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Propulsion Control

Part 1 of 2

Øyvind Smogeli and Asgeir J. Sørensen,

Department of Marine Technology, Norwegian University of Science and Technology,Otto Nielsens Vei 10, NO-7491 Trondheim, Norway

E-mail: Oyvind.Smogeli@ntnu.noAsgeir.Sorensen@ntnu.no

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OutlinePart 1: Modelling• Motivation and problem formulation• Mathematical modelling• Propeller characteristics• Propeller losses and dynamics• Experimental results

Part 2: Local control• Conventional local thruster control• Combined power and torque control• Sensitivity to thrust losses• Propeller load torque observer• Torque loss calculation• Anti-spin control• Simulation results• Experimental results

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• Thrust and power allocation (over/under actuated systems)

• Pitch/rpm/torque/ power control

• Combined torque and power control

• Anti-spin thruster control

• Combined rudder and propeller control

POWER

PITCH or RPM CONTROL

VARIABLETORQUE

CONSTANTTORQUE

TORQUE CONTROL

POWER

Propulsion and Thruster Control

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Dynamic Positioning and Transit

• Demand for vessels to conduct all-year operation in harsh environment and extreme conditions

• High positioning accuracy required

• DP system and propulsion system must be robust to any single failure

• It’s a trend towards physical and functional integration between the power and automation systems

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Motivation

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Thrusters affected by waves, current and vessel motion:

– Rapidly changing operating conditions

– Load fluctuations

Motivation

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Thrusters affected by waves, current and vessel motion:

– Rapidly changing operating conditions

– Load fluctuations

Effects of bad low-level thruster control:

– Danger of blackout

– Wear and tear of the propulsion system

– Increased fuel consumption

– Reduced thrust capability

Motivation

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Propeller Types: FPP and CPP

Two controllable parameters: Shaft speed and pitch– Shaft speed: Fixed pitch propellers (FPP)

• No hydraulics needed to control the pitch• Preferable for electric motors with variable speed• Optimized for one advance speed

– Pitch: Controllable pitch propellers (CPP)• Used for direct-driven shafts when varying thrust is

needed• Fast response, produces thrust in two directions• Better hydrodynamics for varying advance speed

– Consolidated control: Combination also possible (CPP)• Typically two or three speed setpoints• Varying pitch dynamically

Pitch P measured at 0.7R, commonly given as pitch/diameter ratio P/D

Shaft speed given as RPM, n = RPS = RPM/60 or ω = 2n

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Conventional Propulsion Control

RPM INPUT

POWER

RPM

PITCH INPUT

FPP:Fixed Pitch Propeller with controllable speed (RPM)

CPP:Controllable Pitch Propeller withfixed speed

POWER

PITCH

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Propulsion

Courtesy to Rolls-Royce Marine: http://www.rolls-royce.com/marine/default.jsp

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Electric Motor

Ventilation Unit

Bearing

Slipring Unit (Power/Data Transmission)

Air Cooling

Bearing, Shaft Seals

FP Propeller Shaft Line

Installation Block

Hydraulic Steering Unit

Propulsion

Courtesy to ABB Marine: http://www.abb.com/

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Functionality: Control Modes• Station keeping models• Marine operation models• Slender structures• Multibody operations

0 1 2 3 4 5 6 7 ….. Speed [knots]

Station keeping

Marked position

Low speed tracking

High speed tracking/Transit

• Manoeuvring models • Linearized about some Uo

• Sea keeping • Motion damping

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Control Modes: - Speed range and actuation

Control actuation: • Main propellers/pods • Tunnel thrusters• Azimuthing thrusters

0 1 2 3 4 5 6 7 …..Speed [knots]

Station keeping

Marked position

Low speed tracking

High speed tracking/Transit

Control actuation: • Main propellers/pods • Rudders

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Propeller Blade Model

Integrate over propeller blades to get total thrust and torque

i

arctanVa r

i arctanVa 1

2UA

r 12UT

arctanUT

UA

arctan PD

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Propeller Characteristics

is the density of wateris the propeller diameteris the propeller speed (RPS)

The non-dimensional thrust and torque coefficients are given as:

