Patent Abstract:
An integrated three function valve (ITFV) that combines the functions of a bypass valve (e.g.  183   a,    185   a,    161   a ) into a single assembly is disclosed. The ITFV allows continued operation after any two failures. With the integrated three function valve, no combination of electrical or hydraulic component failures, regardless of expected failure rate, will prevent an actuator from continued safe operation

Full Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to actuation systems for aircraft. In particular, the present invention relates to fly-by-wire rotor actuation systems for tiltrotor aircraft.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    Compared with conventional fixed wing commercial aircraft, the task of designing helicopter flight control systems to avoid flight critical failure modes is considerably more challenging. For typical fixed wing commercial aircraft, flight control system redundancy can be provided through the application of multiple independently actuated surfaces. An example of this methodology is the use of two or three ailerons per wing. Configured in this manner, continued safe flight is achieved in the event of an aileron becoming uncontrollable or seized.  
           [0003]    On the other hand, in helicopters and tiltrotor aircraft, application of multiple independently actuated rotors to provide flight control system redundancy is not a viable option. Typically in rotor systems, multiple flight control actuators are structurally or mechanically linked together to provide redundancy of actuation. This methodology provides for continued safe flight following the failure of a system or actuator, except for cases where a failed actuator cannot be freely backdriven, or bypassed, by the remaining actuators. For these failure modes, the result will be loss of rotor control. Therefore, it is a critical requirement for rotor control actuators that their designs incorporate devices that can reliably ensure that a failed actuator can be overridden. For hydraulic actuators, this implies ensuring a bypass condition.  
           [0004]    In conventional fly-by-wire (FBW) rotor control actuation, triple hydraulic redundancy is achieved by employing a dual tandem configuration, i.e., two rams end to end. A switching valve is used to connect two independent hydraulic sources to one of the tandem rams. This ram is controlled by dual redundant electrohydraulic valves and dual redundant signals from a flight control computer (FCC). Typically, the tandem ram configuration is supported with spherical bearings or universal joints to minimize structural bending loads.  
           [0005]    Referring to FIG. 1 in the drawings, another type of FBW rotor control system, one having three hydraulic rams arranged side-by-side in a triangular pattern, is illustrated. As is shown in FIG. 1, a rotor control system  10  for the left hand nacelle of a tiltrotor aircraft includes a triplex collective actuator  11  in which each of three rams  13 ,  15 ,  17  is hydraulically powered and controlled from one of three independent manifolds, hydraulic systems, and FCC&#39;s. By using three rams instead of two, this configuration has the advantage of eliminating the need for hydraulic switching valves, control systems, and their associated failure modes. Degradation of actuator load/rate capacity following a single failure and the severity of transient motions from control failures are also improved by having two rams continuing to operate instead of only one following a single failure.  
           [0006]    Referring now to FIG. 2 in the drawings, each collective actuator hydraulic manifold comprises an electrohydraulic servo-valve (EHSV)  21 , a bypass valve  23  controlled by a solenoid valve  25  to disengage a cylinder  27  in the event of a fault, a differential pressure sensor  29  to allow force balancing across the triple active cylinders to minimize bending loads (a load control concept used in various configurations on other aircraft), and a pressure relief valve  31  to limit cylinder pressures during adverse failure conditions. For each system, four linear variable displacement transducers (LVDT)  33   a ,  33   b ,  33   c ,  33   d  are fitted for control and monitoring of the spool of EHSV  21 , the spool of bypass valve  23 , the spool of differential pressure sensor  29 , and a ram piston  35  of cylinder  27 , respectively.  
           [0007]    Differential pressure sensor  29  includes spring centered pistons  37  with one side ported to extend pressure and the other to retract pressure. The displacement of sensor pistons  37  is measured by LVDT  33   c  and is proportional to the delta pressure acting on ram piston  35 .  
           [0008]    Although the foregoing designs represent considerable advancements in the area of rotor control systems, significant shortcomings remain.  
