Selection and simulation of marine propulsion control system

2 2 SEP 1982

ARCHla

SELECTION AND

SIMULATION OF MARINE

PROPULSION CONTROL

SYSTEMS

By C. Pronk, M.Sc.

Lab. v.

Scheepsbouwkunde

Technische Hogeschool

Delft

o

erez,,E

p

SELECTION AND SIMULATION OF MARINE

PROPULSION CONTROL SYSTEMS Abstract

A brief mathematical description is given of the

propulsion system,consisting of a power source and a controllable pitch propeller. The selection of con-trol parameters is discussed for both diesel engines and gas turbine installations.

After selection and design of the control system,

the behaviour of the entire plant during manoeuvres can be simulated by means of a mathematical model

programmed for a computer. This is illustrated by

some typical examples.

PAR. 1 INTRODUCTION

The necessity of integrated control systems for the

entire propulsion system of ships equipped with controllable pitch propellers has already been

re-cognized for years. Several types of control systems have been applied with various kinds of hardware.

Descriptions of control systems already installed

can be found in references t 1 J, [2), 13), [4].

The type of control system to be chosen usually

depends on the ship's mission and the kind of prime mover used. This paper deals with the selection of

the software of the control system. Before a

se-lection can be made, the available possibilities should be specified.To this end the static behaviour

of the propulsion system will be represented by a simple mathematical model consisting of two alge-braic equations. On the basis of this model con-clusions can be drawn about the possible types of

control systems.

Once the control system has been selected the de-Environment mean values (long term) i.a. waves draught System ship propellers prime movers alternators disturbances (short term) i.a, waves wind control signal Servo mechanisms

Fig. 1. Scheme of control process

measured system parameter values

Controllers

sign requirements can be formulated. These require-ments must be satisfied both statically and

dynam-ically. The latter condition applies in particularly to those parts of a control system whose purpose is not to obtain and maintain a static position, but to control a dynamic condition such as a crash stop. All the means for the development of the software

model are then available. The next step in the design

sequence will be the conversion of the software model into hardware.

This paper, however, is limited to a consideration of the software. Once the control system has been

designed, there still remains the problem of checking

its performance and stability. Computations with the linearized model are a good yardstick in this

respect.

Nevertheless, simulation of the performance of the entire control process by means of an electronic

computer offers the possibility for fast and accurate

computations of the non-linearized model.

We have therefore developed a program for our digital computer in which the hardware of any

control system can be presented in a mathematical model in, as detailed, a form as desired. The program also contains the mathematical model of the propul-sion system, so that the simultaneous performance of both systems is simulated.

In this way any control system can be tested and

optimized before it is installed aboard a ship.

PAR. 2 STATIC CONSIDERATIONS

A scheme of the control process is given in Figure 1 and each part of it will now be dealt with separately.

Sensors

Measuring discrepancy controlled parameters

actual parameter values

2.1 The controlled system

The controlled system consists of a translating mass the ship's mass and one or more rotating

mass-es the various shaft lines with their connected

masses. Translation is caused and maintained by hy-drodynamic and aerodynamic forces, namely thrust and resistance. The torques which cause and

main-tain rotation are of the following kinds: hydrody-namic torque of the propeller, mechanical torque of the prime mover and electrical torque of the shaft driven alternators. The system is assumed to experience only these two motions that is,

translation and rotation. All other motions, such as pitching and yawing, will be considered as dis-turbances which generate forces and torques of hydrodynamic nature influencing the translation and rotation.

The controlled system is now defined and can be

presented by a mathematical model.

translation of ship:

T(P,n,Vs,w,E) R(Vs,t,E) = 0 (1)

props

rotation of single shaft line: Fig. 2. Equilibrium positions for 2 different environmental

conditions

E Op, (F,n,E) _{-77R Qprop (P, n,} w, E)

pm

Qf (n) _Qalt(n,E) =0 (2)

Where:

propeller thrust

ship resistance, hydrodynamic and aerodynamic

Pm prime mover torque, related to propeller

shaft in case of reduction gear

prop propeller torque

Qf frictional torque

Qalt = alternator torque, related to propeller

shaft in case of reduction gear

propeller pitch

rotational speed of propeller shaft

Vs ship's speed

wake fraction

= thrust deduction factor 77 R relative rotative coefficient

fuel rate

= environmental variable

Except for the frictional torque, each force and torque depends on the environmental condition

(see Fig.2).To indicate this dependency the symbol E is introduced. Table 1 gives the environmentai

condition corresponding to each force and torque. Table 1: Environmental conditions influencing the forcesand torques which act on the propulsion system.

