Abstract:
A control valve in fluid communication with an actuator may include a pressurized pilot supply and a fluid reservoir fluidly connected to a passageway defined by a valve body. A pilot operator is selectively moveable within the passageway, and a flow metering operator is moveably disposed within the passageway. A fluid make-up operator, disposed in the passageway and in fluid communication with the fluid reservoir and the actuator, directs pressurized pilot supply fluid in response to a decrease in actuator pressure coinciding with a cavitation condition within the actuator. A control chamber within the valve body contains pressurized fluid from the pilot supply and is in fluid communication with the flow metering operator. The flow metering operator fluidly connects the actuator with the fluid reservoir under the influence of the pressurized fluid within the control chamber through activation by either the pilot operator or the fluid make-up operator.

Description:
TECHNICAL FIELD 
     The invention relates generally to an electrohydraulic valve assembly and, more particularly, to an independent metering valve having a fluid make-up function. 
     BACKGROUND 
     An independent metering valve includes a first pair of independently controlled electrohydraulic displacement controlled spool valves for controlling pump-to-cylinder communication between an inlet conduit and a pair of control conduits and a second pair of independently controlled electrohydraulic displacement controlled spool valves for controlling cylinder-to-tank fluid flow between the pair of control conduits and an outlet. Each of the spool valves has a displacement controlled solenoid valve for controlling the position of the spool valve. The spool valves are normally biased to a closed position and are selectively actuated to provide several modes of actuation. 
     This system can provide many functions normally requiring separate valves simply by actuating one or more of the four independently controlled electrohydraulic displacement controlled spool valves. However, one problem that arises is that the pressure control functions requiring fast response, such as pressure relieving and fluid make-up, typically require a line relief valve and a fluid make-up valve, respectively, to be installed on an actuator supply conduit. Both the line relief valve and fluid make-up valve are, many times, large in size and capacity. 
     In a more recent development, as disclosed in U.S. Pat. No. 5,868,059, the valve element of an independent metering valve includes both the fluid relief function and the fluid make-up function. A fluid make-up means provides communication between the control chamber of the valve and an actuator supply conduit so that the valve element moves to an open position when the fluid pressure in the supply conduit drops below a predetermined level. However, this independent metering valve has a relatively complicated valve structure. 
     The present invention is directed to overcoming one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a control valve in fluid communication with an actuator to controllably move an output member of the actuator may include a valve body defining a passageway, a pressurized pilot supply fluidly connected to the passageway, and a fluid reservoir fluidly connected to the passageway. A pilot operator is selectively moveable within the passageway, and a flow metering operator is moveably disposed within the passageway. A fluid make-up operator is disposed in the passageway and in fluid communication with the fluid reservoir and the actuator. The fluid make-up operator operates to direct an amount of pressurized pilot supply fluid in response to a decrease in actuator pressure coinciding with a cavitation condition within the actuator. A control chamber within the valve body is structured and arranged to contain pressurized fluid from the pressurized pilot supply. The control chamber is in fluid communication with the flow metering operator. The flow metering operator is urged to fluidly connect the actuator with the fluid reservoir under the influence of the pressurized fluid within the control chamber through activation by either the pilot operator or the fluid make-up operator. 
     In another aspect of the invention, a method for controllably moving an output member of an actuator includes supplying a pressurized pilot supply fluid to a passageway defined by a valve body. The passageway includes a control chamber. The method also includes selectively moving a pilot operator within the passageway and controllably moving a flow metering operator within the passageway to provide fluid communication between the actuator and a fluid reservoir. A fluid make-up operator is provided in the passageway and in fluid communication with the fluid reservoir and the actuator. The method includes operating the fluid make-up operator to direct an amount of pressurized pilot supply fluid in response to a decrease in actuator pressure coinciding with a cavitation condition within the actuator, and activating either the pilot operator or the fluid make-up operator to urge the flow metering operator to fluidly connect the actuator with the fluid reservoir under the influence of the pressurized fluid within the control chamber 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a diagrammatic and schematic illustration of an embodiment of the present invention with portions shown in cross-section for illustrative convenience; 
     FIG. 2 is a cross-sectional view taken along line II—II of FIG. 1; and 
     FIG. 3 is a cross-sectional view taken along line III—III of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     In accordance with the present invention, an electrohydraulic valve assembly is provided. Referring to FIG. 1, an electrohydraulic valve assembly  110  is shown in combination with a main pump  112 , a fluid reservoir such as a tank  114 , and an actuator such as a hydraulic cylinder  116 . The main pump  112  may include, for example, a high pressure pump. The hydraulic cylinder  116  may include, for example, a rod end chamber  118 , a head end chamber  120 , and an output member  119 . The valve assembly  110  includes a valve body  122  having a plurality of passageways  130 ,  132 ,  134 ,  136 . The diameter of each passageway varies along its length. The valve assembly also includes a plurality of independently-operated, electronically-controlled metering valves  140 ,  142 ,  144 ,  146  individually seated in the passageways  130 ,  132 ,  134 ,  136 , respectively. 
