US3395667A

US3395667A - Control system for ship roll stabilization

Info

Publication number: US3395667A
Application number: US558033A
Authority: US
Inventors: Wayne E Kohman
Original assignee: Curtiss Wright Corp
Current assignee: Curtiss Wright Corp
Priority date: 1966-06-16
Filing date: 1966-06-16
Publication date: 1968-08-06
Anticipated expiration: 1985-08-06

Description

Aug. 6, 1968 w. E. KOHMAN CONTROL SYSTEM FOR SHIP ROLL STABILIZATION Filed June 16, 1966 5 Sheets-Sheet 1 TRUE VERTICAL ROLL 9 @En V 1DRIZDNTAL WAVE ELDF'E SHIP MUTIUN RATE AND ALIIILELER ATIEIN EENEINE:

RATE SENEIINE:

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5P55 RRS SENSING INVENTOR. WAYNE EMDHMAN awfim HIS ATTORNEY Aug. 6, 1968 W. E. KOHMAN CONTROL SYSTEM FOR SHIP ROLL STABILIZATION Filed June 16, 1966 5 Sheets-Sheet 2 (K2 E 41% 9 a NATURAL LIST EDNTRUL I3 SHIP SPEED MG SENSDR I 6 In DRAIN /\\J:\ I q] RATE. ur' HULL I EENSDR v I EUPPLY" RV FL 9 I DRAIN'- I FIN I 5 3E PDSITIDN 23 um? 7:13? FEED-BA/E/K DHAIN 5" I aup w a A FE: l FE: AND LUAD um I K5,PRELEIAD I IFRETRAET I NEUTRAL:

| EXTEND-J SHAFT(B)TRAVELI I I I I I INVENTOR. WAYNE E. KEIHMAN HIE ATTEIR'NEY Aug. 6, 1968 w. E. KOHMAN CONTROL SYSTEM FOR SHIP ROLL STABILIZATION Filed June 16, 1966 6 Sheets-Sheet 3 INVENTOR WAYNE E. KEIHMAN 04 Pm Hi5 ATTORNEY United State 3,395,667 CONTROL SYSTEM FOR SHIP ROLL STABILIZATION Wayne E. Kohman, Morris Plains, N.J., assignor to Curtiss-Wright Corporation, a corporation of Delaware Filed June 16, 1966, Ser. No. 558,033 Claims. (Cl. 114-122) ABSTRACT OF THE DISCLOSURE A roll stabilization system includes hydromechanical actuating means for positioning the fins of a ship according to roll angle and rate of roll, and feedback mechanism providing a force feedback signal which changes at a low rate in response to movement of the actuating means when fin angles are greater than a predetermined angle and changes at a higher rate when fin angles are less than the predetermined angle to thereby render the system effective to control roll of the ship in both high and low frequency waves.

transverse to said longitudinal axis, generally in the mode of aircraft ailerons. When so tilted, the surfaces or foiis of the fins define angles-of-attack of opposite sense, respectively, for producing in well-known manner hydrodynamic forces tending to oppose the roll and to right the ship. The restoring couple so established by these forces in order to be effective must be adjusted and timed properly in relation to the wave motion for damping-out a given roll with minimum disturbance; with the control properly applied, the succeeding roll angles of opposite sense can be decreased progressively for a constant wave condition until the ship is, for practical purposes, stabilized at negligible or tolerable roll angle. This condition obtains until the ship encounters larger disturbing waves (or wind conditions), for example, that require additional control activity for establishing a new condition of stabilization.

Roll stabilization sytems using power-driven stabilizing fins have heretofore been proposed for the above purposes. The known systems, however, rely on refined and involved multiple sensing means and are therefore comparatively complicated and expensive. Furthermore, the known system now in use depend on electronics control circuitry which in practice is susceptible to deterioration and maintenance down-time under ordinary sea-going conditions.

A principal object of this invention therefore is to provide an improved, simplified and comparatively inexpensive roll stabilization system for the purposes described above, that is highly responsive, precise and positive in operation and that is capable of consistently reliable and efficient performance with a wide range of water and ship conditions, including low and high frequency waves, and list due to ship loading (natural list) and to wind.

A further object of the invention is to provide fully automatic proportional roll stabilization system control with a minimum of primary sensing controls, and that combines 3,395,667 Patented Aug. 6, 1968 rugged mechanical and hydromechanical components with a minimum of basic electrical components for obtaining the inherent advantages of mechanical equipment operating under severe sea-going conditions.

The basic factors involved in the stabilization of ship roll will first be briefly considered. When an unstabilized ship under motion is among sizable waves, rolling about the longitudinal or roll axis results. The extent of roll depends primarily on the wave slope and the period between waves, i.e., the wave frequency as well as the ships design characteristics.

