Abstract:
A fast-acting valve includes an annular valve seat that defines an annular valve orifice between the edges of the annular valve seat, an annular valve plug sized to cover the valve orifice when the valve is closed, and a valve-plug holder for moving the annular valve plug on and off the annular valve seat. The use of an annular orifice reduces the characteristic distance between the edges of the valve seat. Rather than this distance being equal to the diameter of the orifice, as it is for a conventional circular orifice, the characteristic distance equals the distance between the inner and outer radii (for a circular annulus). The reduced characteristic distance greatly reduces the gap required between the annular valve plug and the annular valve seat for the valve to be fully open, thereby greatly reducing the required stroke and corresponding speed and acceleration of the annular valve plug. The use of a valve-plug holder that is under independent control to move the annular valve plug between its open and closed positions is important for achieving controllable fast operation of the valve.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was jointly made by an employee of the United States Government and an inventor having no contractual obligations to the Government who has elected not to retain title. The invention may be used by or for the Government for governmental purposes without the payment of any royalties thereon for therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention is directed to a fast-acting, high-flow valve. In particular, the invention relates to a valve with an annular valve plug and an annular valve seat. 
     2. Description of the Related Art 
     A very fast-acting valve is desirable in many applications including aircraft control systems, pulsejet engines, and chemical and pharmacological processes. Desirable features of a good fast-acting valve include: minimal leakage in the closed state, rapid switching between the closed state and the fully open state, accurate definition of closed and open states, short cycle time for repetitive applications, and large fluid flux through the valve in the open state. 
     Meyer describes a fast-acting valve in U.S. Pat. No. 4,344,449. In this valve, a specially shaped valve stem acts as a sliding gate to the pressurized fluid. An electromagnetic actuator drives the valve stem axially, which opens the valve. A sealed air chamber cooperates with the specially shaped valve stem to form a nonlinear gas spring that helps to return the valve to its closed state. This approach results in a rapid release of a short blast of pressurized gas. However, to reduce friction, sliding gate valves typically have small contact pressures, which leads to leakage when the differential fluid pressure across the valve is large. 
     A fast-acting high-output valve is disclosed by Jaw et. al. in U.S. Pat. No. 5,485,868. This valve comprises a series of pieces that are separated by motion guards and which are arranged so that all of the pieces come together at a common central location. The edges and radial periphery of each piece seal against a valve seat when the valve is closed. A hinge is provided for each piece such that a downward actuating force at the common central location causes the periphery of each piece to move upward, thereby providing an opening for fluid to flow. Concerns about the possibility of substantial leakage with this valve design motivated a continued search for an appropriate fast-acting valve. 
     A review of the various valve designs discussed by Burmeister, Loser and Sneegas in “NASA Contributions to Advanced Valve Technology” (NASA SP-5019) revealed no designs that satisfactorily achieved all of the desirable features of a fast-acting valve. 
     Although a well-designed plug valve could eliminate the leakage problem, a standard plug valve is difficult to open rapidly without long-term adverse effects. For instance, for a valve with a 100 mm 2  orifice area, a circular valve seat will have a diameter slightly greater than 11 mm. The fully open valve state requires that the gap between the valve plug and the valve seat be about half the distance between the edges of the valve seat, or approximately 5.5 mm in this case. To move the valve plug from a closed state to the fully open state in 0.5 ms requires the valve plug to have an average speed in excess of 10 m/s, thereby requiring an acceleration greater than 4000 g (where g is Earth&#39;s acceleration of gravity) during the valve opening. Such a large acceleration is difficult to achieve for a large number of cycles without inelastic deformation. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to overcome the difficulties associated with prior valves by using an annular plug valve. The annular plug valve includes an annular valve seat that defines an annular valve orifice between the edges of the annular valve seat, an annular valve plug sized to cover the valve orifice when the valve is closed, and a valve-plug holder for moving the annular valve plug on and off the annular valve seat. The use of an annular valve orifice reduces the characteristic distance between the edges of the valve seat. Rather than this distance being equal to the diameter of the orifice, as it is for a conventional circular orifice, the characteristic distance equals the distance between the inner and outer radii (for a circular annulus). The reduced characteristic distance greatly reduces the gap required between the annular valve plug and the annular valve seat for the valve to be fully open, thereby greatly reducing the required stroke and corresponding speed and acceleration. Although annular valve seats and plugs have been used previously (for instance, see the concentric disk valves shown in FIG. 14.3.8 of  Mark &#39;s Standard Handbook for Mechanical Engineers , Ninth Edition, McGraw-Hill Book Company, New York), their use has been confined to check-valve applications. In a check valve, opening and closing is not controlled directly, rather differential fluid pressures on opposing sides of the valve plug are responsible for opening and closing the valve. In the current invention, a valve-plug holder that is independently controlled moves the annular valve plug to open and close the valve. The independently controlled opening and closing of the valve is important for achieving controllable fast operation of the valve. 
