Patent Publication Number: US-6659247-B2

Title: Pneumatic volume booster for valve positioner

Description:
PRIORITY 
     This application is a continuation-in-part of and claims priority to the patent application Ser. No. 09/471,921 now U.S. Pat. No. 6,357,335 entitled, “Pneumatic Volume Booster for Valve Positioner, ” filed Dec. 23, 1999, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to control systems for the pneumatically powered actuators of valves such as those often used in the pulp and paper, chemical and petroleum industries. More particularly, it relates to a system that provides for controlling the position of a pneumatically operated valve with an electrical control signal. 
     BACKGROUND OF THE INVENTION 
     It is desirable to utilize piezo valves as part of a control system for a pneumatically powered valve actuator because piezo valves provide an extremely long cycle life and reliability, extremely low power requirements, and fast on/off times. However, the volume of air they pass is relatively small compared to the volume required to quickly move a control valve into a new adjusted position. In some industrial applications, a relatively significant force is required to achieve movement of the valve, necessitating a correspondingly large surface area on the part of the actuator exposed to air pressure. Piezo valves are typically unable to provide the sustained volume of air necessary over the surface area of the movable part. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a valve control system that utilizes piezo valves to take advantage of their inherently desirable characteristics, yet the volume of air supplied to the valve actuator of the system is boosted to such an extent that the actuator quickly repositions the valve into the desired adjusted position. 
     In carrying out the foregoing object, the present invention contemplates a system in which multiple piezo valves are subject to an electrical control signal to either initiate or terminate a pilot pneumatic output at the established control pressure. Such pilot output is in turn directed to appropriate valve assemblies of a volume booster circuit which is likewise connected to a source of pressurized gas at the same pressure as that supplied to the piezo valves. Thus, the pneumatic pilot signal from the piezo valves is used to determine the opened or closed state of valve assemblies in the booster mechanism. These booster valve assemblies are capable of passing much greater volumes of gas to the actuators than the small pilot signals produced by the piezo valves. Consequently, the instantaneous action of the piezo valves is obtained, along with sufficiently large volumes of gas to quickly move the operating part of the actuator. 
     The control system of the present invention utilizes pistons in the booster valve assemblies that operate on the principle of unequal piston areas on opposite operating surfaces thereof. Thus, both surfaces of the piston may be simultaneously exposed to the same operating pressure in the form of a pneumatic signal from the piezo valve and an operating volume from the gas source. However, even though the pressures are the same on both faces of the piston, the piston will be moved in a direction generally toward the smaller surface area because the total force on the piston is greater on the side with the larger surface area. In the preferred form of the invention, the side of the piston exposed to the pneumatic signal from the piezo valve is the side with the large surface area, while the side exposed directly to the gas source is the smaller. 
     In preferred forms, the control system may be either single-acting or double-acting. In a system configured for a single-acting actuator, the actuator has only one operating gas chamber on one side of its movable part, while a spring is disposed on the opposite side. In a double-acting version, pressure chambers are located on opposite sides of the movable part of the actuator so that pressurized gas is used to move the part in both of its adjusting directions of movement. In both versions, the control system is capable of moving the actuator in valve opening or valve closing directions, plus holding the actuator in a selected stationary position. 
     In its preferred form, each booster valve assembly utilizes a piston confined between a pair of diaphragms as the shut-off valve component within the assembly. The piston responds to the pneumatic pilot pressure to push one of the diaphragms into sealing engagement with the valve seat and thus close the operating flow path through the valve assembly. When pilot pressure on the large face side of the piston is absent, the source pressure on the small face side of the piston shifts it away from the diaphragm to permit the diaphragm to uncover and open the flow path. 
     A second embodiment of the valve assembly uses an integral piston and valve wherein one face of the piston is directly exposed to pilot pressure from the piezo valve and the other face of the piston is both directly exposed to operating pressure from the gas source and has a sealing surface that, when engaged with the valve seat, functions to close the valve assembly. 
     A third embodiment of the valve assembly uses a piston confined between a ball valve and a diaphragm. The ball valve directly opens and closes the valve seat rather than the piston, although the ball valve is physically moved by the piston as the piston responds to the presence or absence of pressure on its opposite sides. 
     A fourth embodiment of the valve assembly uses a smaller piston mounted within the larger piston for controlling minute air volumes or pulses. This smaller piston actuates a response to small pulses of air which allows for finer control of the valve&#39;s movement. Thus, this actuation results in a more accurate control leading to better overall performance of the valve and the system. For example, with 80 psi supply air to the system, the pilot air has to bleed down to perhaps 25 psi before the piston opens. Because of the large size of the pistons that is required for speed on large valve moves, the minimum amount of movement is relatively large, however the smaller piston is sized to open up when the pilot air drops below 40 psi instead. Therefore, when small signal changes to the pilot air being bled off is between 40 to 25 psi, only the smaller piston will open up and not the larger piston. This allows for a much smaller volume of air to flow into and out of the actuator with a considerably smaller or finer valve movement. When larger signal changes occur, the pilot pressure is dropped below 25 psi and the larger piston will open up to allow for faster valve response. 
