Patent Publication Number: US-8975566-B2

Title: Fin buzz system and method for assisting in unlocking a missile fin lock mechanism

Description:
FIELD OF THE INVENTION 
     The invention relates to a mechanism for locking in place the steering fins of a missile, particularly when the missile is not in use, and more particularly to a system and method for assisting in unlocking the fin lock mechanism. 
     BACKGROUND 
     A typical missile includes pairs of controllable steering fins disposed on opposite sides of a missile fuselage. The fins are rotatable to provide yaw, pitch, and roll control during missile flight. The fins are coupled to rotatable shafts that extend into the fuselage and engage corresponding control systems, generally through motors and associated gear linkages, that control the rotation of the fins. 
     Accurate flight of the missile depends on the proper function of the steering fins, and it is desirable to avoid damage to the control systems when the missile is carried external to an aircraft or during handling prior to mounting on the aircraft. Locking the steering fins in place when the missile is not in use prevents control fin rotation and reduces the possibility of damage and wear on the steering fins and related fin control systems. At the same time, the steering fins must be quickly and reliably released so that they can perform their steering function when the missile is launched. 
     SUMMARY OF THE INVENTION 
     The same aerodynamic forces on the control fins, that the fin lock mechanism prevents or minimizes transfer to the steering control system, can generate forces in the fin lock mechanism that make the fin lock mechanism much more difficult to unlock. The present invention removes the aerodynamic fin forces from the fin lock mechanism by actuating the fin control system to apply a controlled force that counters the aerodynamic forces acting on the control fins. Consequently, the system and method provided by the invention reduce the forces acting on the fin lock mechanism, thereby making the fin lock mechanism easier and more reliable to unlock. The system and method provided by the invention includes a sensor for monitoring the position of the fin control shaft, and thus the fin, to confirm whether the fin has been unlocked. 
     More particularly, the system and method provided by the invention “buzz” the fins when the fin lock mechanism is asked to unlock the fins. This means that a control signal is sent to a motor in the control system that controls rotation of the fin, which causes the motor to attempt to rotate the fin alternately clockwise and counterclockwise with limited torque for a short period. A sensor is used to monitor the position of the motor shaft. The fin has been successfully unlocked if the motor shaft rotates more than a predetermined amount. If after a predetermined time the motor shaft has not rotated more than the predetermined amount, the fin has not unlocked and the missile is deemed to be inoperative. This can mean that the missile should not be launched, should be disabled, or that testing has failed and the missile requires maintenance. 
     Accordingly, the present invention provides a method for unlocking a fin lock mechanism that releasably holds one or more missile control fins in a locked position where the control fins are prevented from rotating. The method includes the steps of (i) applying an alternating positive and negative rotational force to a control fin; (ii) monitoring the position of the control fin during the applying step; and (iii) while the position of the control fin does not exceed a predetermined value, repeating the applying step a predetermined number of times or for a predetermined period. 
     The present invention also provides a system for assisting in unlocking a fin lock mechanism that releasably holds one or more missile control fins in a locked position where the control fins are prevented from rotating. The system includes (i) means for applying an alternating positive and negative rotational force to a control fin (such as a motive device and a control shaft coupled to the control fin, the motive device being operative to selectively rotate the control shaft); (ii) means for monitoring the position of the control fin (such as a rotational position sensor); and (iii) means for controlling the applying means to apply the rotational force while the position of the control fin does not exceed a predetermined value, and controlling the applying means to apply the rotational force a predetermined number of times or for a predetermined period (such as a microprocessor-based programmable controller). 