     Each metering valve  140 ,  142 ,  144 ,  146  includes a proportional electromagnetic device  121 ,  123 ,  125 ,  127 , respectively, at a proximal end  124  of the valve body  122 . Throughout the description of the invention, the term “proximal” will refer to a position or a direction toward the proximal end  124  of the valve body  122 . The term “distal” will refer to a position or a direction toward the distal end  126  of the valve body  122 , which lies opposite the proximal end  124 . 
     The plurality of metering valves  140 ,  142 ,  144 ,  146  control fluid flow between the pump  112 , the tank  114 , and the hydraulic cylinder  116 . The metering valves are referred to individually as a cylinder-to-tank head end (CTHE) metering valve  140 , a pump-to-cylinder head end (PCHE) metering valve  142 , a pump-to-cylinder rod end (PCRE) metering valve  144 , and a cylinder-to-tank rod end (CTRE) metering valve  146 , as shown in FIG.  1 . 
     Each metering valve  140 ,  142 ,  144 ,  146  includes a flow metering operator, for example, a metering spool. For example, the metering valve  140  includes a metering spool  150  slideably disposed within the passageway  130  for controlling fluid communication between a pair of annular cavities  160 ,  170 , which are axially spaced along and open into the passageway  130 . Similarly, a metering spool  152  of the metering valve  142  controls fluid communication between a pair of annular cavities  162 ,  172 , a metering spool  154  of the metering valve  144  controls fluid communication between a pair of annular cavities  164 ,  174 , and a metering spool  156  of the metering valve  146  controls fluid communication between a pair of annular cavities  166 ,  176 . 
     A head end cylinder conduit  180  provides fluid communication between the annular cavities  160 ,  162  and the head end chamber  120  of the hydraulic cylinder  116 . A rod end cylinder conduit  182  connects the annular cavities  164 ,  166  with rod end chamber  118  of the hydraulic cylinder  116 . An inlet conduit  184  provides communication between the pump  112  and the annular cavities  172 ,  174  and contains a load-hold check valve  186 . Tank conduits, for example, annular cavities  170 ,  176  are fluidly connected to the tank  114 . 
     Each metering valve  140 ,  142 ,  144 ,  146  also includes a pilot operator, for example, a control spool. For example, the metering valve  140  includes a control spool  151  slideably disposed within the passageway  130  between a pair of control chambers  161 ,  171 , which are axially spaced along the passageway  130  and configured to hydraulically balance the control spool  151 . Similarly, a control spool  153  of the metering valve  142  is hydraulically balanced between a pair of control chambers  163 ,  173 , a control spool  155  of the metering valve  144  is hydraulically balanced between a pair of control chambers  165 ,  175 , and a control spool  157  of the metering valve  146  is hydraulically balanced between a pair of control chambers  167 ,  177 . 
     In addition, the metering valve  140  includes a second pair of annular cavities  181 ,  191  located in the valve body  122 . The annular cavities  181 ,  191  are axially spaced along and open into the passageway  130  within the axial range of the control spool  151 . Annular cavities  183 ,  193  are similarly configured in the metering valve  142 , annular cavities  185 ,  195  are similarly configured in the metering valve  144 , and annular cavities  187 ,  197  are similarly configured in the metering valve  146 . 
     A pilot supply  190  provides a low pressure fluid to the proximal annular cavities  181 ,  183 ,  185 ,  187  about the control spools  151 ,  153 ,  155 ,  157 . The pilot supply  190  may include the main pump with an associated pressure reducing valve, a separate pilot pump with an associated relief valve, or any other conventional source of pressurized fluid known in the art. The distal annular cavities  191 ,  193 ,  195 ,  197  about the control spools  151 ,  153 ,  155 ,  157  are in fluid communication with the tank  114  via a common drain passage  192 . 
