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Propulsion Control Part 2 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.no Asgeir.Sorensen@ntnu.no. Outline. Part 1: Modelling

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Propulsion ControlPart 2 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.no

Asgeir.Sorensen@ntnu.no


Outline

Part 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


Propulsion and Thruster Control

  • 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

PITCH or RPM CONTROL

POWER

TORQUE CONTROL

POWER

VARIABLE

TORQUE

CONSTANT

TORQUE


Thruster Modelling for Control: A summary

Thruster dynamics:

first order motor model

rotational dynamics with friction

propeller load torque

propeller thrust

propeller power

Desired thrust and torque:

Thrust and torque loss factors:


Propeller Block Diagram

q

x

Thruster

p

p

unit

.

.

( )

f

q

x

Q

p

p

Q

a

T

T

Q

Q

n

Shaft

Motor

ref

.

m

a

c

( )

f

Control

Dynamics

Dynamics

T

n


Controller Structure

Office Systems

Business enterprise/

Fleet management

Office Network

Ship 3:

Ship 2:

Ship 1:

Operational management

Real-Time Control

Local optimization (min-hour)

Control layers

Fault-Tolerant Control

High level

(0.1-5 s)

Plant control

Real-Time Network

Low level

(0.001-1 s)

Actuator control


DP Controller Structure

Plant control

Low level control


Disturbances

T

n

T

a

ref

ref

g

(

)

T

ref

Relation Between Plant Control and Actuator Control

Typical thrust and moment characteristics for a speed controlled fixed pitch propeller:

Tref = ρwD4KT0|nref|nref

Qref = ρwD5KQ0|nref|nref

is the actual developed thrust subject to disturbance and losses

T

a


Thrust Allocation

Relation between 3 DOF control vector - surge, sway and yaw and r number of thruster/propellers is:

is the thruster configuration matrix

Ý

Þ

T

J

3

r

×

u

is the commanded control input vector - often (pitch)2 or (speed)2

ref

is the commanded control input vector - in Newton

T

ref

is the thruster force coefficients (NB! will be different for CPP or FPP)

á

â

K

diag

k

=

i

Commanded control input provided by r thrusters is:

The thrust produced by thruster unit i is:


Effect on Actual Power for Different Low Level Controllers

Alternative 1:

Propeller speed controlled or pitch controlled

Alternative 2:

Propeller torque or power control


Low Level Controller Designs

Control in moderate seas

  • Shaft speed control

  • Torque control

  • Power control

  • Combined power/torque control

    Anti-spin control in extreme seas

    Combined power/torque control with spin detection and setpoint optimization due to severe thrust losses:

  • Ventilation

  • Water exits


Shaft Speed Controller Design

  • Conventional control scheme

  • Mapping from thrust to shaft speed

  • Setpoint controlled by e.g. PID controller

Weakness with speed control:

  • Severe power fluctuations in waves

  • Wear and tear of propulsion equipment


Torque Control

  • Mapping from thrust to torque

  • Motor torque setpoint given by desired torque

Qref = (DKQ0/KT0 )Tref

Weakness with torque control:

  • Power fluctuations for high thrust

  • Speed transients during ventilation and in-and-out of water effects


Inner Torque Algorithm

If the shaft inertia Isis high, feedforward terms from the reference torque/power may be needed to speed up the response of the propeller

The torque is limited by the maximum available torque and the power

α = 1.1 - 1.2

is the rated (nominal) torque and power


Power Control

  • Mapping from thrust to power

  • Motor torque setpoint found by shaft speed feedback

Power algorithm:

Weakness with power control:

  • Singular for zero shaft speed


Numerical Simulations Applying RPM, Torque and Power Control

  • Propeller:

  • Wageningen B-470

  • D = 4m

  • Z = 4

  • EAR = 0.7

  • nmax = 3.5rps

  • Is = 5000kgm2

  • Tref = 400kN

  • Sea state:

  • JONSWAP spectrum

  • Wave height: 4m

  • Wave period: 10s

  • Simulations for a supply vessel with main particulars [L,B,T]=[80,18,5.6]m


Open Water Experiments Set-up: MCLab

Propeller data:

Shaft Speed: 0-10 rps

Thrust characteristics


Experimental Results: Local Thruster Control

  • Wave height: 8cm

  • Wave period: 1s

Actual Thrust

Actual Thruster

Torque

Shaft Speed

Motor Torque

Motor Power


Sensitivity Functions to Thrust Losses

Sensitivity function - ratio of actual to reference thrust:

T

a

Ý

6

Þ

ª

s

T

ref

Shaft speed feedback control:

Torque feedback control:

Power feedback control:


Sensitivity to Changes in Advance Ratio

KT and KQ as functions of Ja or Va, ignoring all other loss effects:

Sensitivity function

Open water propeller diagram


Experiments Results: Sensitivity tests

Actual Thrust

Actual Thruster

Torque

Shaft Speed

Motor Torque

Motor Power


Experiments: Sensitivity to Changes in Advance Ratio


Sensitivity as Function of Ventilation

1

Reduced propeller disc area

0.8

Propeller torque loss function

h

0.6

Q

0.4

Propeller thrust loss function

h

T

0.2

Submergence/propeller radius

h/R

-0.5

0

0.5

1

1.5

2

h/R

Torque

s

Q

1

Power

s

0.8

P

0.6

RPM

s

n

0.4

Notice that maintaining thrust may lead to unacceptable high values of propeller speed (rps) during ventilation

0.2

-0.5

0

0.5

1

1.5

2

h/R


New Methods in Propulsion Control

  • Combined power/torque control

  • Load torque observer for performance monitoring

  • Anti-spin thruster control for extreme conditions


Combined Power/Torque Control

  • Torque control:

Qcq = Qref = ρwD5KQ0|nref|nref

  • Power control:

Qcp = Qref = Pref / (2π |n|)

Qc = α(n)Qcq + (1- α(n)) Qcp

  • Combined control:

  • Weight function:

α(x) = e-k|pxIr

  • k,p,r tuning factors

  • Avoids singularities for n=0


Combined Power/Torque Control Result

Result:

- Utilize the best properties of both power and torque control.

