Abstract:
A Pendulum Valve having Independently and Rapidly Controllable Theta- and Z-axis Motion. The valve actuator used in the present invention provides the benefit of wide open unrestricted flow of a pendulum valve coupled with the high-resolution and wide dynamic range flow throttling of a ball or butterfly valve. The actuator mechanism will include motor drives and associated control system so that the drives are closely coupled to give highly controlled motion. The drive assembly introduces a concentric shaft arrangement that, when coupled with the highly controllable motor drives, exploits a cam-follower arrangement to make the relative rotation between the two concentric shafts result in highly controlled theta and z-axis motion. Finally, the plate to seal spacing afforded is greater than previously possible with prior valve actuator mechanisms, thereby substantially reducing flow turbulence through the valve as the valve plate eclipses the valve ports.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to valves and actuators therefor and, more specifically, to a Pendulum-type Valve having Independently and Rapidly Controllable Theta- and Z-axis Motion. 
         [0003]    2. Description of Related Art 
         [0004]    Pendulum valves, also known as gate or slide valves are particularly suited for systems mandating large diameter flow conduits such as semiconductor manufacturing, thin film and vacuum process equipment. Specifically, in many such processes, the process chamber is placed under a vacuum condition prior to, during and after engaging in whatever process is conducted in the chamber. This typically involves the metered introduction of small amounts of certain gases into the rarified internal atmosphere (of the chamber) to achieve the required chemistry and pressure conditions within the chamber. In any such process, the ability to rapidly evacuate (empty) the process chamber of gaseous molecules such as process byproducts and other chemicals, is of critical importance. In order to achieve such precise control and rapid evacuations, a high-throughput-volume vacuum pump is connected to the exhaust of the process chamber by large diameter piping; the vacuum pump is “connected” and “disconnected” from the process chamber by a valve capable of opening as wide as the piping bore (to eliminate any flow restrictions), and then close very tightly to precisely increase or maintain desired pressure, and to completely isolate the vacuum pump from the chamber. 
         [0005]    The pendulum valve has historically been the valve best suited for isolating the vacuum pump from the process chamber because it can open wide until the valve plate is completely out of the process flow path (to allow for unrestricted flow and maximum conductance), and can then be closed and sealed tightly to achieve a secure and complete isolation between the vacuum pump and the vacuum chamber. 
         [0006]    But as critical as a fully opening and closing (sealing) valve is to the aforementioned vacuum processes, of even higher importance is the ability to precisely control (throttle) the vacuum level (pressure) in the vacuum chamber within certain desired parameters. These parameters primarily include time, accuracy, stability, and flow symmetry in the vacuum chamber, all of which are strongly influenced by the actuation ability and flow symmetry achieved through the pendulum valve. 
         [0007]    In order to clarify valve plate positioning for later reference herein, the valve plate, while having a multitude of optional positional locations within the valve housing, can be described as having three cardinal locations: a first open position where the valve plate is completely removed from the flow path through the valve housing, and the plate has moved as far away from the valve seat in the Z-axis direction as it can; a second open position where the valve plate is completely eclipsing the flow path, with the valve plate remaining at maximum z-axis stroke away from the valve seat in the Z-axis direction as it can; and a third closed position where the valve plate is completely eclipsing the flow path and the valve plate is being pressed against the valve seat in the Z-axis direction with all available sealing force. Moving from the first open position to the second open position involves movement of the valve plate solely in the theta direction, while moving from the second open position to the third closed position involves movement of the valve plate solely in the Z-direction. 
         [0008]    There are several drawbacks inherent to the conventional “throttling” or control pendulum valve design and actuator mechanism, several of which make meeting all control and sealing parameters particularly challenging. In the conventional pendulum valve, there are essentially two discrete valve positions—full open and full closed (sealed). In addition, a multitude of intermediate positions can be effected by using a variable position valve actuator, such as a motor, which can position the valve plate in positions between full open and full closed so as to achieve the desired flow throttling. In such a manner, the valve plate swings open and closed in what is sometimes referred to in the “theta” direction. Once the valve plate is fully covering the flow path, it then moves in the “z” direction, which is a direction in line with the flow path, until the valve plate seals against the valve housing. It is in this small axial motion that the majority of the process control at low absolute pressure (high vacuum) and low flow of metered gases occurs. 
