US3516377A - Variably damped passive tank stabilizer - Google Patents

Description

June 23, 1970 s, FlELD ET AL 3,516,377

VARIABLIY DAMPED PASSIVE TANK STABILIZER Filed July 3, 1968 6 Sheets-Sheet 1 I \2 2 :WK

F8 -=r I13; r0

L. 22 5 N r: 0% 2 LL la I INVENTORS'. 5Q SHELDON B. FIELD,

g FRANS V. PANGALILA,

RICHARD L. MILLER, 2 NICHOLAS M.GRIGORAKI BY m $2M, M

A TORNEYS.

June 1970 s. B. FIELD ETAL 3,516,377

VARIABLY DAMPED PASSIVE TANK STABILIZER Filed July 3. 1968 6 Sheets-Sheet 2 SHELDON B FIELD,

FRANS V. PANGALILA,

RICHARD L. MILLER, NICHOLAS M.GRIGORAKI ATTORNEYS June 23, 1970 s. B. FIELD ETAL 3,516,377

VARIABLY DAMPED PASSIVE TANK STABILIZER Filed July 3. 1968 6 Sheets-Sheet 5 FIG.4

CURVE FOR UNSTABILIZED SHIP RESONANCE FRAHM TYPE STABILIZER AMPLITUDE OF ROLL IN DEGREES IDEAL CURV FREQUENCY OF ROLL, {2% A =CURVE usmc STABILIZER 0F u.s. SHELDON i i PATENT N0. 5,054,373

NICHOLAS M. GRIGORAKI ORNEYI FRANS V. PANGALILA June 23, 1970 5, FIELD ETAL 3,516,377

VARIABLY DAMPED PASSIVE TANK STABILIZER Filed July 3, 1968 6 Sheets-Sheet 4.

6 SYSTEM POWER SUPPLY 73' L 74 ROLL. RATE DOOR SENSING COMPUTER PRIME TRANSDUCER (GYRO) MOVER SETTING CONTROL /76 AND OVERRIDE PANEL FIG. 5

UNSTABILIZED SHIP AMPLITUDE OF ROLL IN'DEGREES (P) RESONANCE WT FREQUENCY OF ROLL, s INVENTORS SHELDON B. FIELD, FRANS v. PANGALILA, RICHARD L.M|LLER, NICHOLAS M.GRlGORAKI WAQW WQ ORNEYS.

Julie 23, 1970 s HELD ET AL 3,516,377

VAHIABLY DAMPED PASSIVE TANK STABILIZER Filed July 5, 1968 6 Sheets-Sheet 6 ////fl F0 I l 0 l v 3 I I 5 oo :2 L0 96 1| s9 5 f 84 .69 e4 7} 0 835 I0 us |4| |5| 20 N ||9 1.45 I56 COMPUTER COUNTS 000 4|6 500 am 704 759 999 l l l 55.95 I "W LEGEND.

AVERAGED l0 CYCLE FREQUENCY OF SHIP'S MOTION. /v NATURAL FREQUENCY OF SHIP.

I0 PERIOD AVERAGE 0F SHIP'S PERIOD NATURAL PERIOD OF SHIP.

: STABLE STATES.

------ TRANSITIONAL STATES.

QWWMMZM ORNEYS United States Patent 3,516,377 VARIABLY DAMPED PASSIVE TA'NK STABILIZER Sheldon B. Field, Floral Park, N.Y., Frans V. Pangalila, Matawan Township, N.J., and Richard Lawrence Miller, Hempstead, and Nicholas Mark Grigoraki, Westbury, N.Y., assiguors to Flume Stabilization Systems, Inc., Hoboken, N.J., a corporation of New Jersey Filed July 3, 1968, Ser. No. 742,398 Int. Cl. B63b 43/06 US. Cl. 114--125 4 Claims ABSTRACT OF THE DISCLOSURE A passive tank stabilizer for ships including an elongated tank partially filled with a body of liquid in a free surface condition, and an automatically controlled door operative between open, closed and intermediate positions so as to variably and selectively control the amount of damping imparted to transferring liquid, said door operatively permitting maximum liquid passage when the vessel rolls near natural frequency and closing by degrees as the ship rolls at frequencies further away from resonance and being closed to deactivate the tank when the ship is forced rolled above at frequencies at which the stabilizer would otherwise destabilize the ship.

Reference to other patent applications Reference is made to the copending US. patent application Ser. No. 636,780, filed May 8, 1967, now US. Pat. No. 3,422,781 bearing the same title and assignee.