         SUMMARY OF THE INVENTION  
         [0009]    There is a need for a rotor control system for an aircraft that provides full triple redundancy.  
           [0010]    Therefore, it is an object of the present invention to provide a rotor control system that provides full triple redundancy.  
           [0011]    This object is achieved by providing an integrated three function valve (ITFV) that combines the functions of a bypass valve, a pressure relief valve, and a differential pressure transducer into a single assembly. The ITFV of the present invention allows continued operation after any two failures. With the integrated three function valve of the present invention, no combination of two electrical or hydraulic component failures, regardless of expected failure rate, will prevent an actuator from continued safe operation.  
           [0012]    The present invention provides for safe operation of an aircraft by ensuring that electrical and hydraulic component failures do not lie dormant in the actuator, such as when a component is not used during normal operation, or when a component is not capable of being periodically tested. For example, the present invention provides for safe operation of an aircraft in the presence of the following dormant failures:  
           [0013]    1. An EHSV failure, hardover or at null position, in combination with failure to bypass the cylinder. The degradation in load rate capacity resulting from the combination of driving flow across the relief valve on the failed cylinder and increased friction from bending is not acceptable.  
           [0014]    2. An EHSV sticking at null position combined with the sticking of the pressure relief valve (PRV), a dormant failure. With this type of failure, acceptable ram pressures can be exceeded. Although this condition can be overridden by a bypass valve, it is considered that the total time required between failure detection and achieving bypass is not adequate to protect the failed ram from a spike of overpressurization. This condition is a greater problem for the triplex ram than for the duplex ram, as the peak load can be three times the design stall, i.e., two active rams at stall plus air load; compared to twice the stall, i.e., one active ram at stall plus air load. Therefore, with an aircraft system operating pressure of 21.68 MPa (3,000 psi), this failure combination can generate a pressure spike of 62.05 MPa (9,000 psi). This type of failure on a triple ram system can exceed normal design burst conditions of 1.71 MPa (7,500 psi) burst pressure required for 21.68 MPa (3,000 psi) system actuators, unless excessive weight is added to accommodate the failure mode.  
           [0015]    3. A loss of two hydraulic or two FCC systems combined with the dormant failure of a PRV. The ram controlled by the one remaining functioning system will be required to react all flight loads. If the ram on the remaining system contains a PRV that opens at pressures below system operating pressure, control of the actuator can be lost. A preflight built-in test (PFBIT) can be incorporated into the FCC&#39;s to load each ram to stall in order to confirm that the PRV&#39;s do not open below system pressure. However, this subjects the actuator and structure to severe fatigue loads.  
           [0016]    4. In the event of the loss of one ram due to FCC or hydraulic system failure, the two functioning rams should equally support actuator flight loads. However, if a failure mode of a delta pressure sensor results in a pressure indication opposite in direction of the actual ram load, a force fight between the remaining two actuators can result in the frequency response of the actuator becoming severely degraded. In addition to these double failure modes, a change of sensitivity of a differential pressure sensor can lead to increased fatigue due to an increased force fight between the triple actuators.  
           [0017]    The present invention provides many significant benefits and advantages, including: (1) bypass valve function is redundant and independent; (2) confirmation of redundant bypass valve operation is available during PFBIT, and continuous health monitoring is possible in certain applications; (3) PRV function is redundant and independent; (4) confirmation of PRV operation is available during PFBIT, and continuous health monitoring is possible in certain applications (5) redundant delta pressure sensors are provided to permit continuous cross checking of accuracy; (6) delta pressure sensors are robust and free from common mode changes in accuracy; and (7) additional actuator redundancy does not require any additional wiring or changes to existing FCC interfaces.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIGS. 1A and 1B are perspective views of a prior-art FBW rotor control system having three hydraulic rams arranged side-by-side in a triangular pattern.  
         [0019]    [0019]FIG. 2 is a schematic of the collective actuator hydraulic manifold for the FBW rotor control system of FIG. 1.  
         [0020]    [0020]FIGS. 3A-3F are orthographic views of a ITFV collective actuator according to the present invention.  