Assuming a stable design, i.e., wherein the ships metacenter is above the center of gravity, a ship at a moderate roll angle is inherently subject to a righting or restoring couple, depending in magnitude on the instant position of the center of buoyancy and the roll angle. Where due to large wave disturbance the roll angle is high, more time is required for the restoring couple to overcome the ships inertia and bring the ship back to even keel. This same inertia, of course, causes continuation of the roll in the opposite direction to set up an oscillation, which eventually is damped-out by the ships drag, absent further water disturbance.

The main purpose in roll stabilization therefore is to supplement the ships natural couple by an auxiliary control couple that is timed and varied in magnitude according to requirements, for minimizing and quickly damping the ships roll. A slight roll, such as 12 is tolerable control for severe wave conditions; in fact, a small amount of roll is unavoidable in general under practical conditions.

In accordance with the present invention, highly satisfactory and acceptable roll stabilization is obtained by maintaining certain balance and time relationships between but two basic control adjustments, namely, those made primarily in response to (1) the ships vertical axis position with respect to true vertical, i.e., roll angle, and (2) rate of roll. Such stabiliaztion is obtained without sacrificing flexibility and desirable control characteristics and without complex and costly acceleration sensing.

The invention will be more fully set forth in the following description referring to the accompanying drawings, and the features of novelty will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Referring first to the drawings:

FIG. 1 is an explanatory diagram indicating a normally stable ship in a severe roll, viewed as from the stern;

FIG. 1A is a starboard side view of a portion of FIG. 1 illustrating corrective control action of a starboard fin;

FIG. 2 is a block-schematic illustration of a roll stabiiization system embodying the present invention;

FIG. 3 is a partly diagrammatic illustration of the essential hydromechanical and related system equipment indieated generally in FIG. 2;

FIG. 3A is a detailed view for better illustrating appli cation of control forces at the rate gyro of FIG. 3;

FIG. 3B is a sectional View of the fin angle limiter, taken along the line 3B3B of FIG. 3;

FIG. 4 is a graphical illustration for comparing different types of roll control; and

FIG. 5 graphically illustrates feed-back characteristics for the dual control system.

Referring to FIG. 1, a water-borne ship can be considered as a rigid body with six degrees of freedom; roll, yaw, pitch, heave, surge and sway. Of these, roll is the most objectionable; it is however, the most feasible to control as there is negligible cross-coupling between roll and any of the other motion systems. Accordingly, where roll stabilization is solely involved, a ship can for practical purposes be considered as having but one degree of freedom, namely roll. That is, the forces and moments to be considered in roll stabilization can be regarded as in a plane transverse to the ships roll axis.

In FIG. 1, a ship S is shown in a starboard roll on the front of a large wave W having an inclination or slope 'y with respect to horizontal. The ships roll angle is determined by the ships vertical axis V and the true vertical, shown as passing through the ships center of gravity CG. A stable design is assumed wherein the ships metacenter M is well above CG on the vertical axis. For a given roll condition, the metacenter is by definition, at the intersection of the vertical axis and a line drawn through the newly established center of buoyancy B so as to make an angle (0'y) with said axis. It will be seen by inspection that a natural righting or counter-clockwise moment consisting of the buoyancy (or displacement) force F acting through a transverse distance d with respect to CG not only tends to oppose further roll of the ship but also upon decrease of the wave slope, to cause counter-clockwise roll of the ship toward level position.

For supplementing and modifying this inherent corrective action in the most effective manner, the ship is provided with starboard and port fins 1 and 2 respectively, FIGS. 1 and 1A, that are inter-connected by a common drive means generally indicated at 3 for simultaneous rotation or tilting of the fins in opposite directions. The drive means is controlled according to roll angle and rate of roll as hereinafter described for deflecting the fins from a normally horizontal neutral position and so defining posi tive and negative angles of attack 0: respectively, FIG. 1A. The fin deflections in the example shown, produce hydro-' dynamic forces F opposite in direction and depending in magnitude on fin area, the speed of the ship and the fin deflection for establishing a couple tending to rotate the ship in a left roll, i.e., counter-clockwise about its longitudinal axis.

As the ship begins its left or port roll under influence of these corrective forces, angle 0c is proportionately decreased to zero as the ship levels off, and is full-reversed for damping continuation of the inertia roll toward port. At the end of the now abbreviated port roll, the roll damping control cycle is repeated as long as any significant roll exists. As explained more fully below, the inertia roll is quickly damped out, a new roll condition due to ordinary wave disturbance is immediately resisted and the ship in the absence of a severe transient condition as abrupt increase in wind and/ or water disturbance, is maintained is stabilized, practically level condition with negligible roll.

