WO1991006466A1

WO1991006466A1 - Roll damping system

Info

Publication number: WO1991006466A1
Application number: PCT/SE1990/000709
Authority: WO
Inventors: Claes KÄLLSTRÖM
Original assignee: Sspa Maritime Consulting Ab
Priority date: 1989-11-03
Filing date: 1990-11-02
Publication date: 1991-05-16
Also published as: SE8903683D0; SE8903683L; AU6711490A; SE465029B

Abstract

System for reducing wave and wind induced roll motion of a ship intended to be connected into the regular control system of the ship comprising a rudder motor (SG) for driving the rudder of the ship, which rudder is used both as steering means and for generating a rolling moment, said system comprising a central unit (C) which receives rudder angle signals from a steering wheel (H) and which central unit emits rudder driving signals to the rudder motor (SG) in response to said control and rudder angle signals. The central unit (C) comprises a roll sensor (RS) for generating a roll angle velocity signal (Ζ) which is brought to an adaptive discrete filter (F) for generation of reconstructed state variables for sampled roll angle velocity (Ζ(n)) and sampled roll acceleration (Ζ(n)) which by reconstruction in the filter estimated state variables (Ζ(n), Ζ(n)) are transferred to a predicting regulator (RG) for the rudder angle whereby said regulator in response to the filtered roll angle velocity signals generates a rudder driving signal to the rudder motor (SG) which gives the ship a roll moment of such amplitude and phase that the roll moment induced by wind and waves of the ship is substantially neutralized.

Description

ROLL DAMPING SYSTEM

Technical field

The present invention refers to motion stabilising systems and in particular to systems for reducing rolling motions of ships by utilizing active rudder control.

Background

By utilizing active rudder control most of the drawbacks are eliminated of conventional rolling damping systems such as active fins and tank systems. The technique to use the rudder of a ship for damping of wave and wind induced rol- ling is well known and early tests were carried out with simpler control systems constructed in analogue technique.

The results which were accomplished with this type of sys¬ tems have not always been successful owing to, the fact that more complex steering systems could not be implemented with the technique available at the present. In order to avoid any substantial influence on the yaw dynamics, the rudder can be moved outwards only during a short time, since other¬ wise the ship starts to yaw. The limited available rudder velocity also makes it necessary not to use too large rudder angles which in other case results in that the steering system comes out of phase and increases the roll angle.

The normally non linear properties of the steering system of every individual ship also constitutes a problem when desig¬ ning a rolling damping system with active rudder control as well as the adaption to different weather and loading situa¬ tions. Of the collected experiencies up to now of rudder stabilised systems, the importance of integrating the auto- pilot in the system has been recognized in order to obtain effective rolling damping and course keeping at the same time. Even in smaller ships such as yachts, fishing boats and the like there is a need to increase the comfort at rough seas. Experiments have shown that by means of active rudder con¬ trol a reduced roll from a state corresponding to Beaufort 5 down to only Beaufort 3 can be achieved. One of the pro¬ blems being the basis of the present invention is to provide a cost effective and reliable roll damping system which is combined with an autopilot comprising a direction sensor.

The state of art

The US patent specification 4 380 206 describes a ship stabilization system, which utilizes the rudders to compen- sate for wind and wave induced roll motions, including a hydraulically actuated mechanism for controlling the posi¬ tion of the rudders; a pump coupled to the hydraulic mecha¬ nism for controlling the flow of hydraulic fluid; and a flow control device connected to the pump for controlling the flow rate of fluid through the pump.

The ship stabilization system also includes a first actua¬ tion device for translating helm steering command signals into control impulses for the the flow control device and a second actuation device for translating helm steering com- mand signals and roll reduction control signals into control impulses for the flow control device. A coupler is connected to the first and second actuation devices and the flow control device for disengaging the first actuation device from the flow control device when the second actuation device is activated.