Background The present invention relates to passive tank stabilizers and more particularly to substabilizers of the free surface type in which by virtue of the tank geometry and the liquid level of the stabilizing medium therein the tank liquid oscillates with a natural frequency matched to the roll of the ship.

At the turn of the twentieth century, important advancements were made in the art of roll stabilizing ships at sea primarily by the contributions of Herman H. Frahm. Frahms stabilizer, commonly known as a U-tube type passive tank, was characterized by a pair of opposite wing tanks interconnected by a narrow duct which extended from the bottom of the wing tanks down around the bottom of the ship to the bottom of the opposite tank. In later designs, the entire tank was confined between decks of a ship but the interconnecting duct continued to have a height to confine the liquid in the crossover duct. With this arrangement, the Frahm tanks oscillated at a frequency tuned to the roll of the ship so that the stabilizing moment was applied first in one direction and then in another depending upon the angular position of the ship. In view of Frahms basic assumption that a ship rolls at its own frequency regardless of sea, the Frahm approach operated generally satisfactory when the ship rolled at its natural or designed frequency; however, the primary reason that the Frahm tanks fell into disuse was that when the ship was forced rolled at a frequency somewhat removed from the natural roll frequency, the tanks increased or introduced rolling and thereby created an unsafe condition at sea. Corrective changes were made in the design by incorporating a manually or automatically controlled valve in a crossover air duct communicating with the upper parts of the wing tanks. An operator, upon sensing a variation in the roll frequency, or upon visually detecting a change in ice the oncoming wave frequency, would operate the valve to throttle the air passage and in this way attem t to dampen liquid transfer between the wing tanks. This technique not only proved unsatisfactory from the stabilization aspect, but it was also unbearably noisy in operation.

Following the Frahm period, the art developed away from tank stabilizers and turned to the use of activated fins such as the type installed on the Queen Mary which although extremely expensive to build and install operated satisfactorily when the ship was in motion. Because of the excessive installation and maintenance expense and the undesirable effect on speed, fins have enjoyed only limited use.

With the advent of missile tracking ships in the late fifties, there developed a need for a stabilizer which would operate when the ship is substantially at rest. To meet this need, a new passive tank stabilizer was developed and patented in the United States. See US. Pat. No. 3,054,373. Unlike the Frahm tanks, this later development relies only on hydrodynamic damping to control the passage of liquid from one end of the tank to the other. The tank is characterized by having substantially the same height throughout with a liquid body partially filling the tank so that an air space is formed above the liquid thus giving the liquid a free surface condition throughout the tank. Tuning control is accomplished by adjusting liquid height. Unlike the Frahm tanks, the response curve for the stabilizer is substantially flat. When designing the tank parameters, some sacrifice is made in performance near resonance to achieve suitable results when the ship is force rolled above or below resonance. Thus, although a ship stabilized by the free surface tank has a slightly greater roll amplitude at resonance than the same ship stabilized with a Frahm type stabilizer, the conventional free surface tank does not destabilize the vessel nearly as much as the Frahm tank at high and low frequencies removed from the resonant frequency.

It is generally known that the free surface tank is currently setting the standards for the industry and tanks of this type have been installed in over three hundred vessels since 1960. However, a free surface tank with its fixed geometry suffers from the disadvantage that although its performance characteristics are much better than the Frahm type system at high and low frequencies, the conventional tank still introduces slightly more roll amplitude to the ship at these frequencies than is the case with the tank deactivated. In addition, because the tank must be designed to introduce substantial amounts of damping t0 the tank liquid for suppression of destabilization at the roll frequency extremities (particularly the low frequency), the stabilization moment magnitude at resonant frequency is unduly suppressed.

The present invention improves on the conventional free surface tank by providing for the first time a variably damped, free surface tank system which senses the roll frequency and amplitude of the ship and automatically controls damping so that damping is reduced when the ship rolls near resonance and the damping is maximized to completely deactivate the tank when the ship rolls beyond a frequency limit. The damping may be variably set to intermediate degrees at specified frequency ranges between the frequency limits and resonance.