         [0021]    [0021]FIG. 4 is a partial perspective view of a left hand nacelle of a tiltrotor aircraft having ITFV&#39;s according to the present invention.  
         [0022]    [0022]FIGS. 5-12 are schematics of the preferred embodiment of the ITFV according to the present shown in various operating states.  
         [0023]    [0023]FIG. 13 is a table illustrating ITFV component parameters for bypass time for the present invention.  
         [0024]    [0024]FIG. 14 is a table illustrating calculated time to bypass at selected operating temperatures for the present invention.  
         [0025]    [0025]FIG. 15 is a table illustrating delta pressure sensor accuracy for the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    Referring now to FIGS. 3A-3F in the drawings, the preferred embodiment of an ITFV  111  according to the present invention is illustrated. ITFV  111  utilizes two hydraulic spools to combine the functions of bypass valve, pressure relief valve, and delta pressure transducer into a simple and compact assembly. When used as a matched pair in a collective actuator, ITFV&#39;s  111  provide redundant bypass valve, pressure relief valve, and delta pressure transducer functionality. This allows certain redundancy and monitoring requirements to be met with fewer springs, hydraulic spools, and pistons than other actuator designs.  
         [0027]    Referring now to FIG. 4 in the drawings, three collective actuators  113  are shown installed in the left hand nacelle of a tiltrotor aircraft. Each collective actuator  113  utilizes a matched pair of ITFV&#39;s  111 , and is configured to create unequal extend and retract areas to better match predicted flight loads and reduce transient effects. Flight loads are predominantly in tension. In order to minimize ram bending and associated frictional effects, collective actuators  113  are positioned in a plane side-by-side, and are interconnected by a rigid bracket  115  on the control surface via spherical bearings  117  on each collective actuator  113 . This configuration and structural attachment permits ITFV&#39;s  111  to fit within the available aircraft space envelope.  
         [0028]    Referring now to FIGS. 5-12 in the drawings, a dual ITFV manifold assembly  151  having a matched pair of ITFV&#39;s  111   a  and  111   b  according to the present invention is shown schematically to illustrate the operation of the present invention. Although it will be appreciated that other porting configurations through ITFV&#39;s  111   a  and  111   b  are possible, the configuration shown in FIGS. 5-12 is particularly well suited for use in the tiltrotor aircraft application of the present invention. Dual ITFV manifold  151  contains an EHSV  153  with an LVDT  155  to monitor spool position. ITFV  111   a  is comprised of a pilot spool  157   a , a primary spool  159   a , an LVDT  161   a  that senses primary spool position, and two spool centering springs  163   a  and  165   a . ITFV  111   b  is comprised of a pilot spool  157   b , a primary spool  159   b , an LVDT  161   b  that senses primary spool position, and two spool centering springs  163   b  and  165   b . In the tiltrotor aircraft application used herein, this configuration is possible without requiring any new wiring or FCC interface changes.  
         [0029]    Two solenoid valves  167  and  169  are utilized to control the bypass function for the pair of ITFV&#39;s  111   a  and  111   b  pair. Solenoid valve  167  is deenergized “off” to drive the primary spool engaged position, and solenoid valve  169  is deenergized “on” to control chip shear pressure. The coils of each of the solenoid valves  167  and  169  are wired in parallel. Thus, no FCC or wiring changes are required. Solenoid valves  167  and  169  are configured this way to achieve a high chip shear capability, to bypass, when no failures exist, while ensuring that bypass can be achieved should either solenoid valve  167  or  169  fail to deenergize.  
         [0030]    In FIG. 5, ITFV assembly  151  is shown in a bypass mode, i.e., solenoid valves  167  and  169  in the “off” position. In the bypass mode with the hydraulic system at operating pressure, system flow is ported to the left side of primary spools  159   a  and  159   b , through primary spool solenoid valve  167 , to the pilot solenoid valve  169 , and to EHSV  153 . With solenoid valves  167  and  169  deenergized, primary spools  159   a  and  159   b  and pilot spools  157   a  and  157   b  are held in bypass position by springs  163   a ,  163   b ,  165   a , and  165   b . Primary spools  159   a  and  159   b  are also held in bypass position by system pressure.  