     Similarly, the present invention provides a system that includes a motor operatively connected to the control fin to selectively rotate the fin about a fin axis to provide steering capability under the control of a motor control signal, and a controller that generates the motor control signal by executing a motor control logic routine. The motor control signal includes a series of sequential values corresponding to instructions to the motor to apply an alternating positive and negative rotational force to the control fin. 
     Such a system may further include a fin lock mechanism; a sensor for detecting the position of the control fin; and a control shaft coupled to a control fin for controllably rotating the control fin about a fin axis. The fin lock mechanism includes a locking piston that is axially movable along a piston axis transverse the fin axis, and the control shaft and the piston include corresponding features that cooperate to lock the control shaft to prevent the control fin from rotating. The controller is in communication with the motor and the sensor. The controller generates the motor control signal to directionally oscillate the control shaft while attempting to unlock the control shaft by causing the locking piston to move axially, away from the control shaft, to minimize the force required to move the locking piston to unlock the control fin. 
     The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail certain illustrative embodiments of the invention, these embodiments being indicative, however, of but a few of the various ways in which the principles of the invention may be employed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a missile incorporating a system provided by the present invention. 
         FIG. 2  is a schematic representation of an exemplary system provided in accordance with the present invention. 
         FIG. 3  is a graphical representation of a sensed motor shaft position and fin output shaft position over time. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings in detail, and initially to  FIGS. 1 and 2 , an example of a missile  10  is shown in which a fin lock mechanism or fin lock assembly  12  provided by the invention may be employed. The missile  10  generally has a cylindrical body  14  with a longitudinal axis  16 . Multiple fins  20  and  22  extend from the surface of the body  14 , typically paired on opposing sides of the body  14 , to help control the missile&#39;s path during its flight. In particular, the missile  10  includes a plurality of movable steering control fins  20  toward a rear end of the missile  10  that are rotatable about a fin axis  24  transverse the longitudinal axis  16 , and typically perpendicular to the longitudinal axis  16 . 
     A typical steering control fin  20  has an output shaft  26  that extends from the fin  20  and into the missile body  14 . The output shaft  26  defines the fin axis  24 . Rotating this shaft  26  controls the attitude of the steering control fin  20  relative to the longitudinal axis  16  of the missile  10 . In other words, the control shaft  26  coupled to a control fin  20  for controllably rotating the control fin  20  about the fin axis  24 . 
     The plurality of steering control fins  20  can each be held in a locked, unmoving condition by the fin lock mechanism  12 . The fin lock mechanism  12  includes a fin lock piston  34 . The control shaft  26  and the piston  34  include corresponding features that cooperate to lock the control shaft  26  to prevent the control fin  20  from rotating. In the illustrated embodiment, referring now to  FIGS. 2 and 3 , each of the control fins  20  are connected to the fin lock mechanism  12  by a respective fin lock bracket  30  secured to or incorporated into the output shaft  26 . The fin lock bracket  30  has a locking recess or detent  32  for receipt of a corresponding portion of the fin lock piston  34 . When the piston  34  extends into the recess  32  in the fin lock bracket  30 , the output shaft  26 , and thus the control fin  20 , is locked in place and prevented from rotating. Alternatively, the piston may have a notch or recess for receipt of a protrusion formed by the fin lock bracket  30 , the output shaft  26 , or the fin  20  itself. The piston  34  is retractable to allow the fin lock bracket  30 , and thus the output shaft  26  and the control fin  20 , to rotate. Examples of fin lock mechanisms that can use the present invention are disclosed in commonly-owned U.S. patent application Ser. No. 13/554,032, filed on Jul. 20, 2012, titled RESETTABLE MISSILE CONTROL FIN LOCK ASSEMBLY, and which is incorporated herein by reference. 