     The annular plug valve requires a suitable actuator for imparting movement to the annular valve plug. Actuators that impart movement through an impact are desirable for this application because maximum velocity is reached over a very short time interval. A variety of impact actuators have been devised to meet the needs of the annular valve plug. These actuators comprise a shaft that is impacted at one end by an impactor. The acceleration of the shaft takes place only over the time interval in which the impactor maintains contact with the end of the shaft. The shaft achieves its maximum velocity at the end of the impact interval, which is very short. The ability to reach maximum velocity in a very short time interval is highly desirable for the valve actuator. 
     To close the annular plug valve rapidly, a short braking stroke is required. The dissipation of kinetic energy associated with the motion of the valve in a short braking stroke represents difficulties for known damping devices. Hence, a method of shock braking was devised. A shock brake can be considered as a spring with large internal damping, which is achieved by friction between bodies. This process requires first and second bodies to have locally substantially parallel contact surfaces that are inclined to a translation direction. Preferably the first body is shaped as a truncated cone and the second body is annular. As the first body translates axially, it impacts the second body. The bodies deform elastically. Mutual sliding of the respective contact surfaces occurs. During the sliding, frictional forces dissipate much of the kinetic energy. The remainder of the energy is stored in the elasticity of the bodies, most of which is dissipated frictionally as the bodies return to their original shapes. Multiple additional bodies can be used to enhance the performance of the shock brake. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings. 
     FIG. 1 shows a cutaway view of an annular valve seat. 
     FIG. 2 shows a perspective view of an annular valve plug. 
     FIG. 3 illustrates a cross-section of an annular valve seat with an annular valve plug seated thereon. 
     FIG. 4 illustrates a cross-section of an annular valve seat with an annular valve plug lifted off the annular valve seat. 
     FIG. 5 shows a cross section of a conventional circular valve seat and plug. 
     FIG. 6 shows a cross section of an annular valve seat with slit edges that are uneven. An annular valve plug is shown flexing to seal against the annular valve seat. 
     FIG. 7 illustrates a cross section of an annular valve seat and an annular valve plug being supported by a valve-plug holder. The valve is shown in an open position. 
     FIG. 8 shows the valve of FIG. 7 in a closed position. 
     FIG. 9 shows the valve with a wire spring as the valve-plug-biasing member. 
     FIG. 10 shows a velocity-displacement trajectory for the valve-plug holder. 
     FIG. 11 illustrates an actuator with an impactor propelled by an explosion. 
     FIG. 12 illustrates an actuator with an impactor propelled by an electromagnet. 
     FIG. 13 shows an actuator wherein the impactor is a cam mounted on a rotating cylinder. 
     FIG. 14 shows an assembly of the valve with a return spring and a shock brake. 
     FIG. 15 shows a schematic cross-section of a shock brake. 
     FIG. 16 shows a cross-section of a shock brake together with a valve holder body. 