Typical characteristics of actual propeller thrust and torque :

wDn

is the advance speed

P/D is the pitch ratio

Ae/AO

is the propeller expanded blade area ratioZ

is the number of blades

Rn is the Reynolds number

t is the max. blade thickness

c is the propeller chord length

VaKT f1Va ,n,D,P/D,Ae/A0 ,Z,Rn , t/c

KQ f2Va ,n,D,P/D,Ae/A0 ,Z,Rn , t/c

Ta wD4KT|n|n

Qa wD5KQ |n|n

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Desired Thrust and Moment for Speed Controlled Propellers

The desired/reference thrust and torque coefficients for zero advance speed, KT0 and KQ0, are used for control since Va is unknown to the control system:

Tref kuref wD4KT0 |nref |nref

Qref wD5KQ0 |nref |nref

nref is the desired propeller speed (RPS)

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Open Water Tests as Function of Advance Ratio

0 0.2 0.4 0.6 0.8 1 1.20

0.2

0.4

0.6

0.8

Advance ratio

o

KT

10KQ

P/D 0.7, 0.89 and1.1

Rn =2 6106, Z=4, D =3.1 m, and AE/Ao =0.52

J Va

nD

Open water propeller efficiency in undisturbed water:

Work done by propeller in producing a thrust/work required to overcome shaft torque

o TaVa

2 nQa J

2KT

KQ

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Linear Thruster Characteristics

KT 0 1J,

KQ 0 1J.

KT0 KTVa 0 KTJ 0 0

KQ0 KQVa 0 KQJ 0 0

Common simplification:

The thrust and torque are then expressed as:

The nominal thrust and torque coefficients, used for control since Va is unknown to the control system (simplest possible representation):

J Va

nD

Advance ratio:

Ta wn|n|D4 0 1Va

nD wD4 0n|n| D3 1|n|Va Tnnn|n| Tnv |n|Va

Qa wn|n|D5 0 1Va

nD wD5 0n|n| D4 1|n|Va Qnnn|n| Qnv |n|Va

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4 Quadrant Thrust Model

1st quadrant: Va 0,n 0

2nd quadrant: Va 0,n 0

3rd quadrant: Va 0,n 0

4th quadrant: Va 0,n 0

Fixed pitch propeller in the Wageningen series:

CT Ta

12 Va

2 0. 7 nD2 4D2

CQ Qa

12 Va

2 0. 7 nD2 4D3

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Controllable Pitch PropellerFor a given pitch, the propeller

behaves like a fixed pitch propeller:

A “cut” along one P/D value gives the conventional KT curve as function of advance ratio.

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Propulsion Efficiency (1)Axial water inflow velocity to the propeller Va due to vessel velocity U and wake fraction number w is:

The hull reduces the inflow to the propeller 0 < w < 0.4

Va U1 ww wp wv U1 w

is the wake fraction caused by the wave motion of the water particles

ww

is the wake fraction caused by so-called potential effects for a hull advancing forward in an ideal fluid,

is the wake fraction caused by viscous effects due to the effect of boundary layers

wp

wv

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In steady state the effective thrust is equal to the total resistance R:

Overall propulsion efficiency is given by the ratio of useful work done by the product of drag and ship speed divided by the required work to overcome the shaft torque:

Propeller suction on the aft ship given by the thrust-deduction coefficient 0 < td < 0.2 increase resistance (drag)R Ta1 td ds Tatd

r B o

J

2KTKQB

o

KQ

KQB

p RU2 nQa

h o r m

h 1 td1w

where

Propulsion Efficiency (2)

Relative rotational efficiency

Hull efficiency in the range of 1-1.2

m Mechanical efficiency in the range of 0.8-0.9

Open water propeller efficiency in undisturbed water o TaVa

2 nQa J

2KT

KQ

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Propeller and Thruster Losses

• The vessel hull– Coanda effect– Tunnel thruster suction

losses– Thrust deduction

• Velocity fluctuations– In-line change of advance

velocity– Cross-coupling drag

• Thruster-thruster interaction• Ventilation and in-and-out-of

water effects

The actual thrust Ta and torque Qa are affected by:

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Thrust Losses: General formulation

xp

p

The actual thrust Ta and torque Qa may be expressed as:

where:

represents dynamic states (vessel motion, propeller submergence, environmental conditions).

represents propeller dependent parameters.

hT and hQ are termed the thrust and torque reduction functions

Ta hTn,xp , p1 tdTref fTn,xp , p,

Qa hQn,xp , pQref fQn,xp , p,

#

#

Thrust and torque loss factors:Ta

Tref hTn,xp , p1 td T KT

KT0

Qa.