         [0009]    Conventionally, there can be no z-direction control of the plate until the theta direction of motion has distinctly terminated with the valve plate in exactly the near-closed (theta) position, because there needs to be enough of a gap between the valve plate and the valve housing to allow for the plate to swing freely through the entire theta path. Since the two motions cannot conventionally be actuated simultaneously and independently, there is a transition point between the theta motion path and the z motion path that is characterized by a sharp and sudden change in the valve&#39;s flow throttling capability (valve conductance), and the transition also typically includes an undesirable non-controllable flat regime through which active flow throttling cannot be effected. This non-linear valve conductance is shown in  FIG. 9  and depicts the relationship between the pendulum valve plate position and the resultant vacuum chamber pressure. From this, it is evident that effective vacuum chamber pressure control relies heavily on three factors. They are:
       a) the ability to move the valve plate quickly across the theta range of the valve stroke, since chamber pressure is highly insensitive to valve position in the theta valve stroke range;   b) the ability to produce active control along the z-axis, since this is where the majority of the controllable conductance can be realized. In addition, control along this axis has to be highly precise since chamber pressure is highly sensitive to valve position in the z-axis valve stroke range; and   c) the ability to reduce or eliminate the non-controllable flat regime near the transition between the theta- and z-motion respective stroke ranges.
 
The inability of conventional valve geometry and actuation design to independently and simultaneously control theta and z-axis motion creates a severe limitation on the dynamic control range of the valve and/or linearity of control across the addressable stroke of the valve.
       
 
         [0013]    With the conventional pendulum valve actuation and geometry, then, the user must accept nonlinear control characteristics and/or limited dynamic control range (especially when near-sealed in the z-axis direction) common with these types of valves that transition where the theta motion sequences to the z-axis motion and eventually seals. 
         [0014]    What is needed is an improved pendulum valve and actuator mechanism and methodology that combines the high open conductance characteristics of a convention pendulum valve with improvements in its throttling capability garnered by simultaneous yet independent theta path and z-axis motion control. Further improvements should include a light and nimble plate design allowing for the fastest possible theta path motion, as well as valve plate and body design leading to maximum possible stroke length in the z-axis direction (and the ability to throttle there within). 
         [0015]    Furthermore, it is desirable that the z-axis stroke (plate-to-flange travel distance when theta is in the closed or fully eclipsed position) be sufficiently large that conductance is uniform around the plate and that the majority of the control range be in the z-axis; since theta-path control yields non-uniform flow through the throat of the valve housing. Also, control in the z-axis direction produces less vibration than controlling in the theta-path direction because the moment of inertia about the z axis (torque=inertia*angular acceleration) is substantially greater than inertia created in the z axis (force=mass*linear acceleration). 
       SUMMARY OF THE INVENTION 
       [0016]    In light of the aforementioned limitations and inherent problems associated with the prior devices and systems, it is an object of the present invention to provide a Pendulum Valve having Independently and Rapidly Controllable Theta- and Z-axis Motion. The valve actuator used in the present invention should provide the benefit of wide open unrestricted flow of a pendulum valve coupled with the high-resolution and wide dynamic range flow throttling of a ball or butterfly valve. The actuator mechanism should include motor drives and associated control system to closely couple the drives to give highly controlled motion. The drive assembly should introduce a concentric shaft arrangement that, when coupled with the highly controllable motor drives, can exploit a cam-follower arrangement to make the relative rotation between the two concentric shafts result in highly controlled theta and z-axis motion. Finally, the plate to seal spacing should be greater than previously possible with prior valve actuator mechanisms, in order to substantially reduce turbulence in the flow through the valve as the valve plate eclipses the valve ports. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The objects and features of the present invention, which are believed to be novel, are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages, may best be understood by reference to the following description, taken in connection with the accompanying drawings, of which: 
           [0018]      FIG. 1  is a top view of a preferred embodiment of the pendulum valve assembly of the present invention; 
           [0019]      FIG. 2  is a perspective view of the plate actuator assembly of the valve assembly of  FIG. 1 ; 
           [0020]      FIG. 3  is a perspective view of the plate actuator assembly of  FIG. 2  further depicting the actuating shafts; 
           [0021]      FIG. 4  is a partial cutaway perspective view of the assembly of  FIG. 3 ; 
           [0022]      FIG. 5  is a cutaway side view of the plate actuator assembly of  FIGS. 1 and 2 ; 
           [0023]      FIG. 6  is a perspective view of the valve plate of the valve assembly of  FIG. 1 ; 
           [0024]      FIG. 7  is a side view of the plate actuator assembly of  FIGS. 1 ,  2  and  5 ; 
           [0025]      FIGS. 8A and 8B  depict the valve plate motion in the prior art valve and the valve assembly of the present invention; and 
           [0026]      FIG. 9  is a graph depicting the performance benefits of the valve assembly of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    The following description is provided to enable any person skilled in the art to make and use the invention and sets forth the best modes contemplated by the inventors of carrying out their invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the generic principles of the present invention have been defined herein specifically to provide a Pendulum Valve having Independently and Rapidly Controllable Theta- and Z-axis Motion. 