     There has thus been outlined, rather broadly, the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an exemplary installation in which a control system in accordance with the principles of the present invention is utilized; 
     FIG. 2 is an enlarged, fragmentary cross-sectional view of one of the booster valve assemblies utilized in the control system; 
     FIGS. 3,  4  and  5  are schematic illustrations of a preferred embodiment of the control system for a single-acting actuator, FIG. 3 illustrating the system in such a state that the actuator is moving in a direction to close a valve with which it is associated, FIG. 4 illustrating the system in a state for moving the actuator in a valve opening direction, and FIG. 5 illustrating the system in a state for holding the valve stationary in a selected position; 
     FIGS. 6,  7  and  8  disclose a second embodiment of a system for use with a double-acting actuator, FIG. 6 illustrating the system in a state where in the actuator is moving in the valve closing direction, FIG. 7 illustrating the system in a state wherein the actuator is moving in the valve opening direction, and FIG. 8 illustrating the system in a state wherein the actuator is held stationary at a selected position; 
     FIG. 9 is an enlarged, fragmentary cross-sectional view of an alternative embodiment for the booster valve assembly; 
     FIG. 10 is an enlarged, fragmentary cross-sectional view of another alternative embodiment for the booster valve assembly; 
     FIG. 11 is an enlarged, fragmentary cross-sectional view of another alternate embodiment for the booster valve assembly; 
     FIGS. 12-15 are schematic illustrations of a preferred embodiment of the control system utilizing the single-acting actuator which uses a spring, with FIG. 12 illustrating the system in a state of partial movement of the actuator in a valve opening direction, FIG. 13 illustrating the system in a state for fully opening the valve, FIG. 14 illustrating the valve in a stopped or steady-state configuration, and FIG. 15 illustrating the valve in a steady-state configuration with control system failure; and 
     FIGS. 16-19 are schematic illustrations of a preferred embodiment of the control system utilizing the double-acting actuator which lacks a spring, with FIG. 16 illustrating the system in a state of partial movement of the actuator in a valve opening direction, FIG. 17 illustrating the system in a state for fully opening the valve, FIG. 18 illustrating the valve in a stopped or steady-state configuration, and FIG. 19 illustrating the valve in a steady-state configuration with control system failure. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     FIG. 1 illustrates a typical pipe line  10  having a valve  12  that may be adjustably positioned within a number of different positions opening or closing the pipe line  10 . A system for controlling the position of the valve  12  is broadly denoted by the numeral  14  and includes a pneumatically powered actuator  16  mechanically coupled with the valve  12  via a mechanical connection  18 . The system  14  also includes a valve booster circuit  20 , a pilot circuit  22  for the booster circuit  20 , a source of pressurized gas  24  common to both the booster circuit  20  and the pilot circuit  22 , and a controller  26  electrically connected to the pilot circuit  22 . The control system  14  may also include a sensor  28  that is connected to the pipe line  10  in a suitable manner for detecting a flow condition within the pipe line  10  and reporting that condition to the controller so that a comparison can be made with a set or desired condition known to the controller. An appropriate adjustment of the valve  12  can be made accordingly. 
     The system  14  in FIG. 1 utilizes a single-acting actuator  16  that is pneumatically actuated in only one direction, the opposite direction of movement being provided by mechanical spring means or the like. However, as described below, the present invention also contemplates a double-acting system. 
     The pilot circuit  22  of system  14  includes pilot mechanism in the form of a pair of piezo valves  30  and  32 . Depending upon the desired position for the valve  12  to assume in the event of an electrical failure, i.e., “fail open”, “fail closed”, or “fail at last position”, the piezo valves may be selected to be normally open, normally closed, or a combination of both. The manner in which the actuator  16  is mechanically coupled to the valve  12  is also obviously a factor in determining the direction of movement of the valve  12 , if any, to its failed position upon electrical failure. Although one particular set of normal states for the piezo valves  30 ,  32  has been disclosed herein, it is to be understood that such disclosure is made for the purpose of example only and not with the intent of limiting the scope of the present invention. 
     In the illustrated embodiment, the valve  30  is normally open and the valve  32  is normally closed. The piezo valves may take the form, for example, of the valve disclosed in U.S. Pat. No. 5,343,894 in the name of Frisch, et al. Piezo valve  30  receives an electrical control signal from the controller  26  via a conductor  34 , while the normally closed piezo valve  32  receives an electrical signal from the controller  26  via a conductor  36 . The pilot circuit  22  further includes a pneumatic input line  38  to the normally open piezo valve  30  and a second pneumatic input line  40  to the normally closed piezo valve  32 , both of such inputs  38  and  40  being connected to the source of pressurized gas  24  which, in its preferred form, is air. Pilot circuit  22  further includes a pneumatic output line  42  from the normally open piezo valve  30  and a similar pneumatic output line  44  from the normally closed piezo valve  32 . Both output lines  42  and  44  are operable to output a pneumatic signal at system pressure. Each piezo valve  30 ,  32  is operable to connect its output line  42  or  44  to atmosphere via exhausts  31 ,  33  when the valve is closed. 
     The booster circuit  20  comprises booster mechanism that includes a pair of identical booster valve assemblies  46   a  and  46   b  constructed as illustrated by the valve assembly  46  in FIG.  2 . Valve assembly  46   a  is a supply valve assembly connected to source  24  and valve assembly  46   b  is an exhaust valve assembly connected to atmosphere. Assemblies  46   a  and  46   b  may be contained within a common valve block or the like. In addition to the valve assemblies  46   a  and  46   b,  the booster circuit  20  includes an operating pressure input line  50  connected between the source of pressurized air  24  and the supply valve assembly  46   a.  Additionally, the booster circuit  20  includes an output line  52  from the valve  46   a  to the actuator  16 , an exhaust line  54  from the exhaust valve assembly  46   b  to atmosphere, and an interconnecting line  56  between supply valve assembly  46   a  and exhaust valve assembly  46   b.  The two valve assemblies  46   a  and  46   b  also have vents  58  and  60  as will be described in more detail below. 
