Abstract:
A parachute control system is actuated primarily by drogue parachute drag force. Drag force is used as a proxy for vehicle airspeed. The control system uses altitude and force sensors combined with a chronograph to determine the state of a deployed drogue parachute. It then compares the sensed condition with a condition defined by preset altitude, force, and time values. Once both altitude and drogue parachute drag force are below certain maximum values and within pre-determined time windows, an event trigger signal is generated.

Description:
BACKGROUND 
       [0001]    This invention relates to parachute systems and methods of controlling parachute deployment. 
         [0002]    Drogue parachutes are commonly deployed for stabilization or airspeed retardation. Often, the drogue parachute is deployed to decelerate an object to an airspeed at which a larger parachute can be safely inflated. Drogue parachutes are commonly used in aerial cargo and personnel delivery systems, spacecraft landing systems, and on aircraft and aircraft ejection seats. 
         [0003]    It is often necessary while towing a drogue parachute to provide a trigger signal that initiates further events once certain parameters are met. Drogue parachutes typically are not themselves directly used to trigger any subsequent events. Rather, subsequent events are triggered by independent sensing of predetermined parameters. For example, an air pressure threshold corresponding to a target altitude triggers release of the drogue parachute and deployment of the main parachute as shown in U.S. Pat. No. 5,899,415. Alternative systems trigger release after a predetermined time since drogue deployment, or once measured airspeed drops below a predetermined value. Examples are shown in U.S. Pat. Nos. 5,474,257, 5,884,863 and 7,059,570. 
         [0004]    Other systems employ more sophisticated triggering schemes incorporating combinations of altitude, time, and/or airspeed data. One common scheme triggers drogue parachute release and main parachute deployment only when both altitude and airspeed have dropped below predetermined values, as shown in U.S. Pat. No. 4,505,444. Another scheme releases the drogue parachute once the drogue parachute has both remained deployed for a predetermined time interval and dropped below a predetermined altitude threshold. U.S. Pat. Nos. 5,064,151 and 6,889,942 use this system. 
         [0005]    Parachute systems capable of incorporating multiple parameters into the event triggering scheme enable superior system reliability and performance. Prior art systems in which event triggering is a function of airspeed, however, require independent means for directly sensing airspeed. The autonomy and utility of such systems is therefore limited. 
       SUMMARY 
       [0006]    The present invention provides a system and method in which a towed drogue parachute autonomously triggers subsequent events as a function of sensed drogue parachute drag force. Such events may include drogue parachute release, main parachute deployment, or both. This system is capable of generating an event trigger signal at targeted airspeeds without requiring direct airspeed sensing. Furthermore, it can trigger further events as a function of multiple trigger conditions with complete autonomy. 
         [0007]    In one embodiment, an event trigger signal is generated as a function of altitude, drag force, and time. Preset maximum altitude, maximum force, and time interval values define the desired triggering condition. Maximum altitude is set as necessary to render drag force assumptions valid. A maximum force value is calculated from a target airspeed threshold and the physical and performance characteristics of the drogue parachute. Using target force as a proxy for achieving a desired airspeed threshold eliminates reliance on vehicle sensors. A chronograph regulates the sequence of trigger events. It ensures that the drogue parachute is allowed sufficient time to inflate and that the event is triggered only by a “true” reading of force below the trigger level. When sensed altitude, drag force, and time values all satisfy the stored triggering condition, the event is triggered. 
         [0008]    The present invention may provide redundant safety systems. For example, the system may be configured so that the triggering condition cannot be reached without first achieving a certain minimum drag force. This may be necessary in such circumstances when the drogue parachute, upon release, acts as a pilot chute to extract the main parachute and it being more desirable to not continue with subsequent events in case of a drogue failure. In addition, event triggering independent of the sensed drag force may be enabled if a certain time interval has passed since drogue parachute deployment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram showing the components of a parachute system that includes a parachute control system. 
           [0010]      FIG. 2  is a block diagram illustrating the method by which an event trigger signal is generated as a function of drogue parachute drag force, altitude, and time. 
           [0011]      FIG. 3  illustrates graphically the logical parameters triggering the event signal as drogue parachute drag force varies over time. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  is a block diagram of parachute system  10 , which includes drogue parachute  12 , main parachute  14 , drogue parachute deployment mechanism  16 , drogue parachute release mechanism  18 , main parachute deployment mechanism  20 , and control system  22 . Components of control system  22  include force sensor  24 , chronograph  26 , altitude sensor  28 , and event trigger signal generator  30 . 
