MARINE FIN STABILIZER CONTROL CIRCUIT
United States Patent 3727572
A fin stabilizer control circuit accepts signals from a roll angle sensor, a roll rate sensor and a ship's speed sensor. The signals are first processed in a lift order computer which includes an automatic gain control circuit in which the port and starboard lift orders are compared with the square of the ship's speed so as to vary the sensitivity of the control circuits with changes in ship speed and ship residual roll motion. The output signals from the lift order computer are applied to a stroke order computer along with feedback signals representing fin angle and lift. Either feedback signal may be selected for summing with the signal from the lift order computer to produce a stroke order signal that determines the rate of deflection of the stabilizing fins. Limiting circuits in the stroke order computer restrict the stroke order signal in accordance with the larger of the two feedback signals.
US Patent References:
Control system for stabilizing fins
Chadwick, Jr. - June 1959 - 2890673
Roll stabilization system for marine vessels
Chadwick, Jr. et al. - November 1960 - 2960959
Ship stabilization system
Braddon et al. - April 1961 - 2979010
Control system for stabilizing fins
Chadwick, Jr. - June 1959 - 2890673
Roll stabilization system for marine vessels
Chadwick, Jr. et al. - November 1960 - 2960959
Ship stabilization system
Braddon et al. - April 1961 - 2979010
Inventors:
Nelson, Louis W. (Upper Marlboro, VA)
Wesner, Charles R. (Crozet, VA)
Wesner, Charles R. (Crozet, VA)
Application Number:
05/191676
Publication Date:
04/17/1973
Filing Date:
10/22/1971
View Patent Images:
Export Citation:
Assignee:
Sperry Rand Corporation (New York, NY)
Primary Class:
Other Classes:
114/122, 244/177, 318/585
International Classes:
B63B39/06; G05D1/08; B63B39/00; B63B39/06
Field of Search:
114/122,126 244/77R,77E 318/489,585,588
Primary Examiner:
Buchler, Milton
Assistant Examiner:
Kunin, Stephen G.
Claims:
We claim
1. A marine roll stabilization system of the type employing adjustable attitude fin stabilizers, said system comprising means to provide electrical signals representative of the roll angle, roll rate, and speed of the vessel; means to derive an electrical signal representative of the speed squared from said speed signal and an electrical signal representative of the roll acceleration from said roll rate signal; a lift order computer having a first channel for modifying said roll rate signal and a second channel for combining and modifying said roll angle, roll rate, and acceleration signals, each of said channels including an adjustable gain amplifier for varying the amplitude of the signal in that channel; summing means for adding the signals from said first and second channels; means responsive to the output of said summing means for producing direct and inverted lift order signals; comparator means for comparing the magnitudes of each of said lift order signals with said speed squared signals; circuit means responsive to the output of the comparator means for adjusting the gain of said adjustable gain amplifiers; said circuit means being constructed to increase the gain of said amplifiers when the magnitude of the speed squared signal exceeds the magnitude of both lift order signals and to decrease the gain of said amplifiers when the magnitude of either lift order signal exceeds the magnitude of the speed squared signal; and stroke order computer means for driving said adjustable attitude fin stabilizers in response to said lift order signals.
2. The system of claim 1 wherein said circuit means includes integrating means arranged so that the output of said comparator means may be applied to the adjustable gain amplifiers through said integrating means.
3. The system of claim 2 wherein the output of said integrating means is applied to the adjustable gain amplifiers in said first and second channels through first and second gain control amplifiers respectively, said first gain control amplifier having a gain greater than said second gain control amplifier so that said roll rate signal in said first channel reaches a maximum before the corresponding signal in said second channel in response to an increasing voltage from said integrating means and remains at a maximum level longer than the signal in said second channel in response to a decreasing voltage from said integrating means.
4. The system of claim 3 further including means to provide lift feedback signals indicative of the instantaneous lift being provided by said stabilizing fins and means to provide angle feedback signals indicative of the instantaneous deflection angle of said stabilizing fins, said stroke order computer means including means to combine either of said feedback signals with said lift order signals in a negative feedback relationship.
5. The system of claim 4 in which said stroke order computer means includes diode limiting means arranged so that the output signal from the stroke order computer means is restricted to fixed maximum positive and negative values.
6. The system of claim 5 in which said stroke order computer means further includes variable limiting means responisve to the value of said feedback signals, said variable limiting means being adjusted to further restrict the output signal from said stroke order computer means to values within the range of said fixed values.
7. The system of claim 6 in which said variable limiting means responds to the larger of said feedback signals.
8. The system of claim 7 in which the stroke order computer means further includes a varaible gain amplifier connected to amplify the applied lift order signal by a gain factor which is a function of said speed squared signal.
