Abstract:
It is an object to provide a servo valve that can be manufactured at low cost by simplifying adjustment of the relative position between a nozzle and a flapper and simplifying the configuration of a valve-element driving circuit. Provided is a servo valve ( 1 ) including a spool ( 5 ) mounted so as to be movable back and forth; a first chamber ( 7 ) and a second chamber ( 9 ) that mutually push the spool ( 5 ) in opposite directions by means of fluid pressure; and a spool driving circuit ( 3 ) that supplies oil to the first chamber ( 7 ) and the second chamber ( 9 ) and that adjusts the pressure of supplied oil to move the spool ( 5 ) back and forth, wherein the spool driving circuit ( 3 ) maintains the fluid pressure of the first chamber ( 7 ) at a substantially constant level and includes, at an oil outlet of the second chamber ( 9 ), a nozzle flapper mechanism ( 27 ) that adjusts the fluid pressure of the second chamber ( 9 ).

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a servo valve. 
       BACKGROUND ART 
       [0002]    Servo valves are widely used for controlling driving of a hydraulic or pneumatic actuator. 
         [0003]    Some servo valves use a spool that is driven back and forth as a valve element. For such servo valves, a nozzle flapper mechanism, as disclosed in Patent Literature 1, for example, is proposed as a mechanism for driving the spool. 
         [0004]    This is configured such that a variable orifice is formed of a pair of nozzles and a flapper disposed between the nozzles, deriving back pressures of the nozzles, which change depending on the position of the flapper, and the spool is driven by the pressure difference between the derived back pressures. 
         [0005]    A mechanism that uses an electromagnetic coil for this displacement of the flapper is used; however, a mechanism that uses a compact, high-speed, high-generative-power piezoelectric element (a layered piezoelectric element or a bimorph piezoelectric element) has been proposed because size reduction and high performance of servo valves have been required recently. 
       CITATION LIST 
     Patent Literature 
       [0006]    Japanese Unexamined Patent Application, Publication No. 2001-82411 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    With the configuration in which a variable orifice is formed of a pair of nozzles and a flapper disposed between the nozzles, the flapper needs to be mounted in an orientation such that it opposes the nozzles or in an orientation in which a uniform influence is exerted thereon in order to improve the operating accuracy of the spool. This therefore poses the problem of difficulty in adjusting the position of the flapper during mounting. 
         [0008]    Furthermore, since it is necessary to accurately move the flapper to both sides, for example, in the case of layered piezoelectric elements, large layered piezoelectric elements should be provided at both sides of the flapper. Therefore, this increases the size of the servo valve and makes the control of a control system for moving the flapper difficult, thus making it difficult to put it to practical use. 
         [0009]    Furthermore, in the case where a piezoelectric element is used in adjusting the position of the flapper, if an electrode and a body (valve main body) make contact, an excess current flows, thus hindering driving of the flapper, and the occurrence of such a situation must be assuredly prevented. 
         [0010]    In consideration of such circumstances, an object of the present invention is to provide a servo valve that can be manufactured at low cost by simplifying adjustment of the relative position between a nozzle and a flapper and simplifying the configuration of a valve-element driving circuit. 
       Solution to Problem 
       [0011]    The present invention adopts the following solutions to solve the problems described above. 
         [0012]    Specifically, one aspect of the present invention is a servo valve including a valve element mounted so as to be movable back and forth; a first pushing portion and a second pushing portion that mutually push the valve element in opposite directions by means of fluid pressure; and a valve-element driving circuit that supplies fluid to the first pushing portion and the second pushing portion and adjusts the pressure of the supplied fluid to move the valve element back and forth, wherein the valve-element driving circuit maintains the fluid pressure of the first pushing portion at a substantially constant level and includes a nozzle flapper mechanism, at a fluid outlet of the second pushing portion, that adjusts the fluid pressure of the second pushing portion. 
         [0013]    Since the valve element mounted so as to be movable back and forth is mutually pushed in opposite directions by means of the fluid pressures of the first pushing portion and the second pushing portion, the valve is moved back and forth due to the difference in fluid pressure between the first pushing portion and the second pushing portion. That is, the valve element moves in the direction in which the fluid pressure of a pushing portion having a higher fluid pressure of the fluid pressures of the first pushing portion and the second pushing portion acts. 
         [0014]    According to this aspect, since the first pushing portion fluid pressure is maintained at a substantially constant level, the valve element moves back and forth by adjusting the fluid pressure of the second pushing portion to a higher or lower level than the fluid pressure of the first pushing portion. 
         [0015]    Since the nozzle flapper mechanism is provided at the fluid outlet of the second pushing portion, the pressure of the fluid in the second pushing portion can be adjusted by adjusting the distance between the end of the nozzle provided at the fluid outlet and the flapper. When the pressure of the fluid in the second pushing portion fluid pressure can be adjusted, the fluid pressure in the second pushing portion can be adjusted, so that the fluid pressure of the second pushing portion can be made higher or lower than the constant fluid pressure of the first pushing portion. 