         [0028]    The present invention can best be understood by initial consideration of  FIG. 1 .  FIG. 1  is a top view of a preferred embodiment of the pendulum valve assembly  10  of the present invention. The assembly  10  consists of two major functional parts, the pendulum valve  12 , and the plate actuator assembly  14 . 
         [0029]    The pendulum valve  12  has a large housing  16  defined by a first channel opening  18  and a second channel opening (not shown). These two openings are in axial alignment on the two sides (top and bottom in this depiction) of the housing  16 , and are each bounded by a flange  20 . The flanges provide a valve attachment point for the exhaust system piping and process chamber interface. 
         [0030]    The valve plate  22  is shown here in the closed position. In this position, the plate  22  completely covers the two flow channel openings  18  but does not seal or completely isolate the vacuum pump from the chamber. When being opened wide, the valve plate  22  will travel through theta movement path  24  until the plate  22  does not cover the openings  18 . The z-direction is that movement path wherein the plate  22  moves closer to or further away from the housing  16  (closer to or away from the reader in the depiction of this  FIG. 1 ). The simultaneous or sequential movement of the plate  22  through both the theta and the z directions is made possible via the plate actuator assembly  14  of  FIG. 2 . 
         [0031]      FIG. 2  is a perspective view of the plate actuator assembly  14  of the valve assembly of  FIG. 1 . The assembly  14  has a motor base plate  26  attached to the valve housing  16  via a shaft assembly housing  25 . The shaft assembly housing  25  houses the shaft(s) that actually move the valve plate. 
         [0032]    First and second drive motors  28 A and  28 B (not shown) are attached to the motor base plate  26  such that their respective drive shafts extend through first and second pinion apertures  32 A and  32 B. A first pinion gear  30 A is attached to the shaft of the first drive motor  28 A. A second pinion gear  30 B is attached to the shaft of the second drive motor  28 B. As used herein through this specification and claims, the term “pinion” is intended to denote a gear that is the “driving” gear in a gear train. The term “spur” is intended to denote a gear that is the “driven” gear in a gear train. Neither of these terms denotes a relative size difference between any of the gears described herein. 
         [0033]    The first pinion gear  30 A is oriented so that it engages a first spur gear  34 A. A second spur gear  34 B is located adjacent to, and in axial alignment with, the first spur gear  34 A. The second pinion gear  30 B is oriented so that it engages the second spur gear  34 B. It should be apparent that the first drive motor  28 A drives the first spur gear  34 A and the second drive motor  28 B drives the second spur gear  34 B. The reader should note that the gear teeth of the second pinion gear  30 B are actually wider than the width of the gear teeth of the second spur gear  34 B. The reason for this feature will become apparent below in connection with the descriptions of other drawing figures. For now, we will turn to  FIG. 3  to continue to examine the unique features of this invention. 
         [0034]      FIG. 3  is a perspective view of the plate actuator assembly  14  of  FIG. 2  further depicting the actuating shafts. The assembly  14  utilizes a unique arrangement of shafts, with each shaft being independently rotatable. This unique arrangement of shafts is the concentric shaft assembly  44 . The concentric shaft assembly  44  is a sleeve shaft  36  with a center shaft  38  inserted therein. The sleeve shaft  36  is attached to, and driven by, the first spur gear  34 A. The center shaft  38  is attached to, and driven by, the second spur gear  34 B. 
         [0035]    What is very unique about the concentric shaft assembly  44  is the simultaneous interdependence and independence of the motions of the two shafts  36  and  38 . This interdependence/independence is created via the operation of the follower roller  42  riding within the cam groove  40  formed through the wall of the sleeve shaft  36 . The follower roller  42  is attached to the center shaft  38 . 
