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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: [email protected] [email protected] Outline. Part 1: Modelling

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Propulsion control part 2 of 2

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: [email protected]

[email protected]


Outline
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
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
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
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
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
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
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
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
Open Water Experiments Set-up: MCLab

Propeller data:

Shaft Speed: 0-10 rps

Thrust characteristics


Experimental results local thruster control
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
Experiments Results: Sensitivity tests

Actual Thrust

Actual Thruster

Torque

Shaft Speed

Motor Torque

Motor Power



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
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
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
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
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
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
Simulation Results: - Ventilation incident


Anti spin control concept
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



Propeller load torque estimation
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
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
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
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
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
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
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 simulations
Ventilation Detection Simulations

Other detection schemes:

  • Monitoring of motor torque

  • Monitoring of shaft speed

  • Combined detection (several criteria)


Primary anti spin control action
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
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
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
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
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
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
Experimental Results: Anti-spin main data

Thrust

Torque

Shaft speed

Motor torque

Power


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