         [0031]    Primary spool LVDT&#39;s  161   a  and  161   b  provide confirmation to the FCC (not shown) of bypass mode. Flow from a “retract” EHSV control port  171  to the retract side  173  of an actuator cylinder  175  is blocked by the primary spool  159   a . Flow from an “extend” EHSV control port  177  to an extend side  179  of actuator cylinder  175  is blocked by the primary spool  159   b . This configuration isolates EHSV  153  from actuator cylinder  175  in bypass mode, but permits EHSV  153  to be cycled for PFBIT and independent performance checks. In bypass mode, primary spools  159   a  and  159   b  connect both extend and retract cylinder ports  171  and  177  to return and, indirectly, to each other. This allows a shaft  181  of actuator cylinder  175  to be moved freely by other actuators in bypass mode. Because the unequal area pistons are utilized in ITFV&#39;s  111 , makeup flow from return prevents cavitation.  
         [0032]    Referring now to FIG. 6 in the drawings, ITFV assembly  151  is shown with EHSV  153  at a null, or center, position with solenoid valves  167  and  169  in the engaged “on” mode. With EHSV  153  at null, system flow to the ram ports is blocked by EHSV  153 . In the engaged mode with pilot solenoid valve  169  energized, system flow is ported from pilot solenoid valve  169  to the right side of pilot spools  157   a  and  157   b , driving pilot spools  157   a  and  157   b  left to an engaged position. Engagement of primary solenoid valve  167  replaces system pressure with return pressure on the left side of primary spools  159   a  and  159   b . This allows primary spools  159   a  and  159   b  to be centered by the equal preload forces of springs  163   a ,  165   a ,  163   b , and  165   b , respectively.  
         [0033]    LVDT&#39;s  161   a  and  161   b  attached to primary spools  159   a  and  159   b  provide confirmation to the FCC that the engaged mode has been achieved. In engaged mode, flow from the retract EHSV control port  171  is connected through a left side differential area chamber  183   a  of primary spool  159   a  to the retract side  173  of actuator cylinder  175 . Left side differential area chamber  183   a  of primary spool  159   a  is also ported to a right side differential area chamber  185   b  of primary spool  159   b . Symmetrically, flow from extend EHSV control port  177  is connected through left side differential area chamber  183   b  of primary spool  159   b  to the extend side  173  of actuator cylinder  175 . Left side differential area chamber  183   b  of primary spool  159   b  is also ported to a right side differential area chamber  185   a  of primary spool  159   a.    
         [0034]    Left and right side differential area chambers  183   a ,  185   a ,  183   b , and  185   b  of primary spools  159   a  and  159   b  are created by the difference in diameter between three center lands and smaller end lands of primary spools  159   a  and  159   b . The smaller end lands of primary spools  159   a  and  159   b  are equal in diameter. Therefore, the differential areas on the left and right sides of primary spools  159   a  and  159   b  are equal. When EHSV  153  is at null, pressures at left and right differential area chambers  183   a ,  185   a ,  183   b , and  185   b  are equal. Therefore, hydraulic forces on primary spools  159   a  and  159   b  are balanced, and primary spools  159   a  and  159   b  remain at the spring centered positions. These positions, as indicated by primary spool LVDT&#39;s  161   a  and  161   b , are interpreted by the FCC as zero differential pressure between extend and retract sides of actuator cylinder  175 .  
