Abstract:
A double valve having inlet, outlet, and exhaust ports maintains an outlet pressure below one percent of inlet pressure during a faulted state while maintaining a relatively small exhaust for reduced overall valve size. Crossover passages receive inlet pressure through main crossover poppets when the respective valve units are not in a deactuated position. When the valve units are in a deactuated position, then the crossover passages receive inlet pressure through respective bypass passages whose flow rate can be controlled independently from the size of the crossover poppets. The use of bypass passages provides particular benefits for double valves for non-press applications which have a ratio of exhaust flow coefficient to inlet flow coefficient that is less than about 2.5.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to control valves for fluid power systems, and, more specifically, to a double valve for simultaneously achieving a high input to output flow coefficient during its actuated state and a very low outlet pressure during a faulted state. 
     Machine tools of various types operate through a valving system, which interacts with a fluid power-controlled clutch and/or brake assembly. The control valves used to operate these machine tools require the operator to activate two separate control switches substantially simultaneously to ensure that an operator&#39;s hands or other obstructions are away from the moving components of the machine tool when an operating cycle is initiated. Typically, an electronic circuit responsive to the two control switches generates a control signal applied to the actuators for switching the main fluid circuit of the valve to control delivery of compressed air or other fluid to the machine tool to perform its operating cycle. 
     Double valves having two separate valve units operating in parallel within one main valve body have been developed to ensure that a repeat or overrun of a machine tool operating cycle cannot be caused by malfunction of a single valve unit. For example, if one valve unit fails to deactuate at the proper time or if the valve units actuate in a non-synchronous manner, the double valve assumes a fault configuration that continuously diverts the source of fluid power away from the machine tool. A double valve is shown, for example, in commonly assigned U.S. Pat. No. 6,840,258 and U.S. Pat. No. 6,840,259, which are incorporated herein by reference. 
     In known double valves for operating presses and other machine tools, it is desirable to quickly exhaust the pressurized fluid from the outlet when the valve is deactuated so that the outlet pressure rapidly drops to the pressure that allows the brake mechanism to actuate. The coefficient of flow, C v , is a measure of a device&#39;s efficiency in permitting fluid flow, and is calculated based on measured fluid flow rate and the pressure differential across an orifice. The C v  measured in a double valve from the inlet to the outlet is not typically equal to the C v  measured from the outlet to the exhaust. A double valve utilized in press applications typically has a higher outlet-to-exhaust C v  than an inlet-to-outlet C v . 
     Fluid flow through crossover passages within the double valve cause the movement of one valve unit to influence the movement of the other valve unit. The crossover passages are normally pressurized in both the actuated and deactuated states of the valve. In a faulted state of the valve, one crossover is pressurized and the other is opened to the outlet and to the exhaust port through the outlet. Inlet pressure flowing into the crossover that is open to the outlet causes a certain amount of pressure to continue to be present in the outlet during the faulted state. Industry standards state that such pressure should be maintained at less than one percent of the pressure of the fluid supply. Due to the relatively large flow capacity of the exhaust, prior art double valves for press and machine tool applications met the objective of less than one percent. 
     It would be desirable to utilize the lockout capability and dynamic monitoring features of double valves in non-press applications. Such applications would typically employ a continuous and relatively greater C v  between the inlet and the outlet, which is achieved by scaling up the sizes of the inlet, outlet, and valve units. The scaling up of the inlet-to-outlet C v , however, would tend to increase the flow into the outlet during a faulted state which is undesirable for the reasons stated above. This increase is due in part to the pressurization of the crossover passages through respective flow restrictors provided between the crossover passages and the inlet. The flow restrictors are also part of the main flow paths between the inlet and the outlet. When the inlet, outlet, and valve units are scaled up to provide a higher C v , the flow restrictor passages feeding the crossovers likewise are scaled up so that it is not possible to effectively limit or control the flow from the crossover into the outlet during a fault state by scaling these various valve elements. 