     Referring to FIG. 2, the valve assembly  46  comprises three primary sections stacked on top of one another, namely a bottom section  62 , an intermediate section  64 , and a top section  66 . The bottom section  62  includes an internal chamber  68  having a supply port  70  and an exhaust port  72 . The supply port  70  has a flat valve seat  74 . A first flexible diaphragm  76  within the chamber  68  is movable between a closed position engagably covering the valve seat  74  so as to close the same and an open position shown in FIG. 2 in which the diaphragm  76  is up off the seat  74 . When the diaphragm  76  is disengaged from the seat  74 , a flow path is established between the supply port  70  and the exhaust port  72  through the chamber  68 ; when the diaphragm  76  is engaged with the seat  74 , such flow path is closed. 
     The diaphragm  76  is sandwiched between the sections  62  and  64  and seals the chamber  68  of section  62  from a chamber  80  within the intermediate section  64 . A piston  82  is confined within the chamber  80  but is shiftable axially thereof generally toward and away from the valve seat  74 . In FIG. 2, the piston  82  is illustrated in its valve opening position in which it is spaced away from the valve seat  74  so as to allow the diaphragm  76  to likewise be spaced above the seat  74 . 
     Sandwiched between the intermediate section  64  and the top section  66  is another flexible diaphragm  84  overlying the piston  82 . When the piston  82  is lowered, a pilot chamber  86  (see FIG. 3) is defined on the upper side of the piston  82  between the diaphragm  84  and the top section  66 , the pilot chamber  86  receiving a supply of pilot air at the operating pressure via an inlet  88  in the top section  66 . 
     It will be noted that the piston  82  has an upper surface  82   a  that is substantially larger than its bottom surface  82   b.  Upper surface  82   a  is exposed to the effects of a pilot signal in the pilot chamber  86 , while the lower surface  82   b  is exposed to an operating volume of air, at the same pressure as the pilot pressure, in the operating chamber  68 . The supply port  70  and the exhaust port  72  are capable of passing air at a substantially higher volumetric rate than the piezo valves  30 ,  32  and their pilot inlets  88 . A vent port  90  in the chamber  80  below the enlarged pilot end of the piston  82  is adapted to communicate with the atmosphere. 
     As illustrated in FIGS. 3,  4  and  5 , which correspond to FIG. 1, the output line  42  from the normally open piezo valve  30  connects to the pilot inlet  88  of supply valve assembly  46   a.  Input line  50  from the air source  24  connects to the supply port  70  of supply valve assembly  46   a,  and the outlet line  52  connects to the exhaust port  72  of supply valve assembly  46   a.    
     On the other hand, the output line  44  from normally closed piezo valve  32  connects to the pilot inlet  88  of exhaust valve assembly  46   b,  the interconnect line  56  is connected to the supply port  70  of exhaust valve assembly  46   b,  and the exhaust line  54  is connected to the exhaust port  72  of exhaust valve assembly  46   b.  Output line  52  from supply valve assembly  46   a  connects to the operating chamber  92  of actuator  16  so as to supply variable amounts of pressurized air to the chamber  92 . A movable part of the actuator  16  illustrated in the preferred embodiment in the nature of a piston  94  is responsive to the increase or decrease of air within chamber  92  to move leftwardly or rightwardly as viewed in the figures so as to extend or retract the rod which forms the mechanical connection  18  with the control valve  12  as designated in FIG. 1. A spring  96  housed within the actuator  16  yieldably biases the piston  94  leftwardly viewing FIGS. 3,  4  and  5 . 
     FIG. 3 illustrates the condition of the control system  14  for moving the piston  94  leftwardly in the direction of the arrow  98  to adjust the valve  12 . This may be selected as the direction for closing the valve  12 , if desired. In this condition, no electrical signal is supplied from the controller  26  to the piezo valves  30  and  32  such that they remain in their normal, de-energized condition. Thus, the normally open valve  30  allows a pneumatic pilot signal to pass along the output line  42  and into the pilot chamber  86  of supply valve assembly  46   a.  Supply pressure is also present in input line  50  and at supply port  70  of the supply valve assembly  46   a,  but the pressures are the same in both pilot chamber  86  and operating chamber  68 . Consequently, due to the larger surface area on the upper side of the piston  82 , the piston  82  is moved downwardly, forcing the diaphragm  76  against the seat  74  and closing off the supply port  70 . This closes supply valve assembly  46   a  and no pressurized air passes to the actuator  16 . 
     The pilot valve  32  is closed at this time, with output line  44  connected to atmosphere. Thus, only atmospheric pressure is applied to the upper side of the piston  82  of exhaust valve assembly  46   b  at this time. Consequently, pressurized air at supply port  70  of exhaust valve assembly  46   b  lifts the diaphragm  76  of exhaust valve assembly  46   b  off the seat  74 , thereby opening exhaust valve assembly  46   b,  as air is forced out of the operating chamber  92  by the piston  94  under the influence of the return spring  96 . With the diaphragm  76  off its seat, an open flow path is established between the interconnect line  56  and the exhaust line  54  via the supply port  70 , the chamber  68 , and the exhaust port  72  of exhaust valve assembly  46   b.    
     FIG. 4 illustrates the condition of the control system  14  for moving the rod  18  of the actuator  16  rightwardly in the direction of the arrow  100 . This may be selected as the direction for opening the valve  12 , if desired. In this condition, the controller  26  has supplied electrical control signals to the piezo pilot valves  30  and  32  such that they are both in their energized positions. Thus, the piezo valve  30  becomes closed, while the piezo valve  32  becomes opened. 