     The present invention provides a system  40  for assisting in unlocking a fin lock mechanism  12  that releasably holds one or more missile control fins  20  in a locked position. In the locked position, the control fins  20  are prevented from rotating in such a manner as to control the flight of the missile  10 , which in practical terms, due to tolerance variations, for example, generally means preventing the control fins  20  from rotating more than a predetermined value. An exemplary predetermined value is approximately 0.83 degrees. This can mean, for example, that the control fin  20  cannot rotate more than about 0.83 degrees. In this example, if the control fin  20  can rotate more than 0.83 degrees, then the control fin  20  generally will be in an unlocked position. 
     The system  40  includes (i) means for applying an alternating positive and negative rotational force to the control fin  20  (such as a motive device  42 ); (ii) means for monitoring the position of the control fin  20  during the applying step (such as a position sensor  44 ); and (iii) means for controlling the applying means to apply the rotational force while the position of the control fin  20  does not exceed the predetermined value, and controlling the applying means to apply the rotational force a predetermined number of times or for a predetermined period (such as a controller  46 ). 
     The applying means includes the motive device  42  and the control shaft  26  coupled to the control fin  20 , the motive device  42  being operative to selectively rotate the control shaft  26 . The motive device  42  can be a solenoid or an electric motor, for example. The monitoring means includes a rotational position sensor  44 , which can monitor the position of the output shaft  26  directly, or can monitor the position of a shaft of the motor  42  as an estimate of the position of the output shaft  26 . Such a latter type of sensor  44  can be incorporated into the motor  42 . The controlling means includes the controller  46 , such as a microprocessor-based programmable controller. The controller  46  signals the fin lock mechanism  12  to unlock the control fin  20 . This includes outputting a signal to the motive device  42  to attempt to rotate the control fin  20 . During the unlocking process the controller  46  signals the motive device  42  to rotate the control fin  20  with predetermined torque, typically a torque that is less than the torque applied to rotate the control fin  20  during flight of the missile  10 . 
     Accordingly, the system  40  provided by the invention can be described as including (i) a motor  42  operatively connected to the control fin  20  to selectively rotate the fin  20  about the fin axis  24  to provide steering capability under the control of a motor control signal, and (ii) a controller  46  that generates the motor control signal by executing a motor control logic routine. The motor control signal can include a series of sequential values corresponding to instructions to the motor  42  to apply an alternating positive and negative rotational force to the control fin  20 . 
     The system  40  can further include one or more of (iii) the fin lock mechanism  12 , and (iv) the position sensor  44  for detecting the position of the control fin  20 . The fin lock mechanism  12  includes the locking piston  34 , which is axially movable along a piston axis  50  transverse the fin axis  24  to engage the control fin  20  and prevent it from rotating. The controller  46  is in communication with the motor  42  and the sensor  44 . The controller  46  generates the motor control signal to directionally oscillate the control shaft  26  while attempting to unlock the control shaft  26  by causing the locking piston  34  to move axially, away from the control shaft  26 . The control shaft  26  is rotated to reduce the aerodynamic forces acting on the control fin  20 . When the control shaft  26  is rotated counter to the forces acting on the control fin  20 , this reduces or minimizes the force required to move the locking piston  34  to unlock the control fin  20 . 
     To unlock the control fins  20 , the controller  46  outputs a signal directing the motor  42  to move the fins  20  for a predetermined time while monitoring the fin position via the sensor  44 . In other words, the controller  46  controls the control fin  20  in accordance with one or more inputs from the sensor  44 . 
     Accordingly, a method provided by the invention generally includes the steps of applying a rotational force to the control fin  20  while monitoring the position of the control fin  20 . The controller  46  “buzzes” the control fins  20  when attempting to unlock the fin lock mechanism  12 . This means that a control signal is sent to the motor  42  in the control system that controls rotation of the fin  20 , which causes the motor  42  to attempt to rotate the fin  20  alternately clockwise and counterclockwise until the control fin  20  rotates a predetermined distance or a predetermined period has elapsed. The control signal is referred to as a buzz profile, an example of which is shown in the following Table. 
     