In FIG. 2 which outlines schematically the roll stabilization system of the present invention, the two primary sensing means for establishing control are indicated as SHIP POSITION SENSOR (SP5) and RATE OF ROLL SEN- SOR (RRS). The former consists of a reversible, on-off controlled motor M that is mounted so as to move with the ship, and is mechanically connected through a gear reducer GR to the system control equipment presently described at 5.

The operation of motor M is null-controlled by a gravity-type (mercury) switch having its tilt axis parallel with the ships longitudinal axis. The switch which is open at level position and closed when tilted in either direction for respective operation of the motor, is mechanically connected to the gear reducer output for automatic leveling purposes. When due to roll action, the motor is energized through the tilted switch, it rotates in a direction restoring the switch to level position. At level or null position, the motor is de-energized and remains so until further change in ship position.

Accordingly, it will be seen that the rotation of the motor output shaft in either direction indicates the ships vertical axis relative position with respect to the true vertical, i.e., the magnitude and sense of the ships roll angle 0. It will, of course, be understood that the SPS can be of different forms, such as a position sensing gyro or pendulum for example. The motor-mercury switch combination, however, is preferred as it uniformly provides a level reference that is not disturbed by roll action of the ship and other environmental factors.

The rate sensor, RRS, consists of a conventional rate sensitive gyroscope having a gyro motor M for rotating a mass about a common vertical axis, initially aligned with the true vertical. The base of this sensor is secured to the ship and the gyro is positively restrained by connecting linkage or the like 6, to hydraulic flow control valve equipment HC that, jointly with the SPS as indicated by the connection at 7, primarily causes adjustment of the stabilizing fins. In brief, the gyro precession torque which is proportional to the ships roll rate, exerts a corresponding input force on the hydraulic control above.

Outlining briefly the essential elements of the roll stabilization system, the hydraulic valve control HC controls a main actuator ACT that is connected through a feed-back device FB to thrust-transmitting structure 8 that in turn controls the common drive means 3, FIGS. 1 and 2, for deflecting the fins 1 and 2. A feed-back force from PE is transmitted by a connection at 9 to the gyro for modifying the precession torque. The device FB also includes a load limiter for absorbing excess force from the actuator ACT that otherwise might damage the control linkage 8, etc.

As the hydrodynamic forces acting on the fin surfaces during control action increase proportionately with the square of ship speed, fin deflection for a given roll condition is decreased for obvious reasons. To this end, a fin position limit device 10 is interposed in the linear path of movement of the thrust transmitting linkage 8 so as to bracket the movement thereof within distances that vary in inverse relation to the square of ship speed. This control may originate at the ships propeller shaft PS for example, as propeller r.p.m. can reasonably be assumed to represent ship speed. A power take-off from PS indicated at 11 drives a gear pump GP, the output of which is delivered to

parallel lines

12 and 13 having different flow characteristics that feed into a differential pressure device DP. The line 12 above, has a selected flow-characteristic restriction at 14. The resulting differential pressure at DP actuates a reversing pressure valve RV for controlling a fin limiter actuator FL, that in turn controls, in combination with a spring K and thrust linkage 39a, the vertical position of a fin limit device 10. Assuming for the moment that increased propeller r.p.m. results in downward movement of the limit device, it will be apparent that the to-and-fro movement of the offset pin 15 (rigidly secured to the thrust linkage 8) becomes more restricted, and vice versa. As the ship speed increases, a feed-back connection at between the limiter linkage and spring K gradually raises and restores the valve RV to null position. This establishes the desired range of fin deflection at the increased ship speed.

Where natural list due to cargo loading, prevailing wind, etc. is involved, a new fin neutral position is established corresponding to the list so as to minimize ship drag and consumption of fuel. For this purpose a fin position feed-back signal is transmitted by linkage 16 to a switching control SC for the NATURAL LIST CONTROL (NLC) motor M The motor is reversible in direction in response to signals representing positive and negative values respectively of 0:, as represented by closing of the corresponding switch at SC. The motor M is mechanically connected through a high reduction gearing GR and linkage 17 to the SPS linkage 7 above, so as to apply at a very slow build-up rate, a modifying force to the main control valve HC through the RRS gyro. The net effect, over a period of time, is to shift the neutral point of control from the dead-level position of the ship to that of natural list so that for the stabilized no-roll condition, fin deflection is zero-centered at natural list.