         [0016]    Since the nozzle flapper mechanism is disposed only at the outlet of the second pushing portion, that is, the flapper is opposed to just one nozzle, as described above, the positional adjustment of the flapper to the nozzle can be performed easily. This allows accurate placement of the flapper in a short time. 
         [0017]    Furthermore, since the circuit configuration of the valve-element driving circuit can be simplified, the machining costs of the valve main body can be reduced. 
         [0018]    This allows the servo valve to be manufactured at low cost. 
         [0019]    In addition, to maintain the first pushing portion at a substantially constant pressure, the pressure of the fluid should be maintained substantially constant by, for example, providing a orifice in a fluid passage to the first pushing portion. 
         [0020]    In the above aspect, in the valve element, a first pressure-receiving area where the fluid in the first pushing portion acts on the valve element and a second pressure-receiving area where the fluid in the second pushing portion acts on the valve element may be set to substantially the same area. 
         [0021]    The fluid force of the first pushing portion is obtained by multiplying the first pressure-receiving area by the pressure of fluid in the first pushing portion. The fluid force of the second pushing portion is obtained by multiplying the second pressure-receiving area by the pressure of the fluid in the second pushing portion. 
         [0022]    Since the first pressure-receiving area and the second pressure-receiving area are set to substantially the same area, the relative levels of the fluid pressure of the first pushing portion and the second pushing portion are determined by the pressures of the individual fluids. 
         [0023]    The pressure of the liquid in the first pushing portion is set to the level of an intermediate pressure in an intermediate portion in the pressure range of the fluid in the second pushing portion adjusted by the nozzle flapper mechanism. Since the pressure of the fluid in the second pushing portion, in other words, the fluid pressure of the second pushing portion, can be set higher or lower than the pressure of the fluid in the first pushing portion that is maintained constant, in other words, the fluid pressure of the first pushing portion, the valve element can be moved back and forth. 
         [0024]    In addition, in view of ease of adjustment, it is desirable that the pressure of the fluid in the first pushing portion be set so as to be equal to a substantially intermediate level between that of the pressure of the fluid in the second pushing portion in a state in which no voltage is applied to the nozzle flapper mechanism and that of the pressure of the fluid in the second pushing portion in a state in which the maximum voltage is applied to the nozzle flapper mechanism. 
         [0025]    In the above aspect, in the valve element, a first pressure-receiving area where the fluid in the first pushing portion acts on the valve element and a second pressure- receiving area where the fluid in the second pushing portion acts on the valve element may be set to substantially different areas. 
         [0026]    The fluid force of the first pushing portion is obtained by multiplying the first pressure-receiving area by the pressure of fluid in the first pushing portion. The fluid force of the second pushing portion is obtained by multiplying the second pressure-receiving area by the pressure of the fluid in the second pushing portion. 
         [0027]    The pressure of the fluid in the first pushing portion is set to a level obtained by multiplying an intermediate pressure in an intermediate portion in the pressure range of the fluid in the second pushing portion adjusted by the nozzle flapper mechanism by the second pressure-receiving area/first pressure-receiving area. When the pressure of the fluid in the second pushing portion is set higher than the intermediate pressure, the fluid pressure of the second pushing portion becomes higher than the fluid pressure of the first pushing portion, so that the valve element is moved in the direction of the first pushing portion. When the pressure of the fluid in the second pushing portion is set lower than the intermediate pressure, the fluid pressure of the second pushing portion becomes lower than the fluid pressure of the first pushing portion, so that the valve element is moved in the direction of the second pushing portion. 
         [0028]    The pressure of the fluid in the first pushing portion is set to a level obtained by multiplying the intermediate pressure of the fluid in the second pushing portion by the second pressure-receiving area/first pressure-receiving area in this way; therefore, for example, in the case where fluid is supplied from the same supply source, setting the intermediate pressure to a level obtained by multiplying the pressure of the supplied fluid by the first pressure-receiving area/second pressure-receiving area allows the supplied fluid and the intermediate pressure to be the same pressure even if the supplied fluid is directly introduced from the supply source to the first pushing portion. 
         [0029]    In other words, by setting the intermediate pressure of the fluid to be supplied to the second pushing portion to a level obtained by multiplying the pressure of the supplied fluid by the first pressure-receiving area/second pressure- receiving area, the pressure of the fluid in the first pushing portion can be made the same as the pressure of the supplied fluid, and thus, a member for adjusting the pressure of the fluid supplied to the first pushing portion can be eliminated. 
         [0030]    This can further simplify the circuit configuration of the valve-element driving circuit, thereby further reducing the machining costs of the valve main body, thus allowing the servo valve to be manufactured at low cost. 
         [0031]    In the above aspect, a flapper of the nozzle flapper mechanism may be driven by a bimorph piezoelectric element. 
         [0032]    Since a bimorph piezoelectric element that has a relatively large deformation amount and that can be driven at a low voltage is used, a small nozzle flapper mechanism including a power supply can be configured. Furthermore, the relatively low cost of the bimorph piezoelectric element can reduce further the manufacturing costs of the servo valve 
         [0033]    In the above aspect, a flapper of the nozzle flapper mechanism may be driven by a layered piezoelectric element. 