- Valid for all setpoints and operating conditions

  • Propeller:

  • Wageningen B-470

  • D=4m

  • Z=4

  • EAR=0.7

  • nmax=3.5rps

  • Is=5000kgm2

  • Sea state:

  • JONSWAP spectrum

  • Wave height: 4m

  • Wave period: 10s

  • High thrust reference => high shaft speed => power control

  • Low thrust reference => low shaft speed => torque control

  • Shaft speed zero-crossing => singularity for power control


Ventilation and Water Exits

A sudden loss of thrust and load torque may occur when operating with high propeller loading close to the surface.

  • Low propeller loading: Loss of effective disc area

  • High propeller loading: Ventilation

  • High submergence: Conventional thrust curve

  • Wagner effect gives rise to hysteresis in thrust production


Anti-spin Thruster Control

A vessel operating in extreme environmental conditions may experience sudden losses of thrust capability due to ventilation and in-and-out-of water effects.

Idea: Formulate anti-spin control

as used in car wheel anti-spin

  • Detect ventilation

  • Reduce shaft speed

  • Detect ventilation termination

  • Increase shaft speed to old operating point

    Effect:

  • Reduced wear and tear

  • Less power transients

  • Better thrust production


Simulation Results: - Ventilation incident


Anti-spin Control Concept

The general ideas of anti-spin control are to:

  • Reduce the wear and tear of the propulsion unit by preventing uncontrolled propeller racing

  • Limit the load transients on the power system

  • Optimize the thrust production and efficiency in transient operation regimes


Anti-spin Thruster Control


Propeller Load Torque Estimation

For a propeller operating in rapidly changing environmental conditions, can we estimate the propeller loading?

Current velocities Vc

Wave velocities Vw

Propeller velocities Vp

Submergence h

Varying loading Qa

With some system knowledge, the answer is yes.


Propeller Load Torque Observer Equations

The control plant model of the shaft control dynamics is:

Propeller load torque

Rotational inertia

Friction coefficient

Markov time constant

Shaft speed

White noise

Motor torque

Measurement

The propeller load torque has been modeled as a 1st order Markov process.

The observer equations are:

Observer gains

Input

The linear observer is easily proven to be stable (GES)


Propeller Load Torque Observer Analysis

Write on matrix form:

The error dynamics become:

where

The error dynamics are UGES if F is Hurwitz, which is implied by positive definiteness of (-F). By Sylvester’s theorem (-F) is positive definite if:


Propeller Load Torque Observer Simulations

Ventilation

Normal conditions

Fast response during rapid load changes: Estimation delay 0.15-0.2 seconds

Robust towards modelling errors


Torque Loss Calculation

The torque loss factor is defined as the ratio of actual to nominal propeller load torque:

The nominal torque may be calculated from the measured shaft speed:

An estimate of the torque loss factor is then calculated from the estimated propeller load torque:

Avoid the singularity for n = 0 by a weighting function:


Torque Loss Calculation Simulations

Normal conditions

Ventilation

May be used for monitoring, ventilation detection, thrust re-allocation

and as feedback to an anti-spin thruster controller.


Ventilation Detection

  • Monitor the estimated torque loss factor:

  • Define limits for beginning and termination of ventilation:

  • Set the ventilation flag high or low:


Ventilation Detection and Anti-Spin


Ventilation Detection Simulations

Other detection schemes:

  • Monitoring of motor torque

  • Monitoring of shaft speed

  • Combined detection (several criteria)


Primary Anti-spin Control Action

  • Reduce the commanded torque from the combined controller with modification factor :

  • Use the estimated torque loss as modification factor during ventilation:

  • Result: a different shaft speed controller:


Secondary Anti-spin Control Action

  • Reduce the thrust command in order to lower the shaft speed to some optimal value .

  • Modified thrust command during ventilation:


Experimental Set-up: MCLab

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


Experiments: Controller comparison

Slow-motion sinusoidal submergence with period T= 10s and thrust reference 140N

Thrust

Green: RPM control

Blue: Combined control

Red: Primary Antispin

Black: Primary+Secondary Antispin

Torque

RPS

Motor

torque

Power


Experiments: Controller comparison #2

Slow-motion sinusoidal submergence with period T= 5s and thrust reference 180N

Thrust

Green: RPM control

Blue: Combined control

Red: Primary Antispin

Black: Primary+Secondary Antispin

Torque

RPS

Motor

torque

Power


Experimental Results: Anti-spin details

Blue: Measured propeller torque

Red: Estimatied propeller torque

Blue: Estimated torque loss factor

Red: Ventilation detection flag

Submergence

Primary antispin action: Gamma

Secondary antispin action: nras


Experimental Results: Anti-spin main data

Thrust

Torque

Shaft speed

Motor torque

Power


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