Reference is now rnade to FIG. 3 for describing in more detail the interaction of control factors and underlying concepts of the present roll stabilization system. As explained in general above, the output of the ship stabilizer control system constitutes a fin position command signal that is represented by a linear displacement of a fin control thrust rod or shaft 8. The primary inputs to the control system for determining the shaft displacement are signals representing ship roll position, i.e., roll angle 0, and ship roll rate; these signals are derived as explained above from the primary sensors SP8 and RRS, FIGS. 2 and 3. A preferred mechanism for adequately deriving the SPS signal with least complexity and cost is a conventional gravity operated mercury switch MS that senses the direction of gravity at all times. The switch may be suitably mounted on a pivoted arm 20 that is connected for movement with the output shaft of the SPS gear reducer GR The mercury switch is of the single pole, double throw type as shown, and is connected in well-known manner to the terminals of the split-coil 21 of the bidirectional motor M so that the motor can be driven in either direction according to the direction of switch tilt. When a change in the direction of gravity is sensed, i.e., at the start of a roll, the mercury makes a bridging contact between the voltage supply terminal and a motor coil terminal to drive the motor in the direction to re-level the switch. At level position, FIG. 3, the switch is open and coil 21 is de-energized for stopping the motor. The output of the motor is geared down so that a very small motor can generate ample torque at a speed slightly higher than the maximum roll rate anticipated. Thus, the position motor and mercury switch combination provides a comparatively stable level reference that is not disturbed by roll motion of the ship.

The SPS output linkage 5, shown conveniently as a wire or cable, includes a spring K that is attached through a bell crank 22 and the RRS gyro to the HC control lever 23. Since the SPS control is rigidly attached to and rolls with the ship, relative motion between the control and the level reference of the position sensor produces a spring force at K that is proportional to roll angle for controlling the valve HC. This hydraulic valve is of the conventional, proportional flow, 4-way type for controlling reciprocal piston drive and is commercially available, for example, as Pegasus Model -E Hydraulic Servo Valve.

Ship roll rate is sensed as explained above by a conventional rate sensitive gyroscope RRS, FIG. 3A. The gyroscope is rigidly restrained by connecting linkage 6, FIG. 3, to the HC valve lever 23 so that the gyroscope precession torque that is proportional to ship roll rate exerts a control force thereon. The gyro gimbal mount at 25, FIGS. 3 and 3A, confines the tilt of the gyro motor M and rotating mass (disc D) to an axis transverse to the ships longitudinal axis. That is, the gyro precession torque acts at right angles to the plane of roll motion for producing, according to rate and direction of roll, a corresponding tilt from vertical such as 6 or 6 FIG. 3A. For better illustration only, the plane of tilting movement of the SPS level reference (MS) is shown as in the plane of the drawing. It is understood, of course, that the axes of tilt at SP8 and RRS are :at right angles to each other due to the direction of precession torque.

The connections between the gyro and other elements of the control system may include a transverse bar 26, FIGS. 3 and 3A, secured to the lower end of the gyro motor to which the various linkages (or cables) are fastened. Accordingly, there is a summation of forces at the gyro and the resultant force is transmitted by linkage 6 to the servo control valve HC, FIGS. 2 and 3. Valve controlled fiow through the lines 27 and 2-8 from or to the ports 27a and 28a, respectively, of the piston actuator ACT is therefore proportional to the resultant of the forces generated by the position and rate sensors; thus, the sensors jointly produce a resultant control signal that is amplified through hydraulic pressure applied to the actuator piston 30*, for in turn controlling the position and velocity of the fin positioning shaft 8.

A feed-back spring K attached to the unit PB ensures that the displacement of shaft 8 is proportional to the force applied to the flow control valve. For example, a

force applied to the valve that admits pressure to actuator port 27a causes shaft 8 to extend for stretching spring K The extension of shaft 8 is limited when the force exerted by spring K produces a null force balance on the input lever 23 of the flow control valve.

Specifically, the output piston shaft 30a of the actuator is connected to and bodily displaces the feed-back unit PB that comprises a housing 31 with recesses for containing springs K K and K respectively. Spring K functions as a bungee or spring cartridge between the actuator and shaft 8 and acts as a force limiter. The optimum pressure level and flow range for the flow control valve determines the actuator size and stall force capability. This force magnitude is normally much higher than required for the control function. To limit the force as shaft 8 in order to prevent damage to control linkage, the bungee K capable of absorbing displacement in either direction is therefore provided. The force applied at shaft 8 is limited to the preload of spring K DUAL GAIN CONTROL Fin stabilizing systems now in use generally use proportional type control that provides adequate roll damping with limited fin activity under low frequency conditions, but is least effective in a confused sea where fairly high frequency waves are encountered. In the present invention, dual gain control is used in such manner that the advantages of improved proportional control for low frequency conditions are retained, and are combined with advantages of the added lead of on-off control for more effective damping under intermediate and high frequency conditons.