         [0034]    Since the distance of the flapper relative to one nozzle is adjusted, only one layered piezoelectric element is needed to move it. This allows a smaller configuration as compared with a mechanism having large layered piezoelectric elements at both sides of the flapper, thus allowing the servo valve to be made more compact. Furthermore, this also allows the control of a control system for moving the flapper to be simplified. 
         [0035]    Thus, a practical servo valve can be provided 
         [0036]    In the above aspect, a flapper of the nozzle flapper mechanism may be driven by a torque motor. 
         [0037]    This allows a servo valve capable of stable adjustment to be configured using a proven torque motor. 
       Advantageous Effects of Invention  
       [0038]    With the servo valve according to the present invention, since the pressure of the first pushing portion is maintained at a substantially constant level, and a nozzle flapper mechanism that adjusts the pressure of the second pushing portion is provided at a fluid outlet of the second pushing portion, positional adjustment of the flapper relative to the nozzle by the nozzle flapper mechanism can easily be performed. This allows accurate placement of the flapper in a short time. 
         [0039]    Furthermore, since the circuit configuration of the valve-element driving circuit can be simplified, the machining costs of the valve main body can be reduced. 
         [0040]    This allows the servo valve to be manufactured at low cost. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0041]      FIG. 1  is a circuit diagram illustrating a spool driving circuit of a first embodiment of the present invention. 
           [0042]      FIG. 2  is a partial sectional view illustrating part of a nozzle flapper mechanism of the first embodiment of the present invention. 
           [0043]      FIG. 3  is a cross-sectional view illustrating, in outline, the configuration of a flapper unit of the first embodiment of the present invention. 
           [0044]      FIG. 4  is a cross-sectional view illustrating a flapper manufacturing process of the first embodiment of the present invention. 
           [0045]      FIG. 5  is a cross-sectional view illustrating a flapper unit manufacturing process of the first embodiment of the present invention. 
           [0046]      FIG. 6  is a cross-sectional view illustrating a flapper unit curing process of the first embodiment of the present invention. 
           [0047]      FIG. 7  is a circuit diagram illustrating another form of the spool driving circuit of the first embodiment of the present invention. 
           [0048]      FIG. 8  is a circuit diagram illustrating yet another form of the spool driving circuit of the first embodiment of the present invention. 
           [0049]      FIG. 9  is a circuit diagram illustrating a spool driving circuit of a second embodiment of the present invention. 
           [0050]      FIG. 10  is a partial sectional view illustrating part of a nozzle flapper mechanism of the second embodiment of the present invention. 
           [0051]      FIG. 11  is a cross-sectional view taken along line X-X in  FIG. 9 . 
           [0052]      FIG. 12  is a cross-sectional view taken along line Y-Y in  FIG. 9 . 
           [0053]      FIG. 13  is a circuit diagram illustrating another form of the spool driving circuit of the second embodiment of the present invention. 
           [0054]      FIG. 14  is a cross-sectional view taken along line Y-Y in  FIG. 13 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0055]    Embodiments of the present invention will be described hereinbelow with reference to the drawings. 
       First Embodiment  
       [0056]    A servo valve  1  for controlling driving of a hydraulic actuator according to a first embodiment of the present invention will be described hereinbelow using  FIGS. 1 to 6 . 
         [0057]      FIG. 1  is a circuit diagram illustrating a spool driving circuit (valve-element driving circuit)  3  of the servo valve  1 .  FIG. 2  is a partial sectional view illustrating part of a nozzle flapper mechanism. 
         [0058]    The servo valve  1  is configured such that a spool (valve element)  5  that controls driving of a hydraulic actuator (not shown) can be moved in the axial direction. 
         [0059]    The spool  5  has the function of switching a working-oil supply direction to the hydraulic actuator depending on the position in the axial direction. 
         [0060]    The axial position of the spool  5  can be detected by a position detector (not shown). 
         [0061]    A first chamber (first pushing portion)  7  and a second chamber (second pushing portion)  9 , which are spaces that open at the spool  5  side, are provided at both ends of the spool  5 . 
         [0062]    The spool driving circuit  3  is provided with a pump  11  that supplies oil (fluid). The oil from the pump  11  is divided into a first passage  13  and a second passage  15 . The oil that passes through the first passage  13  is supplied to the first chamber  7  and is also returned to a tank  17 . 
         [0063]    The oil that passes through the second passage  15  is supplied to the second chamber  9  and is thereafter discharged to a pipe  19 . The oil discharged to the pipe  19  is returned to a tank  17 . 
         [0064]    The pressure receiving areas of the spool  5  on which the oil in the first chamber  7  and the second chamber  9  acts are set to substantially equal areas. The difference between fluid pressures that the oil in the first chamber  7  and the second chamber  9  exerts on the spool  5  is in proportion to a difference in oil pressure. 