     When the piezo valve  30  is closed, output line  42  to supply valve  46   a  is connected to atmosphere. Thus, air at supply pressure unseats the diaphragm  76  via input line  50  and supply port  70 , opening supply valve assembly  46   a.  A flow path is opened between supply port  70  and exhaust port  72  of supply valve assembly  46   a,  causing significant volumes of pressurized air to enter the operating chamber  92  of actuator  16  via output line  52 . The exhaust valve assembly  46   b  will be closed at this time due to the fact that supply pressure is applied to its pilot chamber  86  via the opened pilot valve  32 . Consequently, the piston  82  of exhaust valve assembly  46   b  keeps the diaphragm  76  seated at this time, blocking access of the interconnect line  56  to the exhaust line  54 . Therefore, the piston  94  moves rightwardly as the volume within chamber  92  increases, against the resistance of the return spring  96 . 
     FIG. 5 illustrates the condition of the system  14  for holding the actuator  16  stationary in any selected position. The piezo valve  30  is not receiving a signal from the controller  26  at this time; thus, the de-energized valve  30  remains open to pass a pneumatic pilot signal into the pilot chamber  86  of supply valve assembly  46   a  via output line  42 . Consequently, the piston  82  of supply valve assembly  46   a  pushes the diaphragm  76  against seat  74  to close the supply port  70 , even though supply pressure is present in the input line  50  and at supply port  70 . 
     On the other hand, the normally closed piezo valve  32  receives a control signal from the controller  26  and becomes energized so as to allow passage therethrough of air at supply pressure. Supply pressure is therefore applied to the pilot chamber  86  of exhaust valve assembly  46   b  via output line  44 . This shifts piston  82  of exhaust valve assembly  46   b  against the diaphragm  76 , causing it to close the supply port  70 , closing exhaust valve assembly  46   b.  Such closure of supply port  70  has the effect of closing off access of the operating chamber  42  to the exhaust line  54  such that the piston  94  of actuator  16  cannot move leftwardly. Moreover, the return spring  96  prevents it from moving rightwardly, and as there is no incoming air at supply pressure, the operating rod  18  of the actuator  16  stays in a fixed position. 
     Double-acting System 
     FIGS. 6,  7  and  8  illustrate a double-acting control system for performing the same functions as the single-acting system of FIGS. 3-5. Instead of a return spring for the piston of the actuator, however, both sides of the piston have access to gas under supply pressure and atmosphere at appropriate times such that, depending upon which valves are opened and which are closed, the piston either moves to the right or left in response to changes in the volume of pressurized air introduced into the chambers on opposite sides of the piston. The double-acting system of FIGS. 6-8 utilizes the same booster valve assembly  46  as the single-acting system, but five of such valve assemblies  46  are utilized in the double-acting system and they are arranged differently. Similarly, although the same type of piezo valves are utilized, three of such valves are utilized instead of two, one of them being normally open and two being normally closed. 
     The double-acting system  200  of FIGS. 6-8 includes a source of pressurized air  202  (or other gas), a double-acting actuator  204 , a pilot circuit  206 , and a booster circuit  208 . In this example, the pilot circuit  206  includes pilot valve mechanism in the form of three piezo valves  210 ,  212  and  214 . As with pilot circuit  22 , depending upon the desired position for the valve  12  to assume in the event of an electrical failure, i.e., “fail open”, “fail closed”, or “fail at last position”, the piezo valves of pilot circuit  206  may be selected to be normally open, normally closed, or combinations of both. Although one particular set of normal states for the piezo valves  210 ,  212 , and  214  has been disclosed herein, it is to be understood that such disclosure is made for the purpose of example only and not with the intent of limiting the scope of the present invention. 
     In the illustrated embodiment, the piezo valve  210  is normally open, while piezo valves  212  and  214  are normally closed. Valve  210  has an input line  216 , the valve  212  has an input line  218 , and valve  214  has an input line  220 , all in direct communication with the air source  202 . Valve  210  has an output line  222 , valve  218  has an output line  224 , and valve  214  has an output line  226 . Piezo valves  210 - 214  are operable when closed to connect their respective output lines  222 - 226  to atmosphere. 
     The booster circuit  208  includes booster valve mechanism in the form of five of the booster valve assemblies  46 . There is a supply valve assembly and an exhaust valve assembly for each end of the actuator  204 , as well as a single control valve assembly that is common to the two exhaust valve assemblies and determines when the exhaust valve assemblies will be open to atmosphere. In the double-acting system  200 , the five valve assemblies have been designated as supply valve assembly  228   a,  exhaust valve assembly  228   b,  control valve assembly  228   c,  exhaust valve assembly  228   d,  and supply valve assembly  228   e.  Each of the booster valve assemblies  228   a-e  is identical to the valve assembly  46  illustrated in FIG. 2, and the internal parts of the booster valve assemblies  228   a-e  will be described using the same reference numerals for such parts as used with respect to FIG.  2 . 