         [0035]    Referring now to FIG. 7 in the drawings, ITFV assembly  151  is shown engaged with solenoid valves  167  and  169  energized, and with EHSV  153  responding to an “extend” computer command from the FCC. EHSV  153  directs system flow to extend side  173  of actuator cylinder  175  and connects retract side  179  of actuator cylinder  175  to return. Pressure generated at extend side  173  of actuator cylinder  175  will be proportional to any load restricting actuator movement. Increasing extend side pressure in left side differential area chamber  183   b  of primary spool  159   b  and return pressure in right side differential chamber  185   b  of primary spool  159   b  results in a net force that displaces primary spool  159   b  to the right, as is indicated by arrow A. Because primary spool  159   b  is centered by fixed rate identical springs  163   b  and  165   b , spool displacement is proportional to the differential pressure between extend side  173  and retract side  179  of actuator cylinder  175 .  
         [0036]    Displacement of primary spool  159   b  resulting from increased extend pressure causes LVDT  161   b  to generate an indication to the FCC of the compression load acting on the actuator. Under increasing extend side pressure, primary spool  159   a  reacts identical to primary spool  159   b , with the exception of direction of displacement, as indicated by arrow A. Because right side differential area chamber  183   a  of primary spool  159   a  is ported to extend pressure and left side differential area chamber  185   a  is connected to return pressure, primary spool  159   a  displaces to the left with increasing differential pressure between extend side  173  and retract side  179  of actuator cylinder  175 .  
         [0037]    Referring now to FIG. 8 in the drawings, the relief valve functions of ITFV&#39;s  11   a  and  111   b  will be described. Because ITFV&#39;s  111   a  and  111   b  operate in opposite directions, when acting as delta pressure sensors, the possibility of a common mode failure affecting both sensors accuracy equally is extremely remote. Except for a reversal of the direction of spool displacement and indicated load resulting from differential pressure between extend side  173  and retract side  179  of actuator cylinder  175 , response of primary spools  159   a  and  159   b  with increasing retract side cylinder pressure is the same as described for increasing extend pressure. In the event actuator cylinder  175  is subjected to external loads exceeding acceptable structural limits, primary spools  159   a  and  159   b  function as relief valves to vent excessive extend or retract cylinder pressure to return.  
         [0038]    Referring now to FIG. 9 in the drawings, ITFV assembly  151  is shown engaged with solenoid valves  167  and  169  energized, and with EHSV  153  in the null position blocking extend port  177  and retract port  171  from return or system pressure. When actuator cylinder  175  is subjected to excessive external compressive load, pressure generated at extend side  173  of actuator cylinder  175  exceeds the relief valve function opening pressure of 27.58 MPa (4,000 psi). At 27.58 MPa (4,000 psi) extend side pressure, left side differential area chamber  183   b  of primary spool  159   b  generates a net force that displaces primary spool  159   b  sufficiently to the right to uncover ports and vent excessive pressure to return. Primary spool  159   a  reacts identically to primary spool  159   b , with the exception of the direction of displacement.  
         [0039]    Referring now to FIG. 10 in the drawings, response of primary spools  159   a  and  1 - 59   b  with 27.58 MPa (4,000 psi) retract side cylinder pressure caused by excessive tension load is the same as described for 27.58 MPa (4,000 psi) extend pressure, except for a reversal of the direction of spool displacement resulting from differential pressure between extend side  173  and retract side  179  of actuator cylinder  175 . Cavitation protection during pressure relief is provided when paired ITFV&#39;s  111   a  and  111   b  are incorporated on unequal area cylinders. Primary spools  159   a  and  159   b , venting excessive cylinder pressure in the same direction as the normal bypass function, connect both cylinder ports to return to prevent cavitation. Because the same centering springs  163   a ,  165   a ,  163   b , and  165   b  and hydraulic components that support the delta pressure measurement function are also used to provide relief of excessive pressure, the integrity of the ITFV relief valve function is continuously monitored in flight.  