The dual gain function is primarily accomplished by means of the system feed-back arrangement including the spring K and the bungee or spring cartridge K FIG. 3. As the piston actuator shaft 30:: moves toward the right, the pivot 29a of the K spring lever 29 mounted on the housing 31, moves bodily therewith. Accordingly, as the spring force at K increases, it tends to urge lever 29 in counter-clockwise direction and force inwardly the bungee guide rod 33, carrying with it the slide collar 32. The force exerted by collar 32 on spring K therefore increases until it exceeds the spring preload force; therefore, further extension of the housing 31 tends to cause compression of the spring K and movement of the slide collar 32 within the spring recess 31a. Since spring K is preloaded so that the direction of force must never reverse, the spring K in the housing recess 31b aligned with recess 31a, is initially adjusted at 40 to balance the preload of spring K At the point where the spring force at K exceeds the K preload, the bungee rod 33 (through to opposite slide collar 41) starts to move against spring K now permitting the lever 29 to move counter-clockwise so that spring K is not stretched as far as the housing 31 has moved. This force feed-back fed through the gyroscope gimbal to the control valve HC behaves as though the rate of spring K were reduced to a lower rate, as determined by the rates of springs K K and the associated linkage ratios. This mechanism provides the bidirectional, non-linear characteristic shown by the graph of FIG. 5, i.e., low grain at small 0: and high gain at large 00.

The effect of reducing the rate of feed-back spring K at approximately 40 percent of displacement for example, has the effect of increasing amplifier gain in the same proportion. The effect on system performance is that the fin moment is linear and proportional to ship motion disturbance up to the disturbance magnitude that causes the amplifier gain to increase. At disturbance magnitude greater than this, a small additional increase in disturbance causes the fins to go to full deflection. This change is operating characteristic makes the control more effective in reducing the ship roll amplitude under the higher frequency conditions as compared with conventional proportional control alone.

FIN LIMIT CONTROL Fin angle limit control relates the maximum displacement of the fins to a function of ship speed. Thus, the fins may have sutficient area to be effective at reduced speed without producing excessive moments and disruptive stresses at maximum speed. Ship speed represented by propeller r.p.rn. is sensed by the propeller shafts PS and a positive displacement gear pump GP. The pump forces fluid in volume proportional to speed through a selected orifice 14 in the flow output line 12. This line exhausts to SUMP at an intermediate point downstream from the orifice 14 and terminates in a differential pressure housing at DP. The housing contains a bellows 35 in direct internal communication through a parallel line 13 with the gear pump output, and is otherwise sealed. The down-stream pressure drop in line 12, which is approximately proportional to the square of propeller speed, represents the differential pressure between

lines

12 and 13, i.e., the differential between the bellows internal static pressure and external or housing pressure. Increased propeller speed, for example, increase the differential pressure so that the bellows expands through a very short distance to extend the stem of valve RV and overcome the feedback spring force at K from the limiter cam 10. The spool or reversing valve RV having lands 42 and 43 moves downward slightly so as to close the pressure supply port 36 and open the drain port 37. The space 38 between the valve lands, now open to drain, communicates with the lower side of the fin limiter piston 39 in cylinder FL through line 40; the pressure across the limiter piston 39 then drops and the limiter cam spring K forces downward the piston rod 39:: and the attached limiter cam 10. At reduced ship speed the spring K moves valve RC upward to admit pressure to the FL piston 39 and raises cam 10 against the pressure of spring K As the cam raises, the feed-back at 10c decreases the tension on spring K until the spring and bellows forces are balanced at a null position of valve RV.

The cam is shown as a fiat block suitably guided for vertical linear movement by side guides 1011, FIG. 3B. The lower end of the block is recessed to form an inverted V-shaped cam surface 10b that may be suitably contoured to the ship speed and/or fin characteristics. The V cam is off-set from shaft 8 and straddles a projection or pin secured to the shaft, FIGS. 3 and 3B. It will be apparent that as the cam is moved downward, for example, the lateral freedom of pin movement becomes less so as to define a maximum travel range of shaft 8, and therefore fin angle deflection for each vertical adjustment of the cam. Downward motion of the cam in turn, increases the spring force at K through the feed-back lever 10c and adjustment 10d until the spool valve RV is again restored to a null position.

Upon shutdown or loss of hydraulic pressure, spring K drives the cam 10 to the lower limit of travel. This restores by camming action the shaft 8 to center position and there holds it against further movement. This centers and locks the fins at neutral position.

The spring K connected to the HC valve lever 23 serves as a bias control or zero adjustment. That is, when all other inputs are held at center position, force of spring K, can be adjusted at Z for centering the fins on neutral.

