Abstract:
A hydraulic valve having a hollow valve body configured with a supply port and a supply port. The hollow valve body has several different internal diameters. An elongate spool is reciprocally received in the body and has a large diameter annular spool ring oriented adjacent a sensing land inside the hollow body forming a gap with a sensing land to cause a pressure drop across the gap in response to a flow of fluid from the pressure port through the gap to the control port. A servomotor reciprocally drives the spool lengthwise of the valve body against a spring force. When flow of fluid occurs in response to servomotor movement of the spool, the pressure drop across the gap combined with the spring force determines a pressure balanced location to which the spool is moved.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This claims the benefit of U.S. Provisional Application No. 62/063,074, filed Oct. 13, 2014, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a hydraulic valve, and more particularly, a hydraulic valve having a pressure compensated flow control feature. 
     BACKGROUND OF THE INVENTION 
     A typical flow control valve today consists of a direct drive spool valve used to vary a controlled flow area and an additional spool valve which senses the flow pressure drop across the first valve and restricts or relieves flow in order to maintain a constant flow to a work port regardless of the required working pressure. 
     It is an object of the invention to provide a low flow, easily assembled, low cost flow control valve that enables the aforementioned valve to be replaced with the valve embodying the invention. 
     It is a further object of the invention to provide a direct drive spool valve that incorporates both functions into one spool, that is, both the controlled flow area and the restricting and relieving area functions are controlled simultaneously to maintain constant flow. 
     SUMMARY OF THE INVENTION 
     The objects and purposes of the invention are met by providing a hydraulic pressure compensated flow control valve having a hollow valve body with a supply port and a control port. The hollow valve body has several different internal diameters. An elongate spool is reciprocally received in the hollow body and includes a large integral annular spool ring oriented adjacent a sensing land to form a gap between a perimeter of the spool ring and the sensing land to cause a pressure drop across the gap in response to a flow of fluid from the pressure port through the gap to the control port. A servomotor drives the spool lengthwise within the valve body against a spring force. A magnitude of the pressure drop across the gap combined with the spring force determining a pressure balanced location to which the spool is moved in response to the output force from the servomotor to thereby control the rate of flow of hydraulic fluid from the pressure port to the control port. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and purposes of the invention will be apparent to persons acquainted with apparatus of this general type upon reading the following specification and inspecting the accompanying drawings, in which: 
         FIG. 1  illustrates a longitudinal sectional view of a hydraulic valve embodying a first embodiment of my new invention in a dormant state; 
         FIG. 1A  is an enlarged fragment of  FIG. 1 ; 
         FIG. 2  is a view similar to  FIG. 1  with valve in a first operative state; 
         FIG. 3  is a view similar to  FIG. 2  with the valve in a second operative state; 
         FIG. 4  is a front, right end isometric view of the spool is the aforesaid first embodiment; 
         FIG. 5  illustrates a longitudinal sectional view of a hydraulic valve embodying a second embodiment of my new invention in a dormant state; 
         FIG. 6  is a view similar to  FIG. 5  with valve in a first operative state; 
         FIG. 7  is a view similar to  FIG. 62  with the valve in a second operative state; 
         FIG. 8  is a front, right end isometric view of the spool is the aforesaid second embodiment; 
         FIG. 9  illustrates a longitudinal sectional view of a hydraulic valve embodying a third embodiment of my new invention in a dormant state; 
         FIG. 10  is a view similar to  FIG. 9  with valve in a first operative state; 
         FIG. 11  is a view similar to  FIG. 10  with the valve in a second operative state; 
         FIG. 12  is a front, right end isometric view of the spool is the aforesaid first embodiment; and 
         FIG. 13  illustrates a partial sectional view of a selected one of the aforementioned embodiments of the hydraulic valve controlled by a hydraulic operated pilot. 
     
    
    
     DETAILED DESCRIPTION 
     Certain terminology has been used above and will be used in the following description for convenience in reference only and will not be limiting. The words “up”, “down” “right” and “left” will designate directions in the drawings to which reference is made. The words “in” and “out” will refer to directions toward and away from, respectively, the geometric center of the device and designated parts thereof. Such terminology will include derivatives and words of similar import. 