    
    
     Reference numerals in the figures correspond to the following items: 
       100  valve 
       105  valve orifice 
       106  fluid particle path 
       107  inner volume 
       108  outer volume 
       109  edge of circular seat 
       110  annular valve seat 
       120  inner slit edge 
       125  canted region 
       130  outer slit edge 
       140  annular valve plug 
       142  inner surface 
       144  outer surface 
       148  free surface 
       160  valve-plug holder 
       200  inner lip 
       210  inner lip ring 
       220  outer lip 
       230  outer lip ring 
       240  hard stop 
       250  d 2    
       260  d 1    
       290  holder body 
       292  inclined surface of holder body 
       300  holder-body flow-through hole 
       310  wire-spring hole 
       320  valve-plug biasing member 
       340  wire spring 
       370  shaft 
       380  first shaft end 
       390  second shaft end 
       400  impactor 
       402  attractor 
       405  impactor guide 
       406  upper shaft bearing 
       408  lower shaft bearing 
       410  electromagnet 
       411  core 
       412  wire coil 
       414  magnet holder 
       416  impactor return spring 
       417  impactor O-ring 
       420  anvil 
       428  shaped protuberance of the anvil 
       430  cam 
       440  rotating cylinder 
       450  return mechanism 
       455  spring support 
       460  return spring 
       470  shock brake 
       471  upper inclined surface of first annular ring 
       472  first annular ring 
       473  lower inclined surfaces of first annular ring 
       474  second annular ring 
       475  upper inclined surface of second annular ring 
       476  clearance between second annular ring and shaft 
       510  receiver 
       520  first body 
       525  inclined surface of first body 
       530  translation direction 
       540  second body 
       545  inclined surface of second body 
       560  large, essentially rigid mass 
       570  inclination angle 
       580  lower bearing 
       585  upper bearing 
       590  support shaft 
       600  inlet connector 
       610  inlet plenum 
       620  outlet plenum 
       630  outlet connector 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A cutaway view of an annular valve seat  110  is shown in FIG.  1 . The annular valve seat  110  comprises an inner slit edge  120  and an outer slit edge  130  that define a valve orfice  105 . The valve orifice  105 , which will be considered as the opening between the inner slit edge  120  and the outer slit edge  130 , separates an inner volume  107  from an outer volume  108 . The outer volume  108  includes the volume in the vicinity of the annular valve seat  110  that is radially interior to the inner slit edge  120  and radially exterior to the outer slit edge  130 . The canted portions  125  of the inner slit edge  120  and the outer slit edge  130  are included in preferred embodiments of the annular valve seat  110 . The canted portions  125  improve the flow through the orifice  105  and reduce the sealing area of the annular valve seat  110 , thereby increasing the surface contact pressure and improving the seal. The most preferred embodiments include a small, but finite sealing area, as indicated in FIG.  1 . 
     FIG. 2 shows a perspective view of an annular valve plug  140  that has an inner surface  142 , an outer surface  144 , a sealing surface (obscured in FIG.  2 ), and a free surface  148 . As shown in cross-section in FIG. 3, the annular valve plug  140  cooperates with the annular valve seat  110  to form a valve  100 . The configuration shown in FIG. 3 illustrates the valve  100  in a closed state. In the closed state, the annular valve plug  140  is in sealing contact with the annular valve seat  110  such that little or preferably no fluid communication occurs between the inner volume  107  and the outer volume  108 . FIG. 4 shows the valve  100  in an open state. In the open state, fluid communication between the inner volume  107  and the outer volume  108  is facilitated. 
     In the preferred mode of operation, fluid is supplied at high pressure to the outer volume  108 . With the valve in the open state, the fluid flows through the valve orifice  105  into the inner volume  107 . Example fluid particle paths  106  are shown to suggest some typical trajectories of fluid particles. The flow through the valve orifice  105  is accompanied by viscous and turbulent losses. A well-designed valve minimizes these losses for a given flow rate through the valve. One approach for reducing losses is to increase the fluid communication area across which the fluid can flow from the outer volume  108  to the inner volume  107  when the valve  100  is in the open state. Such an approach has been employed in this invention. The use of an annular valve seat  110  rather than a more conventional disk serves to increase the fluid communication area without changing the gap size or the geometrical size of the orifice  105 . 
     As an example of the increased fluid communication area available with the present invention, consider a preferred embodiment in which the inner slit edge  120  is circular with a radius R l  and the outer slit edge  130  is circular with a radius R o  and concentric with the inner slit edge  120 . For the preferred embodiment, the gap G o  between the annular valve plug  140  and the outer slit edge  130  is constant around the circumference and is equal to the gap G i  between the annular valve plug  140  and the inner slit edge  120 , hence G i =G o =G. Therefore, the fluid communication area is 
     
       
         2ΠG(R o +R i ). 
       
     
     In contrast, FIG. 5 shows a cross sectional view of a conventional valve with a solid disk used as a valve plug and a valve seat with a single continuous edge  109  of radius R c ={square root over ((R o +R i )(R o −R i ))}. Such a relationship between the radius of the conventional valve and the radii of the annular valve is necessary to maintain the same geometrical area of valve orifice  105 . Using the same gap G between the valve plug and the valve seat, the fluid communication area is 
     
       
         2 πGR   c =2 πG {square root over (( R   o − R   i )( R   o + R   i ))}. 