1. A marine roll stabilization system of the type employing adjustable attitude fin stabilizers, said system comprising means to provide electrical signals representative of the roll angle, roll rate, and speed of the vessel; means to derive an electrical signal representative of the speed squared from said speed signal and an electrical signal representative of the roll acceleration from said roll rate signal; a lift order computer having a first channel for modifying said roll rate signal and a second channel for combining and modifying said roll angle, roll rate, and acceleration signals, each of said channels including an adjustable gain amplifier for varying the amplitude of the signal in that channel; summing means for adding the signals from said first and second channels; means responsive to the output of said summing means for producing direct and inverted lift order signals; comparator means for comparing the magnitudes of each of said lift order signals with said speed squared signals; circuit means responsive to the output of the comparator means for adjusting the gain of said adjustable gain amplifiers; said circuit means being constructed to increase the gain of said amplifiers when the magnitude of the speed squared signal exceeds the magnitude of both lift order signals and to decrease the gain of said amplifiers when the magnitude of either lift order signal exceeds the magnitude of the speed squared signal; and stroke order computer means for driving said adjustable attitude fin stabilizers in response to said lift order signals.
2. The system of claim 1 wherein said circuit means includes integrating means arranged so that the output of said comparator means may be applied to the adjustable gain amplifiers through said integrating means.
3. The system of claim 2 wherein the output of said integrating means is applied to the adjustable gain amplifiers in said first and second channels through first and second gain control amplifiers respectively, said first gain control amplifier having a gain greater than said second gain control amplifier so that said roll rate signal in said first channel reaches a maximum before the corresponding signal in said second channel in response to an increasing voltage from said integrating means and remains at a maximum level longer than the signal in said second channel in response to a decreasing voltage from said integrating means.
4. The system of claim 3 further including means to provide lift feedback signals indicative of the instantaneous lift being provided by said stabilizing fins and means to provide angle feedback signals indicative of the instantaneous deflection angle of said stabilizing fins, said stroke order computer means including means to combine either of said feedback signals with said lift order signals in a negative feedback relationship.
5. The system of claim 4 in which said stroke order computer means includes diode limiting means arranged so that the output signal from the stroke order computer means is restricted to fixed maximum positive and negative values.
6. The system of claim 5 in which said stroke order computer means further includes variable limiting means responisve to the value of said feedback signals, said variable limiting means being adjusted to further restrict the output signal from said stroke order computer means to values within the range of said fixed values.
7. The system of claim 6 in which said variable limiting means responds to the larger of said feedback signals.
8. The system of claim 7 in which the stroke order computer means further includes a varaible gain amplifier connected to amplify the applied lift order signal by a gain factor which is a function of said speed squared signal.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems for stabilizing marine vessels against excessive rolling motion and more specifically to roll stabilizing systems employing adjustable fins projecting from either side of the vessel's hull.
2. Description of the Prior Art
Fin stabilizers for roll stabilization are well known in the art. U.S. Pat. No. 2,979,010 issued to F. D. Braddon et al. on Apr. 11, 1961 and assigned to the present assignee, for instance, concerns a fin stabilization system in which the fins are positioned in accordance with a lift order command signal. Typically, the lift order command signal actuates a hydraulic system through a servo mechanism.
In prior art systems, the servo operates through a slip clutch. Large command signals drive the servo to its limit stops whereupon the slip clutch prevents further motion in the hydraulic stroke control system. In previous systems, the limit stops are manually adjusted to 100 percent stroke. The slip clutches in such systems, however, may experience excessive wear. Furthermore, in prior art systems, mechanical off-stroking is typically used for maximum fin angles only. The limits cannot be programmed to meet changing requirements.
SUMMARY OF THE INVENTION
The fin stabilizer circuit of the present invention receives signals representing the ship's roll angle, roll rate, and speed. The roll acceleration is computed from the roll rate signal and speed squared is computed from the ship's speed. By modifying the lift orders in accordance with the speed squared and the comparative value of the various signals, the circuit compensates for changes in fin activity caused by changes in ship's speed and weather, and permits the fins to be used as resonance dampers in severe seas.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sketch illustrating the operation of a fin stabilizer device using the principles of the invention, and
FIGS. 2a and 2b are schematic representations of a circuit useful in practicing the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG 1 represents a transverse section through a ship employing a fin stabilizing system and illustrates the environment in which the invention is intended to operate. Port and starboard stabilizing fins project laterally from the sides of the vessel and function in a manner analogous to ailerons in an aircraft. Sensors detect the roll angle, roll rate and speed of the ship. Various types of sensors are known in the art. In many applications, a rate gyro may be used to detect roll rate. This value may then be integrated to derive the value of roll angle and differentiated to derive the value of roll acceleration. Signals from the various sensors are applied to a lift order computer which processes the signals from the sensors to provide a lift order signal and a signal representing the square of the ship's speed. The lift order signal and the signal representing the square of ship's speed is applied to a stroke order computer. Signals from the stroke order computer are applied to the port and starboard fin actuating mechanisms 11 and 13 respectively. Feedback signals from the fin actuating mechanisms are all applied to the stroke order computer. The structure and mode of operation of the lift order computer and the stroke order computer will be described in detail. The fin actuating mechanisms 11 and 13 are conventional devices. They may, for instance, be hydraulic devices similar to those illustrated in the aforementioned Braddon et al. patent. Feedback signals representing the instantaneous fin angles as well as the vertical forces or "lift" to which the fins are being subjected may be derived in a straightforward manner. Fin angle transducers are well known in the art. Lift transducers are described in detail in the aforementioned Braddon et al. patent.