     First Embodiment of FIGS.  1  to  4   
       FIGS. 1 to 4  illustrate a first embodiment of a hydraulic valve  10  embodying my invention, with  FIG. 1  illustrating the valve in its dormant state, that is, no hydraulic pressure is being applied thereto and there is no supply of electrical energy. This embodiment of the valve  10  includes a conventional electric servomotor  11  having a reciprocal output member  12  having a distal end  13 . Since the electric servomotor is well known to those skilled in this art, see examples thereof in U.S. Pat. Nos. 6,021,876 and 7,628,378, a further detailed discussion thereof is deemed unnecessary. Since servomotor  11  will produce a nonlinear output force characteristic at the distal end of the output member  12  and will be a function of a resistance to movement load applied to it at the distal end  13 . 
     The valve  10  includes a valve body  16  having an elongate and cylindrical internal bore  17  therein opening outwardly at least at one end  18  of the valve body. The valve body  16  includes a supply port P configured to be connected to a pressurized fluid source and an axially spaced control port C configured to be connected to a load. The internal bore  17  has first segment  19  with a first inner diameter D 1  and a second segment  21  with a second inner diameter D 2  that is larger than the diameter D 1 . The first and second segments are both coaxial with a longitudinal axis  22  of the internal bore  17 . The two segments are located between the supply port P and the longitudinally spaced control port C. The internal bore  17  also has a third segment  23  located between the first and second segments and having a third internal diameter D 3  that is larger than the diameter D 2 . The radially outer peripheral surface of the valve body  16  is provided with structure that forms plural and axially spaced annual grooves  24  that each house an O-ring seal member  26  to facilitate the sealed placement of the valve body  16  inside a conforming bore provided in a further structure not shown. 
     The internal bore  17  of the valve body  16  has an axially facing abutment surface  27  defined by a plate  28  resting against a spring clip  29  received an annular groove in the surface of the first segment  19 . 
     An elongate and hollow spool  30  ( FIG. 4 ), circular in cross section, is reciprocally received in the open end  18  of the internal bore  17  of the valve body  16 . A cylindrical surface  32  is provided on the spool and is received in the first segment  19  of the internal bore  17 , the surface  32  having a diameter that is conformed to the diameter D 1  to form thereat a close sliding tolerance fit relation to the first diameter segment. The spool  30  also has an integral annular spool ring  31  that is axially spaced from the surface  32  and which is configured to be located, in the dormant state of the valve  10 , within the second segment  21  of the internal bore  17 . The spool ring  31  has a cylindrical radially outer circumferential surface  33  having an outer diameter D 4  that is slightly less than the second diameter D 2  to form a gap  34  between the surface of the second segment  21  and the external periphery of the annular spool ring  31  as shown in an enlarged  FIG. 1A . As the spool  30  is shifted rightwardly to, for example, the  FIG. 2  or  FIG. 3  position, the periphery of the spool ring  31  will move to a location wholly within the third segment  23  of the internal bore  17  resulting in an enlargement of an gap  33  between the periphery of the spool ring  31  and the surface of the second segment  21  as well as an increase in the axial spacing between the periphery of the spool ring  31  and the internal surface of the third segment  23  having the diameter D 3  as shown in  FIGS. 2 and 3  to define a sensing orifice  45  ( FIGS. 2 and 3 ). 
     The cylindrical surface  32  is of a finite longitudinal length with an edge  36  thereof ( FIG. 4 ) adjacent the third segment  23  and facing the spool ring  31  being provided with at least one notch  37  having a tapered bottom wall  38  increasing in depth between a mid-section of the surface  32  and the edge  36 . If desired, plural identical notches can be provided at circumferentially spaced locations in the surface  32  and the edge  36 . 
     The spool  30  has an axial end face  39  located at a longitudinal end of the spool that is remote from the surface  32  and is engaged by the distal end  13  of the output member  12  of the electric servomotor  11 . The end face  39  closes the end of the hollow spool thereat with two vent holes  35  for hydraulic balancing and, in some cases, hydraulic damping. The opposite end of the spool  30  has an axially facing abutment surface  41  opposing the abutment surface  27 . A spring  40  is located between the surfaces  27  and  41  and is configured to oppose rightward movement of the spool  30 . 