         [0036]    As the first spur gear  34 A is moved through rotation R 1 , the sleeve shaft  36  (which is directly connected to the first spur gear  34 A) will also move through rotational motion R 1 . Similarly, when the second spur gear  34 B is moved through rotation R 2 , the center shaft (which is directly connected to the second spur gear  34 B) will also move through rotational motion R 2 . Since the valve plate  22  is connected to the center shaft  38 , the valve plate  22  will move through rotational movement R 2  when the center shaft  38  and second spur gear  34 B move rotationally. What is unique is that the center shaft  38  can also move in the translational movement direction T 2  as will now be described. 
         [0037]    If the sleeve shaft  36  and the center shaft  38  are rotated synchronously and at the same speed and same rotational direction, then the follower roller  42  will exhibit no motion in relation to the cam groove  40  (since both are moving in the same direction at the same rate). If, for example, the sleeve shaft  36  is prevented from rotating while the center shaft  38  is rotated, there will be relative motion between the follower roller  42  and the cam groove  40 . This relative motion will cause the roller  42  to move along the length of the groove  40 . As the roller  42  moves along the groove  40 , it will drive the center shaft  38  to move in translational direction T 2 . Of course, the second spur gear  34 B will also be caused to move in the translational direction T 2 . In this scenario, the valve plate  22  will not only rotate (R 2 ), but will also translate (T 2 ). 
         [0038]    If, alternatively, the center shaft  38  is prevented from rotating (R 2 =0) while the sleeve shaft is rotated (R 1 &gt;0 or R 1 &lt;0), there will be no rotational movement of the valve plate  22  (since R 2 =0), and there will only be translational movement T 2 . It is through the infinite available combinations of R 1  and R 2 , together or independently, that the high level of control of the valve plate theta and z-direction movement is achieved. 
         [0039]    An additional benefit and enabling technology of this innovation is obtained through the advanced, closed-loop motor control of the two bi-polar stepper drive motors. Not only is a high level of precision and motor synchronization available for the movement of the valve plate  22 , but the drive motors can actually be independently controlled to slightly oppose one another in order to eliminate the negative control effects of valve backlash and hysteresis on the performance of the plate&#39;s movement. 
         [0040]      FIG. 4  is a partial cutaway perspective view of the assembly of  FIG. 3 . As shown, the follower roller  42  extends perpendicular to the axis of the center shaft  38 . It should be appreciated that in other embodiments, a worm screw-type arrangement could be created between the center shaft  38  and the sleeve shaft  36  (i.e. a threaded inner surface in the sleeve shaft and a corresponding threaded outer surface of the center shaft  38 ). In such an arrangement, both shafts  36  and  38  will move in the translational direction, but otherwise the same interdependence (and resultant control benefits) between the two shafts will result.  FIG. 5  is a cutaway side view of the plate actuator assembly  14  of  FIGS. 1 and 2 . 
         [0041]    The shaft assembly housing  42  attaches and seals at its top end at the motor base plate  26 , and at the bottom end via the flange ring  41  and shaft assembly sealing ring  35 . The voids within the internal volume of the shaft assembly housing  25  are in fluid communication with the exterior of the housing  25  (generally atmospheric pressure). Of course, for safety and purity, the valve housing (see  FIG. 1 ) must be sealed from both the exterior of the valve housing and the internal volume of the shaft assembly housing  25 . This is accomplished via a combination of bellows and elastomeric seals. 
         [0042]    The lower end of the center shaft  38  (i.e. within the valve housing) encircled by a keeper ring  33 . There are one or more center shaft seals  39  between the keeper ring  33  and the center shaft  38  to prevent leak-by. A bellows sleeve  31  is attached (typically welded) at its lower end to the keeper ring  33 , and at its upper end to a shaft assembly sealing ring  35 . The center shaft  38  is encased within a bellows-type sleeve so that the center shaft  38  can move up and down along the z-axis direction, but without the need for a sliding/rotating sealing surface. The bellows sleeve  31  allows the center shaft to move up and down, and confines the sealing elements to be simple O-rings. 