--AIIIIIIIIII

MEM

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-" 3 E P e c E "7!-AIDN:

WAWA

Witarn

Wag.

CMGTANT

.

Apr

WAS

A

Filir

.,..,

RE sisTANc&

;FAZZIAMV

.. . SHAFT SPEED

force or torque enviroonnental condition

thrust sea state

ship's motions

resistance sea state

draught water depth ship's motions

fouling wind force

prime mover torque ambient temperature

propeller torque sea state

ship's motions

alternator torque electrical power absorption =

=

=

EN

In the static formulation the environmental con-ditions are supposed to be known and should be

considered as a given input into the system.

The parameters w, t and ri R , representing the

inter-action between the propeller and hull, depend for a particular ship on the propeller loading and are

therefore to be taken as dependent variables (see

Fig. 3, which has been derived from ref [51). For

the sake of simplicity we will limit our consider-ation to only one shaft line.

OR: t

PROPELLER LOADING CT

Fig. 3. Wake fraction and thrust deduction factor as a

function of propeller loading

Thus in equations (1) and (2) there are 4 variables left to consider:

ship's speed

rotational speed

propeller pitch fuel rate

It is obvious that only two of these variables are

in-dependent and the others are determined by the

relations (1) and (2).

The 6 possible combinations of independent vari-ables are given in Table 2. For each subsequent shaft line to be considered, 3 more independent

variables, and consequently 2 more degrees of

freedom, appear.

Table 2 Combination of independent variables for single-screwship.

2.2 Input of desired parameter values into the

control system

In the preceeding section it has been shown that the

propulsion system of a single-screw ship allows us to feed 2 desired parameter values into the control system. But since equations (1) and (2) are non-linear, more than one pair of roots will result. Par-ticularly in the case of equilibrium position in the

vicinity of zero pitch angle, the roots will differ

little in absolute value.

It is only if the pitch angle and/or ship's speed are

selected as independent variables, that the

depend-ent variable functions will have single values. This means that in the so-called manoevring range of a

ship either pitch or ship's speed should be con-sidered as independent input parameters in order to ensure that the desired sailing direction is ob-tained. If combinations 5 or 6 of Table 2 are used in the manoeuvring range, the actual and desired values of independent variables can be equal for

sailing both ahead and astern.

In equations (1) and (2) the ship's speed Vs has been considered as an independent variable. For given environmental conditions, however, it is also

possible to treat the thrust T as independent instead of the ship's speed Vs and consequently consider Vs as dependent on T (see Table 2, column 3). This

re-placement of Vs by T is correct when the thrust deduction factor and wake fraction are assumed to be a function of propeller loading (see Fig. 2). In this case equation (1) can be transformed into:

R(Vs, CT) = T (3)

in which: CT =

T r D2

p V ,2 _{(1 - \ A1)2}

2 - 4 (4)

with : D = propeller diameter

When equation (4) is inserted into equation (3) it is obvious that T is a function of Vs only and can therefore be treated as an independent variable

in-stead of Vs. 0.4

\

1 DEDUCTION WAKE FRACTION

- - -,

THRUST FA w 0,3 0,2 N

'

t 0,1 -13 -13 -5 10 15_.-No ,3 Ns _0,4 combination of indepent variables existing type of control system combination of alternative variables name of control system Lips trademark 1

: V,

n T n 2 : Vs P T P 3 : Vs F T F

4: n

P + programmed control P.R.C.

5: n F + _QPM n load control

IPLOT 1

6: P F + load control

IPLOT 2

1 w -0,1 -0,2 I.combination I '

An alternative combination of independent variables

also exists for the couple n F. This pair gives a

single value of prime mover torque. Conversely, when Qpm and n are given, F has a single value. The type of control system in which Qpm and

are input parameters, has been installed on several

ships with a diesel engine as prime mover and is called load control.

Load control has also been applied on ships

equipped with turbines. However, this type of

control belongs to the category of control systems

with P and F as input parameters.

In this case, F has to be considered as the energy flow into the free turbine. This energy flow can be compared with the loading of the compressor part

of the gas turbine. The subset of the control system

which controls the fuel rate itself in order to ob-tain the desired loading is not dealt with in this

paper.