     A peripheral surface  42  at an end of the spool  30  remote from the surface  32  is, in this embodiment, of the same diameter as the surface  32  and is located in an extension segment  21 A of the second segment  21  of the bore  17  in the valve body  16  which has the same inner diameter as the second segment  21 . This will facilitate the installation of the spring  40  and spool  30  with its larger diameter spool ring  31  into the bore  17  through the open end  18  so that the spring  40  is oriented between the abutment surfaces  27  and  41 . To accommodate the radial spacing between the peripheral surface  42  and the inner surface of the extended second segment  21 A, a hollow sleeve  43  is force fit or press fit into the axial end of the extension segment  21 A of the second segment adjacent the open end  18  so as to be fixedly secured to the valve body  16 . An interior of the hollow sleeve  43  has an internal surface  44  with a diameter conforming to the external diameter of the surface  42  to form a close sliding tolerance fit relation to the hollow sleeve  43 . 
     Operation of the FIGS.  1 - 4  Embodiment 
     While the operation of the valve  10  will be understood by those skilled in the art, the below set forth description of the operation is being provided only for convenience in understanding. 
     As stated above, the hydraulic valve  10  illustrated in  FIGS. 1 and 1A  is in a dormant state. As depicted in  FIG. 1 , the spring force of the spring  40  initially locates the surface  32  on the spool  30  so as to block the pressure supply port P and orient the spool ring  31  wholly within the second segment  21  of the internal bore  17  as illustrated in  FIG. 1A . 
     The following explanation of the operation of the valve  10  assumes that the supply pressure at the port P is greater than the required pressure at the control port C. 
     During operation the servomotor  11  is given an electrical input command for a flow rate. The servomotor  11 , in response to this command, pushes the distal end  13  of the output member  12  so that the spool  30  is pushed against the bias spring  40 , opening a communication, via the notches  37  between port P and C. In addition, an area (delta P sensing orifice  45  shown in  FIGS. 2 and 3 ) is established between the spool ring  31  and the inner diameter D 3  related to an area of the notches  37  and a flow of hydraulic fluid will begin passing from the port P across the sensing orifice  45  to port C creating a differential pressure from one side of the spool ring  31  to the other with the port P side being higher. This higher pressure applied to the spool ring area creates a load in opposition to the output load from the servomotor  11 . The spool ring load plus the bias load provided by the spring  40  will strive to obtain a load balance against the output load of the servomotor  11 . 
     The spool  30 , in response to any load imbalance, will automatically move to a position which either increases or decreases the flow from port P to port C until a stabilized flow rate is achieved in response to the load balance with the output load of the servomotor  11  being achieved. 
     For example, if the load from the servomotor  11  is greater than the sum of the load applied to the spool ring  31  and the load from the spring  40 , the servomotor will continue to push the spool  30 , increasing the area of the notches  37  and communication from the port P to the port C. This increased area allows more flow from the port P to the port C and across the spool sensing orifice  45 , increasing the pressure drop and increasing the opposition load from the servomotor  11  until the loads become balanced. The spool  30  stops at this point and maintains that flow value. 
     As another example, if the load from the servomotor  11  is less than the sum of the load applied to the spool ring  31  and the load from the spring  40 , this opposition load will push the spool  30  against the distal end  13  of the output member  12  on the servomotor, reducing the exposed area provided by the notches  37  and fluid communication from the port P to the port C. This reduction in the notch area reduces the flow rate from port P to port C and across the spool sensing orifice  45 , reducing the pressure drop and reducing the opposition load until the servomotor load and the opposition load become balanced. The spool  30  stops at this point and maintains this flow rate. 
     Once the required flow rate is achieved, it will remain at the established level until commanded otherwise. If the electrical input command to the servomotor  11  is either increased or decreased from this point on, the change in servomotor load sets up an imbalanced load condition. The spool  30 , in response to the change in servomotor output, is moved as described above until the new required flow rate is achieved. 
     If for some reason the pressure requirement (working load) changes at port C, the following will occur. For example, if the pressure requirement increases at port C, the flow rate will tend to slow down. In response to this reduction in flow rate, the pressure drop across the spool sensing orifice  45  will start to lower. The opposition load against the output from the servomotor will become reduced resulting in the servomotor output moving the spool  30  against the force from the spring  40  and an increase in the exposed area of the notches  37 . This increase in notch area allows flow to pass with less restriction from port P to port C allowing the load balance to be reestablished with a minimal reduction in flow rate. 