FIG. 2a represents, in functional form, the lift order computer and the associated components necessary to produce the lift order signals. The computer pre-amplifier receives the various signals from the sensors and converts these signals into voltages that may be accepted by the lift order computer itself. Typically, the roll angle signal may be an a.c. signal having a frequency of 400 Hertz and an amplitude of 75 millivolts per degree of roll. This a.c. signal may be first passed through a demodulator 15 and a calibrating network 17 adjusted to produce a d.c. roll angle output signal from the signal pre-amplifier having a magnitude of 500 millivolts per degree. The roll rate signal may typically be a d.c. signal having an amplitude of 3 volts per degree per second. This roll rate signal may be calibrated in a calibrating unit 19 to produce a roll rate output signal typically in the range of 2 volts per degree per second. The signal representing ship's speed is also calibrated in a calibrating unit 21. The output of the calibrating unit 21 is then applied to a conventional squaring circuit 23 so as to derive a signal representing the square of a ship's speed. The calibrating unit 21 is adjusted to provide signals having magnitudes that can be accepted by the remaining components in the system.
The roll angle and roll rate signals from the computer pre-amplifier are applied to the lift order computer. The lift order computer derives a first signal representing roll rate and a second signal representing roll angle, plus roll rate, plus acceleration. These signals are summed in a lift order limiter so as to provide port and starboard lift order output signals. The amplitudes of the signals from the lift order computer are modified by a lift order automatic gain control circuit as will be described.
The automatic gain control circuit and the lift order limiter cooperate with the lift order computer to combine the various control signals for optimum performance. It is known in the art to combine angle signals, acceleration, and rate signals for specific purposes. If a ship is displaced about its roll axis by a momentary torque, the ship will heel over to a maximum displacement; then return to a level position. In returning to the level position, the ship may oscillate several times before finally coming to rest. Stabilization circuits may be used not only to minimize the maximum displacement of the ship, but also to eliminate the oscillations by proper damping. Roll stabilization might be used by applying a stabilizing torque which is exactly equal to and opposite the instantaneous displacement of the ship. It is known in the prior art, however, that by introducing control components corresponding to the acceleration and the rate of displacement, superior stabilization may be realized.
A typical damped displacement curve is a single-peaked curve somewhat resembling a normal distribution curve. The corresponding rate curve rises from zero in the same direction as the displacement curve, passes through a maximum before the maximum of the displacement curve, then decreases through zero to a negative maximum near the end of the displacement curve, whereupon the rate curve again approaches zero from the opposite direction of the displacement curve. The corresponding acceleration curve rises from zero in the same direction as the displacement curve, quickly approaches a maximum, then decreases through zero at a time corresponding to the maximum of the rate curve and proceeds to a negative maximum at a time corresponding to the positive maximum of the displacement curve. The acceleration curve then proceeds through zero to a secondary maximum near the end of the displacement curve and finally approaches zero from the same direction as the displacement curve.
The more a control is responsive to acceleration, and to a lesser extent, the more the control is responsive to rate, the nearer the control will approach the ideal of completely neutralizing the disturbance. As was mentioned previously, the rate curve is opposite in sense to the displacement curve near the end of the recovery period. For this reason, the rate control is valuable for its positive damping effect. It will also be remembered that the acceleration control reaches a maximum before either the displacement or rate curves. For this reason, the acceleration control is valuable for its anticipatory effect in reducing displacement. The automatic gain control circuit, the lift order limiter, and the lift order computer circuits of the present invention make use of these observations to provide optimum stabilization regardless of ship speed or weather conditions.
In the automatic gain control circuit, the port and starboard signals from the lift order limiter are compared with a positive signal representing the square of the ship's speed in the comparator amplifiers 25 and 27. The amplifier 25 supplies a positive input signal to the integrator 29 when the port lift order is negative and greater in magnitude than the positive speed squared signal. Similarly, amplifier 27 supplies a positive input to the integrator 29 when the starboard lift order is negative and greater in magnitude than the positive speed squared signal. A mixing network 31 is adjusted to produce a voltage at the output of the integrator that is positive if the output of either amplifier 25 or 27 is positive and negative if both amplifier outputs are negative. The lift order signals are applied to the automatic gain control circuit in a negative feedback fashion. If the magnitude of the speed squared reference exceeds both lift orders, the output of the integrator 29 increases which acts to increase the gain of the amplifiers in the lift order computer and thereby to increase the magnitude of the lift order signals. If either lift order becomes more negative than the speed squared reference is positive, the integrator 29 output decreases so as to decrease the magnitude of the lift order signals. In general, the average input to the integrator 29 is zero only when the integrator output raises the gain of the lift order computer to the point at which the absolute value of the lift order exceeds the speed squared signal for more than 50 percent of the time. Since the amplifiers 25 and 27 constantly compare the lift order signals to the speed squared signal, the sensitivity of the system varies with the changes in the ship's speed and ship residual roll motion.