Once the input parameters for the control system have been selected from the above possibilities, their values must be assigned throughout the com-plete operating range of the propulsion system.

These values are not completely optional, since the

parameter values must fulfil the following require-ments in descending order of priority:

safety requirements operational requirements economic requirements

These requirements apply not only to the input

pa-rameters but also to all other dependent papa-rameters.

Obviously the safety and operational requirements have to be fulfilled under every environmental condition. This also implies that for multi-engine

drive, safety requirements should be observed when

one or more engines are disconnected from the

system.

For example, a safety requirement is

that the

maximum allowed rotational speed and shafttorque should not be exceeded.

When the safety of the propulsion system has been

considered, the avoidance of unfavourable

ope-rational conditions is the next criterion to be tested. To illustrate this, some examples are given below:

Fouling of two-stroke diesel engines should be avoided by refraining from certain combinations

of torque and shaft speed.

When shaft-driven alternators are installed a

con-stant rotational speed is often required.

A constant thermal load might be necessary for

the lifetime of gas turbines.

High adjusting forces for the propeller blades

may call for low rotational speeds combined with small pitch angles.

Maintaining maximum ship's speed in head seas makes it essential to insert a range of decreased

pitch angles into the control system in combina-tion with maximum rotacombina-tional speed.

A minimum rotational speed is normally required

for a gas turbine.

Obviously some operational requirements influ-ence the choice of the type of control system directly.

For instance, if a constant rotational speed is

re-quired one of the independent input parameters

must be the rotational speed.

If the operational requirements still leave freedom to assign values to the input parameters, economic

requirements can be formulated.

If we disregard maintenance costs etc., the

op-timum follows from the

desire to achieve

maximum total propulsion efficiency. This ef-ficiency can be expressed as:

ntOt = X nhX ripm X Tit (5)

in which : _{tot =} total propulsion efficiency

7711 = efficiency of propeller-hull

interaction

7713 M = efficiency of prime mover

= efficiency of power transmission from prime mover to propeller

shaft.

The total efficiency will be

ritot

V, x R

F x B

where B is the calorific value of the fuel.

Consequently ntot is a function of Vs, n,P,F and E.

Still considering E as known, the values of Vs, n,P

and F can be obtained for the maximum value of

/hot. This is

an extremum problem with two

boundary conditions (equations (1) and (2).).

When the forces and torques are presented as

poly-nominals, the solution can be obtained by the mul-tiplicator method of Lagrange.

Since E is supposed to be given, the values obtained

are apparently dependent on environmental

con-ditions.

This case is treated in reference [61 and it is pointed

out that with load control the deviations from the optimum are minimum.

2.3 Environmental conditions

The influence of the environment can be divided into two parts:

A part which is constant during a time interval of the order of hours and more.

A part which varies continuously; this part will be treated as disturbances in the dynamic

be-haviour.

Forces and torques which are hydrodynamic and aerodynamic in nature are influenced by both parts of the environmental conditions.

mean value and a time-varying value.

The difference between this mean value and the

smooth water resistance represents the first part of E, the time-varying value the second part.

The constant part of these forces and torques is to be treated statistically according to the method

described in reference [7].

The influence of displacement on maximum pro-pulsion efficiency can also be dealt with by means

of statistics (see ref [81).

2.4 Sensors and measuring discrepancies

It is essential that each input parameter and each

parameter which is subject to safety requirements, is measured and fed into the control system.

Furthermore, it should be noted that errors can be

introduced by each sensing device. The influence of

these errors in obtaining the desired value should be investigated for each control system.For example,

if the propeller pitch is measured by means of a

long sensing rod, discrepancies between actual and

measured pitch might occur due to a temperature

gradient along the sensing rod.

2.5 Controller

The above considerations may serve as tools for

designing a basic form of the control system. Using

the measured parameter values the first task of the

controller is to determine whether or not parameter restraints (obtained from safety requirements) are

exceeded. In this case, the measured and the desired

value of the relevant parameter must be fed into

one or two servo-mechanisms.

These servo-mechanisms are:

A mechanism which gives a correction signal for

the propeller pitch.

A mechanism which gives a correction signal for the fuel rate.

The designer of the control system has to deal with the problem of selecting a servo-mechanism (3

pos-sibilities) and of establishing a desired value which is

not necessarily the boundary value. Likewise, the

boundary value of the parameter subject to safety

requirements and the value at which the controller should react are not necessarily the same. For in-stance, in the case of overspeeding due to wind-milling the controller should react before the

maxi-mum rotational speed is reached.