     If, on the other hand, the pressure requirement decreases at the port C, the flow rate will tend to speed up. In response to this increase in flow rate, the pressure drop across the spool sensing orifice  45  will start to increase. The opposition load against the load applied by the servomotor  11  increases and the spool  30  will move against the output of the servomotor decreasing the exposed area of the notches  37 . This decrease in notch area resists a change in flow rate from port P to port C allowing the load balance to be reestablished with a minimal increase in flow rate. 
     Second Embodiment of FIGS.  5  to  8   
       FIGS. 5 to 8  illustrate a second embodiment of a hydraulic valve embodying my invention. Since the second embodiment of  FIGS. 5 to 8  includes many instances of structure identical to the first embodiment, the same reference numbers will be used in describing the second embodiment but will have 100 added thereto. 
       FIG. 5  illustrates the valve  110  in its dormant state, that is, no hydraulic pressure is being applied thereto and there is no supply of electrical energy. This embodiment of the valve  110  includes a conventional electric servomotor  111  having a reciprocal output member  112  having a distal end  113 . 
     The valve  110  includes a valve body  116  having an elongate and cylindrical internal bore  117  therein opening outwardly at least at one end  118  of the valve body. The internal bore  117  has, just like in the first embodiment, a first segment  119  with a first inner diameter D 1  and a second segment  121  with a second inner diameter D 2  that is larger than the diameter D 1 . The first and second segments are both coaxial with a longitudinal axis  122  of the internal bore  117 . The two segment are located between a supply port P and a longitudinally spaced control port C, both being provided in the valve body  116 . The internal bore  117  also has a third segment  123  located between the first and second segments and having a third internal diameter D 3  that is larger than the diameter D 2 . The radially outer peripheral surface of the valve body  116  is provided with structure that forms plural and axially spaced annual grooves  124  that each house an O-ring seal member  126  to facilitate the sealed placement of the valve body  116  inside a conforming bore provided in a further structure not shown. In this particular embodiment, the valve body  116  is longer than the valve body  16 ; specifically has an extended portion  151  which is extended in a direction away from the servomotor  111  to provide a tank port T to facilitate a connection to a tank and a low pressure. The extended portion  151  of the valve body  116  includes an inner diameter conforming to the diameter D 1 . 
     The internal bore  117  of the extended portion  151  of the valve body  116  has an axially facing abutment surface  127  defined by a plate  128  resting against a spring clip  129  received in an annular groove in the inner surface of the bore  117  located in the extended portion  151 . 
     An elongate and hollow spool  130 , circular in cross section, is reciprocally received in the open end  118  of the internal bore  117  of the valve body  116 . A cylindrical surface  132  is provided on the spool and is received in the first segment  119  of the internal bore  117 , the surface  132  having a diameter that is conformed to the diameter D 1  to form thereat a close sliding tolerance fit relation to the first diameter segment. The spool  130  also has an integral annular spool ring  131  that is axially spaced from the surface  132  and which is configured to be located, in the dormant state of the valve  110 , within the second segment  121  of the internal bore  117 . The spool ring  131  has a cylindrical radially outer circumferential surface  133  having an outer diameter D 4  that is slightly less than the second diameter D 2  to form a gap identical to the gap  34  between the surface of the second segment  21  and the external periphery of the annular spool ring  31  as shown in the enlarged  FIG. 1A . As the spool  130  is shifted rightwardly to, for example, the  FIG. 6  or  FIG. 7  position, the periphery of the spool ring  131  will move to a location wholly within the third segment  123  of the internal bore  117  resulting in an enlargement of the gap between the periphery of the spool ring  131  and the surface of the second segment  121  as well as an increase in the spacing between the periphery  133  of the spool ring  131  and the internal surface of the third segment  123  having the diameter D 3  as shown in  FIGS. 6 and 7 . 
     The spool  130  includes an extended portion  152  ( FIG. 8 ) that extends into the extended portion  151  of the valve body  116 . The extended portion  152  of the spool  130  includes a cylindrical surface  153  having a diameter that is conformed to the diameter D 1  to form thereat a close sliding tolerance fit relation to the first diameter segment within the extended portion  151  of the valve body  116 . 