A selector switch 33 permits operation in the automatic mode or the manual mode. Although the switch 33 has been illustrated as a simple mechanical switch, it will be understood that the switch ordinarily would be constructed from a network of solid state switching devices. A voltage selecting network shown as a potentiometer 35 for use in the manual mode of operation provides operator selected voltages having magnitudes comparable to those available from the automatic mode section of the circuit.
Signals from the selector switch are applied through a signal level-adjusting circuit, functionally represented as a potentiometer 37, to gain control amplifiers 39 and 41.
The amplifier 39 provides an output signal that is used in controlling the gain of an amplifier in the roll rate channel of the lift order computer. Similarly, the amplifier 41 provides an output signal that is used in controlling the gain of an amplifier in the roll angle, plus acceleration, plus roll rate channel of the lift order computer. The amplifiers 39 and 41 are constructed so that the gain of amplifier 39 is larger than the gain of the amplifier 41. Thus, when an increasing voltage is applied to the amplifiers 39 and 41, the gain of the amplifier in the roll rate channel of the lift order computer will reach a maximum before the gain of the corresponding amplifier in the roll angle plus roll rate, plus roll acceleration channel. Conversely, when the voltage applied to the amplifiers 39 and 41 is decreasing, the gain control acts to first reduce the roll angle and roll acceleration components in the lift order before the roll rate component is reduced. Since the primary signal for stabilization is roll rate, better stabilization in rough seas is obtained than would be possible if all signals were reduced proportionately. Thus, in severe seas, fin activity can be reduced by using the fins as resonance dampers while retaining full stabilization capability for milder weather.
It will also be noted that the same shaping of the gain curve is present whether the device is being operated in the manual or automatic gain control mode.
The lift order computer receives the calibrated roll angle signal and calibrated roll rate signals from the computer preamplifier. The roll rate signal is applied to a potentiometer 43 and through a capacitor 45 to a selector switch 47. These components permit the module to be matched to the ship at installation by allowing cancellation of an unwanted acceleration signal that may be generated due to mounting the roll angle sensor above or below the center of roll.
The roll rate signal is applied to an amplifier 49. The output of the amplifier 49 is applied to a potentiometer 51 which is adjusted in accordance with the natural period of roll of the vessel. The output of the potentiometer 51 is applied through a multiplier 53 whose gain is controlled by the automatic gain control circuit previously described. The output of the multiplier 53 constitutes the roll rate signal to be applied to the lift order limiter.
The output of the amplifier 49 is also applied to a differentiating circuit 55. The output of the differentiating circuit represents the roll acceleration signal.
As explained previously, the potentiometer 43, together with the capacitor 45, and the switch 47 are used to match the module to the ship at installation. The potentiometer 43 is adjusted to compensate for the magnitude of the unwanted acceleration signal. The switch 47 is thrown to the left as indicated when the sensor is mounted below the center of roll of the ship. In this position the switch causes the signals from the potentiometer 43 to pass through the inverting amplifier 57 in the roll angle plus rate, plus acceleration channel. When the sensor is mounted above the center of roll of the ship, the switch 47 is thrown to the right so that the signal from the potentiometer is applied directly to an amplifier 59 in the same channel. The roll rate signal from the amplifier 49 is applied to the roll angle plus rate, plus acceleration channel through the resistor 63. Thus the signal at the input to the amplifier 57 represents the sum of the roll angle and the roll rate. This signal is applied to the amplifier 59 whose output passes through a potentiometer 61 which may be adjusted in accordance with the period of roll of the ship. The signal from the potentiometer 61 is added to the acceleration signal from the differentiating circuit 55 and applied to an amplifier 64. The signal from the amplifier 64 is applied to the multiplier amplifier 65 whose gain is controlled by the automatic gain control circuit as previously described. The output of the multiplier 65 represents a roll angle plus roll rate, plus roll acceleration signal which is applied to the lift order limiter.
Thus the signals from the lift order computer represent roll rate and a combination of roll angle, roll rate, and acceleration having relative magnitudes determined by the lift order automatic gain control circuit.
The signals from the lift order computer are applied to the lift order limiter where they are summed in a summing network 67 and applied to an amplifier 69 whose output represents the port lift order signal. The output of the amplifier 69 is also applied to an inverter 71 whose output constitutes the starboard lift order signal. The output of the inverter 71 is applied to a calibrating circuit functionally illustrated as a potentiometer 73. The output signal from the potentiometer 73 is integrated in an integrator 75 and fed back to the amplifier 69 through the resistor network. The integrator reduces the continuous d.c. offset at the output of the amplifier 71 to zero. Such d.c. offset could result from a permanent list of the ship or from any circuitry between the sensors and the lift order output that might provide an undesired bias.
The lift order limiter also receives the speed squared signal from the computer pre-amplifier. The speed squared signal is applied to a first amplifier 77 and then to a second amplifier 79 arranged in a resistor network such that first and second diodes 81 and 83 are back-biased by the signal to a level determined by the magnitude of the speed squared signal. Thus the output of the amplifier 69 is limited in accordance with the back bias applied to the diodes 81 and 83 which in turn is a function of the speed of the vessel.