     Maintaining an outlet pressure below one percent of inlet pressure could be achieved by scaling up of the exhaust port and the exhaust poppets, but these steps are undesirable because of added cost and an increased-package size for the double valve. 
     SUMMARY OF THE INVENTION 
     The present invention provides modified interaction between the inlet and the crossover passages in a double valve leading to advantages of maintaining an outlet pressure below one percent of inlet pressure during a faulted state while maintaining a relatively small exhaust for reduced overall physical valve size. It provides particular benefits for the use double valves in non-press applications and for the use of double valves having a ratio of exhaust C v  in the deactuated state to inlet C v  in the actuated state that is less than about 2.5. 
     In one aspect of the invention, a valve system comprises a body defining an inlet, an outlet, and an exhaust. A first valve unit is movable to a deactuated position, wherein the deactuated position comprises the first inlet poppet being in its closed position and the first exhaust poppet being in its open position. A second valve unit is movable to a deactuated position, wherein the deactuated position comprises the second inlet poppet being in its closed position and the second exhaust poppet being in its open position. The valve system is in a faulted state when one of the first and second valve units is in a deactuated position when the other of the first and second valve units is not in a deactuated position. 
     The valve system further comprises first and second crossover passages and first and second crossover poppets. A first bypass passage couples the inlet and the first crossover passage, wherein the first bypass passage has a predetermined cross-section to provide a predetermined flow such that when the valve system is in the faulted state and the second valve unit is not in the deactuated position then pressurized fluid passing through the first bypass passage pressurizes the outlet to no more than 1% of the pressure of the source of pressurized fluid. A second bypass passage couples the inlet and the second crossover passage, wherein the second bypass passage has a predetermined cross-section to provide a predetermined flow such that when the valve system is in the faulted state and the first valve unit is not in the deactuated position then pressurized fluid passing through the first bypass passage pressurizes the outlet to no more than 1% of the pressure of the source of pressurized fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a prior art double valve having flow restrictors for feeding the crossovers, with the valve in its deactuated state. 
         FIG. 2  is a cross-sectional view of the prior art double valve of  FIG. 1  in its actuated state. 
         FIG. 3  is a cross-sectional view of the prior art double valve of  FIG. 1  in a faulted state. 
         FIG. 4  is a schematic diagram showing the fluid flow paths through the prior art double valve. 
         FIG. 5  is a graph showing pressure in the crossover passages and the timing chambers during prior art double valve operation. 
         FIG. 6A  is a side, cross-sectional view of the prior art flow restrictor. 
         FIG. 6B  is a top view of the flow restrictor of  FIG. 6A . 
         FIG. 7  is a side, cross-sectional view of a double valve according to a first embodiment of the invention. 
         FIG. 8  is a side, cross-sectional view of a poppet valve and bypass passage according to a second embodiment of the invention. 
         FIG. 9  is a cross-sectional view of a two-position double valve of the present invention using a spring return in a deactuated state. 
         FIG. 10  is a cross-sectional view of the double valve of  FIG. 9  in a faulted state. 
         FIG. 11  is a cross-sectional view of a three-position double valve of the present invention in a faulted state. 
         FIG. 12  is a cross-sectional view of a three-position double valve of the present invention with modified reset-pistons, wherein the valve is in a faulted state. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , a control valve system known in the art and shown in the form of a double valve  10  includes a body  11  having an inlet port  12  leading to an inlet chamber  13 , an outlet port  14  leading to an outlet chamber  15 , and an exhaust port  16  leading to an exhaust chamber  17 . 