     The cylindrical surface  132  is of a finite longitudinal length with an edge  154  thereof ( FIG. 8 ) remote from the spool ring  131  being provided with at least one notch  156  having a tapered bottom wall  157  increasing in depth between a mid-section of the surface  132  and the edge  154 . If desired, plural identical notches  156  can be provided at circumferentially spaced locations in the surface  132  and the edge  154 . 
     The spool  130  has an axial end face  139  located at a longitudinal end of the spool that is remote from the surface  132  and is engaged by the distal end  113  of the output member  112  of the electric servomotor  111 . The end face  139  closes the end of the hollow spool thereat with two vent holes  135  for hydraulic balancing and, in some cases, hydraulic damping. The opposite end of the spool  130  is open and has an axially facing abutment surface  141  opposing the abutment surface  127 . A spring  140  is located between the surfaces  127  and  141  and is configured to oppose rightward movement of the spool  130 . 
     A peripheral surface  142  at an end of the spool  130  remote from the surface  132  and adjacent the open end  118  of the valve body  116  is, in this embodiment, of the same diameter as the surface  132  and is located in an extension segment  121 A of the second segment  121  of the bore  117  in the valve body  116  which has the same inner diameter as the second segment  121 . This will facilitate the installation of the spring  140  and spool  130  with its larger diameter spool ring  131  into the bore  117  through the open end  118  so that the spring  140  is oriented between the abutment surfaces  127  and  141 . To accommodate the radial spacing between the peripheral surface  142  and the inner surface of the extended second segment  121 A, a hollow sleeve  143  is force fit or press fit into the axial end of the extension segment  121 A of the second segment adjacent the open end  118  so as to be fixedly secured to the valve body  116 . An interior of the hollow sleeve  143  has an internal surface  144  with a diameter conforming to the external diameter of the surface  142  to form a close sliding tolerance fit relation to the hollow sleeve  143 . 
     In this particular embodiment, the hollow interior  158  of the spool  130  is connected to the tank port T through a radial extending passageway  159  oriented between the surfaces  132  and  153  on the spool  131  as shown in  FIGS. 5 to 8 . The open end and interior region of the hollow spool  130  is also connected to tank. 
     Operation of the FIGS.  5  to  8  Embodiment 
     While the operation of the valve  110  will be understood by those skilled in the art, the below set forth description of the operation is being provided only for convenience in understanding. 
     As stated above, the hydraulic valve  110  illustrated in  FIG. 5  is in a dormant state. As depicted in  FIG. 5 , the spring force of the spring  140  initially locates the surface  132  on the spool  130  so as to not block the pressure supply port P and its connection to the tank port T while simultaneously blocking the connection between the pressure port P and the control port C. In addition, the spool  130  is initially oriented so that the spool ring  131  is located within the second segment  121  of the internal bore  117  as illustrated in  FIG. 5 . 
     During operation, the servomotor  111  is supplied with an electrical input command for a flow rate. The servomotor, in response to this command, causes the distal end  113  of the output member  112  to push the spool against the force of the spring  140  to thereby open communication between ports P and C. The land or surface  132  between P and T begins to partially block the connection between the port P and the port T except for the area provided by the notches  156  which serve to provide a limited connection between the port P and port T. At this stage, the notches  156  on the land or surface  132  provide the only communication between the port P and the port T. 
     In addition, an area (delta P sensing orifice or gap  145 ) between the spool ring  131  and the third segment  123  in the valve body  116 , related to the notch area, will be established as shown in  FIGS. 6 and 7 . 
     As stated above, the flow of hydraulic fluid passing from the port P to the tank port T, which was available prior to activation, is restricted by the notches  156  and this restriction causes the pressure at port P to rise. If the pressure at port P cannot rise to a level adequate to overcome the pressure requirement to pass fluid to port C, there will be no flow across the spool sensing orifice  145 . With no flow across the sensing orifice  145 , there will not be enough load applied to the spool ring  131  to offset the load from the servomotor to thereby cause the servomotor  111  to continue to push the spool  130  to restrict the notch area and the flow from port P to port T even further. 