If no parameter restraint is exceeded, the controller should supply the servo-mechanisms with the

mea-sured and the desired values of the independent variables. If neither the pitch nor the fuel rate are

selected as the independent variables, the designer

must combine one or two servo-mechanisms with

the independent variables.

In an alternative form of the controller, the desired value of only one independent variable is fed into

it. The controller contains a function generator which determines the desired value of the one

in-dependent variable as a function of the measured value of the other one.

PAR. 3 DYNAMIC CONSIDERATIONS

As already mentioned, the equilibrium condition

can be disturbed by three different sources: change of desired parameters

mutations of the environment

short-frequency disturbances

During the transient conditions the parameter values are time-dependent. This dependency can be

illus-trated by a mathematical model of the propulsion system. The model consists of two simultaneous

non-linear differential equations:'

translational acceleration of deceleration

T(P,n,Vs,w,E) R(Vs,t, E) =

props

M dVs

rotational acceleration or deceleration of single shaft line

CIpm (F,n,E) -71 prop (P,n,Vs,w,E)

pm Qf (n) Qalt (n,E) = 27.1 dn + 27r n dJ dP (7) dr dP dr

in which : M = ship's mass including the added mass of surrounding water,

J moment of inertia

of rotating

parts including added inertia of water surrounding the propeller, = time

The description of and methods for calculating

added mass and added inertia are given in references

[9] and [10].

Again, when the state of the environment, either

constant or time-dependent, is assumed as known,

the four variables are functions of time and their

values can be obtained by solving four simultaneous

differential equations.

Equations (6) and (7) are two such equations, the other two being obtained by the mathematical

model of the two servo-mechanisms. These equations have the form:

dP

pitch servo-mechanism = f1 (measured and dr

desired parameter values) dF

fuel rate servo-mechanism : = f2 (measured and

dT

desired parameter values)

If the type of control system is such that the pitch

(6)

di

or fuel rate is taken as one of the independent

vari-ables, the function f1 or f2 is equal to zero when

the equilibrium is disturbed by environmental

changes. The disturbances can be presented in the mathematical model as time-dependent forces and

torques which are hydrodynamic or aerodynamic

in nature.

Disturbances due to irregular waves are difficult to formulate mathematically. But in order to estimate their influence on the control system, it is sufficient

to present the hydrodynamic forces and torques by

a sine function. The amplitude is determined by a probability distribution.

If we limit ourselves to the propeller thrust,we may consider the propeller which has a certain vertical

velocity relative to the water as a propeller in

oblique flow.

In references [l fl [12] a quasi-steady method is given for calculating the propeller thrust in oblique flow. The vertical velocity of the propeller can be obtained by superposition of the vertical motion of the water and the motion of the propeller due

to the pitching and heaving of the ship.

Calculations of vertical motions of the propeller

can be made according to the method described in reference [131.

The entrance velocity into the propeller disk is also subject to probability considerations.

PAR. 4 COMPUTER SIMULATIONS

The entire system, consisting of the propulsion

sys-tem and control syssys-tem, is rather complex in

be-haviour.

When a control system has been designed, it is

de-sirable to predict its role in the behaviour of the

entire system. The solution of the four simultaneous differential equations for various manoeuvres gives

a good prediction for full size manoevring. These equations can be formulated for both the software

and hardware model.

These four equations can only be solved numerically

and a program for our digital computer has been made for this purpose. Both the static and the

dynamic model have been programmed.

A flow chart of the program is given in Figure 4,

The performance characteristics of the parts of the propulsion system are represented by polynomials

(see reference[14] ). The accuracy of the results can

be specified in the input into the program, which

adapts the size of the steps used in the Runge-Kutta procedure.

The program requires approximately 5.5 K words

core storage on an IBM 1130 computer.

The running time consists of 2 minutes for loading and initiating and about 6 seconds for computation

and data output of each step.

The number of steps (time intervals) depends on

the manoeuvre, the size of the propulsion system

and the desired accuracy.

Reading and initiating constants

Prescribe manoeuvre

by lever positions

Solve statical equations

to find start values

Form 4 differential

equations

Solve d.e. by means of Runge Kutta

Changes

in or stop

manoeuvre

Plot results

Fig. 4. Flow chart of computer simulation program

Normally each step will be from 0.25 to 0.5 of a

second with an accuracy of 0.1 degrees in pitch

angle and 0.1 revolutions per minute.