To accomplish these results, the present invention pro vides a passive free surface tank with a controlled plate or door movable between fully opened and fully closed positions. An automatic sensing and control unit, preferably a gyroscopic type, senses the rolling of the ship and produces signals of predetermined magnitude corresponding to the rolling frequency. When the ship rolls near resonance (which for the purpose of this disclosure is generally synonymous with natural roll frequency) the unit controls the motor to open the door as wide as possible so as to impart minimum tank liquid damping and to permit maximum liquid passage therethrough resulting in the greatest stabilizing moment. When the ship rolls at a frequency below a predetermined minimum frequency and, in some cases, above a predetermined maximum frequency the unit controls the door thus deactivating the tank end preventing the tank from destabilizing the ship. At this time, the ship is stabilized by virtue of its inherent static stability. At intermediate ranges of frequencies, the

control unit operatively moves the door to intermediate positions so as to impart controlled and predetermined amounts of damping depending upon the sensed rolling frequency.

On example of the control system includes a roll sensing apparatus stepping a ring counter every roll cycle. An output on the first stage of the counter starts a pulse oscillator the frequency of which is set to correspond to the ships G-M. A second counter counts the pulses from the pulse oscillator and stores its count information in a digital memory device. When an output appears on the last stage of the first counter, the stored counter is sampled and fed through a logic stage the output of which controls switches or valves to effect door operation as stated above. In addition, the counters are reset to their initial conditions to be ready for the next operation. Manual override to deactivate the tank and manual door positioning are also designed into the system.

It is therefore a primary object of the present invention to provide a variably damped free surface tank stabilizer which provides the advantages and solves the problems outlined above.

Description of drawings Other and further objects of the invention will become apparent with the following detailed description when taken in view of the appended drawings in which:

FIG. 1 is a horizontal section of a tank in accordance with the present invention and part of the ships hull.

FIG. 2. is a transverse vertical section taken along line 2-2 of FIG. 1.

FIG. 3 is a longitudinal vertical section taken along line 33 of FIG. 1.

FIGS. 4 and 5 are charts illustrating the amplitude-frequency characteristics of an unstabilized ship, and ships stabilized with various stabilizer configurations including the present invention.

FIG. 6 is a block flow diagram of the sensing and control system for the invention.

FIG. 7 is a block diagram of one example of the sensing and control system.

FIG. 8 is a graphical illustration of the operating characteristics of the system of FIG. 7.

Detailed description of the embodiments With reference to FIGS. 13, one embodiment of the invention includes a passive tank stabilizer generally indicated as 10 arranged between adjacent decks 12 and 14 of ship 16 and extending from one side of the hull to the other. The height of tank 10 is generally uniform throughout and a body of liquid 18 partially fills tank 12 to a level adjusted so that the natural frequency of the tank liquid oscillation is generally matched to the roll frequency of the ship. The breadth (fore and aft) of tank 10 is determined by the particular volume capacity and geometry selected to best suit the particular ship being stabilized. The liquid can be of any suitable kind such as fresh Water, sea water, bunker oil, fuel or the like. In some cases drilling mud or other suspensions can be used.

According to the principles of US. Pat. No. 3,054,373, tank 10 is divided into two wing tanks 11 and an interconnecting channel 13 by four upstanding members 32 aligned in pairs and spaced from the center and ends of the tank. Members 32 extend from the bottom of the tank up to a height well above the static surface of body 18 and have mutually facing ends 34 spaced from each other to form passageways 36 and turned outward for hydrodynamic purposes. The cross section area of opening 36 is designed so that members 32 impart some damping to the passage of liquid therethrough. For reasons made clear below, the damping provided by members 32 should be less than that provided by the tanks of U.S. Pat. No. 3,054,373. In some designs it may be sufficient to operate with only two such plates 32 which are diagonally aligned.

According to the invention, a damping and cut-off control assembly 20 is arranged in the center of the tank. The assembly can have any suitable form such as sliding doors, rotating horizontal or vertical flaps, or the like. One preferred assembly is formed by a pair of upstanding plates 22 extending from the floor of tank 10 up the fore and aft walls thereof to a height suitably above the expected static liquid level Within the tank. A valve plate or door 24 supported for rotation about a vertical shaft 26 between plates 22 is operatively movable between perpendicular and generally parallel positions relative to plates 22, i.e., open and closed positions, respectively. The distance D should be selected so that door 24 when in a partially opened position cooperates With members 32 to impart some damping to liquid passage.

