Abstract:
A proportional solenoid  5  comprising: an electromagnetic coil  9 ; a fixed yoke  10  provided immovably inside the electromagnetic coil  9  and having a convex portion  13  formed at an edge portion of an end surface of the fixed yoke; and a movable yoke  20  that is disposed inside the electromagnetic coil  9 , has a tip portion thereof inserted into the convex portion  13  of the fixed yoke  10 , and is provided movably relative to the fixed yoke  10 , the proportional solenoid  5  enabling the position adjustment of the movable yoke  20  by controlling an electric current applied to the electromagnetic coil  9 , wherein a protruding portion  21  that protrudes at the side of the fixed yoke  10  is formed at an edge portion of the surface of the movable yoke  20  on the side of the fixed yoke  10 , and an inner surface  21   b  of this protruding portion  21  is tapered such that the inner surface is located further outside as the inner surface approaches the fixed yoke  10.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is entitled to the benefit of and incorporates by reference essential subject matter disclosed in International Patent Application No. PCT/JP2005/023045 filed on Dec. 15, 2005 and Japanese Patent Application No. 2005-032937 filed Feb. 9, 2005. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a proportional solenoid and a flow control valve employing thereof, and more particularly to thrust force improvement of a proportional solenoid. 
       BACKGROUND ART 
       [0003]    A proportional solenoid is a device that can freely adjust the position of a movable yoke (plunger) by controlling electric current applied to an electromagnetic coil, and proportional solenoids have been used as valve drive means and the like for flow control valves, pressure control valves, direction switching valves and the like (see Japanese Patent Application Publication No. 2004-218816 and Japanese Patent Application Publication No. H9-69432). 
         [0004]    The structure of a flow control valve using a proportional solenoid will be described below with reference to  FIG. 6 . 
         [0005]    A flow control valve  100  adjusts an opening area of a port  6  by moving a spool  3  of a spool valve  2  with a proportional solenoid  50  (valve drive means) and controls the flow rate of fluid flowing through the port  6 . 
         [0006]    The proportional solenoid  50  comprises an annular bobbin  7  and an electromagnetic coil  9  wound on the outer periphery of the bobbin  7 , and a fixed yoke  10 , a fixed guide  11 , and a movable yoke  12  are disposed inside these electromagnetic coil  9  and bobbin  7 . 
         [0007]    The fixed yoke  10  is composed of a magnetic material and comprises a cylindrical insertion portion  10   a  that has an outer diameter somewhat less than the inner diameter of the bobbin  7  and a disk-shaped flange portion  10   b  formed at one end (right side end portion in the figure) of the insertion portion  10   a  and having an outer diameter almost equal to the outer diameter of the electromagnetic coil  9 . The fixed yoke  10  is disposed by inserting the insertion portion  10   a  into the electromagnetic coil  9  and bobbin  7  and abutting the flange portion  10   b  against one side of the bobbin  7 . The fixed yoke  10  is fixed and cannot be moved relative to the electromagnetic coil  9  and bobbin  7 . 
         [0008]    An annular convex portion  13  that protrudes in the axial direction is formed at the edge portion of a tip surface (left side end surface in the figure) of the insertion portion  10   a  of the fixed yoke  10 , and the outer circumferential surface of the convex portion  13  is tapered at a predetermined angle such that a the outer circumferential surface inclines inwardly in the radial direction as the tip thereof is approached (side of the movable yoke  12 ). 
         [0009]    The movable yoke  12  (plunger) is composed of a magnetic material and formed as a cylinder with an outer diameter somewhat less than the inner diameter of the convex portion  13  of the fixed yoke  10 . The movable yoke  12  is disposed opposite to the fixed yoke  10  inside the electromagnetic coil  9  and bobbin  7 , and the tip portion (right side end portion in the figure) of the movable yoke is inserted into the convex portion  13  of the fixed yoke  10 . The movable yoke  12  is provided to be movable relative to the electromagnetic coil  9 , bobbin  7 , and fixed yoke  10  and can move in the axial direction (left-right direction in the figure) along the inner surface of the convex portion  13  of the fixed yoke  10 . 