         [0065]    The first passage  13  is provided with a first orifice  21  upstream of the first chamber  7  and a pressure regulating orifice  23  downstream of the first chamber  7 . 
         [0066]    An example of the first orifice  21  is an orifice, which specifies the pressure of oil supplied to the first chamber  7 . The pressure P 1  of the oil supplied to the first chamber  7  is set, for example, to substantially half of the pressure Ps of the oil discharged from the pump  11 . 
         [0067]    The opening area of the pressure regulating orifice  23  can be changed so as to adjust the pressure of the oil in the first chamber  7 . 
         [0068]    The second passage  15  is provided with a second orifice  25  upstream of the second chamber  9  and a nozzle flapper mechanism  27  at the downstream end. 
         [0069]    An example of the second orifice  25  is an orifice, whose opening area is equal to that of the first orifice  21 . The nozzle flapper mechanism  27  is provided with a nozzle  29  mounted at the downstream end of the second passage  15  and a flapper unit  31  opposed to an opening  33  of the nozzle  29  and constituting a orifice. The nozzle  29  is a orifice mechanism in which the opening area of the opening  33 , at an origin (in a state in which no voltage is applied to the flapper  35 ), is equal to the opening area of the pressure regulating orifice  23 , so that the pressure of the oil in the second chamber  9  is equal to the pressure of the oil in the first chamber  7 . 
         [0070]    When the flapper  35  moves from the origin to come away from the nozzle  29  to increase the area of the opening  33 , the pressure of the oil in the second chamber  9  becomes lower than the pressure of the oil in the first chamber  7 . In contrast, when the flapper  35  moves from the origin to come close to the nozzle  29  to decrease the area of the opening  33 , the pressure of the oil in the second chamber  9  becomes higher than the pressure of the oil in the first chamber  7 . 
         [0071]    Accordingly, the pressure of the oil in the second chamber  9  at the origin is an intermediate pressure located at the intermediate portion of a range in which the pressure can be adjusted by the nozzle flapper mechanism  27 . 
         [0072]      FIG. 3  is a cross-sectional view illustrating, in outline, the configuration of the flapper unit  31 . 
         [0073]    The flapper unit  31  is provided with a flapper  35  and a case  37  that holds the flapper  35 . The case  37  is made of metal and has a hollow, rectangular parallelepiped shape one face of which is open. 
         [0074]    The flapper  35  has a configuration in which two plate-like piezoelectric elements  41  and  43  are bonded to both sides of a metal plate  39 , that is, a bimorph piezoelectric element. 
         [0075]    Electrical wires  45  are attached to the ends of the metal plate  39  and the piezoelectric elements  41  and  43 . The metal plate  39  is grounded, the piezoelectric element  41  carries a positive voltage, and the piezoelectric element  43  carries a negative voltage. 
         [0076]    One end of the flapper  35  is inserted into the inner space of the case  37  and is fixed to the case  37  together with the electrical wires  45  by an adhesive  47 . The adhesive  47  is a resin having electrical insulating properties, for example, a molding agent, such as epoxy resin. 
         [0077]    The lateral area of a cylinder formed by the flapper  35  and the distal outer peripheral end  49  of the nozzle  29  determines the orifice level of the nozzle flapper mechanism  27 . A position at which the lateral area is equal to the opening area of the opening  33  is a limit position at which the nozzle flapper mechanism  27  can offer the orifice function. That is, when the flapper  35  comes away from the nozzle  29  relative to this position, the throttling effect becomes smaller than the throttling effect of the nozzle  29 , and thus, the nozzle flapper mechanism  27  provides no orifice function. 
         [0078]    The flapper  35  is disposed at a midpoint position between this limit position and a position at which the flapper  35  and the nozzle  29  are in contact and, with that position as the center, is configured to be displaced between the limit position and the position at which the flapper  35  and the nozzle  29  are in contact. 
         [0079]    A method for assembling this flapper unit  31  will be described hereinbelow with reference to  FIGS. 4 to 6 . 
         [0080]    First, the plate-like piezoelectric elements  41  and  43  are bonded to both sides of the metal plate  39 . 
         [0081]    Next, the electrical wires  45  are joined to the ends of the metal plate  39  and the piezoelectric elements  41  and  43  by, for example, soldering. 
         [0082]    Next, as shown in  FIG. 4 , the peripheral portion of the contact points between the metal plate  39  and the piezoelectric elements  41  and  43  and the electrical wires  45  is fixed by the adhesive  47  to form the flapper  35 . 
         [0083]    At that time, since the amount of the adhesive  47  is small, problems such as deformation of the electrical wires  45  do not occur. That is, deformation whereby the contact points come off or the electrical wires  45  come into contact with the case does not occur. 
         [0084]    After the adhesive  47  hardens, the insulation resistance of the electric circuit is measured to check that it is properly insulated. 
         [0085]    Next, as shown in  FIG. 5 , this flapper  35  is mounted at a predetermined position of the case  37 , and the adhesive  47  is injected into the inner space of the case  37 . 