Door 24 is operated by a reversible motor 28 receiving control signals from a control unit 31 which senses the frequency of the rolling of the ship and generates a control signal which operates motor 28 to effect rotation of door 24 to the fully opened, fully closed, or one of a number of intermediate positions. Instead of a motor drive, a hydraulic power system or any other suitable type of prime mover can be used. In this way, the assembly 20 operates to vary the damping imparted to the tank liquid and in some instances serves to de-activate the tank completely. It will be appreciated that more than one such assembly 20 and control motor therefor can be provided throughout the tank as desired. Mechanical means (not shown) may be provided to bias the door toward the closed position so that the tank halves are isolated in the event of power failure. Motor 28 may be housed within a space formed in the top of the tank by a fiat plate 30 supported by the tops of plates 22 and 32 and joined rigid with the top of the tank by transverse diaphragms 38 and longitudinal plates 40, the latter of which are provided with openings 42 to permit a free passage of air and to reduce the weight of the assembly.

In order to achieve smooth and reliable operation for the damping and cut-off assembly 20, it is preferred that unit 31 average the frequency of the previous ten (or other suitable number) roll cycles and generate control signals accordingly. In this way, the stabilizer will avoid hunting and will not be adversely affected by transients as would be the case if assembly 20 responded solely to the frequency of each roll cycle.

Unit 31 may comprise any suitable mechanical or electrical arrangement, one example of which is illustrated in FIG. 6. Power supply 70 applies operating power to the system. A sensing transducer stage 72 senses the roll rate and magnitude and develops a sinusoidal signal accordingly. The transducer feeds this signal to the computer stage 74 which is programmed by the manually adjusted switches of the control panel 76 which has override switching capability. The computer automatically develops the door control signals in accordance with the control switch settings, pre-programmed logic circuit design, and roll frequency information received from stage 72 and applies the same to motor 28. In response thereto, motor 28 opens, closes, or moves door 24 to one of a number of intermediate positions. In the example hereinafter referred to, the door can occupy a full open, twothirds open, onethird open and full close positions.

One exampled embodiment of the control system is illustrated in FIG. 7. The ship roll rate is sensed by a conventional sensing unit 80 which may include a gyroscope, pendulum, accelerometers, or the like. Power is conventionally applied by power supply 82 through phase shifter 84 to signal generator 86. The output signal from unit 80 is amplitude demodulated by demodulator 8 8 which receives a single-phase reference signal from power supply 82. The envelope of the demodulator output is developed by detector 90 which controls crossover detector 92 for developing a train of square-wave pulses the fundamental period of which corresponds to the roll period of the ship. A pulseshaping circuit 94 is provided to develop spikes or pulses of short duration, preferably at the leading edge of each pulse received. In this way, the pulses at the output of circuit 94 occur once for every roll cycle of the ship.

For the example in which the door position is to be adjusted once every eleven roll cycles, an eleven-stage ring counter 96 receives the output pulses from circuit 94. A variable frequency pulse oscillator 98 is provided to generate pulses at a frequency determined by dial 100 located at the operating console panel, which dial is set on a reading corresponding to the ships G-M. The operation of oscillator 98 is effected by an output signal occurring on any one of the states 1 through of counter 96. Thus, when counter 96 counts to the eleventh cycle, no signal appears at the output of the first ten stages thereof, and oscillator 98 stops running.

Additional ring counters 102, 104 and 106, preferably formed as units, tens and hundredths ten-stage ring counters, receive the output of oscillator 98 and keep a running count thereof. The on or operating status of counters 102, 104 and 106 are also controlled by an output or any one of the stages 1 through 10 of counter 96 delivered along lead 108. As evident from the figure, whenever an output appears at stage 11 of counter 96, oscillator 98 is immediately turned olf and after a oneunit time delay established by a delay device 114, counters 102, 104 and 106 are reset to initial conditions.

A gate or switch 110 is enabled by the output of oscillator 98 so as to pass a reset signal whenever an auxiliary output of counters 102, 104 and 106 reads a predetermined count determined by the connections to And gate 112. In one example, And gate 112 reads the stages corresponding to a count of 987 which signal, if developed with oscillator 98 still running, is applied through gate 110 to counter 96 so as to immediately step the counter output to stage 11. Therefore, if the ship rolls with amplitude below that which the gyroscopic unit can sense or is programmed to sense, the ship is considered to be suitably stabilized so that repositioning of the door is not desired. Therefore, with this arrangement, if ring counter 96 does not count 11 roll cycles for the ship within the time period determined by the frequency of oscillator 98 and the programmed count number determined by And gate 112, the system will automatically be reset.