     As shown in FIG. 1, pilot valves  194 ,  196  are connected to cylinder conduits  180 ,  182 , respectively. One or both of the pilot valves  194 ,  196  may be configured as a needle valve. An exemplary pilot valve  194  is disposed at the cylinder conduit  180 . The exemplary pilot valve  194  includes a valve spring  106  configured to urge a poppet  105  toward a closed position against a valve seat  107 . A pilot valve passage  108  is configured to provide fluid communication between the cylinder conduit  180  and the distal control chamber  171  when fluid pressure in the cylinder conduit  180  urges the poppet  105  to an open position away from the valve seat  107 . 
     Further, none, one, or both of the pilot valves  194 ,  196  may include an optional proportional electromagnetic device  109 , for example, a solenoid, as shown associated with the exemplary pilot valve  196 . The proportional electromagnetic device  109  provides the capability to adjust the force of the spring  106  acting on the poppet  105 . Thus, the first predetermined pressure may be adjusted easily from an external location at any time and at the option of an operator. 
     While FIG. 2 is a sectional view taken through the metering valve  142 , it discloses the basic structural features of all four metering valves  140 ,  142 ,  144 ,  146 . As shown in FIG. 2, the control spool  153  has a first land  202  axially spaced from a second land  204 . A first limiting collar  206  is disposed at a proximal end  208  of the first land  202  and limits the movement of the control spool  153  in the distal direction. A first spring  210  is disposed in the proximal control chamber  163  between the electromagnetic device  123  and a spring shoulder  212  disposed on the first limiting collar  206 . 
     The control spool  153  and the first limiting collar  206  include a longitudinal throughbore  207  extending the length thereof. The throughbore  207  provides fluid communication between the proximal control chamber  163  and the distal control chamber  173 . As a result, the control spool  153  remains hydraulically balanced. The force of the first spring  210  biases the control spool  153  in a direction away from the electromagnetic device  123  to close communication between the annular cavity  183  and the passageway  132  and to open communication between the passageway  132  and the annular cavity  193 . 
     The control spool  153  comprises a reduced-diameter portion  214  forming an annular chamber  216  between the axially-spaced lands  202 ,  204 . The reduced-diameter portion  214  includes at least one transverse throughbore  218  that opens to the annular chamber  216 , for example, at diametrically-opposed sides of the reduced-diameter portion  214 . Additional throughbores may be provided to meet desired performance criteria. The distal end  220  of the control spool  153  also includes an annular groove  222  forming a distally-facing shoulder  224 . 
     The control spool  153  and the metering spool  152  define the distal control chamber  173  within the passageway  132 . A second spring  226  is disposed in the control chamber  173  between the distally-facing shoulder  224  of the control spool  153  and a proximal end  228  of the metering spool  152 . Thus, the second spring  226  biases the control spool  153  away from the metering spool  152  and against the bias of the first spring  210 . 
     The metering spool  152  comprises a first land  230  axially spaced from a second land  232  and a reduced-diameter portion  234  between the axiallyspaced lands  230 ,  232  and adjacent with the annular cavity  172 . The metering spool  152  also comprises a reduced-diameter distal portion  236  disposed in a spring chamber  138 . An expanded-diameter passageway  240  and the distal end  126  of the valve body  122  define the spring chamber  138 . A groove  242  may be cut into the distal end  126  of the valve body  122 . As shown in FIG. 1, the spring chamber  138  is in communication with the tank  114  so that any fluid leakage into the spring chamber  138  is drained. 
     A second limiting collar  244  is disposed on the distal portion  236  of the metering spool  152  and limits the movement of the metering spool  152  in a proximal direction. A third spring  246  is disposed between a shoulder of the collar  244  and the distal end  126  of the valve body  122 . Thus, the third spring  246  biases the metering spool  152  in a direction toward the control spool  153 . 
     The second land  232  of the metering spool  152  includes metering slots  248  at its proximal end  233 . In one embodiment, the second land  232  comprises four metering slots  248  disposed in two diametrically-opposed pairs. The metering slots  248  may be semi-circular as shown in FIG.  2 . However, it should be appreciated that the second land may include more or less than four metering slots. It should further be appreciated that the metering slots  248  may be shaped and positioned as necessary to achieve desired performance results. The metering slots  248  are configured to provide fluid communication between the annular cavities  162 ,  172  when the metering spool  152  moves distally a sufficient distance for the metering slots  248  to open to the annular cavity  162  . 