         [0013]    Drogue parachute deployment mechanism  16  initiates deployment of drogue parachute  12 . Drogue parachute  12  is attached directly to force sensor  24 . Drogue parachute  12  inflates following deployment, generating a drag force. Force sensor  24  measures the drag force and transmits measured drag force to event trigger signal generator  30 . Force sensor  24  may comprise any mechanical or electrical device capable of measuring tensile force. Chronograph  26  tracks time and inputs time values into event trigger signal generator  30 . Altitude sensor  28  determines altitude of the parachute system and transmits measured altitude to event trigger signal generator  30 . Altitude sensor  28  may comprise a GPS or Radar sensor, a mechanism that measures atmospheric pressure and correlates this into an altitude, a barostat, or any other device capable of measuring altitude. Such altitude sensing devices are well known in the field. Sensors  24 ,  26 , and  28  function autonomously, independent of host vehicle sensors. Event trigger signal generator  30  incorporates values received from sensors  24 ,  26 , and  28  to generate event trigger signal ET as a function of drogue parachute drag force, time, and altitude. Event trigger signal ET may either release drogue parachute  12  by activating drogue parachute release mechanism  18  or deploy main parachute  14  by activating main parachute deployment mechanism  20  or alternatively, it may both deploy main parachute  14  and release drogue parachute  12 . Drogue parachute release mechanism  18  may comprise a pyrotechnic cutter or some other release mechanism well known in the field. ET may also act as a signal that can be used by any other peripheral equipment attached. 
         [0014]    Once the drogue parachute deployment mechanism  16  has been activated by an outside event, parachute system  10  is capable of functioning autonomously, requiring no information input or control from the host vehicle. Furthermore, system  10  enables main parachute  14  deployment, drogue parachute  12  release, or both at a specified airspeed without requiring actual measurement of airspeed. 
         [0015]    Control system  22  is actuated primarily by drogue parachute  12  drag force. Event trigger signal generator  30  stores preset altitude, drogue parachute drag force, and time values defining an event trigger condition. Event trigger signal generator  30  is also capable of comparing sensed altitude, drogue parachute drag force, and time values with the stored trigger condition. Finally, event trigger signal generator  30  is capable of recognizing when current conditions satisfy the stored trigger condition and thereafter generating event trigger signal ET. Storing and comparing altitude, drogue parachute drag force, and time values, as well as generating event trigger signal ET, may be accomplished by either mechanical or electronic means. In an exemplary embodiment, a programmable electronic microprocessor stores values defining the event trigger condition. The programmable microprocessor used is capable of continuously comparing values received from altitude, drogue parachute drag force, and time sensors to stored values. The programmable microprocessor further is capable of producing an electronic event trigger signal ET once it recognizes the event trigger condition has been achieved. 
         [0016]      FIG. 2  shows the method by which control system  22  of  FIG. 1  produces event trigger signal ET as a function of altitude, drogue parachute drag force, and time. Control system  22  is activated by step  40 , deployment of drogue parachute  12 . At this time, drogue parachute drag force sensor  24 , chronograph sensor  26 , and altitude sensor  28  begin to measure values. In step  42 , the event trigger signal generator  30  delays further events until measured time since drogue chute deployment t exceeds the preset value of t inflation . Time t inflation  corresponds to a predetermined time from drogue chute deployment to full drogue chute inflation. The value of this preset inflation time is specific to a chosen vehicle and drogue parachute combination. Once the event trigger signal generator determines measured time is greater than t inflation , step  42  is complete. 
         [0017]    Step  44  shows an optional safety check that may be incorporated into parachute system  10 . In step  44 , the event trigger signal generator prevents passage to step  46  until sensed drogue parachute drag force F is above a threshold value of F min . If correctly deployed, the drogue parachute should exert a drag force of predictable magnitude on the force sensor following completion of t inflation . By setting F min  near this expected value, completion of step  44  verifies successful drogue parachute deployment. 
         [0018]    Step  46  compares sensed altitude A with preset maximum trigger altitude A max  stored in the event trigger signal generator. Step  46  is necessary to ensure that the vehicle and its occupants have reached a breathable atmosphere before initiating further events, such as deploying a main parachute. Furthermore, step  46  is necessary for assumptions of constant air density used in step  48  to be relatively accurate within predefined boundaries. Because air density decreases as altitude increases, step  46  guarantees that step  48  is not initiated until air density reaches a threshold level corresponding to air density at A max . 
         [0019]    Step  48  compares sensed drogue parachute drag force F with preset maximum drag force F max . Drag force F and preset maximum drag force F max  serve as proxy values for airspeed V and target airspeed V max . Airspeed V and drag force F are related according to the following formula, where air density (ρ), drogue parachute coefficient of drag (C D ), and drogue parachute surface area (S) are constants: 
         [0000]        V= √{square root over ((2 F/C   D   ρS ) )}  Equation 1 
         [0000]    As dynamic air pressure decreases with altitude for a given true airspeed, the measured drag force F will be smaller at higher altitudes for a given true airspeed than at lower altitudes. This may cause control system  22  to believe that the vehicle&#39;s airspeed is lower than it actually is. This natural phenomenon has to be offset by choosing the appropriate target airspeed V max  and setting A max  appropriately. For greater accuracies of airspeed V correlating to drag force F throughout the altitude envelope below A max , the current air density at which the vehicle is presently at may be used in the equation if the chosen altitude sensor  28  can provide this data. 