The roll reduction signal processed by the second actuation device consists of the instantaneous roll rate of the ship and the statistical gain factor representing the statistical average roll rate during a predetermined period of time according to: D = K * a'

K = 0,41 D'_LIMIT / w₀ E(a') a'= roll velocity D = rudder angle ^D' _LIMIT ⁼ available rudder velocity w₀= ship natural roll frequency E (a' ) = Expected absolute value of roll rate

The object of the invention is to provide a system with improved signal processing compared to what is evident from the above cited US 4380 206 for reduction of wind and wave induced roll of ships in which system optimal course keeping is combined with optimal reduction of roll.

A second object of the invention is to provide an effective roll reducing system for various wheather situations and ship speeds.

A third object of the invention is to provide a cost effec- tive system which is simple to install on ships without additional reconstruction being necessary.

The above objects are achieved according to the invention with a system with features according to the accompanying patent claims.

Description of the drawing

With reference to the accompanying drawing in the following a preferred embodiment of the present invention is descri¬ bed. In the drawing

Fig. 1 is an outline over forces and moments which are used in a system according to the present invention;

Fig. 2 is a control block diagram over a control system according to an embodiment of the invention;

Fig. 3 is control diagram over a control system according to the invention in which particularly the filter function is illustrated;

Fig 4 is a function flow diagram over a control system according to an embodiment of the invention;

Fig. 5 is a time diagram of a) rudder position δ, b) roll angle _rr and roll rate Φ_rr of a ship caused only by rudder movement, c) roll angle Φ_m of the ship caused only by waves and wind, d) total roll angle of the ship Φ_rr + Φ_w, e) resulting course of the ship;

Fig. 6 shows phase difference between rudder position δ and roll velocity Φ_rr at different frequencies of rudder activa- tion;

Fig. 7 is a block diagram over the electronics in a control system according to the invention.

With reference to Fig. 1 the basic principle is illustrated for roll damping with an active rudder. The moment created by the rudder movement is

M_R = L *

where L = the lifting force of the rudder, d_R= the distance between the lifting force center and the roll center R_c of the ship. In Fig. 1 G_A further is the designation for yaw arm and G_c yaw center. Fig. 1 also illustrates the difference between roll dynamics and the yaw dynamics. This difference makes it possible to use high frequency rudder signals to damp the roll without affecting the course of the ship.

With reference to Fig 2 is shown a block diagram over the control functions in a first embodiment of the invention. In the block diagram the following designations are given:

SP = ship

SG = rudder motor

R = rudder RP = rudder position sensor

S = velocity sensor

CG = course gyro

RR = roll action

The blocks framed constitute the central unit C, of which

AP = auto pilot

RG = regulator

F = filter

RS = roll sensor CU = control unit together constitute the part which is installed in addition to a conventional steering system.

A rudder command δ_c from the auto pilot or the steering wheel

H is modified with respect to roll RR, yaw CG and velocity S, and the modified command δ*_c thereupon is transferred to the rudder motor SG.

A feature of the system is that steering commands are trans¬ mitted directly to the rudder motor if the roll damping system has been switched off maually by means of the switch S (see fig 2) or if a malfunction has been detected in the system. If large manaeuvres are initiated the roll damping system is automatically switched off until the ship has resumed a steady course.

With reference to Fig. 3 the roll reduction regulator is based on state feedback of and reconstruction of state variables (Φ(n), Φ(n)) by means of a Kalman filter where Φ (n) is sampled roll angle velocity and Φ (n) sampled roll acceleration. By means of reconstruction of the state vari- ~ -~- ables in the filter the estimated states (Φ(n),Φ(n)) are obtained with which thereafter a prediction is made of the roll angle of the ship. An essential advantage of the Kalman filter is that it does not give any phase shift. The regula¬ tor is adaptive in the sense that it adapts to the system, that is, it calculates new filter constants in reference to

Φ and Φ.