         [0086]    Injection of the adhesive  47  causes a large force to act on the flapper  35 ; however, since the contact points between the metal plate  39  and the piezoelectric elements  41  and  43  and the electrical wires  45  is protected by the adhesive  47 , which is hardened in advance, they do not come off. Furthermore, since the movable portions of the electrical wires  45  are not long, the electrical wires  45  are not greatly deformed so as to come into contact with the case  37 . 
         [0087]    The added adhesive  47  is cured to be hardened in the state in  FIG. 5 . 
         [0088]    At this time, it is important that the end face  51  of the case  37  and the surface of the flapper  35  intersect at right angles; therefore, the curing may be performed by disposing the flapper unit  31  in a first jig  53  and a second jig  55  that maintain the intersecting state, as shown in  FIG. 6 . 
         [0089]    The first jig  53  has a through-hole  57  having a rectangular cross section. The through-hole  57  has an enlarged portion at one end so that the end face  51  of the case  37  intersects the through-hole at right angles. 
         [0090]    The second jig  55  is formed so that one end thereof can be inserted into the through-hole  57 . The second jig  55  is provided with a through-hole  59  into which the flapper  35  is inserted. 
         [0091]    The vertical center positions of the through-hole  57  and the through-hole  59  are aligned. 
         [0092]    The case  37  is inserted into the through-hole  57  from the flapper  35  side and is fitted into the enlarged portion. Next, the second jig  55  is inserted from the opposite side of the through-hole  57 , and the end of the flapper  35  is inserted into the through-hole  59 . Thus, the end face  51  of the case  37  and the surface of the flapper  35  intersect at right angles. 
         [0093]    When cured in this state, the adhesive  47  is hardened, so that the flapper  35  is fixed to the case  37  in the form in which the end face  51  of the case  37  and the surface of the flapper  35  intersect at right angles. 
         [0094]    The adhesive  47  may be injected while the flapper unit  31  is retained by the first jig  53  and the second jig  55 . 
         [0095]    The operation of the thus-configured spool driving circuit  3  will now be described. 
         [0096]    When the pump  11  is driven to supply oil, the supplied oil is divided and flows into the first passage  13  and the second passage  15 . The oil flowing into the first passage  13  is reduced in pressure by the first orifice  21 , flows into the first chamber  7 , and is also returned to the tank  17  through the pressure regulating orifice  23 . 
         [0097]    The oil flowing into the second passage  15  is reduced in pressure by the second orifice  25  and flows into the second chamber  9 . The oil is discharged from the second chamber  9  to the pipe  19  through the nozzle flapper mechanism  27  and is returned from the pipe  19  to the tank  17 . 
         [0098]    At this time, if the flapper  35  is located at the origin, the opening area of the opening  33  is equal to the opening area of the pressure regulating orifice  23 ; therefore, the pressure of the second chamber  9  becomes the same as the pressure of the first chamber  7 , so that the differential pressure between the first chamber  7  and the second chamber  9  becomes 0. In the state of the differential pressure of 0, the spool  5  is in a halted state. 
         [0099]    When a + (−) voltage is applied to the flapper  35 , the flapper  35  is displaced to the nozzle  29  side, so that the lateral area of the cylinder formed by the flapper  35  and the distal outer peripheral end  49  of the nozzle  29 , that is, the orifice level of the nozzle flapper mechanism  27 , becomes smaller than that of the pressure regulating orifice  23 . 
         [0100]    When the orifice level of the nozzle flapper mechanism  27  becomes lower, the throttling effect of the nozzle flapper mechanism  27  becomes larger than that of the nozzle  29 , so that the pressure of the second chamber  9  becomes higher than that of the first chamber  7 , which causes a differential pressure between the first chamber  7  and the second chamber  9 . This differential pressure moves the spool  5  to the first chamber  7  side. 
         [0101]    When a − (+) voltage is applied to the flapper  35 , the flapper  35  is displaced in a direction away from the nozzle  29 , so that the lateral area of the cylinder formed by the flapper  35  and the distal outer peripheral end  49  of the nozzle  29 , that is, the orifice level of the nozzle flapper mechanism  27 , becomes larger than that of the pressure regulating orifice  23 . 
         [0102]    When the orifice level of the nozzle flapper mechanism  27  becomes higher than that of the pressure regulating orifice  23 , the pressure of the second chamber  9  becomes lower than that of the first chamber  7 , which causes a differential pressure between the first chamber  7  and the second chamber  9 . This differential pressure moves the spool  5  to the second chamber  9  side. 
         [0103]    Thus, since the pressure of oil supplied to the first chamber  7  is maintained at a substantially constant level, the spool  5  moves back and forth by adjusting the pressure of the second chamber  9  to a higher or lower level than the pressure of the first chamber  7  using the nozzle flapper mechanism  27 . 
         [0104]    Since this nozzle flapper mechanism  27  is disposed only at the end of the second passage  15 , that is, at the outlet of the second chamber  9 , the flapper  35  is opposed to just one nozzle  29 . Accordingly, this can facilitate the positional adjustment of the flapper  35  relative to the nozzle  29 , thus allowing accurate placement of the flapper unit  31  in a short time. 