Each of the port and starboard lift order signals from the lift order limiter is applied to a stroke order computer. A typical stroke order computer is illustrated in FIG. 2b of the drawings. The stroke order computer receives the appropriate lift order signal from the lift order limiter on a terminal 85 and the signal representing the speed squared on a terminal 87. A first feedback signal representative of the instantaneous angle of deflection of the fins is applied to the angle feedback terminal 89. A second feedback signal representative of the vertical forces or lift being experienced by the fins is applied to a lift feedback terminal 91. The lift and angle feedback signals are applied to amplifiers 93 and 95 respectively which provide scaling and offset compensation for the various feedback signals. The particular feedback signal desired is chosen by means of the feedback select means which includes switching networks functionally illustrated as simple double throw switches 97 and 99.
If lift feedback is selected, the lift feedback signal is summed with the lift order in a summing circuit 101, and the resulting error signal is amplified in an amplifier 103. The gain of the amplifier 103 is adjusted by mans of a multiplier 105 which, in turn, is responsive to the speed squared signal received at the terminal 87. The signal from the amplifier 103 passes through a second summing network 107 which is also connected to receive a signal from the switch 99. The switch 99, however, is connected to ground when the lift feedback signal is to be received. Thus, the output of the summing network 107 is equal to the output of the amplifier 103. This output is applied in turn to an amplifier 109 whose output represents the stroke order signal.
If angle feedback is selected, the switches 97 and 99 are thrown to their lower positions as illustrated in FIG. 2b. Under these conditions, the output of the summing means 101 is equal to the lift order signal; and the angle feedback signal, after passing through the amplifier 95, is applied directly to the summing means 107. Thus the signal applied to the amplifier 109 from the summing network 107 represents the sum of the signals from the amplifier 103 and the angle feedback signal as modified by the amplifier 95. The output signal from the summing means 107 is again amplified in amplifier 109 whose output again constitutes the stroke order signal.
A diode network 111 connected across the amplifier 109 provides fixed limits restricting the maximum swing of the stroke order signal.
In addition, variable limits are established by means of a limiting circuit 113 connected in shunt with the diode circuit 111 and responding to the lift and angle feedback signals so as to limit maximum swing of the stroke order in accordance with the larger of the two feedback signals. The fixed and variable limit circuits operate so that the variable limit circuit restricts the stroke order signal to values within the range permitted by the fixed limit circuit 111.
The angle feedback signal from the amplifier 95 is applied directly to the variable limit circuit 113. The lift feedback signal from the amplifier 93 is inverted in an amplifier 115 and then applied to the limit circuit 113. A diode circuit 117 in the input of the variable limit circuit 113 applies the larger of the feedback signals to the amplifier 119 or the amplifier 121 depending upon the polarity of the feedback signal. The output signals from the amplifiers 119 and 121 are applied to a resistance network 123 and a pair of limiting diodes 125 and 127. In summary, the fixed limit circuit 111 and the variable limit circuit 113 cooperate to provide a limiting signal which restricts the stroke order signal to a value determined by the smaller of the fixed and variable limits.
In accordance with conventional practice, the stroke order signal may be used to operate a servo which controls the flow rate of a hydraulic pump by positioning a stroke shaft. The output of the pump then controls the speed and position of the fins used to stabilize the ship.
The circuit of the present invention generates a stroke order signal in which all gain changes are completed prior to the signal's entry into the stroke servo. The circuit also provides fixed and variable limits on the stroke order signal wherein the variable limits are set by either the lift or angle feedback.
While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.
1. Field of the Invention
The present invention relates to systems for stabilizing marine vessels against excessive rolling motion and more specifically to roll stabilizing systems employing adjustable fins projecting from either side of the vessel's hull.
2. Description of the Prior Art
Fin stabilizers for roll stabilization are well known in the art. U.S. Pat. No. 2,979,010 issued to F. D. Braddon et al. on Apr. 11, 1961 and assigned to the present assignee, for instance, concerns a fin stabilization system in which the fins are positioned in accordance with a lift order command signal. Typically, the lift order command signal actuates a hydraulic system through a servo mechanism.
In prior art systems, the servo operates through a slip clutch. Large command signals drive the servo to its limit stops whereupon the slip clutch prevents further motion in the hydraulic stroke control system. In previous systems, the limit stops are manually adjusted to 100 percent stroke. The slip clutches in such systems, however, may experience excessive wear. Furthermore, in prior art systems, mechanical off-stroking is typically used for maximum fin angles only. The limits cannot be programmed to meet changing requirements.