         [0010]    The fixed guide  11  is also composed of a magnetic material and comprises a cylindrical insertion portion  11   a  having an outer diameter somewhat less than the inner diameter of the bobbin  7  and an inner diameter somewhat larger than the outer diameter of the movable yoke  12  and a disk-shaped flange portion  11   b  formed at one end (left side end portion of the figure) of the insertion portion  11   a  and having an outer diameter almost equal to the outer diameter of the electromagnetic coil  9 . In the fixed guide  11 , the insertion portion  11   a  thereof is inserted between the bobbin  7  and the movable yoke  12  on the opposite side from the fixed yoke  10 . Further, the flange portion  11   b  of the fixed guide  11  is disposed by abutting against one side of the bobbin  7 . The fixed guide  11  is fixed and cannot be moved relative to the electromagnetic coil  9 , bobbin  7 , and fixed yoke  10 . 
         [0011]    The electromagnetic coil  9 , bobbin  7 , fixed yoke  10 , and fixed guide  11  are connected integrally by a cylindrical case  15  made from a non-magnetic material. 
         [0012]    A member (not shown in the figure) made from a nonmagnetic material may be inserted between the convex portion  13  of the fixed yoke  10  and the insertion portion  11   a  of the fixed guide  11 . 
         [0013]    On the other hand, a spool valve  2  comprises a sleeve  16  having formed therein a port  6  for passing a fluid, a spool  3  disposed so that it can slide in the axial direction inside the sleeve  16 , and bias means (a coil spring in the example shown in the figure)  17  for biasing the spool  3  toward the proportional solenoid  50 . 
         [0014]    A land  3   a  for closing the port  6  is formed in the central portion in the longitudinal direction of the spool  3 , and the opening surface area of the port  6  can be adjusted by moving the spool  3  relative to the sleeve  16 . 
         [0015]    A rod portion  3   b  extending via a through hole  19  formed in the fixed yoke  10  of the proportional solenoid  50  is provided at one end (left end portion in the figure) of the spool  3 , and this rod portion  3   b  is connected to the tip surface of the movable yoke  12 . 
         [0016]    In the flow control valve  100 , where an electric current is applied to the electromagnetic coil  9  of the proportional solenoid  50 , a magnetic circuit is formed via the fixed yoke  10 , fixed guide  11 , and movable yoke  12 , and a magnetic attraction force proportional to the applied current is generated between the fixed yoke  10  and movable yoke  12 . This attraction force acts as a thrust force Ft that biases the movable yoke  12  toward the fixed yoke  10 . Where the movable yoke  12  and the spool  3  connected thereto are moved to the right by the thrust force Ft, as shown in the figure, the coil spring  17  is compressed, and a reaction force Fr is generated that biases the movable yoke  12  in the direction opposite to that of the thrust force Ft. 
         [0017]    As a result, the movable yoke  12  and spool  3  are stopped in a position where the thrust force Ft produced by the proportional solenoid  50  is balanced by the reaction force Fr produced by the coil spring  17 . 
         [0018]    Because the thrust force Ft produced by the proportional solenoid  50  is proportional to electric current applied to the electromagnetic coil  9 , the position of the movable yoke  12  and spool  3  can be adjusted by controlling the current applied to the electromagnetic coil  9 . Therefore, by controlling the current applied to the electromagnetic coil  9 , it is possible to adjust the position of the land  3   a  of the spool  3  and adjust arbitrarily the opening area of the port  6 . 
         [0019]    In such proportional solenoid  50  and flow control valve  100  using thereof, a stroke (range of reciprocating movement) of the movable yoke  12  and spool  3  is typically set to range in which the thrust force Ft is constant regardless of the position of the movable yoke  12  and spool  3 . 
         [0020]    This will be explained with reference to  FIG. 7 . 
         [0021]      FIG. 7  is a graph illustrating the relationship between the thrust force Ft acting upon the movable yoke  12  and the stroke of the movable yoke  12  (a position of the movable yoke  12  where a position closest to the fixed yoke  10  is taken for zero; it can be also called a spacing between the fixed yoke  10  and the movable yoke  12 ) when a predetermined voltage is applied to the electromagnetic coil  9  of the proportional solenoid  50 . 
         [0022]    As follows from the figure, where the stroke of the movable yoke  12  (spacing between the fixed yoke  10  and the movable yoke  12 ) is small in the case the current applied to the electromagnetic coil  9  is constant, the thrust force Ft acting upon the movable yoke  12  rapidly increases from a certain point. Further, where the stroke of the movable yoke  12  increases, the thrust force Ft acting upon the movable yoke  12  rapidly decreases from a certain point. 