     In addition to the booster valve assemblies  228   a-e,  the booster circuit  208  also includes a number of flow lines. Such flow lines include an input line  230  from the air source  202  to the supply port  70  of supply valve assembly  228   a.  Another input line  232  is connected between the air source  202  and the supply port  70  of supply valve assembly  228   e.  An output line  234  connects the exhaust port  72  of supply valve assembly  228   a  with a chamber  236  on the left side of the piston  238  of actuator  204 , while an output line  240  connects the exhaust port  72  of supply valve assembly  228   e  with a chamber  242  on the right side of the piston  238 . The output line  234  from supply valve assembly  228   a  has a branch  234   a  that connects to the supply port  70  of exhaust valve assembly  228   b,  while the output line  240  has a branch  240   a  that connects to the supply port  70  of exhaust valve assembly  228   d.  An interconnect line  244  connects the exhaust port  72  of exhaust valve assembly  228   b  with the exhaust port  72  of exhaust valve assembly  228   d.  Interconnect line  244  has a branch  244   a  connected to the supply port  70  of control valve assembly  228   c,  and an exhaust line  246  communicates the exhaust port  72  of control valve assembly  228   c  with atmosphere. 
     The pilot circuit  206 , in addition to the piezo valves  210 - 214 , the input lines  216 - 220 , and the output lines  222 - 226 , also includes a branch  222   a  of output line  222  that connects to the pilot inlet  88  of exhaust valve assembly  228   d,  and a branch  226   a  of output line  226  that connects to the pilot inlet  88  of exhaust valve assembly  228   b.  Output line  222  from piezo valve  210  connects to the pilot inlet  88  of supply valve assembly  228   a,  the output line  224  of piezo valve  212  connects to the pilot inlet  88  of control valve assembly  228   c,  and the outlet line  226  of piezo valve  214  connects to the pilot inlet  88  of supply valve assembly  228   e.    
     FIG. 6 illustrates the condition of the control system  200  for moving the rod  248  of the actuator  204  leftwardly as indicated by the arrow  250 . This may be selected as the valve closing direction for valve  12 , if desired. In this condition, all three of the piezo valves  210 ,  212  and  214  are de-energized. Thus, pressurized air flows through the normally open piezo valve  210 , but not through the normally closed piezo valves  212  and  214 . This applies supply pressure to output line  222  and branch  222   a,  introducing pilot pressure into the pilot chambers  86  of supply valve assembly  228   a  and exhaust valve assembly  228   d.  Thus, supply valve assembly  228   a  and exhaust valve assembly  228   d  are closed. Consequently, no pressurized air flows along the output line  234  into the left chamber  236 . On the other hand, because pilot valve  214  is in a closed condition, no pilot pressure exists in output line  226  such that diaphragm  76  of supply valve assembly  228   e  is free to be unseated by supply pressure in the input line  232  and supply port  70 . Thus, supply valve assembly  228   e  opens, creating a flow path to the right operating chamber  242  via line  232  and  240 , causing the piston  238  to shift leftwardly. 
     It will be noted that when piston  238  is shifting leftwardly, air in the left chamber  236  needs to be exhausted. This is accommodated due to the fact that closed piezo valve  212  prevents a pilot signal on output line  224 , and closed piezo valve  214  connects branch  226   a  to atmosphere. Thus, exhaust valve assembly  228   b  and control valve assembly  228   c  open as their diaphragms  76  are free to be unseated by the air seeking to exhaust from the left chamber  236  of actuator  204 , opening chamber  236  to atmosphere. Such exhaust path comprises line  234 , branch  234   a,  open exhaust valve assembly  228   b,  line  244 , branch  244   a,  open control valve assembly  228   c,  and exhaust line  246 . 
     FIG. 7 illustrates the condition of the control system  200  for moving the rod  248  of actuator  204  rightwardly as illustrated by the arrow  252 . This may be selected as the valve opening direction for the valve  12 , if desired. In this condition, the normally open piezo valve  210  is energized by a control signal from the controller  26 , the normally closed piezo valve  212  is de-energized, and the normally closed piezo valve  214  is energized. Consequently, no pilot pressure is present on output line  222  for supply valve assembly  228   a,  or on branch line  222   a  for exhaust valve assembly  228   d.  These valve assemblies are therefore free to open. Consequently, pressurized air in supply line  230  and at supply port  70  of supply valve assembly  228   a  unseats the diaphragm  76  of that assembly, causing pressurized air to be introduced into the left chamber  236  of the actuator  204  via output line  234  from supply valve assembly  228   a.  Since trapped air within the right chamber  242  of the actuator  204  must be exhausted by the rightwardly moving piston  238 , chamber  242  becomes open to atmosphere via line  240 , branch  240   a,  open exhaust valve assembly  228   d,  interconnect line  244 , branch  244   a,  open control valve assembly  228   c,  and exhaust line  246 . Due to the fact that no pilot pressure appears in branch  222   a,  the diaphragm  76  of exhaust valve assembly  228   d  can be unseated. Likewise, because no pilot pressure exists in output line  224  to control valve assembly  228   c  due to the closed piezo valve  212 , the diaphragm  76  of control valve assembly  228   c  is unseated by the exhausting air. 
     At this time, although supply pressure exists in input line  232  and at supply port  70  of supply valve assembly  228   e,  the diaphragm  76  thereof cannot unseat because pilot pressure is present in the pilot chamber  86  via open piezo valve  214  and output line  226 . The pilot pressure in output line  226  is also communicated to branch  226   a,  creating pilot pressure within the pilot chamber  86  of exhaust valve assembly  228   b.  This causes the diaphragm  76  thereof to close supply port  70 , despite the presence of pressurized air at port  70  assembly via branch  234   a.    