     FIG. 3 shows additional structural detail specifically related to the metering valves  140 ,  146 . The structural detail of metering valve  140 , as illustrated in FIG. 3, is essentially identical to the structure of metering valve  146 . 
     As shown in FIG. 3, a first land  330  of the metering spool  150  includes an annular groove  350  defining, in combination with the valve body  122 , a pilot pressure chamber  352 . A second land  332  of the metering spool  150  comprises a make-up valve  354  configured to provide make-up fluid in the event the cylinder conduit  180  reaches a cavitation condition. 
     A pair of longitudinal passages  356 ,  358  extend from the make-up valve  354  through the reduced-diameter portion  334  and first land  330  of the metering spool  150  and to the distal control chamber  171 . A plug  360  is disposed at a proximal end of the first longitudinal passage  356  to close off the passage  356  from the distal control chamber  171 . The second longitudinal passage  358  opens to the distal control chamber  171 , thus being capable of providing fluid communication between the control chamber  171  and the make-up valve  354 . The longitudinal passages  356 ,  358  may be positioned radially opposite to one another, substantially the same radial distance from a central longitudinal axis  300  of the metering valve  140 , as shown in FIG.  3 . However, it should be appreciated that the longitudinal passages may be positioned in an asymmetrical fashion as necessary to achieve desired performance results. 
     A first lateral passage  362  provides fluid communication between the first longitudinal passage  356  and the pilot pressure chamber  352 . The first lateral passage  362  may extend diagonally between the first longitudinal passage  356  and the pilot pressure chamber  352 , as shown, or it may extend radially perpendicular to the central longitudinal axis  300  of the metering valve  140 . Again, it should be appreciated that the configuration of the first lateral passage  362  may be varied to achieve desired performance results. 
     The make-up valve  354  comprises a fluid make-up operator, for example, a valve element  364 , slidably disposed in a valve passageway  366 . The valve element  364  includes a first land  368  axially spaced from a second land  370  and a reduced-diameter portion  372  between the first and second lands  368 ,  370 . The valve element  364  further includes a proximal end portion  374 . 
     A head chamber  376  is defined between the proximal end of the valve passageway  366  and a proximal end  378  of the first land  368 . A diagonally-extending second lateral passage  380  provides communication between the head chamber  376  and the annular cavity  170 . 
     The reduced-diameter portion  372  of the valve element  364  and the valve passageway  366  define an annular chamber  382 . A radially-extending notch  384  is formed in the valve passageway  366  and provides fluid communication between the annular chamber  382  and the first longitudinal passage  356 . The valve passageway  366  also includes a first annular groove  386  axially spaced from the radial notch  384  in the distal direction. The first annular groove  386  is configured such that it provides fluid communication between the annular chamber  382  and the second longitudinal passage  358  in response to movement of the valve element  364  in the distal direction. 
     A second annular groove  388  is disposed at a distal end  390  of the valve passageway  366 . A fourth spring  371 , for example, a weakly loaded spring, is disposed between the second land  370  of the valve element  364  and the distal end  390  of the valve passageway  366 . The fourth spring  371  urges valve element  364  in a direction away from the distal end  390  of the valve passageway  366 . 
     A third lateral passage  392  is disposed in the second land  332  of the metering spool  150  and provides fluid communication between the second annular groove  388  and the cylinder conduit  180  through the annular cavity  160  (FIG.  1 ). The third lateral passage  392  may extend radially, perpendicular to the central longitudinal axis  300  of the metering valve  140 . Again, it should be appreciated that the configuration of the third lateral passage  392  may be varied to achieve desired performance results. 
     Industrial Applicability 
     In use, the metering valves  140 ,  146  control cylinder-to-tank fluid flow while the metering valves  142 ,  144  control pump-to-cylinder fluid flow. Conventional extension of the hydraulic cylinder  116  is achieved by substantially simultaneous, operator-controlled actuation of the metering valves  142 ,  146 , and retraction is achieved by simultaneous operator controlled actuation of the metering valves  144 ,  140 . 
     For example, actuation of the valve  142  moves the metering spool  152  distally establishing fluid flow from the pump  112  to the head end chamber  120 , and actuation of the metering valve  146  moves the metering spool  156  distally establishing fluid flow from the rod end chamber  118  to the tank  114 . Similarly, actuation of the metering valve  144  moves the metering spool  154  distally establishing flow from the pump  112  to the rod end chamber  118 , and actuation of the metering valve  140  moves the metering spool  150  distally establishing fluid flow from the head end chamber  120  to the tank  114 . 