         [0023]    On the other hand, in the intermediate stroke region shown by an arrow in the figure, the thrust force Ft acting upon the movable yoke  12  is almost constant, regardless of the position (stroke) of the movable yoke  12 . This region is called “control range”, and usually the stroke (range of reciprocating movement) of the movable yoke  12  is set within this range. 
         [0024]    Next, the relationship between the thrust force Ft created by the proportional solenoid  50  and the reaction force Fr created by the coil spring  17  in the flow control valve  100  shown in  FIG. 6  will be explained below as a reference example based on  FIG. 8 . 
         [0025]    Lines Ft 1  to Ft 7  in the figure indicate the thrust force Ft created by the proportional solenoid  50 , and it is clear that the thrust force increases as the current applied to the electromagnetic coil  9  increases (as the line number increases). 
         [0026]    Line Fr in the figure indicates the reaction force Fr created by the coil spring  17 , and it is clear that the reaction force increases as the stroke of the movable yoke  12  decreases (as the movable yoke  12  approaches the fixed yoke  10 ). 
         [0027]    In the figure, points where the thrust force lines Ft 1  to Ft 7  and reaction line Fr intersect (circles in the figure) are balance points of the two, and the movable yoke  12  stops in these positions. 
       DISCLOSURE OF THE INVENTION 
       [0028]    However, with such proportional solenoid  50  and flow control valve  100  using thereof, friction force between the movable yoke  12  and the fixed yoke  10 , the movable yoke  12  and fixed guide  11  as well as the spool  3  and the sleeve  16  create a hysteresis in the actuation of the movable yoke  12  and spool  3 . In other words, when the reciprocating movement of the movable yoke  12  and spool  3  is caused by controlling electric current applied to the electromagnetic coil  9  of the proportional solenoid  50 , a reciprocating difference in thrust force occurs at a certain current value. Where such hysteresis becomes too large, it leads to ineffective actuation of spool valve  2 . Therefore, the hysteresis has to be eliminated or reduced, but this requires an increase in the thrust force Ft of the proportional solenoid  50 . 
         [0029]    Further, the thrust force Ft of the proportional solenoid  50  may also be required to be increased in order to improve responsiveness of the spool valve  2 . 
         [0030]    In order to increase the thrust force Ft of the proportional solenoid  50 , the proportional solenoid  50  can be increased in size or an electric current applied to the electromagnetic coil  9  can be increased, but in these cases the size and cost of the device are increased. 
         [0031]    Accordingly, it is an object of the present invention to resolve the above-described problems and to provide a proportional solenoid that enables the increase in thrust force without increasing the proportional solenoid in size, and a flow control valve using such proportional solenoid. 
         [0032]    The first aspect of the present invention created to attain the above-described object provides a proportional solenoid comprising: an electromagnetic coil; a fixed yoke provided immovably inside the electromagnetic coil and having a convex portion formed at an edge portion of an end surface of the fixed yoke; and a movable yoke that is disposed inside the electromagnetic coil, has a tip portion thereof inserted into the convex portion of the fixed yoke, and is provided movably relative to the fixed yoke, the proportional solenoid enabling the position adjustment of the movable yoke by controlling an electric current applied to the electromagnetic coil, wherein a protruding portion that protrudes at the side of the fixed yoke is formed at an edge portion of the surface of the movable yoke on the side of the fixed yoke, and an inner surface of the protruding portion is tapered such that the inner surface is located further outside as the inner surface approaches the fixed yoke. 
         [0033]    In the second aspect of the present invention, an outer surface of the protruding portion is formed to extend substantially parallel to an axial line of the movable yoke. 
         [0034]    In the third aspect of the present invention, an angle formed by the axial line of the movable yoke and the inner surface of the protruding portion is within a range of 35 to 60 degrees. 
         [0035]    In the fourth aspect of the present invention, an end surface extending in a direction substantially perpendicular to the axial line of the movable yoke is formed at the tip of the protruding portion. 
         [0036]    In fifth aspect of the present invention, the movable yoke is a cylindrical body having a predetermined outer diameter, and the radial length of the end surface is formed to be equal to or less than 5% of the outer diameter of the movable yoke. 