     FIG. 8 illustrates the condition of the control system  200  for holding the actuator  204  in any selected position. In this condition, the normally open piezo valve  210  is de-energized, the normally closed piezo valve  212  is de-energized, and the normally closed piezo valve  214  is energized by a control signal from the controller  26 . Consequently, pilot pressure is present in the pilot chambers  86  of supply valve assembly  228   a  and exhaust valve assembly  228   d  to close those valve assemblies. Likewise, pilot pressure is present in the pilot chambers  86  of supply valve assembly  228   e  and exhaust valve assembly  228   b  via output line  226  and branch  226   a  to close those valve assemblies. Thus, lines  234  and  240  connected to the actuator chambers  236  and  242  respectively are effectively blocked from both the source of supply  202  and atmosphere. Consequently, the piston  238  is trapped against movement, holding the rod  248  in a fixed position. 
     It will be noted that although the control valve assembly  228   c  is free to be opened at this time due to a lack of a pneumatic pilot signal from closed piezo valve  212 , there is no exhausting air from either side of the actuator  204 , and there is no way such chambers can be communicated with the exhaust line  246  through the opened control valve assembly  228   c.    
     Alternative Embodiments of Booster Valve Assembly 
     FIG. 9 illustrates an alternative embodiment  300  of the booster valve assembly  46 . The primary difference between the booster valve assembly  46  and the booster valve assembly  300  lies in the internal movable valve component. Whereas in the valve assembly  46  the valve component comprises two major parts, i.e., the diaphragm  76  and the separate piston  82 , in the valve assembly  300 , the valve component comprises a single integrated part wherein the two functions of the piston and diaphragm are combined into a single structure. 
     Valve assembly  300  includes a lower section  302  and a upper section  304 . The lower section  302  includes an internal chamber  306  having a supply port  308  and an exhaust port  310 . The supply port  308  has a valve seat  312  at its upper end. The chamber  306  has an upper enlarged portion  306   a  and a lower, smaller diameter portion  306   b.  A piston  314  reciprocates within the chamber  306 , the larger diameter portion  314   a  thereof being received within the enlarged portion  306   a  and the smaller diameter portion  314   b  thereof being received within the smaller diameter portion  306   b  of the chamber  306 . O-ring seals  316  and  318  encircle the enlarged piston portion  314   a  and the smaller piston portion  314   b  respectively so as to seal the chamber portions  306   a  and  306   b  from one another. A sealing pad  320  at the lower end of the piston  314  faces the valve seat  312  and makes sealing engagement therewith when the piston  314  is in its closed position, shifted downwardly from the opened position illustrated in FIG.  9 . 
     The upper section  304  of valve assembly  300  includes a pilot chamber  322  immediately overlying the upper surface of the piston  314 . An inlet  324  in the upper section  304  is used to communicate the pilot chamber  322  with a source of pressurized air. The surface area of the upper face of the piston  314  exposed to pressurized air in the pilot chamber  322  is considerably larger than the surface area of the bottom face of the piston  314  exposed to pressurized air within the chamber  306 . Consequently, when pressures are equal on opposite faces of the piston  314 , it is shifted down into its closed position. A vent  326  within the lower section  302  communicates the larger diameter chamber portion  306   a  with the atmosphere. 
     It will be appreciated that the valve assembly  300  may be substituted for the assembly  46  in either of the applications illustrated in FIGS. 1-8. The functions and operating sequences are the same in either case. 
     FIG. 10 illustrates another alternative embodiment  400  of booster valve assembly. The valve assembly  400  comprises three primary sections stacked on top of one another, i.e., a bottom section  402 , an intermediate section  404 , and a top section  405 . The bottom section  402  includes an internal chamber  406  having a supply port  408  and an exhaust port  410 . The supply port  408  has a beveled valve seat  412 . A valve ball  414  within the chamber  406  is movable between a closed position engaging the valve seat  412  so as to close the same and an open position in which the ball  414  is up off the seat  412 . When the ball  414  is disengaged from the seat  412 , a flow path is established between the supply port  408  and the exhaust port  410  through the chamber  406 ; when the ball  414  is engaged with the seat  412 , such flow path is closed. 
     A diaphragm  416  is sandwiched between the sections  402  and  404  and seals the chamber  406  of section  402  from a chamber  418  within the intermediate section  404 . A piston  420  is confined within the chamber  404  but is shiftable axially thereof generally toward and away from the valve seat  412 . In FIG. 10, the piston  420  is illustrated in its valve closing position in which it pushes the valve ball  414  into engagement with the valve seat  412 . 
     Sandwiched between the intermediate section  404  and the top section  405  is another flexible diaphragm  421  overlying the piston  420 . When the piston  420  is lowered, a pilot chamber  422  is defined on the upper side of the piston  420  between the diaphragm  421  and the top section  405 , the pilot chamber  422  receiving a supply of pilot air at the operating pressure via an inlet  424  in the top section  405 . A vent port  426  in the chamber  418  below the enlarged pilot end of the piston  420  is adapted to communicate with the atmosphere. The valve assembly  400  may be substituted for the assembly  46  in either of the applications illustrated in FIGS. 1-8. The functions and operating sequences are the same in either case. 
     Referring now to FIG. 11, in an exemplary embodiment, according to the present invention, the valve assembly  46  comprises three primary sections stacked one on top of the other. The three primary sections include a bottom section  502 , an intermediate section  504 , and a top section  506 . The bottom section  502  includes an internal chamber  508 , having a supply port  510  and an exhaust port  512 . A first flexible diaphragm  514  is movable between a closed position, which engagably covers the supply port  510  as shown in FIG. 11, and an open position. In an open position, the diaphragm is disengaged from the supply port  510 , and a flow path is established between the supply port  510  and the exhaust  512 . When the diaphragm is engaged with the supply port  510 , the flow path is closed. 