       
     
     Because {square root over ((R o −R i )(R o +R i )&lt;(R o +R i ))} for all positive values of R i , the fluid communication area of the new annular valve is greater than that of a conventional valve. 
     Another advantage of the current annular valve, as compared with a conventional valve, is related to the improved sealing capability for non-ideal conditions. In the preferred embodiments, the annular valve plug  110  is a flat resilient membrane in the shape of a circular annulus with concentric inner surface  142  and outer surface  144 . Most preferably, the membrane is a metallic or ceramic compound, which allows the valve to be operated under extreme temperature conditions. In some embodiments, the annular valve plug  110  is coated with an elastic coating, such as one that includes polytetrafluoroethylene (sold commercially as TEFLON). The coating aids in providing a tight seal to the annular valve seat  110  when the valve is in the closed position. In the preferred embodiments the inner slit edge  120  and the outer slit edge  130  have the same height, so that the annular valve plug  140  remains flat when in sealing contact with the edges of the annular valve seat  110 . However, in practice, deviations from the ideal conditions are expected. In particular, temperature nonuniformities in the annular valve seat  110  can vary the heights of the slit edges, as is shown in FIG.  6 . These height differences can vary around the circumference. The preferred annular valve plug  140  has sufficient flexibility to conform to the height changes and maintain a good seal. In contrast, a conventional valve plug is typically insufficiently flexible to conform to circumferential height variations of the valve seat, thereby leading to valve leaks. 
     An annular valve plug  140  with concentric inner and outer surfaces is most preferred; however, embodiments of the invention with nonconcentric and even noncircular planforms of the inner and outer surfaces are included within the meaning of annular valve plug  140 . Similar changes to the geometry of the annular valve seat  110  are possible and are included within the definition of annular valve seat  110 . Preferably, the geometries of the annular valve plug  140  and the annular valve seat  110  are similar, with the annular valve plug  140  simply scaled to ensure that the annular valve plug  140  can seal against the edges of the annular valve seat  110 . Additionally, although a flat membrane is preferred for the annular valve plug  140 , alternate embodiments with a thick and/or contoured annular valve plug  140  are also included within the meaning of annular valve plug  140 . 
     Although not necessary to the invention, leakage in the valve-closed state is reduced if the annular valve seat  110  and the annular valve plug  140  have their mating surfaces ground flat. Preferably a diamond grinder with an average roughness height between approximately 150 and 300 angstroms is used to achieve the desired smooth surfaces. Sealing is generally improved with the use of a thin flat membrane as the annular valve plug  140 . The thin membrane&#39;s flexibility helps ensure a tight seal. 
     With reference to FIG. 7, movement of the annular valve plug  140  is enabled through the use of a valve plug holder  160 . The ability to independently control the movement of the annular valve plug  140  through an actuator distinguishes the present invention from previous concentric disk valves, which have been used as check valves. In a check valve, the differential pressure of the fluids on either side of the valve plug determines whether the valve is open or closed. Here, a valve plug holder  160 , which is controlled by an actuator, applies a force to the annular valve plug  140  that shifts the valve between its open and closed states. Independent control means that the valve state can be changed without regard to the fluid pressures in the inner volume  107  and the outer volume  108 . Preferably, the valve plug holder  160  lifts the annular valve plug  140  from the annular valve seat  110 . However, alternative embodiments in which the valve plug holder  160  resides in the inner volume  107  and pushes the annular valve plug  140  off the annular valve seat  110  are encompassed within the meaning of valve plug holder  160 . In the preferred embodiments, the valve plug holder  160  is annular; however, the valve plug holder  160  is not necessarily circumferentially continuous. In some embodiments, the valve plug holder  160  engages the annular valve plug  140  continuously around the inner and outer circumferences of the annular valve plug. In other embodiments, the engagement occurs only at discrete intervals. To simplify FIG. 7, only a cross-section showing an engaging portion on the left side of the axis is shown. Therefore, the inner side of the valve plug holder  160  is to the right and the outer side of the valve plug holder  160  is to the left. In a preferred embodiment, the valve plug holder  160  comprises a holder body  290 , an inner ring  210  coupled to the inner surface of the holder body  290 , and an outer ring  230  coupled to the outer surface of the holder body  290 . The inner ring  210  terminates in an inner lip  200  and the outer ring  230  terminates in an outer lip  220 . The movement of the annular valve plug  140  is constrained by the inner and outer lips  200  and  220  from below and by a hard stop  240  of the holder body  290  from above. 