         [0105]    Furthermore, since the circuit configuration of the spool driving circuit  3  can be simplified, the machining costs of the valve main body can be reduced. 
         [0106]    This allows the servo valve  1  to be manufactured at low cost. 
         [0107]    Furthermore, since a bimorph piezoelectric element that has a relatively large deformation amount and that can be driven at a low voltage is used as the flapper  35 , a small nozzle flapper mechanism  27  including a power supply can be constituted. Furthermore, the relatively low cost of the bimorph piezoelectric element can further reduce the manufacturing cost of the servo valve  1 . 
         [0108]    The flapper  35  of the nozzle flapper mechanism  27  may be driven by a layered piezoelectric element  61 , as shown in  FIG. 7 . 
         [0109]    Since the distance of the flapper  35  relative to one nozzle  29  is adjusted, only one layered piezoelectric element  61  is needed to move it. 
         [0110]    This allows a smaller configuration as compared with a mechanism having the large layered piezoelectric elements  61  at both sides of the flapper  35 , thus allowing the servo valve  1  to be made more compact. 
         [0111]    Furthermore, this also allows the control of a control system for moving the flapper  35  to be simplified. 
         [0112]    Thus, the practical servo valve  1  can be provided even with the layered piezoelectric element  61 . 
         [0113]    Furthermore, the flapper  35  of the nozzle flapper mechanism  27  may be driven by a torque motor  63  that performs linear motion, as shown in  FIG. 8 . 
         [0114]    This allows the servo valve  1  capable of stable adjustment to be configured using the proven torque motor  63 . 
       Second Embodiment  
       [0115]    A servo valve  71  for controlling driving of a hydraulic actuator (not shown) according to a second embodiment of the present invention will be described hereinbelow using  FIGS. 9 to 12 . 
         [0116]      FIG. 9  is a circuit diagram illustrating a spool driving circuit (valve-element driving circuit)  73  of the servo valve  71 .  FIG. 10  is a partial sectional view illustrating part of a nozzle flapper mechanism.  FIG. 11  is a cross-sectional view taken along line X-X in  FIG. 9 .  FIG. 12  is a cross-sectional view taken along line Y-Y in  FIG. 9 . 
         [0117]    The servo valve  71  is provided with a body  75  having a space inside and a spool (valve element)  77  disposed in the inner space of the body  75  so as to be movable in the axial direction. 
         [0118]    The spool  77  is provided with a plurality of land portions  79  serving as sliding surfaces and having substantially the same diameter. The spool  77  moves in the axial direction so that the positions of these land portions  79  in the axial direction move. These land portions  79  have the function of switching a working-oil supply direction to the hydraulic actuator (not shown) depending on the positions in the axial direction. 
         [0119]    A land portion  79   a  provided at one end of the spool  77  is provided with a first rod  81  projecting outward. The first rod  81  transmits its motion to a differential transformer  83 . The differential transformer  83  detects the axial position of the spool  77 . 
         [0120]    A first chamber (first pushing portion)  85  is formed at the outer side of the land portion  79   a  so as to surround the first rod  81 . 
         [0121]    A land portion  79   b  provided at the other end of the spool  77  is provided with a second rod  87  projecting outward. A second chamber (second pushing portion)  89  is formed at the outer side of the land portion  79   b  so as to surround the second rod  87 . 
         [0122]    The spool driving circuit  73  is provided with a pump  91  that supplies oil through a main passage  93 . The main passage  93  is provided with a pressure regulating valve  95 , to which oil at a substantially constant pressure is supplied. 
         [0123]    The main passage  93  is divided into a first passage  97  and a second passage  99 . The oil that passes through the first passage  97  is supplied to the first chamber  85 , passes through a pipe  101  and a return passage  103 , and is returned to a tank  105 . The first chamber  85  is directly supplied with the oil that is supplied through the main passage  93 . The pressure of this supplied oil is the pressure Ps at which the pump  91  discharges. 
         [0124]    The oil that passes through the second passage  99  is supplied to the second chamber  89 , thereafter passes through a pipe  107  and the return passage  103 , and is returned to the tank  105 . 
         [0125]    Since the first rod  81  passes through the first chamber  85 , a first pressure-receiving area A 1  where the land portion  79   a  receives pressure from the oil supplied to the first chamber  85  is of a size obtained by subtracting the cross-sectional area of the first rod  81  from the area of the land portion  79   a , as shown in  FIG. 11 . 
         [0126]    Since the second rod  87  passes through the second chamber  89 , a second pressure-receiving area A 2  where the land portion  79   b  receives pressure from the oil supplied to the second chamber  89  is of a size obtained by subtracting the cross-sectional area of the second rod  87  from the area of the land portion  79   b , as shown in  FIG. 12 . 
         [0127]    In this embodiment, the sizes of the first rod  81  and the second rod  87  are set so that the first pressure-receiving area A 1  is substantially half of the second pressure-receiving area A 2 . 