SUMMARY OF THE INVENTION
The fin stabilizer circuit of the present invention receives signals representing the ship's roll angle, roll rate, and speed. The roll acceleration is computed from the roll rate signal and speed squared is computed from the ship's speed. By modifying the lift orders in accordance with the speed squared and the comparative value of the various signals, the circuit compensates for changes in fin activity caused by changes in ship's speed and weather, and permits the fins to be used as resonance dampers in severe seas.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sketch illustrating the operation of a fin stabilizer device using the principles of the invention, and
FIGS. 2a and 2b are schematic representations of a circuit useful in practicing the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG 1 represents a transverse section through a ship employing a fin stabilizing system and illustrates the environment in which the invention is intended to operate. Port and starboard stabilizing fins project laterally from the sides of the vessel and function in a manner analogous to ailerons in an aircraft. Sensors detect the roll angle, roll rate and speed of the ship. Various types of sensors are known in the art. In many applications, a rate gyro may be used to detect roll rate. This value may then be integrated to derive the value of roll angle and differentiated to derive the value of roll acceleration. Signals from the various sensors are applied to a lift order computer which processes the signals from the sensors to provide a lift order signal and a signal representing the square of the ship's speed. The lift order signal and the signal representing the square of ship's speed is applied to a stroke order computer. Signals from the stroke order computer are applied to the port and starboard fin actuating mechanisms 11 and 13 respectively. Feedback signals from the fin actuating mechanisms are all applied to the stroke order computer. The structure and mode of operation of the lift order computer and the stroke order computer will be described in detail. The fin actuating mechanisms 11 and 13 are conventional devices. They may, for instance, be hydraulic devices similar to those illustrated in the aforementioned Braddon et al. patent. Feedback signals representing the instantaneous fin angles as well as the vertical forces or "lift" to which the fins are being subjected may be derived in a straightforward manner. Fin angle transducers are well known in the art. Lift transducers are described in detail in the aforementioned Braddon et al. patent.
FIG. 2a represents, in functional form, the lift order computer and the associated components necessary to produce the lift order signals. The computer pre-amplifier receives the various signals from the sensors and converts these signals into voltages that may be accepted by the lift order computer itself. Typically, the roll angle signal may be an a.c. signal having a frequency of 400 Hertz and an amplitude of 75 millivolts per degree of roll. This a.c. signal may be first passed through a demodulator 15 and a calibrating network 17 adjusted to produce a d.c. roll angle output signal from the signal pre-amplifier having a magnitude of 500 millivolts per degree. The roll rate signal may typically be a d.c. signal having an amplitude of 3 volts per degree per second. This roll rate signal may be calibrated in a calibrating unit 19 to produce a roll rate output signal typically in the range of 2 volts per degree per second. The signal representing ship's speed is also calibrated in a calibrating unit 21. The output of the calibrating unit 21 is then applied to a conventional squaring circuit 23 so as to derive a signal representing the square of a ship's speed. The calibrating unit 21 is adjusted to provide signals having magnitudes that can be accepted by the remaining components in the system.
The roll angle and roll rate signals from the computer pre-amplifier are applied to the lift order computer. The lift order computer derives a first signal representing roll rate and a second signal representing roll angle, plus roll rate, plus acceleration. These signals are summed in a lift order limiter so as to provide port and starboard lift order output signals. The amplitudes of the signals from the lift order computer are modified by a lift order automatic gain control circuit as will be described.
The automatic gain control circuit and the lift order limiter cooperate with the lift order computer to combine the various control signals for optimum performance. It is known in the art to combine angle signals, acceleration, and rate signals for specific purposes. If a ship is displaced about its roll axis by a momentary torque, the ship will heel over to a maximum displacement; then return to a level position. In returning to the level position, the ship may oscillate several times before finally coming to rest. Stabilization circuits may be used not only to minimize the maximum displacement of the ship, but also to eliminate the oscillations by proper damping. Roll stabilization might be used by applying a stabilizing torque which is exactly equal to and opposite the instantaneous displacement of the ship. It is known in the prior art, however, that by introducing control components corresponding to the acceleration and the rate of displacement, superior stabilization may be realized.
A typical damped displacement curve is a single-peaked curve somewhat resembling a normal distribution curve. The corresponding rate curve rises from zero in the same direction as the displacement curve, passes through a maximum before the maximum of the displacement curve, then decreases through zero to a negative maximum near the end of the displacement curve, whereupon the rate curve again approaches zero from the opposite direction of the displacement curve. The corresponding acceleration curve rises from zero in the same direction as the displacement curve, quickly approaches a maximum, then decreases through zero at a time corresponding to the maximum of the rate curve and proceeds to a negative maximum at a time corresponding to the positive maximum of the displacement curve. The acceleration curve then proceeds through zero to a secondary maximum near the end of the displacement curve and finally approaches zero from the same direction as the displacement curve.
The more a control is responsive to acceleration, and to a lesser extent, the more the control is responsive to rate, the nearer the control will approach the ideal of completely neutralizing the disturbance. As was mentioned previously, the rate curve is opposite in sense to the displacement curve near the end of the recovery period. For this reason, the rate control is valuable for its positive damping effect. It will also be remembered that the acceleration control reaches a maximum before either the displacement or rate curves. For this reason, the acceleration control is valuable for its anticipatory effect in reducing displacement. The automatic gain control circuit, the lift order limiter, and the lift order computer circuits of the present invention make use of these observations to provide optimum stabilization regardless of ship speed or weather conditions.