The spring K can serve as a manual fin control in lieu of the primary sensors. For example, the force generated by spring K can be controlled manually as indicated at MAN in any suitable manner. This feature is useful during installation and rigging and can be used to provide some roll damping if the motion sensors were inoperative.

The method of operation by which the control system positions the shaft 8 to indicate desired fin position has been described. The shaft can be coupled to means for positioning the fins in several ways. For example, the shaft can be connected to a floating or equalizing lever 8a intermediate its ends which in turn are connected respectively to cranks 8b and 8c. The crank 8b is connected through linkage indicated at -8d to a reversible variable delivery pump as represented for operating the fin actuators at 3 in the manner described above. The crank 8c is connected through linkage indicated at 16 to a fin shaft to sense actual fin position. Any error between actual fin position and desired fin position will then position the equalizer lever 8a so as to command pressure flow from the pump to the fin actuators in the proper direction and magnitude for eliminating the fin position error. This feed-back control therefore ensures that the fin position is always properly related to the fin position command as represented by the linear position of shaft 8.

NATURAL LIST CONTROL The natural list control (NLC) acts to avoid the problems related to forcing the ship to a level attitude under a natural list condition due to cargo loading or wind pressure. The effect of normal control operation would be deflection of the fins to correct the list in response to position sensing at SPS. Correcting such a list by fin displacement is undesirable because of the resulting increased ship drag and fuel consumption. This condition is always associated with displace-ment of the fins for unequal time periods to either side of neutral position and is readily corrected by the natural list control of FIG. 3.

The crank 8c which senses fin position is provided with a two-face cam 45 for coacting with switches S and S respectively. In the neutral position, the switches are open and when the fins are deflected, the operative cam face closes one of the switches which in turn energizes the associated part of the split coil 46 of NLC motor M When the fins are displaced to command roll left, for example, switch S, is cammed closed. This causes the motor M to rotate in a direction for moving the K spring lever 47 clockwise. This increases the tension of spring K which produces a force that is applied to the valve HC through the bell crank 22, etc. for reducing fin displacement. The natural list motor drives through a high reduction gearbox so that the change in the force of spring K is so slow and limited in magnitude that the effect on any one roll cycle is negligible. When the ship rolls in the opposite direction, the fin position reverses and the switch S is cammed closed. The natural list motor M now drives the lever 47 in the counter-clockwise direction for decreasing the tension of spring K which in turn acts to reduce fin displacement. As the fins during natural list are consistently displaced for greater time periods in one direction, the natural list motor will drive for longer time periods in that direction. Each cycle will produce a net bias increment in the neutralizing direction to equalize the time periods of fin displacement. After a reasonable period of time, say ten minutes, the fins will be biased to displace for equal time periods on each side of the natural list neutral. This condition permits the ship to have stabilized roll about its natural neutral roll position, thereby resulting in minimum fin drag.

OPERATION CYCLE In a typical roll cycle, where a large transient wave has caused a severe roll condition as in FIG. 1, an SPS signal is immediately applied to the control valve HC as the ship starts the starboard roll. The RRS signal at this point is low due to the natural and fin damping and the inertia of the ship. As the roll progresses, the SPS signal increases in magnitude as spring K is further tensioned with continued reference leveling at the mercury switch MS. The RRS signal also increases as the precession torque tilts the gyro through an angle corresponding in sense to 6 FIG. 3A. Due to the magnitude of the disturbance, the fins are subject to a comparatively large command signal within the initial phase of the starboard roll for causing full deflection within the limits imposed :by ship speed.

At the limit of the roll, FIG. 1 a large SPS signal is still applied, but the RRS signal has decreased to zero. Ac-

cordingly, due to the decreased resultant signal, the fin angle has decreased somewhat in the last phase of the starboard roll. As the ship starts the return roll, the RRS signal now reversed in sense, starts to increase as indicated by angle 6 FIG. 3A, and becomes dominate as the ship is in its full heeling roll. This reverse signal causes reverse deflection of the fins prior to actual heeling so as to damp continuation of the roll toward port. As the ship heels, the SPS signal which has decreased in magnitude during the return roll, drops to zero as the ship levels off and then reverses as the damped port roll starts. The RRS signal however remains dominant as the roll rate is still fairly high as the ship continues into the port or inertia roll. The cycle described above is re peated until the ship is again stabilized within two or three cycles at tolerable roll.

SYSTEM CHARACTERISTICS stabilized 6 unstabilized db=20 log against the term ta /w,

where:

w =wave frequency, and

w natural frequency of ship. This base axis term takes into account the range of wave activity and the resonant condition, i.e., where tu /w is unity.