     In FIG. 7, the valve-plug holder  160  has raised the annular valve plug  140  off the annular valve seat  110 , thereby positioning the valve in an open state. Example particle paths  106  are shown. A valve-plug-biasing member  320  biases the annular valve plug  140  towards the inner and outer lips  200  and  220 , respectively. 
     In FIG. 8, the valve is in a closed state because the annular valve plug  140  is in sealing contact with the annular valve seat  110 . The valve-plug-biasing member  320  maintains the annular valve plug  140  on the annular valve seat  110 . The distance from the annular valve plug  140  to the inner and outer lips  200  and  220  is designated d 1  and is indicated in FIG. 8 by reference numeral  260 . Similarly the distance from the annular valve plug  140  to the hard stop  240  is designated d 2  and is indicated in FIG. 8 by reference numeral  250 . The sum of the distances d 1  and d 2  is fixed for any given combination of valve-plug holder  160  and annular valve plug  140 ; however, the proportions of the total distance occupied by d 1  and d 2  are adjustable by varying the elevation of the valve-plug holder  160  in the closed position. The elevation is varied by the use of shims, a screw adjustment, or other means. Adjustments in d 1  and d 2  vary the timing details associated with opening and closing the valve. 
     A variety of different structures are useful as the valve-plug-biasing member  320 . Preferably, a mechanical spring is used. For instance, in FIGS. 7 and 8 a leaf spring is illustrated as the valve-plug-biasing member  320 . The most preferred embodiments employ a wire spring  340  as shown in FIG.  9 . The wire spring  340  is attached to the annular valve plug  140  and passes through a wire-spring hole  310  in the holder body  290  and is attached to the holder body  290 . Also shown in FIG. 9 is a typical holder-body flow-through hole  300 . The holder-body flow-through hole  300  allows fluid to flow to all portions of the outer volume  108 . 
     Ideally the valve of the present invention is either in the closed state or the open state. Time spent in intermediate states (i.e., not fully open nor closed) is generally undesirable. The flow rate through the valve during the transient period between the open and the closed states is difficult to estimate and difficult to control. FIG. 10 shows a velocity-displacement diagram for the valve-plug holder that will result in desirable valve performance. The velocity of the valve-plug holder is indicated on the horizontal axis and the displacement of the valve-plug holder is indicated on the vertical axis. At point O the valve-plug holder is at rest with the valve in the closed state (see FIGS.  8  and  9 ). Rapid acceleration of the valve-plug holder occurs between points O and A. During the time to traverse from O to A, the valve remains in the closed state because the annular valve plug remains sealed against the annular valve seat. At point A, the inner and outer lips of the valve-plug holder will contact the annular valve plug and begin to lift the annular valve plug off the annular valve seat. This event starts the transient opening that continues until point B. At point B, the annular valve plug has been lifted a distance equal to about half the slit width and the flow rate through the valve is approximately at its maximum. The valve is therefore considered to be in its open state (see FIG.  7 ). During the interval BCD, the valve is open. The closing transient is experienced from point D to E. At point E, the annular valve plug seals against the annular valve seat. A rapid deceleration that we shall call “shock braking” occurs from point E to point O. The curve BCDE approximates a parabola as a result of an almost constant return force causing the deceleration over this interval. The trajectory associated with FIG. 10 should be understood to be illustrative, rather than limiting. Other trajectories will also result in acceptable valve performance. 
     To rapidly switch from the open state to the closed state and back again requires a carefully designed actuator to interact with the valve-plug holder. The preferred type of actuator for use in the invention will rapidly accelerate the valve-plug holder from point O to point A of the trajectory shown in FIG.  10 . To achieve this performance, the preferred actuator will take advantage of the dynamics associated with elastic-body collisions or impacts. The principles involved are discussed first with reference to the simple actuator shown in FIG.  11 . Here, an impactor  400  is rapidly accelerated inside an impactor guide  405  by an explosion. The explosion develops high pressure that propels the impactor  400  to high speed. A shaft  370  has a first shaft end  380  that is coupled to a receiver  510 . In the case of the valve, the receiver  510  would be a structure that is linked to the valve-plug holder, (e.g., the holder body) but in general, the receiver can be any structure that requires translation. The shaft  370  includes a second shaft end  390  that is impacted by the impactor  400 , thereby causing the rapid acceleration of the shaft  370 . Preferably, both the impactor  400  and the second shaft end  390  of the shaft  370  are made from materials that will elastically deform with a high coefficient of restitution under the impact loads. The acceleration of the shaft  370  occurs only over the duration of the impact. Because the duration of impact for such collisions is generally quite small, the shaft  370  rapidly accelerates to its maximum speed. 