         [0128]    Note that the area ratio of the first pressure-receiving area A 1  to the second pressure-receiving area A 2  is not limited thereto. 
         [0129]    The second passage  99  is provided with an inlet orifice  109  constituted by, for example, an orifice, upstream of the second chamber  89 . The pipe  107  is provided with a nozzle flapper mechanism  111 . 
         [0130]    The nozzle flapper mechanism  111  is provided with a nozzle  113  mounted to the pipe  107  and a flapper unit  117  opposed to an opening  115  of the nozzle  113  and constituting a orifice. 
         [0131]    The flapper unit  117  is provided with a flapper  119  and a layered piezoelectric element  121  in which a plurality of piezoelectric elements that drive the flapper  35  are layered. 
         [0132]    The lateral area of a cylinder formed by the flapper  119  and the distal outer peripheral end  123  of the nozzle  113  determines the orifice level of the nozzle flapper mechanism  111 . 
         [0133]    A position at which the lateral area is equal to the opening area of the opening  115  (the state in  FIG. 10 ) is a limit position at which the nozzle flapper mechanism  111  can offer the orifice function. That is, when the flapper  119  comes away the nozzle  113  relative to this position, the throttling effect becomes smaller than the throttling effect of the nozzle  113 , and thus, the nozzle flapper mechanism  111  provides no orifice, function. 
         [0134]    The flapper  119  is disposed at a midpoint position between this limit position and a position at which the flapper  119  and the nozzle  113  are in contact and, with the position as the center (origin), is configured to be displaced between the limit position and the position at which the flapper  119  and the nozzle  113  are in contact, that is, in an adjusting range C. 
         [0135]    In this embodiment, the specifications of the nozzle flapper  111  are set so that when the flapper  119  is in the origin, the pressure P 1  of oil in the first chamber  85  is substantially the same as the pressure Ps applied by the pump  91 . 
         [0136]    The operation of the thus-configured spool driving circuit  73  will be described. 
         [0137]    When the pump  91  is driven, oil is supplied from the tank  105  through the main passage  93 . The pressure Ps of the supplied oil is maintained substantially constant by the pressure regulating valve  95 . 
         [0138]    The oil flowing through the main passage  93  is divided and flows into the first passage  97  and the second passage  99 . 
         [0139]    The oil flowing into the first passage  97  flows into the first chamber  85  and is returned to the tank  105  through the pipe  101  and the return passage  103 . 
         [0140]    The oil flowing into the second passage  99  is reduced in pressure by the inlet orifice  109  and flows into the second chamber  89 . The oil is discharged from the second chamber  89  to the pipe  107 , passes through the nozzle flapper mechanism  111 , and is returned to the tank  105  through the return passage  103 . 
         [0141]    At this time, if the flapper  119  is at the origin, the pressure P 1  of the oil in the first chamber  85  is substantially the same as the pressure Ps supplied by the pump  91 , that is, P 1 =Ps. A force (fluid pressure) F 1  that the oil in the first chamber  85  exerts on the land portion  79   a  is expressed as F 1 =A 1 ×Ps. 
         [0142]    On the other hand, the pressure P 2  of the oil in the second chamber  89  is substantially half of the pressure Ps supplied by the pump  91 , that is, P 2 =Ps/2. A force (fluid pressure) F 2  that the oil in the second chamber  89  exerts on the land portion  79   b  is expressed as F 2 =A 2 ×Ps/2. 
         [0143]    Since A 2 =2×A 1 , the force F 2  is expressed as F 2 =2×A 1 ×Ps/2=A 1 ×Ps. Since the force F 1  and the force F 2  become equal, the differential pressure therebetween becomes 0. In the state of the differential pressure of 0, the spool  7  is in a halted state. 
         [0144]    When a voltage is applied to the layered piezoelectric element  121  to displace the flapper  119  to the nozzle  113  side, the lateral area of the cylinder formed by the flapper  119  and the distal outer peripheral end  123  of the nozzle  113 , that is, the orifice level of the nozzle flapper mechanism  111 , becomes lower than that at the origin. 
         [0145]    When the orifice level of the nozzle flapper mechanism  111  becomes low, the throttling effect of the nozzle flapper mechanism  111  increases, so that the pressure P 2  of the oil in the second chamber  89  becomes higher than Ps/2. 
         [0146]    When the pressure P 2  becomes high, a force F 2  that the oil in the second chamber  89  exerts on the land portion  79   b  becomes large, so that the force F 2  becomes larger than the constant force F 1  in the first chamber  85 . 
         [0147]    This differential pressure causes the spool  77  to move to the first chamber  85  side. 
         [0148]    When an opposite voltage is applied to the layered piezoelectric element  121  to displace the flapper  119  at the origin in the direction away from the nozzle  113 , the lateral area of the cylinder formed by the flapper  119  and the distal outer peripheral end  123  of the nozzle  113 , that is, the orifice level of the nozzle flapper mechanism  111 , becomes larger than that when at the origin. 