     Chambers  13 ,  15 , and  17  are joined by various passages to create bores for receiving a first valve unit  18  and a second valve unit  20 . A first crossover-passage  21  is pressurized by fluid supplied from an external source (not shown) to inlet  12  and then through inlet chamber  13  via a first flow restrictor  32  formed by a shoulder  22  of first valve unit  18  receivable in an orifice  23 . A second crossover passage  25  is pressurized by fluid supplied from an external source to inlet  12  and then through inlet chamber  13  via a second flow restrictor  33  formed by a shoulder  26  of second valve unit  20  receivable in an orifice  27 . First valve unit  18  also includes an inlet poppet  30  at the outlet end of crossover passage  25 . Second valve unit  20  includes an inlet poppet  31  at the outlet end of crossover passage  21 . Valve units  18  and  20  are shown in their deactuated positions with shoulders  22  and  26  in mechanical contact with orifices  23  and  27 , respectively, so that pressurized fluid from inlet chamber  13  flows at a reduced rate as compared to the actuated positions into crossover passages  21  and  25 , respectively. The pressure in crossover passages  21  and  25  helps maintain inlet poppets  30  and  31  seated in the deactuated positions. 
       FIG. 2  shows the double valve with valve units  18  and  20  in their actuated positions. Valve units  18  and  20  are pushed downward in  FIG. 2  by pressure supplied by actuators  6  and  7 , which may preferably comprise pilot valves. The actuator pressure is obtained from timing chambers  8  and  9 , respectively. With valve units  18  and  20  in their actuated positions, fluid flows from inlet  12  through inlet chamber  13 , flow restrictors  32  and  33 , crossover passages  21  and  25 , and inlet poppets  30  and  31  to outlet chamber  15 . 
       FIG. 3  shows double valve  10  in a faulted state wherein first valve unit  18  is in an actuated position and second valve unit  20  is in a deactuated position. First crossover passage  21  is open via the flow restrictor  32  to inlet chamber  13  and the resulting pressure maintains second inlet poppet  31  on seat which, in turn, keeps second valve unit  20  in its deactuated position. Second crossover passage  25  is open via first inlet poppet  30  to outlet chamber  15  and to exhaust chamber  17  through the open exhaust poppet  19 B. Pressure in passage  25  is evacuated which results in the loss of pressure to return chamber  24 B and to timing chamber  9 , which would be used by actuator  7  to actuate second valve unit  20 , thereby keeping the valve  10  in the faulted state until it is positively reset. It is to be understood that double valve  10  is also in a faulted state when first valve unit  18  is in a deactuated position and second valve unit  20  is in an actuated position, and in that event, a flow analysis similar to that above applies. 
     Referring to  FIG. 4 , fluid flow paths of the valve are shown schematically between inlet  12  and outlet  14  for an actuated state. In the actuated state, flow passes through both sides of the double valve  10 . Fluid flow into inlet  12  passes through inlet chamber  13  and flow restrictors  32  and  33  into first crossover passage  21  and second crossover passage  25 , respectively. Fluid flow leaves the crossover passages  21  and  25  through inlet poppet  30  and inlet poppet  31  into outlet chamber  15  and through outlet  14  to the controlled device or circuit. Crossovers  21  and  25  are pressurized during unfaulted valve operation. Pressurized fluid flows from inlet  12  through to inlet chamber  13 , and then on to timing chamber  8  and return chamber  24 A. Pressurized fluid also flows from inlet  12  through inlet chamber  13  and then on to timing chamber  9  and return chamber  24 B. Thus, timing chamber  8  maintains a source of pressurized fluid for actuator  6  that controls actuation of first valve unit  18  and for return chamber  24 A which provides a return force against second valve unit  20 . 
     When valve  10  enters a faulted state, one crossover passage  21  or  25  will be pressurized and the other crossover passage  21  or  25  will be depressurized. Consequently, one timing chamber  8  or  9  and one return chamber  24 B or  24 A will not have the pressure needed for actuation and will thereby maintain valve  10  in the faulted state until a reset operation is performed in response to external application of a resetting force to the valve units  18  and  20 . 