Qref hQn,xp , p Q

KQ

KQ0

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Example: Thrust curves for varying KT

-200

0

200

400

800

Thrust Tth [kN]

-3 -2 -1 1 2 3 4 5

Speed n [RPS]

KT = 0.4

KT = 0.36

KT = 0.30

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Thrust Losses: Velocity fluctuations

Axial in-line fluctuations:

Variation in Va

and hence thrust coefficient

Transverse fluctuations:

Cross-coupling drag

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Thrust Losses: - Thruster-thruster interaction

Loss of thrust because of:

V j

Va

Vt

Va V j,a+

V t+ V j,t

• Change of advance velocity due to inline jet velocity component Vj,a, which leads to change in the thrust coefficient

• Cross-coupling drag due to transverse jet velocity component Vj,t

• Other interaction effects, harder to model

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Thrust Losses: Coanda effect• Propeller slipstream is drawn towards the hull and

deflected• Severe loss of thrust for unfortunate thrust angles

F Va

Low-p ressurereg ion

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Propeller:• D = 250 mm• Z = 4• P/D = 1• EAR = 0.55

Duct:• L/D = 0.5• L = 118.8 mm

• Di = 252.1 mm

• Ducted propeller• Varying shaft

speed / loading and submergence

• Measuring thrust and torque

• Steady state

Cavitation Tunnel Experiments: - Ventilated ducted propeller

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Experiments: - Cavitation tunnel results

Thrust for Ja = 0.2

Ventilation

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Experiments: - Cavitation tunnel results

Thrust for Ja = 0.2

Ventilation

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Experiments: MCLab• Same ducted propeller as in the cavitation tunnel• Operating in waves with ventilation• Varying submergence and propeller speed

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Experimental Results MCLab

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Experimental Results MCLab: Waves

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Experiments: MCLab time series

Thrust for increasing shaftspeed in h/R = 1.5

Loss of thrust duringventilation

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Modelling of Ventilation Loss Effects

•Ventilation loss model for simulation

•Experimental results from cavitation tunnel at NTNU

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Propeller Shaft Model

2n

torque generated by the motor

actual torque experienced by the propeller

QmQa

Is moment of inertia of the propeller shaft

angular shaft speed

Is Qm Qa K

Km friction coefficient

Power delivered by the motor:

Actual propeller shaft power accounting for the effect of thrust losses:

Pm Qm 2 nQm

Pa Qa 2 nQa

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Torque Loop in Electrical Motor Drive

MotorModel

PWM

Qc

n

-Qcalc

Isb

~~

InductionMotor

Flux controller Isa

Torquecontroller

-

c

calc

Tacho Tm

Flux weakening

PWMConverter

The closed loop of thrust motor and torque controller is assumed to be equivalent with a 1st order model:

where 20 < Tm < 200 milliseconds

Qm 1Tm

Qc Qm

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Thruster Modelling for Control: A summaryThruster dynamics:

first order motor model

rotational dynamics with friction

propeller load torque

propeller thrust

Desired thrust and torque:

Thrust and torque loss factors:

propeller power

Is Qm Qa K

Qm 1Tm

Qc Qm

Qa wD5KQ |n|n

Ta wD4KT|n|n

Pa Qa 2 nQa

Tref kuref wD4KT0 |nref |nref ,

Qref wD5KQ0 |nref |nref ,

#

#

Ta

Tref hTn,xp , p1 td T KT

KT0

Qa.

Qref hQn,xp , p Q

KQ

KQ0

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Propeller Block Diagram

( )

( ).

.MotorDynamics

Control

.

T

ShaftDynamics

Thrusterunit