         [0149]    When the orifice level of the nozzle flapper mechanism  111  becomes high, the throttling effect of the nozzle flapper mechanism  111  decreases, so that the pressure P 2  of the oil in the second chamber  89  becomes lower than Ps/2. 
         [0150]    When the pressure P 2  becomes low, the force F 2  that the oil in the second chamber  89  exerts on the land portion  79   b  becomes small, so that the force F 2  becomes smaller than the constant force F 1  in the first chamber  85 . 
         [0151]    This differential pressure causes the spool  77  to move to the second chamber  89  side. 
         [0152]    Thus, since the pressure of oil supplied to the first chamber  85 , that is, the force F 1  that acts on the land portion  79   a , is maintained at a substantially constant level, the spool  77  moves back and forth by adjusting the pressure of the oil in the second chamber  89  using the nozzle flapper mechanism  111 . 
         [0153]    Since this nozzle flapper mechanism  111  is disposed only at the pipe  107 , that is, at the outlet of the second chamber  89 , the flapper  119  is opposed to just one nozzle  113 . 
         [0154]    Accordingly, this can facilitate the positional adjustment of the flapper  119  relative to the nozzle  113 , thus allowing accurate placement of the flapper unit  117  in a short time. 
         [0155]    Furthermore, since the circuit configuration of the spool driving circuit  73  can be simplified, the machining costs of the valve main body can be reduced. 
         [0156]    This allows the servo valve  71  to be manufactured at low cost. 
         [0157]    Since the oil supplied from the pump  91  to the first chamber  85  is supplied directly, in other words, the first orifice  21  and the pressure regulating orifice  23  of the first embodiment are omitted, the circuit configuration of the valve-element driving circuit  73  can be further simplified. Since this eliminates adjustment of the pressure regulating orifice  23  etc. adjustment costs can be reduced. 
         [0158]    This can reduce further the machining costs of the servo valve  71  main body, thus allowing the servo valve  71  to be manufactured at lower cost. 
         [0159]    When the first orifice  21  and the pressure regulating orifice  23  are used as in the first embodiment, the space from the first orifice  21  to the pressure regulating orifice  23  including the first chamber  85  constitutes a large voluminous chamber because of separation by the first orifice  21  and the pressure regulating orifice  23 . This increases the spring constant of the oil in this space, thus easily causing resonance. Since this embodiment does not use the first orifice  21  and the pressure regulating orifice  23 , resonance can be avoided, and thus the accuracy when driving at a high frequency can be improved. 
         [0160]    In this embodiment, although the flapper  119  of the nozzle flapper mechanism  111  is driven by the layered piezoelectric element  121 , it is not limited thereto. 
         [0161]    For example, the bimorph piezoelectric element that can be driven at a low voltage, used in the first embodiment, may be used. This allows a small nozzle flapper mechanism  111  including a power supply to be constituted. The relatively low cost of the bimorph piezoelectric element can further reduce the manufacturing cost of the servo valve  71 . 
         [0162]    For example, a torque motor that performs linear motion may be used for driving. 
         [0163]    This allows the servo valve  71  capable of stable adjustment to be configured using a proven torque motor. 
         [0164]    In this embodiment, the first pressure-receiving area A 1  and the second pressure-receiving area A 2  are adjusted depending on the sizes of the respective cross-sectional areas of the first rod  81  and the second rod  87 ; however, it is not limited thereto. 
         [0165]    For example, as shown in  FIGS. 13 and 14 , it is also possible to adjust the areas of the land portion  79   a  and the land portion  79   b , with the cross-sectional areas of the first rod  81  and the second rod  87  set equal. 
         [0166]    In this embodiment, the first pressure-receiving area A 1  is set to substantially half of the second pressure-receiving area A 2 ; however, the ratio of the first pressure-receiving area A 1  to the second pressure-receiving area A 2  is not limited thereto. 
         [0167]    That is, the oil pressures in the first and second chambers  85  and  89  and the sizes of the first pressure-receiving area A 1  and the second pressure-receiving area A 2  should be selected so that the pressure of the oil in the first chamber  85  comes to a level obtained by multiplying the pressure of the oil in the second chamber  89  when the flapper  119  is located at the origin by A 2 /A 1 . 
         [0168]    The present invention is not limited to the embodiments described above, and various modifications may be made without departing from the spirit of the present invention. 
       Reference Signs List 
       [0169]      1  Servo Valve 
         [0170]      3  Spool Driving Circuit 
         [0171]      5  Spool 
         [0172]      7  First Chamber 
         [0173]      9  Second Chamber 
         [0174]      35  Flapper 
         [0175]      61  Layered Piezoelectric Element 
         [0176]      63  Torque Motor 
         [0177]      71  Servo Valve 
         [0178]      73  Spool Driving Circuit 
         [0179]      77  Spool 
         [0180]      85  First Chamber 
         [0181]      89  Second Chamber 
         [0182]      119  Flapper 
         [0183]      121  Layered Piezoelectric Element