     To further illustrate the pressurization of crossover passages  21  and  25  and timing chambers  8  and  9 ,  FIG. 5  shows a plot of pressure at the timing chambers  8  and  9  and crossover passages  21  and  25  during cycles of valve  10 . A curve  34  shows pressure in the crossover passage  21  between full pressure and approximately zero pressure. Likewise, a curve  35  corresponds to timing chamber  8 , a curve  36  corresponds to crossover passage  25 , and curve  37  corresponds to timing chamber  9 . Initially, crossovers  21  and  25  and timing chambers  8  and  9  are at full inlet pressure. An actuation of the valve  10  begins to occur at time t 1 . Pressure in crossover passages  21  and  25  quickly drops when valve units  18  and  20  begin to move because when the inlet poppets  30  and  31  first begin to open, the exhaust poppets  19 A and  19 B have not yet closed and a path from the crossover passages  21  and  25  to exhaust  16  exists. Since timing chambers  8  and  9  are coupled to crossover passages  21  and  25  through respective restrictors  32  and  33 , pressure in timing chambers  8  and  9  drops more slowly beginning at time t 1 . As valve elements  18  and  20  continue to move, exhaust poppets  19 A and  19 B eventually close and the pressure in crossover passages  21  and  25  quickly returns to full pressure. More gradually, pressure in timing chambers  8  and  9  likewise returns to full pressure. 
     Deactuation of double valve  10  begins at time t 2 . The pressure levels shown in curves  34  and  36  for crossover passages  21  and  25  quickly drop to approximately zero since the returning valve units  18  and  20  have their inlet and exhaust poppets  30 ,  31 ,  19 A and  19 B open substantially simultaneously. Once inlet poppets  30  and  31  close at point  39 , crossover passages  21  and  25  repressurize but the repressurization occurs more slowly than following an actuation operation because crossover passages  21  and  25  are now being repressurized through flow restrictors  32  and  33 . Once again, as crossover passages  21  and  25  re-pressurize, the pressure in timing chambers  8  and  9  recovers to a full pressure. 
     An exemplary fault event involving valve unit  18  is shown occurring at a time t 3 . Valve element  18  fails to move and the pressure in crossover passage  25  and timing chamber  9  fed by crossover passage  25  remain at their full pressure. The pressure in crossover passage  21  and timing chamber  8  both fall to approximately zero since valve unit  20  becomes trapped in an actuated position. 
     As made apparent from  FIG. 5 , flow restrictors  32  and  33  for pressurizing crossover passages  21  and  25  during various cycles of valve  10  need to pressurize crossover passages  21  and  25  quickly enough to keep timing chamber pressure from dropping too much but slow enough that fluid escaping through one timing chamber into the outlet  14  during a faulted state does not exceed 1% of the inlet pressure. 
       FIGS. 6A and 6B  show in greater detail a prior art flow restrictor wherein a spool-type valve unit  18  includes a shoulder  22  for selectively blocking an orifice  23  in valve body  11 . When shoulder  22  is fully received within orifice  23 , a gap  38  provides for the restricted flow which is used to pressurize a respective crossover passage  21  while the corresponding valve unit  18  is in its deactuated position. When orifice  23  is not blocked by shoulder  22 , and when the double valve is in an actuated state, orifice  23  supports approximately one-half of the full fluid flow through the valve to outlet  14 . When a higher flow capacity is required for the valve, then orifice  23  is necessarily larger. As orifice  23  becomes larger, the outer periphery of shoulder  22  likewise becomes larger. The cross-sectional area of gap  38  determines the overall flow into crossover passage  21  in the deactuated position. For a larger orifice, it becomes necessary to maintain a very thin gap  38  to obtain the desired area for gap  38 , which may not be easily obtainable at normal manufacturing tolerances and which may result in impeded movement of valve unit  18 . Therefore, prior art double valves have not been utilized in certain high-flow applications as described above. 