     Another embodiment of the actuator is shown in FIG.  12 . Here the impactor  400  is accelerated by the use of an electromagnet  410 . In the preferred embodiment shown in FIG. 12, the impactor  400  is made from a diamagnetic material while a ferromagnetic material is used for an attractor  402  that is coupled to the impactor  400 . A wire coil  412  is wound inside a core  411 . Both the core  411  and an impactor return spring  416  are maintained in appropriate relationship with a magnet holder  414 . Upper and lower shaft bearings  406  and  408 , respectively, help guide the shaft  370  on which is mounted the receiver  510 . In operation, when an electrical current is activated in the wire coil  412 , it magnetizes the core  411 , thereby generating a force that draws the attractor  402  towards the core  411 . The motion of the attractor  402  forces the impactor  400  upward so it can impact the second shaft end  390  of the shaft  370 . The impactor return spring  416  bears against the impactor O-ring  417  to return the impactor  400  and the attractor  402  approximately to their original positions when the electrical current is deactivated. 
     In the embodiments illustrated in FIGS. 11 and 12, the mass of the impactor  400  (and the attractor  402  if one is used) is preferably similar to the combined mass of the shaft  370  and the receiver  510 . Controlling the event that initiates the acceleration of the impactor with a computer or a microprocessor is straightforward to one skilled in the art. After the impactor  400  achieves its maximum speed, the impact dynamics of the impactor  400  with the shaft  370  governs the acceleration of the shaft  370  and thereby of the receiver  510 . 
     Another embodiment of an impact actuator is shown in FIG.  13 . Here a cam  430  is fixed on a rotating cylinder  440 . An anvil  420  with a shaped protuberance  428  is fixed to the shaft  370 . Preferably the protuberance  428  is shaped to provide a large impact area with the cam  430 . The cam  430  rotates with the rotating cylinder  440  and impacts the protuberance  428  of the anvil  420 . To endure a large number of impacts, the cam  430  and the protuberance  428  of the anvil  420  preferably include hard pads. These hard pads preferably are made from a pressurized mixture of the powders of several metals, including tantalum, titanium, cobalt, and tungsten. An example of such a mixture is the Russian-made material designated TT7K12. Preferably, the motion of the shaft  370  is restricted axially so that only the protuberance  428  of the anvil  420  can be struck by the cam  430 . In addition, because the impact of the cam  430  with the anvil  420  imparts momentum that is perpendicular to the axis of the shaft  370  as well as momentum that is along the axis of the shaft  370 , a bearing  580  is used to reduce any lateral movement of the anvil  420  and subsequently to the shaft  370 . As with the other actuators discussed above, the impact accelerates the shaft  370 , thereby accelerating the receiver  510 . In this embodiment the impactor is the cam  430  and the protuberance  428  of the anvil  420  corresponds to the second shaft end. This embodiment is particularly well suited for periodic acceleration of the shaft  370 . The speed of the rotating cylinder  440 , and thereby the frequency of impacts can be computer controlled. 
     To be used in a cyclic mode, the actuator must have a means for returning the receiver  510  to its pre-actuation position. A return mechanism  450  is shown generically above the receiver  510  in FIG. 13. A preferred return mechanism  450  in combination with preferred embodiments of other portions of the valve are shown in FIG.  14 . In this figure, the return mechanism comprises a spring support  455  and a return spring  460 . In the embodiment shown in FIG. 14, the return spring  460  is a stack of Bellville springs. However, different types of springs may be used as the return spring  460 . Here, the receiver is the holder body  290 . In this embodiment, the holder body  290  includes holder-body flow-through holes  300 . These holes are spaced periodically about the axis of the holder body  290 . The holder-body flow-through holes  300  provide access of fluid in the inlet plenum  610  to the central portion of the valve (i.e., the volume  108  inside the inner slit edge  120  in FIG.  1 ). When the valve is in an open state, fluid flows from the inlet plenum  610  to the outlet plenum  620  where an outlet connector  630  facilitates the transit of the fluid to its desired destination. 