Specifically, graph I represents the characteristic performance of the present roll stabliization system, wherein the degree of stabilization throughout the entire wave frequency range that would be encountered is well within the limit of 90% quenching of the resonant peak, i.e., the quenched or stabilized roll with reference to the unstabilized roll of a given ship. This is an arbitrary standard for roll stabilization and is typical of that now specified by ship builders. At the point of resonance (1.0), it will be noted that stabilization is affected to but a minor extent in graph I, and the high point of the curve which is still well within the quenching standard, is beyond the point of resonance and in the higher wave frequency area that is not so commonly encountered. In this system, the SPS gain is made lower than the RRS gain in order to offset the position signal sufficiently to maintain stability at high wave frequencies. As would be expected, stabilization becomes much more effective at the higher limits of wave frequency where the inertia of the ship, which in general is lowly damped with comparatively low natural frequency tends by itself to oppose a high rate of roll.

Graph II represents the characteristic of a known, comparatively complicated and expensive system. Although this more sophisticated system using acceleration sensing with modern electronic control stabilizes within the quenching standard above, it should be noted that stabilization near resonance is not only greatly diminished but significantly, is closer to the low frequency area than in the ease of graph 1. Any improvement in stabilization over graph I takes place only in the rarely encountered higher range of wave frequencies. From the practical standpoint, the improvement is insignificant for reasons mentioned above. I

Graph III illustrates combined rate and acceleration sensing and indicates especially poor stabilization well in advance of resonance, and barely acceptable control only at and beyond resonance, i.e., in the high frequency area.

Graph IV shows roll rate sensing alone, wherein a resonant condition is not apparent; however, as in graph III barely acceptable control is not accomplished within the main control area, i.e., prior to resonance.

The roll stabilization control system of this invention has, as described above, the installation advantages inherent in a compact, simplified and rugged control arrangement that is essentially hydromechanical in construction and involves an optimum of components. In addition, it has practical operational advantages including highly acceptable, reliable stabilization for all ordinarily encountered wind and wave conditions, and flexibility for automatic adaptation to other conditions such as widely varying ship speed, natural list and excess actuator force, not heretofore found in uncomplicated, lowcost roll stabilization systems.

In brief, the basic stabilization control equipment of this invention can be considered as consisting of the SP5 and RRS systems for deriving the primary control signals which in turn are summed at the RRS gyro. The resultant signal is fed to force amplifying apparatus comprising the hydraulic control valve EC and actuator ACT. The force amplifier output is in turn modified by the dual gain feed-back, the characteristics of which are described above in the interaction of the springs K K and K and associated linkages, referring also to FIG. 5. The expressed purpose of this feed-back is to vary the gain of the RRS sensor amplifier as a function of the amplified output amplitude and maintain the fine moment in linear and proportional relationship to roll disturbance up to a certain disturbance magnitude, beyond which the amplifier gain is increased. This causes the fins to move to full deflection upon further significant increase in magnitude of the disturbance, thereby significantly reducing the roll amplitude.

It should be understood that this invention is not limited to specific details of construction and arrangement thereof herein illustrated, and that changes and modifications may occur to one skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. A roll stabilization control system for a ship having power operated lateral fins that are adjustable from neutral positions to angles of attack for applying a righting couple of hydrodynamic forces incident to the ships forward motion, comprising:

(a) ship position sensing means for deriving a force signal variable in sense and magnitude according to the direction and degree of ship roll from a neutral position;

(b) rate of roll sensing means for deriving a second force signal variable in sense and magnitude according to the roll rate;

(c) means for summing said force signals and applying the resultant force signal to proportional control means;

(d) hydromechanical means responsive to said control means for amplifying said resultant force signal;

(e) operating means controlled by the amplified force signal for adjusting said fins to opposite angles of attack, respectively, for establishing said righting couple; and

(f) means for deriving a force feedback signal from the output of said hydromechanical amplifying means for modifying the application of the resultant force signal to said proportional control, said feedback signal deriving means including mechanism for changing the feedback signal at one rate in response to movement of the operating means within a range wherein the fin angles are less than a predetermined magnitude as measured from said neutral positions, and means for modifying the operation of said mechanism at fin angles greater than said predetermined magnitude to thereby cause such mechanism to change the feedback signal at a reduced rate in response to movement of the operating means when the predetermined angle is exceeded.

2. A roll stabilization control system as specified in claim 1, wherein the force feedback deriving means produces a low gain for the force amplifying means incident to ship disturbance at and below a given magnitude, and a comparatively high gain for a greater magnitude of disturbance for causing full fin deflection.