       FIG. 7  shows an inventive structure wherein valve unit  18  is in mechanical contact with a crossover poppet  40 . A seal  41  abuts a sealing surface  42  of valve body  11 ′ to seal an orifice  43  when valve unit  18  is in its deactuated position. In order to supply pressurized fluid to crossover passage  21 , a bypass passage  44  is formed through valve body  11 ′ to connect inlet chamber  13  and crossover passage  21 . Bypass passage  44  can be manufactured with any desired cross-section (e.g., a flow profile such as a circular diameter or other cross-sectional area) for proper pressurization of crossover passage  21  within the time constraints and without introducing excess fluid that would over-pressurize outlet  14  during a fault state. Any time that inlet chamber  13  is pressurized and crossover passage  21  is at a lower pressure, pressurized fluid will flow through bypass passage  44  into crossover passage  21  whether the valve  10 ′ is in an actuated state, a deactuated state, or a faulted state. 
     Referring still to  FIG. 7 , valve unit  20  is in mechanical contact with crossover poppet  153 . A seal  154  abuts a sealing surface  150  to seal orifice  151  when valve unit  20  is in its deactuated position. To supply pressurized fluid to crossover passage  25 , a bypass passage  152  is formed through valve body  11 ′ to connect inlet chamber  14  and crossover passage  25 . Bypass passage  152  can be manufactured with any desired cross-section for proper pressurization of crossover passage  25  within the time constraints and without introducing excess fluid that would over-pressurize outlet  14  during a fault state. Any time that inlet chamber  13  is pressurized and crossover passage  25  is at a lower pressure, pressurized fluid will flow through bypass passage  152  into crossover passage  25  whether valve  10 ′ is in an actuated state, deactuated state, or a faulted state. 
     Operation of the inventive double valve  10 ′ is as follows. During an actuation, valve unit  18  shifts so that fluid flows past an open crossover poppet  40  into crossover passage  21 . If double valve  10 ′ actuates properly, then fluid flows from inlet chamber  13  to crossover passage  21  and then to outlet chamber  15 , and fluid also flows to timing chamber  8  and return chamber  24 A. During a deactuation, valve unit  18  shifts so crossover poppet  40  closes. Fluid then flows from inlet chamber  13  into crossover passage  21  via bypass passage  44 . Fluid continues to flow to timing chamber  8  and return chamber  24 A until they are fully pressurized. 
     Also during an actuation, valve unit  20  shifts so that fluid flows past an open crossover poppet  153  into crossover passage  25 . If double valve  10 ′ actuates properly, then fluid flows from inlet chamber  13  to crossover passage  25  and then to outlet chamber  15 , and fluid also flows to timing chamber  9  and return chamber  24 B. During a deactuation, valve unit  20  shifts so crossover poppet  153  closes. Fluid then flows from inlet chamber  13  into crossover passage  25  via bypass passage  152 . Fluid continues to flow to timing chamber  9  and return chamber  24 B until they are fully pressurized. 
     In the event that valve  10 ′ enters a faulted state with valve unit  18  in its actuated position, then fluid from inlet chamber  13  flows past crossover poppet  40  into crossover-passage  21  so that valve unit  20  is held in a deactuated position by 1) the fluid in crossover passage  21  acting on inlet poppet  31 , and 2) the pressurization of return chamber  24 B. 
     If valve  10 ′ enters a faulted state with valve unit  20  in its actuated position, then fluid from inlet chamber  13  flows past crossover poppet  153  into crossover passage  25  so that valve unit  18  is held in a deactuated position by 1) the fluid in crossover passage  21  acting on inlet poppet  30 , and 2) the pressurization of return chamber  24 A. 
     In the event that valve  10 ′ enters a faulted state with valve unit  18  in its deactuated position, then fluid is blocked at crossover poppet  40 . However, fluid continues to flow through bypass passage  44  into crossover passage  21 . This fluid exits crossover passage  21  to atmosphere via inlet poppet  31  and exhaust poppet  19 B. As a result of the venting to atmosphere, pressure drops in timing chamber  8  so that actuator  6  is unable to move valve unit  18  out of its deactuated position. Since crossover poppet  40  is closed, the main source of fluid flowing into outlet chamber  15  is through bypass passage  44 , and the pressurization of outlet  14  is held to below 1% of the pressure in inlet chamber  13 . 