     Numerous less conventional operating modes can be achieved by actuation of a single metering valve or actuation of various combinations of two or more metering valves. However, an understanding of the primary features of the present invention can be achieved by describing the general operation of the metering valve  142  shown in FIG. 2 combined with the additional features of the metering valve  140 , more specifically shown in FIG.  3 . 
     When a proportional electromagnetic device  123 , for example a solenoid, of PCHE metering valve  142  is energized, the first spring  210  is compressed. The control spool  153  is urged toward a proximal end  124  (FIG. 1) of the valve body  122  by the force of the second spring  226 . As a result, the first land  202  moves axially toward the proximal end  124  such that the annular chamber  216  is opened to the pilot supply  190  (FIG.  1 ). The pilot supply  190  is then in fluid communication with the proximal and distal control chambers  163 ,  173  by way of the transverse throughbore  218  and the longitudinal throughbore  207 . 
     The pressure of the fluid in the distal control chamber  173  acts on the proximal end  228  of the first land  230  urging the metering spool  152  in the direction toward the distal end  126  of the valve body  122 . As a result, the compressed load of the second spring  226  is reduced, and the control spool  153  is urged toward the distal end  126  of the valve body  122  by the force of the first spring  210 . As the control spool  153  moves axially in the distal direction, the first land  202  of the control spool  153  reduces the opening between the annular chamber  216  and the pilot supply  190 . The opening between the annular chamber  216  and the pilot supply  190  and the opening between the annular chamber  216  and the tank  114  are reduced until the control chambers  163 ,  173  hydraulically balance the control spool  153 . 
     As the opening between the annular chamber  216  and the pilot supply  190  is reduced, the metering spool  152  is urged in the direction of the proximal end  124  by spring  246  and the metering slots  248  provide fluid communication between the annular cavities  172 ,  162 . Then, pump  112  provides pressurized fluid, via the load-hold check valve  186  and the supply conduit  184 , to the annular cavity  172 . From there, the pressurized fluid is metered to the annular cavity  162 , which directs the fluid to the cylinder conduit  180 , which in turn supplies the fluid to the head end chamber  120  of the hydraulic cylinder  116 . 
     Likewise, a CTHE metering valve  140  may also be controlled with the aid of a proportional electromagnetic device, for example a solenoid. In the CTHE metering valve  140 , the metering slots  248  provide communication between the annular cavities  160 ,  170 . As a result, fluid in the cylinder conduit  180 , received from the head end chamber  120 , is supplied to the tank  114 . The PCRE metering valve  144  and CTRE metering valve  146  function similarly to the PCHE metering valve  142  and CTHE metering valve  140 , respectively, but in relation to the rod end chamber  118  of the hydraulic cylinder  116 . 
     Referring to the CTHE metering valve  140 , such as that shown in FIG. 3, when pressure of fluid in the cylinder conduit  180  exceeds a first predetermined pressure, an amount of the pressurized fluid must be released from the cylinder conduit  180 . Release of the fluid reduces the pressure of the fluid in the cylinder conduit  180  and prevents potential damaging effects to the hydraulic circuit. If, on the other hand, the pressure of fluid in the cylinder conduit  180  drops below a second predetermined pressure, make-up fluid must be supplied to the cylinder  180  to prevent a cavitation condition. 
     As shown in FIG. 1, a pilot valve  194  is connected to the cylinder conduit  180 . When the pressure of fluid in the cylinder conduit  180  exceeds a first predetermined pressure, the pressurized fluid in the cylinder conduit  180  urges the poppet  105  to an open position away from the valve seat  107  against the force of the valve spring  106 . Pressurized fluid then flows through the pilot valve  194  and through the pilot valve passage  108  to the distal control chamber  171  at the proximal end  228  of the metering spool  150 . The pressurized fluid then passes through the longitudinal throughbore  207  and the transverse throughbore  218  into the annular chamber  216  and out to the tank  114 . 
     Since the proximal and distal pressurized fluid chambers  161 ,  171  are in communication with one another, the control spool  151  will not move since it is hydraulically-balanced. As a result, fluid flowing from the pilot supply  190  and into control chamber  173  is restricted at location  203  (FIG.  2 ). However, rather than flow becoming choked at location  203 , the pressure acts on the proximal end  228  of the metering spool  150  through the distal control chamber  171  and moves the metering spool  150  in a distal direction against the force of third spring  246  and the flow is relieved to the tank  114  through the metering slots  248 . 