     The aforesaid will continue to happen until the pressure at port P rises to a level high enough to open the flow of fluid to port C through the gap  161  between the across flow sensing orifice  145  as shown in  FIG. 6  and  FIG. 7 . The servomotor will continue pushing the spool  130 , restricting the flow to port T, until a flow is achieved through the gap  161  and across the spool sensing orifice  145  to create a differential pressure/load working on the spool ring  131  in opposition to the servomotor output load that is adequate to balance, with the aide of the bias of the spring  140 , the servomotor output load. The spool will stop moving and this flow will be maintained. Once the required flow rate is achieved it will remain at this level until commanded otherwise. 
     If the input command to the servomotor is either increased or decreased from this point on, the change in servomotor load sets up an imbalanced load condition. The spool  130 , in response to this, either moves against the bias spring  140  to increase flow to port C or against the servomotor load to decrease flow to port C. In both cases, a load balance is reestablished when the required new flow rate is achieved. 
     If for some reason, the pressure requirement (working load) changes at port C, the following will occur. If the pressure requirement increases at port C, the flow rate will tend to slow down. In response to this reduction in flow rate, the pressure drop across the spool sensing orifice  145  will start to lower. The opposition load against the servomotor  111  will reduce the ability of the servomotor to move the spool against the force of the bias spring  140 , thereby resulting in a decrease of the openness of the spool notch area. This decrease in notch area openness further increases the restriction from port P to port T causing the pressure at port P to rise to a level adequate to maintain the required flow to port C with a minimal reduction in flow. 
     On the other hand, if the pressure requirement decreases at port C, the flow rate will tend to increase. In response to this increase in flow rate, the pressure drop across the spool sensing orifice  145  will start to increase. The opposition load against the servomotor thereby increases and the spool moves against the force of the servomotor, increasing the spool notch area to port T. This increase in notch area allows the flow passing from port P to port T to do so at a lower pressure. This pressure will continue to reduce until a minimally increased flow rate from port P to port C reestablishes the load balance. 
     Third Embodiment of FIGS.  9  to  12   
       FIGS. 9 to 12  illustrate a third embodiment of a hydraulic valve embodying my invention. Since the third embodiment of  FIGS. 9 to 12  includes many instances of structure identical to the first and second embodiments, the same reference numbers will be used in describing the third embodiment but will be a like  200  series number. 
       FIG. 9  illustrates the valve  210  in its dormant state, that is, no hydraulic pressure is being applied thereto and there is no supply of electrical energy. This embodiment of the valve  210  includes a conventional electric servomotor  211  having a reciprocal output member  212  having a distal end  213 . 
     The valve  210  includes a valve body  216  that is identical in construction to the valve body  116 . More specifically, the valve body  216  has an elongate and cylindrical internal bore  217  therein opening outwardly at least at one end  218  of the valve body. The internal bore  217  has, just like in the first and second embodiments, a first segment  219  with a first inner diameter D 1  and a second segment  221  with a second inner diameter D 2  that is larger than the diameter D 1 . The first and second segments are both coaxial with a longitudinal axis  222  of the internal bore  217 . The two segments are located between a supply port P and a longitudinally spaced control port C, both being provided in the valve body  216 . The internal bore  217  also has a third segment  223  located between the first and second segments and having a third internal diameter D 3  that is larger than the diameter D 2 . The radially outer peripheral surface of the valve body  216  is provided with structure that forms plural and axially spaced annual grooves  224  that each house an O-ring seal member  226  to facilitate the sealed placement of the valve body  216  inside a conforming bore provided in a further structure not shown. In this particular embodiment, the valve body  216  is identical to the valve body  116 ; specifically, it has an extended portion  251  that is extended in a direction away from the servomotor  111  to provide a bypass port BP to facilitate a connection to a bypass circuit. The extended portion  251  of the valve body  216  includes an inner diameter conforming to the diameter D 1 . 
     The internal bore  217  of the extended portion  251  of the valve body  216  has an axially facing abutment surface  227  defined by a plate  228  resting against a spring clip  229  received an annular groove in the inner surface of the bore  217  located in the extended portion  151 . 