     If valve  10 ′ enters a faulted state with valve unit  20  in its deactuated position, then fluid is blocked at crossover poppet  153 . Fluid continues to flow through bypass passage  152  into crossover passage  25 . This fluid exits crossover passage  25  to atmosphere via inlet poppet  30  and exhaust poppet  19 A. As a result of the venting to atmosphere, pressure drops in timing chamber  9  so that actuator  7  is unable to move valve unit  20  out of its deactuated position. Since crossover poppet  153  is closed, the main source of fluid flowing into outlet chamber  15  is through bypass passage  152 , and the pressurization of outlet  14  is held to below 1% of the pressure in inlet chamber  13 . 
       FIG. 8  shows an alternative embodiment for crossover poppet  40 ,  153  and for bypass passage  44 ,  152  wherein a bypass passage  45  is provided in a crossover poppet  46 . Depending upon the presence of a spacer  47  which may be fixedly or slideably mounted to a shaft  48 , a passage  49  may be utilized within spacer  47  to complete the bypass between inlet chamber  13  and crossover passage  21 ,  25 . Any time that inlet chamber  13  is pressurized and crossover passage  21 ,  25  is at a lower pressure, pressurized fluid will flow through bypass passage  45  (and passage  49 , if used) into crossover passage  21 ,  25  whether the valve is in an actuated state, a deactuated state, or a faulted state. 
     Overall valve operation using the embodiment of  FIG. 8  is similar to that as described in connection with the embodiment illustrated in  FIG. 7 . During an actuation, valve unit  18  shifts so that fluid flows from inlet  12  into inlet chamber  13  and past open crossover poppet  46  into crossover passage  21 . Fluid also flows from inlet  12  into inlet chamber  13  to timing chamber  8  and return chamber  24 A. If double valve  10 ′ actuates properly, then fluid from crossover passage  21  flows to outlet chamber  15 . During a deactuation, valve unit  18  shifts so crossover poppet  46  closes. Fluid then flows from inlet  12  into inlet chamber  13  and into crossover passage  21  via bypass passages  45  and  49 , if present. Fluid also flows from inlet  12  into inlet chamber  13  on to timing chamber  8  and return chamber  24 A until they are fully pressurized. A similar fluid flow description during an actuation applies with respect to valve unit  20 . 
     In the event that valve  10 ′ enters a faulted state with valve unit  18  in its actuated position, then fluid from inlet chamber  13  flows past crossover poppet  46  into crossover passage  21  so that valve unit  20  is held in a deactuated position by 1) the fluid in crossover passage  21  acting on inlet poppet  31  and 2) the pressurization of return chamber  24 A. A similar flow description applies in the event that valve  10 ′ enters the faulted state with valve unit  20  in its actuated position. 
     In the event that valve  10 ′ enters a faulted state with valve unit  18  in its deactuated position, then fluid is blocked at crossover poppet  46 . However, fluid continues to flow through bypass passages  45  and  49  into crossover passage  21 . This fluid exits crossover passage  21  to atmosphere via inlet poppet  31  and exhaust poppet  19 A. As a result of the venting to atmosphere, pressure drops in timing chamber  8  so that actuator  6  is unable to move valve unit  18  out of its deactuated position. Since crossover poppet  46  is closed, the main source of fluid flowing into outlet chamber  15  is through bypass passages  45  and  49 , and the pressurization of outlet  14  is held to below 1% of the pressure in inlet chamber  13 . A similar flow description applies in the event that valve  10 ′ enters the faulted state with valve unit  20  in its actuated position. 
       FIG. 9  shows an alternative embodiment of a double valve  100  employing a spring return which may be useful in many non-press applications. The double valve  100  includes first and second valve units  50  and  51  having inlet poppets  52  and  53 , respectively. First and second crossover passages  54  and  55  are fed by bypass passages  56  and  57  through the main valve body. Crossover poppets  58  and  59  are closed when the valve units are in their deactuated position. Springs  60  and  61  are provided in engagement with valve units  50  and  51  to urge them into their deactuated position. With valve  100  in the actuated-state, if the actuators  105 ,  106  become deactuated, valve units  50  and  51  will move to their deactuated positions under spring, force. As in the previous embodiments, any time that inlet pressure is higher than crossover pressure, pressurized fluid will flow-through bypass passages  56  and  57  whether valve  100  is in an actuated state, a deactuated state, or a faulted state. Otherwise, fluid flows within valve  100  in a similar manner as described above with reference to  FIGS. 7 and 8 . 