     The diaphragm  514  is sandwiched between the bottom section  502  and the intermediate section  504 , and seals the chamber  508 , of the bottom section  502 , from a chamber  516  within the intermediate section  504 . The piston  518  is confined within chamber  516 , but is shiftable, axially, toward and away from the diaphragm  514 . 
     Sandwiched between the intermediate section  504  and the top section  506  is another flexible diaphragm  520 . The diaphragm  520  is engagably positioned on the piston  518 . In a closed position, the diaphragm  520  seals off the intermediate section  504  from the upper section  506 . The upper section  506  includes a pilot inlet  524  and pilot chamber  526 . The pilot chamber  526  receives a supply of pilot air at an operating pressure via the inlet  524 . In an open position, as shown in FIG. 11, the diaphragm  520  is positioned to allow a flow path in the pilot chamber. In a closed position, the diaphragm  520  closes the flow path between the pilot inlet  524  and pilot chamber  526 . In addition, a vent  528  may be provided within the intermediate section  504 . The operation of the valve  46  will become clear from the discussion of FIGS. 12-19 to follow. 
     FIGS. 12-19 illustrate two-piezo pneumatic volume booster assemblies for single-acting, as well as double acting, actuators. Specifically, FIGS. 12-15 illustrate two-piezo pneumatic volume booster assemblies for single-acting actuators, and FIGS. 16-19 illustrate two-piezo pneumatic volume booster assemblies for double-acting actuators. As shown in FIGS. 12-19, the valve booster circuit  20  includes four (4) valve assemblies  46   c-   46   f.    
     FIG. 12 illustrates the condition of the control system  14  for moving the piston  94  leftwardly, in the direction of the arrow  530 . This may be selected as the direction for closing the valve  12 , FIG.  1 . In this condition, no electrical signal is supplied from the controller  26 , FIG. 1, to the piezo valves  30  and  32 , such that they remain in their normal, de-energized condition. Thus, the normally open valve  30  allows a pneumatic pilot signal to pass along the output line and into the pilot chamber  526  of valve assemblies  46   c  and  46   d,  which forces the diaphragm  514  of valve assemblies  46   c  and  46   d  to seal the supply port of  46   c  and  46   d.  Supply pressure is also present at the supply port  510  of valve assemblies  46   c  and  46   d.  Because the diaphragm  514  of valve assembly  46   c  is sealed, valve assembly  46   c  does not contribute any pressure to the chamber  92  of the actuator  16  or to valve assembly  46   f.  Valve assemblies  46   e  and  46   f,  in their normally closed state, do not supply any pressure to the chamber  92  of the actuator  16 . The supply pressure at valve assembly  46   e  is blocked because the exhaust  512  of valve assembly  46   e  is plugged, and the supply port  510  of valve assembly  46   d  is sealed by diaphragm  514  of valve assembly  46   d.  Additionally, no supply pressure is input to valve assembly  46   f.  Accordingly, the piston  94  is able to move leftwardly, because there is no opposing pressure. The pressure in chamber  92  enters the supply port  510  of valve assembly  46   f  and is exhausted at the exhaust port  512  of valve assembly  46   f.    
     FIG. 13, illustrates the condition of the control system  14  for moving the piston  94  rightwardly, in the direction of the arrow  532 . This may be selected as the direction for opening the valve  12 , if desired. In this condition, the controller has supplied electrical control signals to the piezo pilot valves  30  and  32 , such that they are both in their energized positions. The only valve assembly that contributes supply pressure to the moving of the piston  94 , is valve assembly  46   c.  Valve assembly  46   c  receives supply pressure at its supply port  510 , which is exhausted, at its exhaust port  512  into the actuator chamber  94 . The pressure is blocked from entering the supply port  510  of valve assembly  46   f  because the supply pressure that enters  46   f  pushes down diaphragm  514  of valve assembly  46   f,  such that it seals the supply port of valve assembly  46   f,  preventing any pressure from being exhausted from valve assembly  46   f.  Consequently, all of the supply pressure that enters  510  of  46   c  is exhausted into chamber  236 , and moves the piston  94  rightwardly. 
     FIG. 14 illustrates the condition of the control system  14  for holding the piston at any location, so as to not move the piston  94  leftwardly or rightwardly. In this condition, the controller  26  has supplied electrical control signals to piezo valves  30  and  32 , such that piezo valve  30  is in its de-energized state and piezo valve  32  is energized. Consequently, supply pressure enters the pilot chamber  526  of each of the valve assemblies  46   c-   46   f.  Although supply pressure is present at supply port  510  of valve assemblies  46   c  and  46   e,  the diaphragm  514  seals the supply port  510  of each of the valve assemblies  46   c  and  46   e,  because of the downward force on the diaphragm  514  of each of  46   c  and  46   e  caused by the pressure in the pilot chamber of each of  46   c  and  46   e.  As a result, the supply pressure at the supply port  510  of valve assembly  46   c  does not flow into the chamber  236  of the actuator  16 . 
     The piston  94  will likewise be held in the last position, if for some reason the control system  14  fails. As illustrated in FIG. 15, for example, if the control system  14  fails, and the normally-closed piezo valve  32  remains open, all of the piezo valve assemblies  46   c-   46   f  will remain open, and the conditions referenced in the description of FIG. 14 will exist. Therefore, the piston will also be held in its last position, and not move either rightwardly or leftwardly. 