         [0020]    Event trigger signal ET cannot be generated unless airspeed V is below target airspeed V max . The value of V max  is set prior to system use, based on a variety of design considerations. Maximum drag force F max  is next calculated by specifying target airspeed V max , inputting the remaining known values, and solving Equation 1 for F max . Event trigger signal generator  30  then stores F max . Step  48  is complete when event trigger signal generator  30  determines measured drag force F is less than F max . Equation 1 shows that completion of step  48  means that airspeed V is also less than target airspeed V max . In this manner, the event trigger signal generator can generate the event trigger signal at a desired airspeed without actually measuring airspeed. 
         [0021]    Due to the complex nature of parachute systems and the environment in which they operate, situations may exist where sensed drogue parachute drag force F never drops below F max . Step  50  shows an optional ultimate redundant safety feature that may be incorporated into the functional method of parachute control system  22 . Step  50  compares sensed time t to a constant value t ultimate . When t exceeds t ultimate , event trigger signal generator  30  generates event trigger signal ET even though F remains greater than F max . Step  50  essentially forces event signal generator  30  to skip directly to step  56 , event trigger signal generation, once a specified amount of time since drogue parachute deployment has elapsed and altitude is below A max . In this manner, event trigger signal ET is generated despite imperfect drogue parachute function, but only after sufficient time has passed to allow drogue parachute  12  sufficient opportunity for normal function. 
         [0022]    Step  52  is complete once event trigger signal generator  30  has stored the value of t 1 . Time t 1  corresponds to measured time upon completion of either step  48  or step  48 A. At step  48 A, a comparison is again made of sensed drogue parachute drag force F to maximum drag force F max . If F is no longer less than F max , t 1  is reset at step  52 . If F is less than F max  at step  48 A, the process continues to step  54 . 
         [0023]    Step  54  compares measured time to a time value corresponding to t 1  plus an additional time interval t f . Time t f  corresponds to a preset time interval for which F must remain below F max . As long as the measured time is less than t 1 +t f , the process returns to step  48 A, so that sensed force F must remain less than F max  throughout the period t 1 +t f  or time t 1  will be reset. Step  54  is complete once t exceeds the value of t 1  plus t f . Steps  48 A and  54  ensure that false force readings produced by oscillations in generated drogue chute drag force do not induce event trigger signal generator  30  to prematurely generate the event trigger signal ET. 
         [0024]    Once step  54  is completed, event trigger signal generator  30  is free to proceed to step  56 . Step  56  generates event trigger signal ET, completing the event triggering scheme. 
         [0025]    Curve  60  on  FIG. 3  graphically represents sensed drogue parachute drag force versus time. From  FIG. 3  it is possible to see the method by which event trigger signal generator  30  generates event trigger signal ET primarily as a function of measured drogue parachute drag force F. Point  62  corresponds to initial drogue parachute deployment. By point  64 , drogue parachute drag force has reached its maximum value. Inflation time t inflation  is a preset value stored by generator  30  corresponding to the predetermined time in which the drogue parachute should be fully open, which may coincide with the point at which maximum drag force is reached. Following point  64 , sensed drogue parachute drag force begins to decrease as drogue parachute  12  acts to slow the airspeed of its attached load. 
         [0026]    Once sensed altitude falls below the preset maximum trigger altitude A max , altitude no longer limits event trigger signal generation. Point  68  in  FIG. 3  is reached when sensed drag force falls below the preset force threshold F max . As discussed earlier, F max  operates as a proxy for a target airspeed at which the event trigger signal is preferably generated. Once generator  30  senses that point  68  has been reached, it stores the corresponding time as t 1 . Generator  30  recognizes point  74  when the sensed drogue parachute drag force has remained below the F max  value for a time corresponding to the preset time interval t f . The value of t 1  plus t f  is labeled as t sg . At point  74 , event trigger signal ET is generated, and the progression is complete. 
         [0027]    The present invention provides a parachute control system in which the event trigger signal is actuated primarily by drogue parachute drag force. This system may be integrated into a parachute system useful for cargo delivery, aircraft rescue, and many other applications. Because the event trigger signal generation system requires only altitude, drag force, and time value inputs, it is capable of functioning completely independent of host vehicle sensors. By using a known relationship between drag force and airspeed, however, the system remains capable of generating the event trigger signal at targeted airspeeds. 
         [0028]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.