       
         
           
               
            
               
                   
               
               
                 Output Shaft Unlock Buzz Command 
               
            
           
           
               
               
            
               
                 CHANNEL NUMBER 
                 COMMAND 
               
               
                   
               
               
                 1 
                 +1.325 × sin(50 × 2π × τ + 25 × π/180) DEG 
               
               
                 2 
                 −1.325 × sin(50 × 2π × τ + 25 × π/180) DEG 
               
               
                 3 
                 |1.325 × sin(50 × 2π × τ | 25 × π/180) DEG 
               
               
                 4 
                 −1.325 × sin(50 × 2π × τ + 25 × π/180) DEG 
               
               
                   
               
            
           
         
       
     
     If the control fin  20  moves a predetermined distance, the fin  20  is unlocked. If the control fin  20  does not move the predetermined distance, the applying step is repeated for a predetermined period or a predetermined number of times or a combination thereof. If the control fin  20  has not moved the predetermined distance after the predetermined period or predetermined number of tries, the attempt to unlock the control fin  20  has failed. 
     Thus if the predetermined distance value is 0.83 degrees, then if the achieved output shaft positions of all axes during the predetermined period are greater than 0.83 degrees or less than −0.83 degrees, then the output shafts  26  are assumed to have been unlocked, the fin lock mechanism  12  is disabled, de-energized, or otherwise maintained in an unlocked position. The controller  46  can then control the orientation of the control fins  20  to control the missile&#39;s roll, pitch, and yaw. But if the predetermined period, such as 500 milliseconds, elapses without the output shaft positions of all fin axes achieving positions greater than 0.83 degrees or less than −0.83 degrees, one or more control fins  20  have not unlocked. The missile  10 , whether mounted on an aircraft, launched, or in a test stand, is considered defective and will not be activated, and if possible will be repaired before being returned to service. 
     More particularly, the present invention provides a method for unlocking a fin lock mechanism that releasably holds one or more control fins in the locked position. One method provided by the invention includes the steps of (i) applying an alternating positive and negative rotational force to a control fin; (ii) monitoring the position of the control fin during the applying step; and (iii) while the position of the control fin does not exceed a predetermined value, repeating the applying step a predetermined number of times or for a predetermined period. 
     Additionally, the applying step can include outputting a signal to or otherwise signaling the motive device  42 , such as a motor, that is coupled to the control fin  20  to rotate the control fin  20  alternately clockwise and counterclockwise. The method can further include the step of indicating a failure after the repeating step is complete and the position of the control fin  20  has not exceeded the predetermined value. If during the monitoring step the position of the control fin exceeds the predetermined value, the method can include the step of stopping the applying step. 
     The applying step includes the controller outputting a predetermined signal profile with a predetermined amplitude. The control signal typically has a varying positive and negative amplitude. An exemplary signal profile is a 50 Hz sine wave with an amplitude of 1.325 degrees. 
     The predetermined period can be calculated to ensure that the repeating step occurs at least three times. Specifically, the repeating step allows the applying step to apply rotational force to cause the control fin to rotate alternately no more than three times clockwise and no more than three times counterclockwise. The repeating step only occurs, however, when the monitoring step detects rotation of the control fin of less than 0.83 degrees, positive or negative. The controller determines that the control fin is unlocked when the sensor detects rotation of at least 0.83 degrees. 
     The applying step includes applying a predetermined torque. After the monitoring step detects movement of the control fin in excess of the predetermined value, the method can further include the step of rotating the control fin to provide flight control using a torque that is greater than the torque applied during the applying step. 
     The method also can include the step of moving a piston  34  to engage the control fin  20 , including via the control shaft  26 , to prevent the control fin  20  from rotating; as well as the step of disengaging a fin lock mechanism  12  from connection to the control fin  20 . 
     A graphical illustration of the sensed motor shaft position and fin output shaft  26  position over time is shown in  FIG. 3 . This graph shows the angular position  52  of the output shaft  26 , representing the position of the fin  20 , and the angular position  54  of the shaft of the motor  42  as reported by the motor&#39;s position sensor  44 . The graph also shows the upper and lower unlocked threshold values  56  and  58 , and typical upper and lower fin lock limits  60  and  62 , based on an estimated worst-case estimate  64  of tolerances that determine how far the output shaft  26  can rotate in the locked condition. 
     As shown in the graph, at about time 0.005 second (indicated by arrow  66 ), the motor  42  pushes against the fin lock piston  34  ( FIG. 2 ), increasing the load on the fin lock mechanism  12 , making it difficult to unlock. At about 0.010 second, the motor  42  reduces the load on the fin lock mechanism  12 , making it easier to unlock the control fin  20 , as shown at  68 . And as shown at  70 , at about 0.015 second, the sensed motor position exceeds the predetermined unlock threshold value of −0.83 degrees, indicating that the control fin  20  is unlocked and available to assist in controlling the flight of the missile  10 . 
     In summary, by removing the aerodynamic fin forces from the fin lock mechanism  12 , achieved by actuating the fin control system to apply a controlled force that counters the aerodynamic forces acting on the control fins  20 , the system can reduce the transmission of aerodynamic forces onto the fin lock mechanism  12 , which makes the fin lock mechanism  12  easier to unlock with less force. Accordingly, a method for unlocking a fin lock mechanism  12  that releasably holds one or more missile control fins  20  in a locked position, where the control fins  20  are prevented from rotating, includes the steps of (i) applying an alternating positive and negative rotational force to a control fin  20 ; (ii) monitoring the position of the control fin  20  during the applying step; and (iii) while the position of the control fin  20  does not exceed a predetermined value, repeating the applying step for a predetermined number of times or for a predetermined period. 
     Although the invention has been shown and described with respect to a certain illustrated embodiment, equivalent alterations and modifications will occur to others skilled in the art upon reading and understanding the specification and the annexed drawings. In particular regard to the various functions performed by the above described integers (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such integers are intended to correspond, unless otherwise indicated, to any integer which performs the specified function (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated embodiment of the invention.