Some example of computed manoeuvresare given

in Figures 5 -7. These manoeuvres are calculated for

a 20,000 tons Freighter equipped with a diesel

Fig. 5. Accelerating manoeuvre.

Fig. 6. Decelerating manoeuvre

8 "lb I 95 L10 85 :20 -25 55 2.. 115 0 110 0 4 100 2 1105 I . 25 120 65 95 140 30 .120 110 .,100 2 3000 3 4. 000 21:5 2500 21 2000 20 1000 8 '90 19 0. 150 22 2500 t 19I 0 95 1500 500 500

DIFFERENCE IN PITCH FROM FULL AHEAD (DEGR) NUMBER OF REVS PER MINUTE,

SHIP'S SPEED (KNOTS) FUEL RATE (KG/HOUR)

DIFFERENCE IN PITCH FROM FULL AHEAD (DEGR.) NUMBER OF REVS PER MINUTE

SHIP'S SPEED (KNOTS) FUEL RATE (KG/HOUR)

$: DIFFERENCE IN PITCH FROM FULL AHEAD,(DEGIR4,

2: NUMBER OF REVS PER MINUTE a SHIP'S SPEED (KNOTS) 4: FUEL RATE (KG/HOUR)

2

TIMEJSEC

O. TIME/SEC Fig. 7. First part of crash stop (desired pitch 6 degrees,, Desired RPM 100 %)1

10 15 20 .25 3C 35 40, 2 4 10 15 20 1 25 25 5 10 15 20 --M.- TIME/SEC.. =5 10 -15 :20 21 2000 1500 1000 9 110 !105 2500 2000 1500 5 - 500 0 3 ,20 4:

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-/ ,15 75 3 0 5 3

PAR. 5 RECOMMENDATIONS

1 Since safety and operational requirements play

an important role in the design of the control

system, its design should be made in close

cooper-ation between the propeller makers, the prime

mover manufacturers and the shipbuilders.

The selection of the type of control system to be used should be based on the ship's mission and type of prime mover.

Before a new control system is installed, com-puter simulations should be made to predict the

full-size behaviour.

During the ship's trial manoeuvres should be re-corded for comparison with the computer

si-mulations.

References:

Klaassen, H.: "Automation and controllable pitch

propellers". International Shipbuilding Progress

1964.

Yonehara, N.: "Controls of Main Steam Turbine connected to a C.P. Propeller". Shipping World and Shipbuilder, July 1969.

"Die KaMeWa Ajfo Lastregelung fur

Diesel-anlagen mit Verstellpropeller". Schiff und Hafen, Heft 7/1969.

Drenth, B.W.: "Principles of the Pneumatic Hydraulic Remote Control System with Load Control". Internal Lips Technical Report,

Feb. 1970.

Bindel, S. et Garguet, M.: "Quelques Aspects du

Fonctionnement des Helices pendant Les Ma-noeuvres d'arret des Navires." Bulletin de l'As-sociation Techniques Maritime et Aeronautique,

1962

Schanz, F.: "Lieber die Gemeinsame Steuerung von Schiffsdieselmotor und Verstellpropeller". Schiffstechnik 1963, Heft 51/52.

Zubaly, R.B.: "Average Power Increase due to Waves for a Ship on a Specific Trade Route",

International Shipbuilding Progress 1970. Bennett, R. et al.: "Controllable Pitch Propellers in Large Stern Trawlers". Transactions of The

Institute of Marine Engineers, August 1968.

Rusetskiy, A.A.: "Hydrodynamics of controllable

pitch propellers". Shipbuilding Publishing House, Leningrad, 1968.

Kruppa, C.F.L.: "High Speed Propellers, Hydro-dynamics and Design". Course for Engineers, University of Michigan, October 1967.

Gutsche, F.: "Untersuchung von Schiffsschrau-ben in Schrager Anstromung".

Schiffbauforschung 3/4/1964

Taniguchi, K.et.al.: "Investigation into the Pro-peller Cavitation in Oblique Flow".

M.T.B. 010045, March 1967.

Gerritsma, J.: "Behaviour of a Ship in a Seaway".

Delft Shipbuilding Laboratory Report no.25a.

Pronk, C.: Schroefkarateristieken ontwikkeld in

Polynomen".

Lips Technical Report no 1120 - 7111.

4. 5. 6. 7. 8. 9. 10., 11,. 112.,