In the automatic gain control circuit, the port and starboard signals from the lift order limiter are compared with a positive signal representing the square of the ship's speed in the comparator amplifiers 25 and 27. The amplifier 25 supplies a positive input signal to the integrator 29 when the port lift order is negative and greater in magnitude than the positive speed squared signal. Similarly, amplifier 27 supplies a positive input to the integrator 29 when the starboard lift order is negative and greater in magnitude than the positive speed squared signal. A mixing network 31 is adjusted to produce a voltage at the output of the integrator that is positive if the output of either amplifier 25 or 27 is positive and negative if both amplifier outputs are negative. The lift order signals are applied to the automatic gain control circuit in a negative feedback fashion. If the magnitude of the speed squared reference exceeds both lift orders, the output of the integrator 29 increases which acts to increase the gain of the amplifiers in the lift order computer and thereby to increase the magnitude of the lift order signals. If either lift order becomes more negative than the speed squared reference is positive, the integrator 29 output decreases so as to decrease the magnitude of the lift order signals. In general, the average input to the integrator 29 is zero only when the integrator output raises the gain of the lift order computer to the point at which the absolute value of the lift order exceeds the speed squared signal for more than 50 percent of the time. Since the amplifiers 25 and 27 constantly compare the lift order signals to the speed squared signal, the sensitivity of the system varies with the changes in the ship's speed and ship residual roll motion.
A selector switch 33 permits operation in the automatic mode or the manual mode. Although the switch 33 has been illustrated as a simple mechanical switch, it will be understood that the switch ordinarily would be constructed from a network of solid state switching devices. A voltage selecting network shown as a potentiometer 35 for use in the manual mode of operation provides operator selected voltages having magnitudes comparable to those available from the automatic mode section of the circuit.
Signals from the selector switch are applied through a signal level-adjusting circuit, functionally represented as a potentiometer 37, to gain control amplifiers 39 and 41.
The amplifier 39 provides an output signal that is used in controlling the gain of an amplifier in the roll rate channel of the lift order computer. Similarly, the amplifier 41 provides an output signal that is used in controlling the gain of an amplifier in the roll angle, plus acceleration, plus roll rate channel of the lift order computer. The amplifiers 39 and 41 are constructed so that the gain of amplifier 39 is larger than the gain of the amplifier 41. Thus, when an increasing voltage is applied to the amplifiers 39 and 41, the gain of the amplifier in the roll rate channel of the lift order computer will reach a maximum before the gain of the corresponding amplifier in the roll angle plus roll rate, plus roll acceleration channel. Conversely, when the voltage applied to the amplifiers 39 and 41 is decreasing, the gain control acts to first reduce the roll angle and roll acceleration components in the lift order before the roll rate component is reduced. Since the primary signal for stabilization is roll rate, better stabilization in rough seas is obtained than would be possible if all signals were reduced proportionately. Thus, in severe seas, fin activity can be reduced by using the fins as resonance dampers while retaining full stabilization capability for milder weather.
It will also be noted that the same shaping of the gain curve is present whether the device is being operated in the manual or automatic gain control mode.
The lift order computer receives the calibrated roll angle signal and calibrated roll rate signals from the computer preamplifier. The roll rate signal is applied to a potentiometer 43 and through a capacitor 45 to a selector switch 47. These components permit the module to be matched to the ship at installation by allowing cancellation of an unwanted acceleration signal that may be generated due to mounting the roll angle sensor above or below the center of roll.
The roll rate signal is applied to an amplifier 49. The output of the amplifier 49 is applied to a potentiometer 51 which is adjusted in accordance with the natural period of roll of the vessel. The output of the potentiometer 51 is applied through a multiplier 53 whose gain is controlled by the automatic gain control circuit previously described. The output of the multiplier 53 constitutes the roll rate signal to be applied to the lift order limiter.
The output of the amplifier 49 is also applied to a differentiating circuit 55. The output of the differentiating circuit represents the roll acceleration signal.
As explained previously, the potentiometer 43, together with the capacitor 45, and the switch 47 are used to match the module to the ship at installation. The potentiometer 43 is adjusted to compensate for the magnitude of the unwanted acceleration signal. The switch 47 is thrown to the left as indicated when the sensor is mounted below the center of roll of the ship. In this position the switch causes the signals from the potentiometer 43 to pass through the inverting amplifier 57 in the roll angle plus rate, plus acceleration channel. When the sensor is mounted above the center of roll of the ship, the switch 47 is thrown to the right so that the signal from the potentiometer is applied directly to an amplifier 59 in the same channel. The roll rate signal from the amplifier 49 is applied to the roll angle plus rate, plus acceleration channel through the resistor 63. Thus the signal at the input to the amplifier 57 represents the sum of the roll angle and the roll rate. This signal is applied to the amplifier 59 whose output passes through a potentiometer 61 which may be adjusted in accordance with the period of roll of the ship. The signal from the potentiometer 61 is added to the acceleration signal from the differentiating circuit 55 and applied to an amplifier 64. The signal from the amplifier 64 is applied to the multiplier amplifier 65 whose gain is controlled by the automatic gain control circuit as previously described. The output of the multiplier 65 represents a roll angle plus roll rate, plus roll acceleration signal which is applied to the lift order limiter.