       FIG. 10  shows valve  100  of  FIG. 9  wherein a malfunction occurs during valve actuation such that a faulted state results. Valve unit  51  is in an actuated position and valve unit  50  is in a deactuated position, so that the pressure in crossover  54  drops and the pressure in a corresponding timing chamber (not shown) continues to drop. When the timing chamber pressure drops to approximately 50% of the inlet pressure, valve unit  50  will remain in the deactuated position since the corresponding actuator  105  no longer has sufficient pressure to create an actuation. When both actuators  105  and  106  are deactuated, the pair of springs  60  and  61  move the internal valve units  50  and  51  to their deactuated positions and valve  100  can automatically begin another cycle since the crossover passages  54  and  55  become re-pressurized via bypass passages  56  and  57 . 
       FIG. 11  shows another embodiment employing a 3-position double valve  101  wherein return springs  65  and  66  are configured to only return the valve units  69 A and  69 B to an intermediate position between the actuated and deactuated positions.  FIG. 11  shows a double valve  101  in a faulted state. When all pressure is removed from double valve  101  in the faulted state, it remains in the faulted state by virtue of a valve unit  69 A or  69 B which is already at the intermediate position or the actuated position becoming balanced at the intermediate position. In order to reset double valve  101 , pistons  67  and  68  are provided in alignment with valve units  69 A and  69 B and are selectively controlled via pressure from a reset pilot  70 , which is supplied through a coupling block  71  to pistons  67  and  68 . More specifically, when double valve  101  has entered a faulted state and it is desired to reset it to a deactuated state, then reset pilot  70  is actuated so that pressurized fluid is supplied against pistons  67  and  68 , forcing them upward against valve units  69 A and  69 B so that they are restored to their deactuated positions. 
       FIG. 12  shows a further embodiment incorporating an additional protection feature wherein a double valve  75  remains in a faulted state even after application of reset pressure unless the reset pressure is turned off prior to subsequent operation of the valve  75 . Double valve  75  shown in a faulted state has a first reset piston  76  and a second reset piston  77 . Reset piston  76  has a surface  78  for receiving reset pressure from a source, such as a reset pilot  85 , through a coupling block  79 . Piston  77  has a surface  80  likewise receiving reset pressure from source  85  through coupling block  79 . Surface  78  has an area which is large compared to a surface of actuating piston  81  so that the first valve unit  82  moves into the deactuated position during a reset even if pressure is applied to actuating piston  81 . Surface area  80  of second reset piston  77  is small compared to the area of an actuating piston  83  of second valve unit  84 . Thus, during a reset operation, if actuators  86  and  87  are turned on and pressure is applied against actuating piston  83 , then reset piston  80  generates insufficient force to move valve unit  84  into its deactuated position and it instead moves to the actuated position. Thus, valve  75  will not cycle with reset pressure from source  85  turned on. Any time that valve  75  is receiving reset pressure and actuators  86  and  87  are also energized, it enters the faulted state. Moreover, if valve  75  is in its deactuated position with a source of pressurized fluid being supplied to inlet  88 , turning off the source and then turning it back on will result in valve  75  remaining in the deactuated position. If valve  75  is in the faulted state, then turning the source off and then back on will result in valve  75  remaining in the faulted state. Consequently, valve  75  cannot be reset by turning off and on the source of pressurized fluid to inlet  88 . Since valve  75  likewise cannot cycle with pressure from source  85  turned on, the user will be required to correct any malfunctioning valve unit  75  before normal operation can continue.