         [0040]    When changing from the engaged to the bypass mode, as illustrated in FIGS. 6 and 5, respectively, solenoid valves  167  and  169  are deenergized. This causes preload in centering springs  163   a ,  165   a ,  163   b , and  165   b  to return pilot spools  157   a  and  157   b  to their disengaged stops. Concurrently, the solenoid valve  167  feeds system pressure, if available, to the left end of both primary spools  159   a  and  159   b . System pressure acting on the end areas of primary spools  159   a  and  159   b  generates an 890 N (200 lb) force to move primary spools  159   a  and  159   b  to the right. This force works in combination with preloaded springs  163   a ,  165   a ,  163   b , and  165   b  to provide primary spool chip shear capability when bypass is commanded. This chip shear capability ensures that debris that is not filtered by a supply line filter  180  does not prevent movement of primary spools  159   a  and  159   b . Filter  180  is preferably a 100-micron filter.  
         [0041]    For the failure mode where one of primary spools  159   a  or  159   b  sticks and fails to move into bypass position, the other primary spool  159   a  or  159   a  provides the conditions for bypass. In the event that solenoid valve  167  fails to port system pressure to primary spools  159   a  and  159   b , the preload of centering springs  163   a ,  165   a ,  163   b , and  165   b  is sufficient to move primary spools  159   a  and  159   b  into bypass position. For the failure modes where solenoid valve  169  fails to open, or a pilot spool  157   a  or  157   b  sticks in the engaged position, the 890 N (200 lb) force from system pressure acting on the left end of primary spools  159   a  and  159   b  is sufficient to compress centering springs  163   a ,  165   a ,  163   b , and  165   b  and move pilot spool  157   a  and  157   b  into the bypass position. Because the same centering springs  163   a ,  165   a ,  163   b , and  165   b  and hydraulic components supporting the delta pressure measurement function are also used to provide bypass, the integrity of the ITFV bypass function is continuously monitored in flight, with the exception of the solenoid valves.  
         [0042]    Because deenergizing either solenoid valve  167  or  169  will cause both ITFV&#39;s  111   a  and  111   b  to enter bypass rnode, failure of one of these solenoid valves  167  or  169  could lay dormant. To permit the PFBIT to identify if either solenoid valves  167  or  169  has failed, ITFV assembly  151  is configured to stop primary spools  159   a  and  159   b  just short of the normal bypass position if either solenoid valve  167  or  169  has failed. The correspondingly incorrect LVDT output for the bypass position, provides the FCC with an indication of a failure of either solenoid valve  167  or  169 .  
         [0043]    Referring now to FIGS. 11 and 12 in the drawings, implementation of this feature is as follows:  
         [0044]    1. As shown in FIG. 11, in the case where solenoid valve  169  has failed, pilot spools  157   a  and  157   b  remain in the engaged position. To halt primary spools  159   a  and  159   b  just short of the full bypass position, the right ends of primary spools  159   a  and  159   b  contact stops  186   a  and  186   b  that extend out from the left ends of pilot spools  157   a  and  157   b . System pressure acting on the left end of primary spools  159   a  and  159   b  is sufficient to compress centering springs  163   a ,  165   a ,  163   b , and  165   b , but not enough to move pilot spools  157   a  and  157   b.    
         [0045]    2. As is shown in FIG. 12, in the case where solenoid valve  167  has failed, pilot spools  157   a  and  157   b  move to the disengaged position. To halt primary spools  159   a  and  159   b  just short of the full bypass position, Bellville spring washers  189   a  and  189   b  are employed on primary spool bypass position stops  187   a  and  187   b . The spring rate of washers  189   a  and  189   b  is sufficient to halt primary spools  159   a  and  159   b  from achieving a normal bypass position, unless system pressure is supplied through solenoid valve  167  to primary spools  159   a  and  159   b.    