     In operation, upward motion of the shaft  370  urges the top of the shaft  370  against the spring support  455 , thereby compressing the return spring  460 , which subsequently forces the shaft  370  back to its pre-actuation position. To obtain a nearly parabolic trajectory of BCDE of FIG. 10, the restoring force provided by the return spring  460  must be nearly constant. This is achieved by precompressing the return spring  460  such that the additional compression of the spring caused by the displacement of the shaft  370  is small compared with the precompression. Other embodiments that employ different return mechanisms, such as, but not limited to, an elastomer or an air piston are also feasible. 
     Also shown in FIG. 14 is a shock brake  470 . The shock brake  470  is designed to rapidly decelerate the valve-plug holder along the EO portion of the trajectory shown in FIG.  10 . The distance over which this rapid deceleration occurs is the braking stroke. The shock brake  470  dissipates the kinetic energy associated with the motion of the shaft  370 . The kinetic energy associated with the motion of the shaft  370  includes not only the kinetic energy of the shaft  370 , but also that of structures connected to the shaft  370 , such as the valve plug holder  160 . The shock brake  470  can be considered as a spring with large internal damping, which is achieved by friction between bodies. 
     In the shock brake, a translating body that translates in an initial translation direction urges a first body in the initial translation direction against a second body. In the preferred embodiments described below, the translating body is the same as the first body, although this equivalence is not required. The process requires the first and second bodies to have substantially parallel contact surfaces that are inclined to the translation direction. Alternative embodiments include one or more additional bodies, wherein pairs of adjacent bodies have substantially parallel contact surfaces that are inclined to the translation direction. Mutual sliding occurs on the respective contact surfaces. During the sliding, frictional forces dissipate much of the kinetic energy. The remainder of the energy is stored in the elasticity of the bodies, which elastically deform as the translating body urges the first body against the second body (and if additional bodies are used, the second against the third, etc.). As the bodies restitute to their original shapes, energy is again dissipated frictionally as sliding occurs on the respective contact surfaces. The shock brake should be understood to include devices that rapidly decelerate the translating body primarily through frictional dissipation of kinetic energy along substantially parallel contact surfaces of adjacent bodies. 
     The principles governing the shock brake are more easily understood with reference to the cross-sectional view in FIG.  15 . In FIG. 15, a first body  520  has an inclined surface  525  with an inclination angle  570  to an initial translation direction  530 . A second body  540  has an inclined inner surface  545 . Preferably, the inclined surface  525  of the first body  520  and the inclined surface  545  of the second body  540  have the same inclination angle  570  and hence are parallel. The second body  540  is restricted from translating in the first direction  530  because it abuts a large, essentially rigid mass  560 . In practice, this restriction is relaxed, but its imposition helps in understanding the mechanism of the shock brake. Although a variety of different shapes can be used for the bodies, visualization of the process is easiest if the first body  520  is considered as a truncated cone and the second body  540  is considered as an annular ring with a sloped inner surface. 
     In the preferred embodiments, the first body  520  is the same as the translating body so the process starts with the first body  520  translating in an initial translation direction  530 . Its inclined surface  525  impacts the second body  540  along the inclined surface  545 , which is inclined to the initial translation direction  530 . The continued translation of the first body  520  causes the bodies to elastically deform. The elastic deformations produce elastic forces that increasingly urge the first body  520  in a direction opposite to the initial translation direction  530 . In addition, a large frictional force develops along the inclined surfaces  525  and  545 , resisting continued translation of the first body  520  and dissipating much of the kinetic energy of the first body  520 . Eventually, the combined elastic and frictional forces bring the first body  520  to rest, whereupon the elastic forces push the first body  520  in a direction opposite to the initial translation direction  530 . During the return motion, additional kinetic energy is dissipated by the frictional force along the inclined surfaces  525  and  545 . In practice, deviations from the ideal operation are to be expected. For instance, in many embodiments, a force (such as that applied by a spring) urges the first body  520  in the initial translation direction  530 , so oscillations might occur until all of the kinetic energy is dissipated. Another deviation from ideal operation might return the first and second bodies  520  and  540  only approximately to their pre-motion condition. 
     The preferred inclination angle  570  for the inclined surfaces  525  and  545  is approximately 45 degrees. With inclination angles  570  substantially different than 45 degrees, the bodies have a tendency to become stuck together because the frictional force exceeds the component of the elastic force that tends to return the first body  520  to its pre-motion position. 