         [0043]    The shaft assembly sealing ring  35  seals to the valve housing (see  FIG. 1 ) via a valve housing seal  37 , which is stationary. These seals and the bellows sleeve  31  result in the components above the dashed line (plus the interior volume of the bellows sleeve  31 ) being at atmospheric pressure, while the components below the dashed line are at the same pressure as the interior of the valve housing (under vacuum). This pressure differential causes the keeper ring  33  to be pulled downward towards the shoulder formed near the bottom end of the center shaft  38 . 
         [0044]    Furthermore, for optimum control purposes, it is desirable to have minimum backlash in the mechanism, that is, no slack in the various linkages transmitting power and motion from the drive motor output shafts to the valve plate  22 . A dynamic preload of all mechanical linkages is afforded by the isolation bellows. The pre-loading of the follower roller  42  toward one side of the cam groove  40  is accomplished by the pressure differential by exploiting the pressure differential between extra-bellows (atmospheric) and inter-bellows (chamber vacuum) cavities. Considering that the concentric shafts are under vacuum while the outer mechanism is at atmospheric pressure, a pressure will be exerted on the inner shaft, forcing it downward (as viewed here), thereby forcing the follower roller toward the lower cam follower groove surface. Now, under static conditions, if spur gears  30 A and  30 B are held in place so that neither can rotate, the downward force of the follower roller onto the lower cam surface will cause a counterclockwise torque on the outer shaft and an equal and opposite (clockwise) torque on the inner shaft. Since the spur gears are being held in place by the two motors  28 A,  28 B, any slack in the gear linkages will also be taken up. Thus, a preload of the entire mechanism is created, resulting in zero hysteresis in conductance characteristics (as a function of motor position) of the valve. 
         [0045]    With complete, calibrated feedforward compensation for the loads reflected back to the motor output shafts (including torques, coulomb and viscous frictions, and inertias), this pressure differential “preload” is extended to include gear meshes, so that ideal, near-zero total backlash is accomplished dynamically. The loads are calibrated during system initialization to optimize control settings.  FIG. 6  indicates how the valve plate  22  is moved by the concentric shaft assembly  44 . 
         [0046]      FIG. 6  is a perspective view of the valve plate  22  of the valve assembly of  FIG. 1 . When the center shaft  38  is caused to move in rotational direction R 2 , the valve plate  22  will move in the theta direction. When the center shaft  38  is caused to move in the translational (not in unison with the sleeve shaft  36 ) direction T 2 , the valve plate  22  will also move in the z-axis direction. Also of note in this drawing is the sealing face  46  located at the distal end of the valve plate  22 . It is the sealing face  46  (or gaskets associated therewith) that create the seal with the valve housing. Further detail regarding the plate actuator assembly  14  is provided below in connection with  FIG. 7 . 
         [0047]      FIG. 7  is a side view of the plate actuator assembly  14  of  FIGS. 1 and 2 . From the side, the differences in tooth width  48  of the second spur gear  34 B and the tooth width  50  of the second pinion gear  30 B is clearly visible. The tooth width  50  is wider than the tooth width  48 . This difference in width is what allows the second spur gear  34 B to move in direction  52  while its teeth remain fully engaged with the pinion gear  30 B teeth. The same effect is achieved by making tooth width  48  of the second spur gear  34 B wider than the too width  50  of the second pinion gear  30 B.  FIGS. 7A and 7B  illuminate the result of this inventive approach to actuating this pendulum valve. 
         [0048]      FIGS. 8A and 8B  depict the valve plate motion in the prior art valve and the valve assembly of the present invention, respectively.  FIG. 8A  depicts the conventional “L-motion” characteristics exhibited by the typical prior pendulum valve and actuator. The macro or gross valve movement  54  is conducted in the theta direction (i.e. angular rotation). The final valve seal  56  is achieved in the z-direction motion. The transition point  58  is that point when theta movement stops and z-direction movement begins (or vice-versa). 
         [0049]    In contrast,  FIG. 8B  depicts the valve plate additional mode of interactive motion  59  in the system of the present invention. The motion depicted in the solid line by  60 A is best described as “J-motion” because there is no division between the theta direction motion and the z-direction motion. Since both movements can be controlled simultaneously and independently, the movement is only L-shaped if the user desires that movement pattern. What is more likely is that the user will prefer to move the plate through theta and z-directions simultaneously in order to achieve superior chamber flow characteristics as well as very rapid valve actuation speed. Curves  60 B and  60 C illuminate how the system can control motion of the plate in virtually to move through virtually any motion path, since z-direction and theta-direction movement are totally independent. 