The logic or computing apparatus is formed by a sevenstate numeric memory 120, including seven inputs, each of which is connected to counters 102, 104 and 106 so as to receive an input whenever the counters indicate a predetermined count. The seven-state numeric memory is of conventional design, including transistors, SCRs or the like, and can assume one of seven stable states. With reference to FIG. 8, each input to memory 120 represents one of the seven counts necessary to effect one of the seven door position changes as described below. The memory 120 has seven outputs supplying respective inputs of a gating circuit 122 which includes a plurality of normally off, solid state gated channels, all of which switch into their saturated on state in response to a sample signal appearing on the output of stage 11 of counter 96. The seven outputs of circuit 122 are fed directly to a selective logic circuit 124 which includes a four-state memory device, receiving the output of gating circuit 122. The output of circuit 124 is applied on one of the four output leads, depending upon the last-received input, which is weighed against the information already stored in the circuit 124.

The capability of the computer and logic stage to develop an output so as to control the door to remain stationary or move to one of its other positions, thus making tank stabilization more efiicient, is best understood with reference to FIGS. 7 and 8. From FIG. 8 it can be seen that the system is programmed to maintain the door position in one of the four positions indicated on the left While the ship is rolling in certain frequency ranges as indicated. The system also has a hysteresis characteristic, so that the system avoids hunting. For example, the door is two-thirds open until the roll average sensed by the system reaches less than .69 normalized frequency, upon which the door is moved to a one-third open position. However, on subsequent readings, the door is moved back to a twothirds open position only when a normalized frequency of .71 or greater is sensed by the system.

As seen in FIG. 8, the normalized frequency values at which the door changes position is correlated to number counts taken from counters 102, 104 and 106. Each input to memory circuit corresponds to the programmed number count designated for one of the door changes. For example, during operation, when the count reaches 500 an input signal appears at P (full open) of memory 120 and the memory is triggered to its first stable state. When the count reaches 581, an input appears at P (full open). triggering memory 120 to its second stable state, and so on, until an output signal appears at stage 11 of counter 96. When this occurs, oscillator 98 stops running and the output of memory 120 is gated through circuit 122 to the logic network 124. After a suitable delay and before counter 96 receives the next pulse from circuit 94, memory 120 and counters 102, 104 and 106 are reset from the signal output of delay device 114. However, by that time logic circuit 124 has reacted and changes states in accordance 'with the stored information and the input from the last reading to develop the appropriate signal on one of the four output leads representing the four door posi tions. Upon the gate through of circuit 122, the input to circuit 124 is compared with previously stored DC. bias signals so as to develop the logic result indicated in FIG. 8. The output of stage 124 controls a solid-state contactor 126 which feeds a signal on the respective line (representing the new position of the door) to a door position sensor which compares the doors existing position and the signal line representing the new position and controlling the open and close valves 130 and 132 associated with prime mover 134, thereby moving the door to the new position. Of course, if the door is in one position and the signal appears on the line from contactor 126 corresponding to that same position, neither valve 130 or 132 will be operated and the door will remain in that position. A manual override switch mechanism 136 permits manual positioning of the door regardless of the output of selective logic 124. Also, an emergency close switch mechanism 138 permits overriding the system to deactivate the tank by fully closing the door.

The stabilizing effect of the invention is best understood With reference to FIG. 4. In that figure there is illustrated the typical amplitude-frequency characteristic curve for the unstabilized ship. Roll amplitude is maximum at resonance, F that is, when the frequency of oncoming waves is matched to the natural roll frequency of the ship. It can be seen that because of the static stability of the ship the amplitude of roll at frequencies well above and below F drops off to only a fraction of that at resonance. Also illustrated in the figure is the curve for the typical Frahm type stabilizer with its characteristic double hump bridging the resonant frequency. It is thus clearly seen that the Frahm type stabilizer introduces more roll amplitude at the high and low frequency regions than for the case of the unstabilized ship. However, the Frahm type stabilizer does achieve good results near resonance.

Also shown in FIG. 4 is curve A for a typical passive free surface tank of the type disclosed in U.S. Pat. No. 3,054,373 which provides a generally flat response curve A showing adequate roll reduction near resonance and much better results at high and low frequencies than in the case of the Frahm type stabilizer.