     FIG. 16 shows an embodiment of the shock brake  470  in combination with a valve-plug holder  160  and a shaft  370 . As in FIG. 13, the holder body  290  extends beyond the valve-plug holder  160  and couples the valve-plug holder  160  with the shaft  370 . The junction between the holder body  290  and the shaft  370  includes an inclined surface  292  that corresponds to the inclined surface  525  of the first body  520  in FIG. 15. A first annular ring  472  serves as the equivalent of the second body  540  in FIG.  15 . The first annular ring  472  includes an upper inclined surface  471  that is substantially parallel to the inclined surface  292  of the holder body  290 . The first annular ring  472  also includes a lower inclined surface  473 . A second annular ring  474  serves as a third body. It includes an upper inclined surface  475  that is substantially parallel to the lower inclined surface  473  of the first annular ring  472 . In this embodiment, as the holder body  290  bears on the first annular ring  472  along the inclined surfaces  292  and  471  the first annular ring  472  elastically expands radially. Similarly, as the first annular ring  472  bears on the second annular ring  474  along the inclined surfaces  473  and  475  the second annular ring  474  elastically contracts radially. Preferably, a clearance  476  between the second annular ring  474  and the shaft  370  permits significant radial contraction of the second annular ring  474  without pinching the shaft  370 . Frictional forces dissipate kinetic energy along inclined surface pairs  292 / 471  and  473 / 475 . With this design, approximately 75% of the kinetic energy is dissipated during the downward motion. Additional annular rings can be added to provide more surfaces along which kinetic energy is dissipated. 
     Referring again to FIG. 14, a preferred assembly of the valve holder  160 , the shaft  370 , the return mechanism  450 , and the shock brake  470  is shown. In this embodiment, four support shafts  590  (two of which are shown) provide structural support of the assembly. Included in the figure is an inlet connector  600 , which connects a source of high-pressure fluid to an inlet plenum  610 . The inlet plenum  610  corresponds to the outer volume  108  in FIGS.  1  and  3 - 9 . The inner volume  107  in FIGS.  1  and  3 - 9  corresponds to an outlet plenum  620 . An outlet connector  630  connects the outlet plenum  620  to the outside recipient of the fluid. The holder-body flow-through holes  300  provide access of inlet fluid to the inner edge of the annular valve seat (obscured). Although the location of the valve-plug holder  160  is shown, details of the valve-plug holder  160  are too small to discern at the scale of the figure and therefore were not rendered. The holder body  290  couples the valve-plug holder  160  to the shaft  370 . In the embodiment illustrated in FIG. 14, any appropriate means can be used to actuate the shaft  370 . Preferably, one of the impact actuators shown in FIGS. 11-13 and discussed above is used. 
     For a valve orifice area of about 100 mm 2 , performance estimates suggest that cyclic operation with a period of 10 ms can be sustained with the valve-plug holder spending a total of about 2.5 ms in motion, approximately 2.0 ms of which will be spent in the open state (gap between the annular valve plug and the annular valve seat being greater than about half the distance between the inner and outer edges of the annular valve seat). To do this, the annular valve seat will have inner and outer radii of approximately 15.5 mm and 16.5 mm, respectively. Because of the small 1 mm distance between the inner and outer edges of the annular valve seat, a gap of approximately 0.5 mm between the annular valve seat and the annular valve plug is sufficient for the valve to be in the open state. The total stroke of the annular valve plug is estimated to be approximately 1.5 mm. The annular valve plug itself can be quite thin, a thickness of approximately 0.3 mm is estimated as being sufficient for most applications for which the inlet plenum gas pressure does not exceed 16 atmospheres. With reference to FIG. 10, approximately 0.1 ms will be spent on the interval OA, during which the valve-plug holder will move from its resting position to the point at which the lips contact the annular valve plug. During this time, the valve-plug holder will have accelerated to its maximum velocity of approximately 3 m/s. The interval AB is estimated to take approximately 0.2 ms and to move the annular valve plug to its open-position. The total cycle time OABCDEO is estimated to take approximately 2.5 ms. The shock braking occurs during the interval EO and is estimated to take approximately 0.1 ms. 
     Although the description above contains specific examples, these should not be construed as limiting the scope of the invention, but as merely providing illustrations of some of the presently preferred embodiments. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.