         [0037]    The sixth aspect of the present invention provides a flow control valve comprising: a sleeve having openly formed therein a port for passing a fluid; a spool that is disposed slidably inside the sleeve and serves to open and close the port; valve drive means for causing the spool to move in one direction; and biasing means for biasing the spool in the opposite direction in which the spool is moved by the valve drive means, wherein the valve drive means is the proportional solenoid according to any of aspects 1 to 5 above, the spool is connected to the movable yoke of the proportional solenoid, and a position of the spool connected to the movable yoke can be adjusted by controlling an electric current applied to the electromagnetic coil of the proportional solenoid. 
         [0038]    In the seventh aspect of the present invention, the movable yoke and the spool move reciprocatingly with a predetermined stroke, and a length of the protruding portion of the movable yoke is about ½ of the stroke. 
         [0039]    The present invention demonstrates an excellent effect of enabling the increase in thrust force, without increasing the proportional solenoid in size. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0040]      FIG. 1  is a cross-sectional view of a flow control valve using the proportional solenoid of an embodiment of the present invention. 
           [0041]      FIG. 2  is an enlarged view of A portion of  FIG. 1 . 
           [0042]      FIG. 3  illustrates schematically a magnetic flux flowing between the movable yoke and the fixed yoke when an electric current is applied to the electromagnetic coil of the conventional flow control valve. 
           [0043]      FIG. 4  illustrates schematically a magnetic flux flowing between the movable yoke and the fixed yoke when an electric current is applied to the electromagnetic coil of the flow control valve shown in  FIG. 1 . 
           [0044]      FIG. 5  is a graph illustrating the relationship between a stroke of the movable yoke and a thrust force acting upon the movable yoke when a predetermined current is applied to the electromagnetic coil of the proportional solenoid. 
           [0045]      FIG. 6  is a cross-sectional view of the conventional flow control valve. 
           [0046]      FIG. 7  is a graph illustrating the relationship between a stroke of the movable yoke and a thrust force acting upon the movable yoke when a predetermined current is applied to the electromagnetic coil of the conventional flow control valve. 
           [0047]      FIG. 8  is a graph illustrating the relationship between a thrust force created by the proportional solenoid and a reaction force created by the coil spring. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0048]    A preferred embodiment of the present invention will be described below with reference to the appended drawings. 
         [0049]    The present embodiment is applied to a flow control valve using a proportional solenoid as a valve drive means.  FIG. 1  is a cross-sectional view of the flow control valve.  FIG. 2  is an enlarged drawing of a portion representing a specific feature of the flow control valve of the present embodiment. 
         [0050]    The basic structure of a flow control valve  1  is identical to that shown in  FIG. 6 , structural elements identical to those of  FIG. 6  are denoted by identical reference symbols, explanation thereof is omitted, and only the difference between the two structures is described. 
         [0051]    As follows from  FIG. 1  and  FIG. 2 , a specific feature of the flow control valve  1  of the present embodiment is in a movable yoke  20  of the proportional solenoid  5 . 
         [0052]    Explaining in greater detail, the movable yoke  20  of the proportional solenoid  5  of the present embodiment has a protruding portion  21  protruding to the side of the fixed yoke  10  at the tip surface of the movable yoke, that is, at the edge portion of the surface facing the fixed yoke  10 . 
         [0053]    The protruding portion  21  is formed to have annular shape over the entire circumference of the movable yoke  20  and, as shown in  FIG. 2 , the outer circumferential surface  21   a  of the protruding portion extends substantially parallel to the axial line AL of the movable yoke  20 . 
         [0054]    On the other hand, the inner circumferential surface  21   b  of the protruding portion  21  is tapered as to be inclined at a predetermined angle θ to the axial line AL of the movable yoke  20 , such as to be located farther outside in the radial direction as the tip side, that is the fixed yoke  10 , is approached. 
         [0055]    Further, an end surface  21   c  extending in the direction substantially perpendicular to the axial line AL of the movable yoke  20  is formed at the tip of the protruding portion  21 . 