     An elongate and hollow spool  230 , circular in cross section, is reciprocally received in the open end  218  of the internal bore  217  of the valve body  216 . The spool  230  is identical to the spool  130  except for the provision of notches  236  and  256  on both laterally spaced edges  236  and  254  of a cylindrical surface  232  ( FIG. 12 ). More specifically, the cylindrical surface  232  is provided on the spool and is received in the first segment  219  of the internal bore  217 , the surface  232  having a diameter that is conformed to the diameter D 1  to form thereat a close sliding tolerance fit relation to the first diameter segment. The spool  230  also has an integral annular spool ring  231  that is axially spaced from the surface  232  and which is configured to be located, in the dormant state of the valve  210 , within the second segment  221  of the internal bore  217 . The spool ring  231  has a cylindrical radially outer circumferential surface  233  having an outer diameter D 4  that is slightly less than the second diameter D 2  to form a gap identical to the gap  234  between the surface of the second segment  221  and the external periphery of the annular spool ring  231 . As the spool  230  is shifted rightwardly to, for example, the  FIG. 10  or  FIG. 11  position, the periphery of the spool ring  231  will move to a location wholly within the third segment  223  of the internal bore  217  resulting in an enlargement of the gap between the periphery of the spool ring  231  and the surface of the second segment  221  as well as an increase in the spacing between the periphery  233  of the spool ring  231  and the internal surface of the third segment  223  having the diameter D 3  as shown in  FIGS. 10 and 11  to define a sensing orifice  245 . 
     The spool  230  includes an extended portion  252  ( FIG. 12 ) that extends into the extended portion  251  of the valve body  216 . The extended portion  252  of the spool  230  includes a cylindrical surface  253  having a diameter that is conformed to the diameter D 1  to form thereat a close sliding tolerance fit relation to the first diameter segment within the extended portion  251  of the valve body  216 . 
     The cylindrical surface  232  is of a finite longitudinal length with both edges  236  and  254  thereof ( FIG. 12 ) being provided with at least one notch  237  and  256 , respectively, each having a tapered bottom wall  238 ,  257  increasing in depth between a mid-section of the surface  232  and the respective edges  236  and  254 . If desired, plural identical notches  236  and  256  can be provided at circumferentially spaced locations in the surface  232  and the respective edges  236  and  254 . 
     The spool  230  has an axial end face  239  located at a longitudinal end of the spool that is remote from the surface  232  and is engaged by the distal end  213  of the output member  212  of the electric servomotor  211 . The end face  239  closes the end of the hollow spool thereat with two vent holes  235  for hydraulic balancing and, in some cases, hydraulic damping. The opposite end of the spool  230  is open and has an axially facing abutment surface  241  opposing the abutment surface  227 . A spring  240  is located between the surfaces  227  and  241  and is configured to oppose rightward movement of the spool  230 . 
     A peripheral surface  242  at an end of the spool  230  remote from the surface  232  and adjacent the open end  218  of the valve body  216  is, in this embodiment, of the same diameter as the surfaces  232  and  253  and is located in an extension segment  221 A of the second segment  221  of the bore  217  in the valve body  216  which has the same inner diameter as the second segment  221 . This will facilitate the installation of the spring  240  and spool  230  with its larger diameter spool ring  231  into the bore  217  through the open end  218  so that the spring  240  is oriented between the abutment surfaces  227  and  241 . To accommodate the radial spacing between the peripheral surface  242  and the inner surface of the extended second segment  221 A, a hollow sleeve  243  is force fit or press fit into the axial end of the extension segment  221 A of the second segment adjacent the open end  218  so as to be fixedly secured to the valve body  216 . An interior of the hollow sleeve  243  has an internal surface  244  with a diameter conforming to the external diameter of the surface  242  to form a close sliding tolerance fit relation to the hollow sleeve  243 . 
     In this particular embodiment, the hollow interior  258  of the spool  230  is not connected to the bypass port BP but is connected to tank. 
     Operation of the FIGS.  9  to  12  Embodiment 
     In this embodiment, it is assumed that the pressure at the port BP is significantly lower than the required pressure at the port C. During operation, the servomotor  211  is supplied with a given an input command for a flow rate. The servomotor, in response to this command, pushes the spool  230  against the force of the bias spring  240  to thereby open a communication between ports P and C. The land or surface  232  between the ports P and BP is engaged into the valve body allowing the notches  256  on that edge of that land to be the only communication between the ports P and BP as shown in  FIG. 10 . In addition, the delta P sensing orifice  245  between the surface  233  on the spool ring  231  and the inner surface  223  defining the diameter D 3  of the bore  217 , related to the notch area, will be established. 