Thus the signals from the lift order computer represent roll rate and a combination of roll angle, roll rate, and acceleration having relative magnitudes determined by the lift order automatic gain control circuit.
The signals from the lift order computer are applied to the lift order limiter where they are summed in a summing network 67 and applied to an amplifier 69 whose output represents the port lift order signal. The output of the amplifier 69 is also applied to an inverter 71 whose output constitutes the starboard lift order signal. The output of the inverter 71 is applied to a calibrating circuit functionally illustrated as a potentiometer 73. The output signal from the potentiometer 73 is integrated in an integrator 75 and fed back to the amplifier 69 through the resistor network. The integrator reduces the continuous d.c. offset at the output of the amplifier 71 to zero. Such d.c. offset could result from a permanent list of the ship or from any circuitry between the sensors and the lift order output that might provide an undesired bias.
The lift order limiter also receives the speed squared signal from the computer pre-amplifier. The speed squared signal is applied to a first amplifier 77 and then to a second amplifier 79 arranged in a resistor network such that first and second diodes 81 and 83 are back-biased by the signal to a level determined by the magnitude of the speed squared signal. Thus the output of the amplifier 69 is limited in accordance with the back bias applied to the diodes 81 and 83 which in turn is a function of the speed of the vessel.
Each of the port and starboard lift order signals from the lift order limiter is applied to a stroke order computer. A typical stroke order computer is illustrated in FIG. 2b of the drawings. The stroke order computer receives the appropriate lift order signal from the lift order limiter on a terminal 85 and the signal representing the speed squared on a terminal 87. A first feedback signal representative of the instantaneous angle of deflection of the fins is applied to the angle feedback terminal 89. A second feedback signal representative of the vertical forces or lift being experienced by the fins is applied to a lift feedback terminal 91. The lift and angle feedback signals are applied to amplifiers 93 and 95 respectively which provide scaling and offset compensation for the various feedback signals. The particular feedback signal desired is chosen by means of the feedback select means which includes switching networks functionally illustrated as simple double throw switches 97 and 99.
If lift feedback is selected, the lift feedback signal is summed with the lift order in a summing circuit 101, and the resulting error signal is amplified in an amplifier 103. The gain of the amplifier 103 is adjusted by mans of a multiplier 105 which, in turn, is responsive to the speed squared signal received at the terminal 87. The signal from the amplifier 103 passes through a second summing network 107 which is also connected to receive a signal from the switch 99. The switch 99, however, is connected to ground when the lift feedback signal is to be received. Thus, the output of the summing network 107 is equal to the output of the amplifier 103. This output is applied in turn to an amplifier 109 whose output represents the stroke order signal.
If angle feedback is selected, the switches 97 and 99 are thrown to their lower positions as illustrated in FIG. 2b. Under these conditions, the output of the summing means 101 is equal to the lift order signal; and the angle feedback signal, after passing through the amplifier 95, is applied directly to the summing means 107. Thus the signal applied to the amplifier 109 from the summing network 107 represents the sum of the signals from the amplifier 103 and the angle feedback signal as modified by the amplifier 95. The output signal from the summing means 107 is again amplified in amplifier 109 whose output again constitutes the stroke order signal.
A diode network 111 connected across the amplifier 109 provides fixed limits restricting the maximum swing of the stroke order signal.
In addition, variable limits are established by means of a limiting circuit 113 connected in shunt with the diode circuit 111 and responding to the lift and angle feedback signals so as to limit maximum swing of the stroke order in accordance with the larger of the two feedback signals. The fixed and variable limit circuits operate so that the variable limit circuit restricts the stroke order signal to values within the range permitted by the fixed limit circuit 111.
The angle feedback signal from the amplifier 95 is applied directly to the variable limit circuit 113. The lift feedback signal from the amplifier 93 is inverted in an amplifier 115 and then applied to the limit circuit 113. A diode circuit 117 in the input of the variable limit circuit 113 applies the larger of the feedback signals to the amplifier 119 or the amplifier 121 depending upon the polarity of the feedback signal. The output signals from the amplifiers 119 and 121 are applied to a resistance network 123 and a pair of limiting diodes 125 and 127. In summary, the fixed limit circuit 111 and the variable limit circuit 113 cooperate to provide a limiting signal which restricts the stroke order signal to a value determined by the smaller of the fixed and variable limits.
In accordance with conventional practice, the stroke order signal may be used to operate a servo which controls the flow rate of a hydraulic pump by positioning a stroke shaft. The output of the pump then controls the speed and position of the fins used to stabilize the ship.
The circuit of the present invention generates a stroke order signal in which all gain changes are completed prior to the signal's entry into the stroke servo. The circuit also provides fixed and variable limits on the stroke order signal wherein the variable limits are set by either the lift or angle feedback.
While the invention has been described in its preferred embodiment, it is to be understood that the words which have been used are words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.