         [0056]    Thus, a specific feature of the flow control valve  1  of the present embodiment is that an annular protruding portion  21  with a tapered inner surface is formed at the tip of the movable yoke  20  of the proportional solenoid  5 . Because of this specific feature, the thrust force acting upon the movable yoke  20  when an electric current is applied to the electromagnetic coil  9  can be increased with respect to that of the conventional flow control valve  100  (proportional solenoid  50 ) shown in  FIG. 6 . This issue will be explained below. 
         [0057]    First, a magnetic flux flowing between the movable yoke  12  and the fixed yoke  10  when an electric current is applied to the electromagnetic coil  9  of the proportional solenoid  50  of the conventional flow control valve  100  ( FIG. 6 ) will be described with reference to  FIG. 3 . 
         [0058]      FIG. 3  shows a state in which the stroke of the movable yoke  12  is minimal, that is, the distance between the movable yoke  12  and the fixed yoke  10  is minimal. A dot line in the figure shows schematically the magnetic flux flowing between the movable yoke  12  and the fixed yoke  10 . 
         [0059]    As follows from the figure, in the conventional flow control valve  100  in which the tip surface of the movable yoke  12  extends substantially perpendicularly to the axial line AL of the movable yoke  12  (extends substantially parallel to the tip surface of the fixed yoke  10 ), an axial magnetic flux φ 1  flowing in the axial direction from the tip of the movable yoke  12  to the tip of the fixed yoke  10  is larger than a radial magnetic flux φ 2  flowing in the radial direction from the outer circumferential surface of the movable yoke  12  to the inner circumferential surface of a convex portion  13  of the fixed yoke  10  (φ 1 &gt;φ 2 ). Therefore, a thrust force (attraction force) F 1  acting upon the movable yoke  12  under the effect of the axial magnetic flux φ 1  is larger than a thrust force F 2  acting upon the movable yoke  12  under the effect of the radial magnetic flux φ 2 . In other words, the thrust force acting upon the movable yoke  12  is generated mainly based on the axial magnetic flux φ 1 . 
         [0060]    Here, because the distance between the tip of the movable yoke  12  and the tip of the fixed yoke  10  changes when the stroke of the movable yoke  12  changes, the thrust force F 1  generated by the axial magnetic flux φ 1  is greatly affected by the stroke of the movable yoke  12 . For this reason, where the stroke of the movable yoke  12  increases in a region where the stroke of the movable yoke  12  is small, the thrust force acting upon the movable yoke  12  decreases abruptly (see  FIG. 7 ). Therefore, the control range is positioned on a larger stroke side from the region in which the thrust force changes (decreases) abruptly. 
         [0061]    Next, a magnetic flux flowing between the movable yoke  20  and the fixed yoke  10  when an electric current is applied to the electromagnetic coil  9  of the proportional solenoid  5  of the flow control valve  1  ( FIG. 1  and  FIG. 2 ) of the above-described present embodiment will be explained with reference to  FIG. 4 . 
         [0062]      FIG. 4  shows a state in which the stroke of the movable yoke  20  is minimal, that is, the distance between the movable yoke  20  and the fixed yoke  10  is minimal. A dot line in the figure shows schematically the magnetic flux flowing between the movable yoke  20  and the fixed yoke  10 . 
         [0063]    As follows from the figure, in the flow control valve  1  (proportional solenoid  5 ) of the present embodiment in which the annular protruding portion  21  with the inclined inner circumferential surface  21   b  is formed at the tip of the movable yoke  20 , an axial magnetic flux φ 1  flowing in the axial direction from the end surface  21   c  of the protruding portion  21  of the movable yoke  20  to the tip of the fixed yoke  10  is smaller than a radial magnetic flux φ 2  flowing in the radial direction from the outer circumferential surface  21   a  of the protruding portion  21  of the movable yoke  20  to the inner circumferential surface of the convex portion  13  of the fixed yoke  10  (φ 1 &lt;φ 2 ). 
         [0064]    This is apparently because the orientation of the magnetic flux flowing in the protruding portion  21  changes toward the outside in the radial direction since the inner circumferential surface  21   b  of the protruding portion  21  is inclined such as to be located farther outside in the radial direction as the fixed yoke  10  is approached, and also because the distance between a central portion  23  of the tip of the movable yoke  20  and the tip  25  of the fixed yoke  10  is increased with respect to that in the conventional flow control valve. 