     As the metering spool  150  moves in an axial direction toward the distal end  126  of the valve body  122 , the metering slots  248  provide fluid communication between the annular cavities  160 ,  170 . Consequently, the pressurized fluid in the cylinder conduit  180  is relieved to the tank  114  through the annular cavities  160 ,  170  and the metering slots  248 . Thus, the pilot valve  194  achieves the relief function for a large amount of pressurized fluid by operating the metering spool  150  to provide a substantial fluid path from the cylinder conduit  180  to the tank  114 . 
     Further, as discussed above, one or both of the pilot valves  194 ,  196  may include an optional proportional electromagnetic device  109 , for example a solenoid, thereby providing the capability to adjust the force of the spring  106  acting on the poppet  105 . Thus, the first predetermined pressure may be adjusted easily from an external location at any time and at the option of an operator. 
     Referring again to FIG. 3, a make-up valve  354  is disposed in the metering spool  150 . When the pressure of fluid in the cylinder conduit  180  drops below the second predetermined pressure, the make-up valve  354  functions in cooperation with the valving element  150  to supply pressurized fluid to the cylinder conduit  180  to prevent a cavitation condition. 
     As shown in FIG. 3, the pilot pressure chamber  352  is in fluid communication with the annular chamber  382  of the make-up valve  354  by way of first lateral passage  362 , first longitudinal passage  356 , and radial notch  384 . 
     The tank  114  communicates with the head chamber  376  of the make-up valve  354  by way of annular cavity  170  and the diagonally-extending second lateral passage  380 . Thus, the pressure of fluid in the tank  114  acts on the proximal portion  374  of the make-up valve element  364 , compressing the fourth spring  371  in a direction toward the distal end  390  of the valve passageway  366 . The cylinder conduit  180  is in communication with the distal end  390  of the valve passageway  366  by way of annular cavity  160 , third lateral passage  392 , and second annular groove  388 . 
     When the cylinder conduit  180  approaches cavitation, the force acting against the second land  370  in a proximal direction becomes less than the force acting on the first land  368  in the distal direction. As a result, the make-up valve element  364  moves in a direction toward the distal end  126  of the valve body  122  against the force of the fourth spring  371 . Consequently, pressurized fluid supplied by the pilot supply  190  flows from the pilot pressure chamber  352 , through the first lateral passage  362 , the first longitudinal passage  356 , the radial notch  384 , the annular chamber  382  of the make-up valve  354 , and the second longitudinal passage  358  into the distal control chamber  171 . 
     From the distal control chamber  171 , the pressurized fluid supplied by the pilot supply  190  can pass through the longitudinal throughbore  207  and the transverse throughbore  218  into the annular chamber  216 . Again, since the proximal and distal pressurized fluid chambers  161 ,  171  are in communication with one another, the control spool  151  will tend to remain in a hydraulically-balanced position. As a result, the opening between the annular chamber  216  and the pilot supply  190  and the opening between the annular chamber  216  and the tank  114  remain minimized. The flow of pressurized fluid is restricted at the opening between the annular chamber  216  and the tank  114 , thus causing a resistant pressure. This resistant pressure acts on the proximal end  228  of the metering spool  150  through the distal control chamber  171  and moves the metering spool  150  in a distal direction against the force of the third spring  246 . 
     As the metering spool  150  moves axially toward the distal end  126  of the valve body  122 , the metering slots  248  provide fluid communication between the annular cavities  160 ,  170 . Consequently, a make-up flow of fluid is supplied to the cylinder conduit  180  by way of the annular cavities  160 ,  170  and the metering slots  248 , thereby eliminating the cavitation condition. Thus, the make-up valve  354  disposed in the metering spool  150  supplies a large amount of make-up fluid by operating the metering spool  150 . 
     In view of the above, it is readily apparent that the structure of the present invention provides an improved and simplified electrohydraulic valve assembly in which the fluid make-up function is integrally formed as part of a metering valve. This provides fast response for pressure relieving and fluid make-up, without special pressure sensors and the need for increased microprocessor computing speed. Moreover, the structure of the assembly is relatively uncomplicated. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the electrohydraulic valve assembly without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.