The amplitude-frequency roll characteristics of a ship stabilized by the present invention is represented by curve B for the case of varying door positions at both ends of the frequency spectrum. When the ship rolls at a frequency near resonance (F control unit 31 operates motor 20 to open door 24 to the fullest extent. With door 24 fully opened, the tank imparts a minimum damping to the liquid passage so that a maximum stabilizing moment is developed in opposition to roll. Because of the operation of the invention at high and low frequencies as described below, the magnitude of this minimum damping (developed by all internal members 32, 22, 24) is less than that developed by the conventional free surface tank of U.S. Pat. No. 3,054,373. In other words, with the door full open the tank should be underdamped. Therefore, for frequencies near F curve B has a lower magnitude than curve A. Like the tank in said patent, the invention is designed so that the tank liquid oscillates at a substantial phase lag relative to the ships roll, i.e., up to a theoretical 90 lag.

When the gyroscopic control unit 31 senses that the ship is rolling below F or above F it generates a signal sufiicient to close door 24 an amount which corresponds to the extent said frequency is removed from F or F Thus, as the ship rolls at decreasing frequencies below and relative to F or at increasing frequencies above and relative to F door 24 is rotated to more closed positions to correspondingly block liquid passage. As evident from FIG. 4, the effect of closing door 24 in the regions of F -F and F -F is to change the response curve toward that for the unstabilized ship at the frequency limits. Examples of the door position-roll frequency are:

Roll frequency Door position F and below Full closed. Near F /3 open. Near F open. F and above Full open. F and below Full open. Near F /3 open. Near F, /3 open. F and above Full closed.

If the control senses the ship rolling at or below F or at or above F door 24 is fully closed to thus deactivate the stabilizer so that the ship rolls with an unstabilized characteristic which is an improvement over conventional free surface tanks and, of course, over the Frahm type stabilizer. When door 24 is completely closed to de-activate the tank, plates 32 assist to quiet the liquid on each respective side of door 24. Of course hysteresis characteristics can be programmed into the system as described in the above example as depicted in FIGS. 7 and 8.

The stabilization effect achieved by the embodiment including FIGS. 7 and 8 represents a somewhat different mode of operation. In this mode, a greater volume of tank liquid is placed in the tank than is required for tuning the tank liquid and ship roll. Normally, such a step would be detrimental to tank operation at low frequencies but beneficial to tank performance near F and high frequencies. See curve C, FIG. 5. But with the invention good results are obtained over the entire frequency range by effecting curve D at frequencies below F To develop curve D, the invention operates as follows:

Roll frequency Door position F and below Full close. Near F /1 open. Near F /2 open. Near F open. F and above Full open.

If desired, the system can fully close the door at F Again hysteresis principles can be designed into the system to achieve smooth operational transition along curve D.

Thus there has been described a new and improved variable damped free surface passive tank stabilizer which is designed for optimum performance near resonant frequency without consequences to the destabilization effect it might have at high or low frequency forced roll conditions. In addition, the invention provides means to impart variable amounts of damping to the tank liquid at prescribed frequency ranges so that the ship operates with improved stability. It will be understood that various modifications and changes can be made to the herein disclosed example of the present invention without departing from the spirit and scope thereof.

What is claimed is:

1. A roll stabilization system for ships comprising an elongated tank, a body of free surface liquid partially filling the tank to a level at which the tank imparts to the ship an oscillating, timed stabilizing moment near the natural frequency of the ship, closure means including a closure assembly located within the tank and spaced from the ends thereof to selectively isolate and permit liquid communication along the liquid path within the tank, and sensing and control means for sensing the average roll frequency of the ship during a predefined interval and completely closing the closure assembly to deactivate the tank when the ship rolls at an averaged first frequency away from its natural frequency and at which the tank liquid would destabilize the ship so that the ship rolls at an amplitude primarily dependent upon the ships static stability, wherein said control means includes oscillator means whose frequency is variable in accordance with the ships G-M and further including first means for counting the output oscillations of the oscillator means and developing the control signals in accordance with the count existing upon the first means receiving an interrogate signal, said sensing means including second means for counting the ships roll cycles and establishing a time zero reference and applying an interrogate signal to said first means upon reaching a predetermined roll cycle count after time zero.

2. A system as set forth in claim 1, wherein the oscillator means is a free running oscillator.

3. A system as set forth in claim 1, wherein said sensing and control means includes sensing means to generate signals representing the ships roll frequency and control means for developing control signals in accordance with the ships programmed GM and the signals from said sensing means to effect positioning of the closure means.

4. A system as set forth in claim 1 wherein said second means also includes means for turning off the oscillator means during interrogation and resetting said first means shortly after interrogation but prior to the next establishment of time zero and energizing the oscillator means and enabling the first means upon the establishment of the next time zero.

TRYGVE M. BLDC, Primary Examiner

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