         [0046]    Although the present invention combines the three separate functions of bypass valve, delta pressure transducer, and PRV into a single valve assembly, the flexibility to refine components to meet selected performance requirements is not lost. For example, it is preferred that primary spools  159   a  and  159   b  fit with very close tolerances in order to minimize internal leakage; however, in order to achieve adequate delta pressure sensor accuracy, a loose spool fit that minimizes frictional effects is desired. Also, although both leakage and friction can be reduced by decreasing the ITFV spool diameter, the desire to have a minimum chip shear force of 890 N (200 lb) during bypass engagement requires a relatively large spool diameter. The following is an analysis of how bypass valve response time and delta pressure transducer accuracy can be selectively tailored by utilizing ITFV&#39;s  111   a  and  
         [0047]    Referring now to FIGS. 13 and 14 in the drawings, selected parameters for bypass time are illustrated in a tables  201  and  301 . The severity of failure transient actuator motions is directly related to the speed in which an actuator can be placed into bypass. It is preferred that ITFV&#39;s  111   a  and  111   b  have a bypass time limit of 30 milliseconds. Although the large size of the primary spools  159   a  and  159   b  would make ITFV&#39;s  111   a  and  111   b  slower to respond than smaller dedicated bypass valve spools, because primary spools  159   a  and  159   b  also function as a delta pressure sensors and PRV&#39;s, force fight loads induced by EHSV  153  or other failures effectively preposition one of primary spools  159   a  or  159   b  closer to the bypass pass position. In addition, full bypass position is not required to disengage the failed actuator cylinder  175 . Because the pressure relief ports are uncovered as primary spools  159   a  and  159   b  move to the bypass position, any ram force fight is significantly reduced at that valve position. Thus, the speed in which the dual ITFV assembly  151  can achieve effective bypass of a failed actuator cylinder  175  under force fight conditions is as fast as, if not faster than, the conventional designs. In FIG. 14, table  301  illustrates calculated times for the selected operating temperatures. The times listed are all inclusive of solenoid valve switching time.  
         [0048]    Referring now to FIG. 15 in the drawings, delta pressure sensor accuracy is illustrated in a table  401 . Accuracy of differential pressure setting is determined by dimensional variations resulting from a combination of manufacturing tolerances, such as LVDT sensitivity, spring rate, spool and sleeve diameters, and differential thermal expansion. Because the LVDT&#39;s  161   a  and  161   b  are also used to indicate the bypass positions of primary spools  159   a  and  159   b , only 60% of the stroke of LVDT&#39;s  161   a  and  161   b  are used to measure delta pressure.  
         [0049]    Generally, thermal effects influencing delta pressure accuracy are considered as absolute values. It is preferred that the sensor have an absolute accuracy requirement of ±2,068 kPa ([±300 psi]; a worst case ±4,137 kPa [±600 psi] between two sensors). However, because the purpose of the delta pressure sensors is to balance ram pressures relative to each other, it is acceptable to allow greater deviation in the absolute accuracy of the sensors, as long as the accuracy of the sensors relative to each other is maintained. According to a thermal analysis of the hydraulic systems, without failures, the worst case maximum difference between the three return system temperatures should never exceed 10° C. (50° F.). Therefore, between actuators, the difference in ITFV delta pressure readings when subjected to fluid temperatures within 10° C. (50° F.) is not allowed to exceed 4,137 kPa (600 psi) up to 20.68 MPa (3,000 psi). This accuracy between lanes falls within a selected requirement of 4,137 kPa (600 psi), even given an adverse buildup of tolerances. Allowing for sensitivity variations between LVDT&#39;s and manufacturing tolerances, ITFV lane-to-lane matching within a manifold is possible to within 13% of reading.  
         [0050]    The integrated three function valve according to the present invention is less complex and more reliable than separately housed pressure transducers, bypass valves, and pressure relief valves. When used as a matched pair in a hydraulic actuator, ITFV&#39;s  111   a  and  111   b  provide redundant bypass valve, pressure relief valve, and delta pressure transducer functionality. This added redundancy is achieved with no additional LVDT&#39;s or wiring over conventional arrangements. This redundancy allows a control linkage or aerodynamic surface driven by multiple actuators to continue to operate safely following most common dual failures.  
         [0051]    It is apparent that an invention with significant advantages has been described and illustrated. Although the present invention is shown in a limited number of forms, it is not limited to just these forms, but is amenable to various changes and modifications without departing from the spirit thereof.

Technology Classification (CPC): 8