         [0065]    Therefore, in the flow control valve  1  of the present embodiment, a thrust force F 2  acting upon the movable yoke  20  under the effect of the radial magnetic flux φ 2  is larger than a thrust force (attraction force) F 1  acting upon the movable yoke  20  under the effect of the axial magnetic flux φ 1 . In other words, in the flow control valve  1  of the present embodiment, the thrust force acting upon the movable yoke  20  is generated mainly based on the radial magnetic flux φ 2 . 
         [0066]    Here, because the distance between the outer circumferential surface  21   a  of the protruding portion  21  of the movable yoke  20  and the inner circumferential surface of the convex portion  13  of the fixed yoke  10  is constant regardless of the stroke of the movable yoke  20 , the thrust force F 2  generated by the radial magnetic flux φ 2  is substantially constant (not affected by the stroke of the movable yoke  20 ) regardless of the stroke of the movable yoke  20 . Therefore, in the flow control valve  1  of the present embodiment, even if the stroke of the movable yoke  12  increases in the region with a small stroke of the movable yoke  20 , the thrust force does not decrease and is maintained at a-substantially constant value. As a result, the thrust force acting upon the movable yoke  20  in the flow control valve  1  (proportional solenoid  5 ) of the present embodiment is larger than in the conventional flow control valve  100  (proportional solenoid  50 ) in which thrust force drops abruptly when the stroke of the movable yoke  12  increases. 
         [0067]    This result will be explained with reference to  FIG. 5 . 
         [0068]      FIG. 5  shows the relationship between the stroke of the movable yoke  20  and the thrust force acting upon the movable yoke  20  when a predetermined electric current is applied to the electromagnetic coil  9  of the proportional solenoid  5  of the flow control valve  1  of the present embodiment. For comparison, the thrust force of the proportional solenoid  50  of the conventional flow control valve  100  shown in  FIG. 6  is also shown by a dot line. 
         [0069]    As follows from the figure, in the flow control valve  1  of the present embodiment, practically no decrease in thrust force is observed in a region with a small stroke of the movable yoke  20 , and the thrust force in the region with a constant thrust force (control range) is increased significantly with respect to that of the conventional flow control valve  100 . Further, the control range of the flow control valve  1  of the present embodiment is wider than the control range of the conventional flow control valve  100  and shifts to the region with a small stroke. 
         [0070]    Therefore, by using the flow control valve  1  of the present embodiment, it is possible to increase the thrust force acting upon the movable yoke  20 , without increasing the proportional solenoid  5  in size, and the hysteresis of the movable yoke  20  and spool  3  can be eliminated or reduced. Further, because the thrust force acting upon the spool  3  also increases, the responsiveness of the flow control valve  1  (spool valve  2 ) is improved. 
         [0071]    Further, in the flow control valve  1  of the present embodiment, the control range is wider than in the conventional flow control valve. Therefore, the strokes of the movable yoke  20  and spool  3  can be set larger than in the conventional flow control valve. As a result, the control range of flow rate can be increased and the maximum flow rate can be raised. 
         [0072]    The applicant carried out a variety of tests by changing the shape of the protruding portion  21  with the object of finding an optimum shape of the protruding portion  21  that produces the above-described effect. 
         [0073]    The results demonstrated that the shape of the protruding portion  21  is preferably set as described hereinbelow. 
         [0074]    First, an angle θ (taper angle) formed by the axial line AL of the movable yoke  20  and the inner circumferential surface  21   b  of the protruding portion  21  shown in  FIG. 2  is preferably set within a range of 35 to 60 degrees. 
         [0075]    The radial length L 1  of the tip surface  21   c  of the protruding portion  21  is preferably set to be equal to or less than 5% of the outer diameter R of the movable yoke  20 . 
         [0076]    The axial length L 2  of the protruding portion  21  is preferably set to about ½ of the stroke (usually, the control range) of the movable yoke  20  and spool  3 . 
         [0077]    The above-described embodiment is presented as an example of the present embodiment, and the present invention is not limited to this embodiment. 
         [0078]    For example, in the present embodiment the flow control valve  1  using the proportional solenoid  5  is explained, but the present invention can be also applied to other means such as a pressure control valve and a direction switching valve, provided that the proportional solenoid is used as the valve drive means. 
         [0079]    Further, the structure of the flow control valve  1  is not limited to that shown in the figures. 
         [0080]    While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention.