of subsequent lines describing the macro-block

connections in the graphical image is started. The procedure

of the file subsyst.m creation after adding the file with

declarations formal file close is finished. In the procedure of

M-file creation the knowledge in form of production rules

took part. The algorithm of the creation of simulation

models of power subsystems in the Fig. 6 is shown.

7. An exemplary session of ship power subsystem design

A power system in configuration: middle speed Diesel

engine working through a coupling on constant pitch

propeller screw is considered. A designer introduces to the

expert system data describing component elements and the

structure of the subsystem. The production rules check a

formal and substantial correctness of collected data of the

designed subsystem. Because there are no mistakes, the

expert system creates a frame structur.frm with data of

the designed subsystem and creates its simulation model

in environment of MATLAB-SIMULINK program (Fig. 7).

An image of middle speed Diesel engine with governor

(macro-block DIESEL1), two-stage coupling (macro-block

COUPL1) and a constant pitch propeller (macro-block

CONSTPROP1) is displayed.

Accepting the following binary input signals:

† 0 s step to nominal value of angle velocity of Diesel

engine,

† 12 s the coupling is switching on (I stage),

† 15 s the coupling begins to work on II stage,

† 30 s the coupling is switching off,

the simulation investigations are carried out.

For a simulation the following parameters are accepted:

† Runge-Kutta 5 method of numerical integration,

† time of simulation 40 s,

† maximum step size 0.01,

Fig. 6. The algorithm of the creation of the simulation models of the

designed power subsystems.

Fig. 7. A structure of a simulation model of the ship power subsystem.

R. Arendt / Expert Systems with Applications 27 (2004) 493–499498

† minimum step size 0.0001,

† tolerance 0.001.

The achieved results of simulation investigations are

presented in Fig. 8. During 4 s an angle velocity of Diesel

engine achieves a value close to nominal one—the difference

depends on a steady-state error of governor. During the

coupling switching on a disturbance of angle velocity in a

shape of dumping oscillations are observed. At 15 s due to

transferred by coupling load torque (plot 3) an angle velocity

of the propeller is growing from null to 0.6 nominal value

(plot 2). In Fig. 8 a nominal value of load torque is multiplied

by 0.3 to achieve more clear plot. At 15 s the II work step of

coupling is switching on; the transferred load torque is

growing and at 16 s the shaft is fully coupled. Plot 4 shows the

moment of equal angle velocities of both halves of coupling

(step from 0 to 1). After uncoupling a great oscillations of

angle velocity and load torque (plots 1 and 3) due to step

change of load torque and shaft flexibility are observed.

Angle velocity of the propeller inertial goes to null.

The presented plots quite good map real phenomena

which are observed during the switching on and switching

off a coupling. It is necessary to correct at the I step of

coupling work a value of transferred load torque and

eventually a time of its work.

8. Concluding remarks

The process of designing ship automation is well

qualified for application of an expert system, as it is a

complex process in which various kinds of information are

processed. This processing is based, to a large extent, on

experience and intuition of experts (designers) and should

be carried out in as short time as possible. On the other hand,

the result of the designing process should have quality, as it

directly influences the operating advantages of the ship,

including its safety.

There is not fully systematised knowledge about the

designing process in designing ship systems automation,

because, often, the only source are the people involved in

solving problems in this area. Apart from that, there is yet

the output of many projects made on ships. This knowledge

should be subject to structuralisation—it should be

systematised and documented, which is, in part, done in

the expert system (in the knowledge database and in the

form of reports). To a large extent, this takes place also in

the database during the generation of technical specifica-

tions using similar ships, and during making result

documents such as: materials and suppliers lists, and at

further stages—the statement of checking and measuring

apparatus for the engine room automation.

The developed application of simulation investigation

makes possible to run very effective simulations of the

designed systems. The process of acquiring data describing

the designed power subsystem and automatic transition to

an investigations session takes only a few minutes. The

investigations allow for an assessment of basic technical

requirements regarding transient processes taking place

during: start-up, shut-down, change of parameters of the

subsystem, as well as the appearance of factors disturbing

the system’s operation.

Acknowledgements

This research was supported by the Polish Scientific

Research Committee under grant No. 4T11A 009 25.

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Fig. 8. Simulated responses of ship power subsystem; angle velocity of

Diesel engine (1), propeller (2), load torque of Diesel (3), a state of coupling

(4). X-axis in seconds, Y-axis in relative values 1 ¼ 100%.

R. Arendt / Expert Systems with Applications 27 (2004) 493–499 499