3. A roll stabilization control system as specified in claim 1, wherein the resultant force signal controls a proportional flow hydraulic valve, and the force feedback signal is algebraically added to said resultant signal for corresponding control of said valve and the hydromechanical means.

4. A roll stabilization control system as specified in claim 1, wherein means responsive to actual fin position with respect to a normally neutral position cause, during a condition of natural list, actuation of motive means at a low rate to produce, by increments over a multiple-cycle time period, a biasing force on said proportional control means, said biasing force acting in a direction for establishing a new neutral position corresponding to said natural list, about which roll is stabilized.

5. A roll stabilization control system as specified in claim 1 wherein resilient, energy storing means are interposed individually between the position sensing and feedback means, respectively, and the proportional control means for applying respective force signals thereto and said rate of roll sensing means comprises a rate sensitive gyroscope having a rigid connection with said proportional control means.

6. A roll stabilization control system as specified in claim 1, wherein means for deriving a force signal that is a function of ship speed controls the position of a tin angle limiter in variable blocking attitude with respect to movement of the fin adjusting means, said limiter causing restriction of said movement within a range that varies in an inverse relation to ship speed.

7. A roll stabilization control system for a ship having power operated lateral fins that are adjustable to angles of attack for applying a righting couple of hydrodynamic forces incident to the ships forward motion, comprising:

(c) means for summing said force signals;

(d) a control valve operably connected with the summing means for operation in accordance with the summed signals;

(e) a hydraulic actuator responsive to the operation of the control valve; and

(f) thrust transmitting structure movable by the actuator for producing fin adjustment, such thrust transmitting structure including as a part thereof feedback means that is operably connected to the valve to apply a force thereto for controlling the output characteristics of the actuator, said thrust transmitting sructure also including a force limiting bungee for precluding the transmission of excess actuating force to the thrust transmitting structure.

8. A roll stabilization control system for a ship having power operated lateral fins that are adjustable to angles of attack for applying a righting couple of hydrodynamic forces incident to the ships forward motion, comprising:

(0) means for summing said force signals;

(e) a hydraulic actuator responsive to the operation of the control valve;

(f) thrust transmitting structure movable by the actuator for producing fin adjustment, said thrust transmitting structure including a shaft for reciprocal movement; and

(g) force feedback deriving means for controlling the output characteristics of the actuator comprising a housing carried by the shaft of the thrust transmitting structure and having preloaded spring means mounted therein, a linkage mounted on the housing and coacting with said preloaded spring means, and means operably connecting said linkage to the valve including oppositely preloaded spring means.

9. A roll stabilization control system for a ship having power operated latera-l tins that are adjustable to angles of attack for applying a righting couple of hydrodynamic forces incident to the ships forward motion, comprising:

(d) hydromechanical means responsive to said control means for amplifying said resultant force signal; (e) operating means controlled by the amplified force signal for adjusting said fins to opposite angles of attack, respectively, for establishing said righting couple;

(f) means for deriving a force feedback signal from the output of said hydromechanical amplifying means for modifying the application of the resultant force signal to said proportional control for producing fin adjustment in non-linear relation to said resultant signal; and

(g) means for obtaining a pressure which is a function of propeller speed, a hydraulic reversing valve positionable in accordance with said pressure, an actuator controlled by the valve, and a dual faced cam connected to the actuator for restricting movement of the operating means within a range that varies in inverse relation to propeller speed, said cam having a feedback connection with the reversing valve for restoring the valve to a null position.

10. A roll stabilization control system for a ship having power operated lateral fins that are adjustable to angles of attack for applying a righting couple of hydrodynamic forces incident to the ships for-ward motion, comprising:

(a) ship position sensing means including a normally open mercury switch and a motor responsible to the operation of said switch for deriving a force signal variable in sense and magnitude according to the direction and degree of ship roll from a neutral position and for automatically leveling said switch to establish a stable level reference;

(b) rate of roll sensing means for deriving asecond force signal variable in sense and magnitude according to the roll rate;

(0) means for summing said force signals and applying the resultant force signal to proportional control means;

((1) hydromechanical means responsive to said control means for amplifying said resultant force signal;

(e) operating means controlled by the amplified force signal for adjusting said fins to opposite angles of 13 14 attack, respectively, for establishing said righting References Cited couple; and UNITED STATES PATENTS (f) means for deriving a force teedbacksignal from 2,979,010 4/1961 Branddon et a] 114126 the output of said hydromechanical amplifying means 3 070 869 2/1962 Beach 114 126 for modifying the application of the resultant force 5 g 2/1965 signal to said proportional control for producing fin adjustment in non-linear relation to said resultant MILTON BUCHLER, Primary Examiner.

signal- TRYGVE' M. BLIX, Assistant Examiner.

Wesner 114-126.