     The flow passing from the port P to the port BP, which was available prior to activation, is restricted by the flow notches  256  to the port BP. This restriction causes the pressure at port P to rise. If the pressure at port P cannot rise to a level adequate to overcome the pressure requirement to pass fluid to port C, there will be no flow across the spool sensing orifice  245 . With no flow across the sensing orifice  245 , there will not be enough spool load to offset the load applied by the servomotor  211  resulting in the servomotor continuing to push the spool  230  and restricting the notch area of the notches  256  from port P to port BP even further. This will continue to happen until the pressure at port P rises to a level high enough to provide flow to port C across the flow sensing orifice  245 . The servomotor  211  will continue pushing the spool  230  and restricting the flow to port BP until a flow across the spool sensing orifice  245  is achieved that creates a differential pressure/load working on the spool ring  231  in opposition to the output load from the servomotor  211  and be adequate to balance, with the aide of the force applied by the bias spring  240 , the servomotor output load. The spool  230  will stop moving and this flow will be maintained. Once the required flow rate is achieved, it will remain at this level until commanded otherwise. 
     If the input command to the servomotor is either increased or decreased from this point on, the change in servomotor load sets up an imbalanced load condition. The spool  230 , in response to this, either moves against the force of the bias spring  240  to increase flow to port C or against the servomotor output to decrease flow to port C. In both cases, a load balance is reestablished when the required new flow rate is achieved. 
     More specifically, and as an example, if the pressure at the port BP is significantly lower than the required pressure at port C and if, for some reason, the pressure requirement (working load) changes at the port C, the following will occur. If the pressure requirement increases at port C, the flow rate will tend to slow down. In response to this reduction in flow rate, the pressure drop across the spool sensing orifice  245  will start to lower. The opposition load against the servomotor will be reduced and the servomotor will move the spool  230  against the force of the bias spring  240  thereby decreasing the spool notch area of the notches  256 . This decrease in notch area further increases the restriction from port P to port BP to thereby raise the pressure at port P to a level adequate to maintain the required flow to port C with minimal reduction in flow. 
     If, on the other hand, the pressure requirement decreases at port C, the flow rate will tend to increase. In response to this increase in flow rate, the pressure drop across the spool sensing orifice  245  will start to increase. The opposition load against the servomotor increases and the spool  230  will be moved against the servomotor force to increase the spool notch area of the notches  256  to the port BP. This increase in notch area allows the flow passing from port P to port BP to do so at a lower pressure. This pressure will reduce until a minimally increased flow rate from port P to port C reestablishes the load balance. 
     Now suppose that the pressure at port BP is significantly higher than the required pressure at port C″ and, during operation, the servomotor is supplied with given input command for a flow rate. The servomotor will push the spool  230  against the force of the bias spring  240  to the  FIG. 10 or 11  position to thus cause an opening of communication, via the notches  237 , between ports P and C. In addition, the delta P sensing orifice  245  will be established between the spool ring  231  and the inner surface  223  defining the diameter D 3 , related to the notch area, and flow will begin passing from port P, across the sensing orifice  245  to the port C to thereby create a differential pressure from one side of the spool ring  231  to the other, with the port P side being higher. This higher pressure, applied to the spool ring area, will create a load in opposition to the servomotor output load. This spool ring load plus the bias spring load will strive to obtain a load balance against the servomotor output load. The spool  230 , in response to any load imbalance, will move to a position which either increases or decreases the flow from the port P to the port C until the required flow rate is achieved. 
     Finally, the delta P sensing orifice in all of the embodiments described above is largely based on the servomotor magnetic design and is made to compensate for the change in magnetic output load as the armature strokes to push the spool. If, as described above, the magnetic loads any given input command decreases as the servomotor strokes or extends, the delta P sensing orifice will increase as the spool is pushed. This is caused by the reduction in load available as the servomotor strokes so that a reduction in the delta P is required at any given flow rate. If the magnetic loads, at any given input command remains relatively constant as the servomotor strokes, the delta P sensing orifice area can remain constant. Therefore, and if desired, a hydraulic fluid operated pilot servomotor as shown in  FIG. 13  can be used to achieve a constant stroke instead of the electric servomotors described above. 
     Although particular preferred embodiments of the invention have been disclosed in detail for illustrative purposes, it will be recognized that variation or modifications of the disclosed apparatus lie with the scope of the present invention.