<?^• ~ I Φ is designated with DFI and Φ with DDFI the filter con¬ stants are given as

CalDFI = konst 1 + w /Ts * konst 2 (1)

CalDDFI = konst 3 + w /Ts * konst 3 ... (2)

where Cal means updating of the filter

Ts = the sampling frequency of the filter w = the present roll frequency * 2π

Since the discrete character of the filter the temporary variable

TempDFI =DFI ...(3) is formed.

The estimation of DFI och DDFI is carried out through the following algorithms

DFI = Cos(w/Ts)* DFI + Sin(w/Ts)/w*DDFI ...(4)

DDFI = - Sin(w/Ts)* w * DDFI + Cos(w/Ts) * TempDFI ...(5)

The control error = Error = R - DFI ... (6)

R = roll angle velocity = Φ is one of the input signals to the summing point SI i fig. 3.

The estimates of DFI = Φ och DDFI = Φ are obtained in the integration loop K2 - S2 - INT see Fig. 3. Corresponding algoritms are

DFI - DFI + CalDFI * Error ... (7)

DDFI = DDFI + CalDDFI * Error ... (8)

The roll frequency is calculated as

w = w + Calw * Error ...(9)

where Calw is calculated by means of DFI och DDFI.

w is used to change the coefficients in the filter and at the prediction of the rudder command.

When the ship runs at calm sea, the phase difference between the roll angle velocity and rudder angle δ by giving the rudder a reciprocating action at a number of different frequencies about the resonance frequency w_ε in the roll direction of the ship.

By recording the roll angle velocity Φ and rudder angle δ as a function of time a phase curve is obtained for the rudder angle - roll angle velocity dependence as a function of the period time, see Fig. 5a and 5b which illustrate the roll of the ship for a rudder frequency 1/T with a resulting phase shift of 0 between Φ_r._r and δ.

If the movement of the ship in the water is regarded as an activated pendulum system in a damping medium the forced oscillations from the rudder results in a amplitude and frequency response concerning roll of the ship corresponding to the system transfer function.

At the phase shift = 0 the natural oscillation period= T_s of the ship is obtained.

By entering the phase curve with the present roll period T an appropriate phase position of the rudder is obtained for the current condition.

The exact time for the rudder order is obtained by calcula¬ ting DFI and DDFI for the present phase angle + a rudder motor constant, Rkonst.

By means of DFI and DDFI the predictor P calculates the rudder order δ_c.

The maximum rudder angle which can be used for roll damping is calculated according the following:

^δ _nai_{MUM RUDDER} = ^τ/⁴ * Rudder angle velocity ...(12)

That the rudder is not used more than the maximum time which the rudder can be used for roll damping without affecting the course is controlled according to the following: If rudder time > Max. rudder time, then rudder order is set = 0

With reference to Fig. 4 is shown a somewhat simplified functional flow diagram over a control system according to a second embodiment of the invention intended for yachts, small fishing ships and the like.

In Fig. 4 the blocks are designated according to the follo¬ wing: SP = Ship

RRS = Roll rate sensor

F s Adaptive filter w = Calculation of filter, filter coefficients

FST = Trimming MRR = recording av roll angle velocity och rudder angle

CPH = calculated phase curve

PHC = estimated phase curve

RPC = predicted rudder phase calculation

RAC = predicted rudder angle calculation RTC = predicted rudder time calculation

AP = auto pilot

ADD = summation point SM = steering motor

FGS = direction sensor type Fluxgate sensor

R = rudder

The function of the system according to Fig. 4 is the same as that in Fig. 3 with the difference that the rudder order has been modified with the auto pilot order from the block AP according

Rudder order = δ_c + Auto pilot order ... (13)

The rudder order is converted to suitable signals in a block not shown for the steering motor on board the present ship.

A practic design in hardware of the control system according to the embodiment in Fig. 4 is illustrated in Fig. 6. The CPU- unit of the roll damping system is based on a 16 bit μ - processor type M 68000. The CPU - unit 20 is pro¬ vided with a series interface type RS-232C by means of which the system can be connected to a portable computer 10 and be programmed for measured (see Fig 5a and Fig 5b) and given ship parameters.

The control unit communicates directly with the I/O ports of the CPU-unit or by the A/D and D/A converter 2. The control unit 1 in addition to the auto pilot contains a maneuvering panel with switch means for the system function and a dis¬ play for indication of course deviation, loading state and yaw radius.

The roll sensor 3 which detects the roll angle velocity of the ship comprises a solid state transducer. The operation is in principle that of a gyro and based on a tuning fork which oscillates at its resonance frequency. When the tuning fork is rotated about its symmetry axis the shanks will be deflected from the plane in which they are oscillating. This results in measurable elongations in the shanks perpendicular to the symmetry plane of the tuning fork generator. The roll angle velocity is then derived from the elongations measured.

The roll sensor 3 of solid state type to a very little extent is affected by moving velocities or accelerations. This means that it can be installed generally in any place of the ship under the condition that the rotation sensitive axis is parallel with the roll axis of the ship. This ad¬ vantageous property allows installation of the sensor in the control box for the system as a module. By providing the sensor module with a mechanical setting facility of the sensor axis it is possible to provide parallel alignment with the roll axis of the ship. The signals from the sensor 3 are analogue to digital con- verted and are signal processed in digital form in the CPU- unit 20 by way of a filter program stored in an EPROM corre¬ sponding to the function diagram in Fig. 3 and related filter calculation algorithms (see page 6).

Analogue signals from a sensor for velocity 7, sensor 8 for rudder position, sensor for direction 11 and rudder command 9, are fed to an isolating unit 4 comprising opto couplers connected to the A/D - converter 2.

Digital rudder control signals δ_c, see fig. 3, are digital- analogue converted in the unit 2 and are converted in the converter 5 to appropriate control impulses for the rudder motor 6.

Claims

1. System for reducing wave and wind induced roll motion of a ship intended to be connected into the regular control system of the ship comprising a rudder motor (SG) for dri¬ ving the rudder of the ship, which rudder is used both as steering means and for generating a rolling moment, said system comprising a central unit (C) which receives rudder angle signals from a steering wheel (H) and which central unit emits rudder driving signals to the rudder motor (SG) in response to said control and rudder angle signals, c h a r a c t e r i z e d t h e r e i n, that said central unit (C) comprises a roll sensor (RS) for generating a roll angle velocity signal (Φ) which is brought to an adaptive discrete filter (F) for generation of recon¬ structed state variables for sampled roll angle velocity (Φ(n)) and sampled roll acceleration (Φ(n)) which by recon¬ struction in the filter estimated state variables' (Φ(n),Φ(n)) are transferred to a predicting regulator (RG) for the rudder angle whereby said regulator in response to the filtered roll angle velocity signal generates a rudder driving signal to the rudder motor (SG) which gives the ship a roll moment of such amplitude and phase that the roll moment induced by wind and waves of the ship is substanti- ally neutralized.

2. System according to claim 1 , c h a r a c t e r i z e d t h e r e i n, that said regulator is connected to one of the inputs of a summing circuit (ADD) and the other input of which is con¬ nected to a direction sensor (CG) which generates signals in response to the movement direction of the ship whereby the output signal from said summing circuit constitutes a rudder drive signal for the rudder motor (SG) which gives the ship a roll moment at yaw of such amplitude and phase that the roll moment induced by wind and waves of the ship is sub¬ stantially neutralized.

3. System according to claim 1 or 2, c h a r a c t e r i z e d i n a CPU-unit (20) comprising a μ - processor which is provided with an interface (21) to receive instructions and data in serial form from a portable computer (10), said data compri¬ sing recorded values of the amplitude and frequency respon¬ se wit respect to roll when the rudder is activated with different frequencies higher and lower than the natural roll frequency of the ship.