         [0050]    Although not depicted here, testing on the valve and actuator of the present invention indicate that in order to optimize valve performance including its ability to throttle flow when nearly closed (“near-closed conductance”), a two-stage sealing ring may be desirable at either the sealing face of the valve plate or at the corresponding valve seat surface on the inside of the valve housing. This “two-stage” valve seal or gasket may define a cross-section that is more complex than a simple circular shape, and further may be made from more than one material of construction such that different sections of the seal (cross-sectional sections) may have different material properties (e.g. elasticity, etc.). The intent of the two-stage seal would be to allow for controllable near-closed conductance as well as sealing to an extremely low conductance when “closed.” 
         [0051]    Furthermore, because the plate actuator assembly can move the valve in the z-axis without slowing down the response time, it enables the valve housing to be wider (interior flange-to-flange), which in turn allows for greater travel (and throttling range) in the z-direction. Z-direction travel is of particular importance when the valve plate is eclipsing the flow channel (i.e. the plate is positioned over the valve seat, but there is a (z-direction) gap remaining between the valve plate and the valve seat/valve housing. The best, most symmetrical flow conditions can be achieved when throttling flow through a pendulum valve when the valve plate is eclipsing the flow channel. This is because theoretically the flow will surround the entire periphery of the valve seating face (symmetric flow), rather than only a portion of the valve plate as the valve plate moves through the theta direction prior to eclipsing the flow channel. Having a greater gap between the valve plate and the valve seat/valve housing will allow for a greater symmetric throttling range in the z-axis direction. The maximum z-distance available in the prior art pendulum valves has been two (2) millimeters. The valve of the present invention has been tested with a z-distance of up to thirteen (13) millimeters, and has demonstrated superior performance, both in speed of actuation and in quality and range of flow throttling. 
         [0052]      FIG. 9  is a graph depicting the performance benefits of the valve assembly of the present invention. This graph shows the relationship between the chamber pressure (conductance) and the position of the valve plate expressed in percent of full open. The auxiliary axis depicted below the valve position axis indicates where the conventional pendulum valve plate transitions from moving along only the Θ path, to then moving in the z-axis direction towards closed and sealed. The dashed line to the right is denotes where the edge of the valve plate begins to pass over the valve seat edge (i.e. the edge of the inlet or outlet port of the valve housing). The dashed line to the left denotes when the valve has completed all movement along the Θ path (i.e. the plate is fully eclipsing the valve seat. 
         [0053]    This graph is presented to make clear the advantages of the valve of the present invention over the conventional pendulum valve having very limited stroke in the z-axis direction. Because the gap between the valve plate and the valve seat is so tight, there is a flat section in the position vs vacuum curve beginning near where the plate begins and completes the eclipse. This is because the movement of the valve plate through this eclipsing range (without also moving the plate in the z-axis direction) does not create a substantial change in flowrate through the valve. So, while the valve is “closing” as a percentage of being open, it isn&#39;t really closing as it relates to throttling flow. 
         [0054]    In contrast, the valve of the present invention does two things: (1) it allows the valve plate to move simultaneously and independently in both (or either) the z-axis and the theta directions over the theta motion path; and consequently (2) it enables the prior art “flat” portion to be effectively eliminated by increasing the z-axis gap between the valve plate and the valve seat in the eclipsing range. Furthermore, the actual z-axis vs theta positions can be optimized (i.e. tuned) in-situ so that the smoothest performance curve possible can be achieved. 
         [0055]    Although not specifically depicted here, the inventors have further discovered that motion of the valve of the present invention in the z-axis can be controlled very effectively in the positive direction, and even in the opposing direction. Conventionally, pendulum valve seats are oriented on the “upstream” side of the valve housing. As such, z-axis motion towards the seat (i.e. in the “positive z-axis direction”) will be opposing any flow through the valve housing. The valve, actuator and control system of the present invention has such high tolerance control of valve plate motion that the valve seat can actually be located on the housing port that is downstream of the valve plate. The benefit of such an orientation is that pressure differential across the valve plate and seat (i.e. in the direction of flow) will work with the valve actuator assembly to provide a more robust seal between the plate and the seat. 
         [0056]    Those skilled in the art will appreciate that various adaptations and modifications of the just-described preferred embodiment can be configured without departing from the scope and spirit of the invention. Therefore, it is to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described herein.