     FIGS. 16-19 illustrate a double-acting control system for performing the same functions as the single-acting system of FIGS. 12-15. Instead of a return spring used for the piston  94  of the actuator  16 , both sides of the piston have access to gas under supply pressure atmosphere at appropriate times such that, depending upon which valves are opened and which are closed, the piston either moves to the right or to the left, in response to changes in the volume of pressurized air introduced into the chambers on opposite sides of the piston. The double-acting system of FIGS. 16-19 utilize the same booster valve assembly  46  as the single-acting system FIGS. 12-15, except for the configuration. 
     FIG. 16 illustrates the condition of the control system  14  for moving the piston  94  leftwardly, in the direction of the arrow  530 . This may be selected as the direction for closing the valve. In this condition, electrical signals are supplied from the controller  26  to piezo valves  30  and  32 , such that they remain in their normal de-energized positions. Thus, no electrical signals may be supplied from the controller  26  to piezo valves  30  and  32 . In their de-energized conditions, valve assemblies  46   c  and  46   d  are normally open, and valve assemblies  46   e  and  46   f  are normally closed. Thus, the supply pressure in the pilot chamber  526  of each of valve assemblies  46   c  and  46   d  forces the piston down, and the diaphragm  514  of each of the valve assemblies  46   c  and  46   d  prevent a flow path from the supply  510  of each of the valve assemblies  46   c  and  46   d  to the exhaust of valve assemblies  46   c  and  46   d.  As a result, there is no supply pressure flowing from valve assemblies  46   c  and  46   f  into chamber  236  of the actuator. The supply pressure at the normally closed valve assembly  46   e  has a flow path to the exhaust  512  of valve assembly  46   e  into chamber  242 . There is no flow path created between valve assembly  46   e  and  46   d  because the diaphragm  514  of valve assembly  46   d  seals the supply  510  of valve assembly  46   d.  Consequently, the supply pressure enters chamber  242  and moves the piston  94  leftwardly. The pressure forced out of chamber  236  flows through the supply  510  of valve assembly  46   f  and is exhausted at the exhaust  512  of valve assembly  46   f.    
     FIG. 17 illustrates the condition of the control system  14  for moving the piston  94  rightwardly, in the direction of the arrow  532 . This may be selected as the direction for opening the valve. In this condition electrical signals are supplied to both piezo valves  30  and  32 , and they are energized. When the normally-open piezo valve  30  is energized the supply pressure at the pilot inlet  524  of valve assembly  46   c  and  46   d,  is prevented from entering the pilot chamber  526 . The supply pressure at the supply  510  of valve assembly  46   c,  pushes the piston  518  upward and the supply pressure flows through the exhaust of  46   c  and into chamber  236  of the actuator  16 . The supply pressure from the exhaust  512  of  46   c  does not flow through  46   f  because, in its energized state, valve assembly  46   f  allows supply pressure to enter its pilot chamber and forces the piston down, and forces the flexible diaphragm  514  to seal the supply port  510  of  46   f.  No supply pressure enters chamber  242  of the actuator  16  because in its energized state, valve assembly  46   e  allows supply pressure to enter the pilot chamber  526  and the diaphragm is pushed downward, thus, preventing the supply pressure at the supply port  510  of  46   e  from flowing to chamber  242  or to valve assembly  46   d.  As a result, no supply pressure enters chamber  242 , and the supply pressure entering chamber  236  moves the piston  94  rightwardly. 
     FIG. 18 illustrates the condition of the control system  14  for holding piston  94  at any location. Thus, the piston  94  does not move rightwardly or leftwardly. In this condition, electrical signals are supplied to piezo valves  30  and  32 , such that piezo valve  30  remains de-energized and piezo valve  32  is energized. As a result, each of the valve assemblies  46   c-   46   f  is in an open state and supply pressure at the pilot inlet  524  of each of the valve assemblies  46   c-   46   f  is allowed to enter the pilot chamber of each of valve assemblies  46   c-   46   f.  The supply pressure forces each piston downward, and the flexible diaphragm  514  of each of valve assemblies  46   c-   46   f  seals the supply port  510  of each of the valve assemblies  46   c-   46   f.  Accordingly, the supply pressure at the supply ports  46   c  and  46   e  is prevented from flowing to valve assemblies  46   f  and  46   d,  respectively. In addition, the supply pressure at the supply ports  46   c  and  46   f  is also prevented from flowing into the chambers  236  and  242 , respectively, of the actuator  16 . Because pressure does not enter either of the chambers  236  and  242 , the piston  94  is not forced in any direction. 
     FIG. 19 illustrates the condition when, for some reason, the control  14  fails in its last state, and the normally-closed piezo valve, for example, remains open, and signals are supplied to piezo valves  30  and  32  such that they remain de-energized. Thus, each of the valve assemblies  46   c-   46   f  will be in an open state, as described with reference to FIG. 18, and the piston will also be held in its last state, and not move in any direction. 
     In particular, under the circumstances depicted in FIG. 19, each of the piezo valve assemblies  46   c-   46   f  allows the supply pressure at the pilot inlet  524  of each of  46   c-   46   f  to enter the pilot chamber  526  of each of the valve assemblies  46   c-   46   f.  The supply pressure in each pilot chamber  521 , causes the piston to push each diaphragm  514  downward, sealing the supply chambers  510  of each of valve assemblies  46   c-   46   f.  Like the circumstances described with reference to FIG. 18, when all of the valve assemblies  46   c-   46   f  are in their open state, the piston does not move either leftward or rightward. 
     The above description and drawings are only illustrative of preferred embodiments which achieve the objects, features, and advantages of the present invention, and it is not intended that the present invention be limited thereto. Any modification of the present invention, which comes within the spirit and scope of the following claims, is considered to be part of the present invention.