Abstract:
A valve assembly couples a plurality of hydraulic actuators to a variable displacement pump and to a tank. A separate valve is associated with each hydraulic actuator and comprises a variable flow source orifice between the supply conduit and a summation node coupled to a pump control port, a variable metering orifice between the summation node and the associated hydraulic actuator, and a variable bypass orifice between the summation node and the tank. As a valve operates to enlarge the metering orifice, the flow source orifice also enlarges, and the bypass orifice shrinks. When the valve operates to shrink the metering orifice, the flow source orifice also shrinks and the bypass orifice enlarges. Those operations vary fluid flow in and out of the summation node, which alters pressure applied to the pump control, thereby causing the pump output to vary as required to drive the associated hydraulic actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention relates to valve assemblies for operating hydraulically powered machinery; and more particularly to such valve assemblies that produce a pressure signal which controls a variable displacement hydraulic pump. 
         [0005]    2. Description of the Related Art 
         [0006]    The speed of a hydraulically driven working member on a machine depends upon the cross-sectional area of principal narrowed orifices of the hydraulic system and the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to eliminate the pressure drop. These previous control systems include load sense conduits which transmit the pressure at the valve workports to the input of a variable displacement hydraulic pump supplying pressurized hydraulic fluid in the system. The resulting self adjustment of the pump output provides an approximately constant pressure drop across a control orifice whose cross-sectional area can be controlled by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of movement of the working member is determined only by the cross-sectional area of the orifice. 
         [0007]    One such system is disclosed in U.S. Pat. No. 5,715,865 entitled “Pressure Compensating Hydraulic Control Valve System” in which a separate valve section controls the flow of hydraulic fluid from the pump to each hydraulic actuator that drive a working member. The valve sections are of a type in which the greatest load pressure acting on the hydraulic actuators is sensed to provide a load sense pressure which is transmitted to the control input port of the pump. The greatest load pressure is determined by daisy chain of shuttle valves that receives the load pressure from all the valve sections. 
         [0008]    Each valve section includes a control valve, with a variable metering orifice, and a separate pressure compensating valve. The output pressure from the pump is applied to one side of the metering orifice and the pressure compensating valve at the other side of the metering orifice, responds to the load sense pressure, so that the pressure drop across the metering orifice is held substantially constant. 
         [0009]    While this system is effective, it requires a separate pressure compensating valve and a shuttle valve in each valve section, in addition to the control valve that has the metering orifice. These additional components add cost and complexity to the hydraulic system, which can be a important consideration for less expensive machines. Thus, there is need for a less expensive and less complex technique for performing this function. 
       SUMMARY OF THE INVENTION 
       [0010]    A control valve assembly is provided for a hydraulic system in which fluid from a variable displacement pump is furnished into a supply conduit for operating a plurality of hydraulic actuators. Fluid from the plurality of hydraulic actuators enters a return conduit through which that fluid flows to a tank. 
         [0011]    The control valve assembly includes a flow summation node and a plurality of control valves. The flow summation node is connected to a control input port of the variable displacement pump. Each of the plurality of control valves is operatively connected so that as it opens, fluid flow from the variable displacement pump to the flow summation node increases, fluid from the flow summation node to a respective one of the plurality of hydraulic actuators increases, and fluid flow from the flow summation node to the return conduit decreases. This operation varies pressure applied to the control input port of the variable displacement pump, which responds by increasing the fluid furnished into the supply conduit, in order to satisfy an increased fluid demand for operating the respective hydraulic actuator. 
         [0012]    In one aspect of the present invention, each control valve further comprises a variable flow path through which fluid flows from the associated hydraulic actuator to the return conduit. 
         [0013]    In another aspect of the present invention, each control valve comprises (1) a variable flow source orifice connected between the variable displacement pump and the flow summation node, (2) a metering orifice connected between the flow summation node and the associated hydraulic actuator for varying the flow of fluid there between, and (3) a variable bypass orifice connected between the flow summation node and the return conduit. Wherein for a given control valve, as the metering orifice enlarges, the variable flow source orifice also enlarges and the variable bypass orifice shrinks; and as the metering orifice shrinks, the variable flow source orifice also shrinks and the variable bypass orifice enlarges in that one valve. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a diagram of a hydraulic system that incorporates the present invention; and 
           [0015]      FIG. 2  is a schematic diagram of the hydraulic system in  FIG. 1  with certain internal components separated from the control valves and rearranged for a better understanding of their functional relationships. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The term “directly connected” as used herein means that the associated components are connected together by a conduit without any intervening element, such as a valve, an orifice or other device, which restricts or controls the flow of fluid beyond the inherent restriction of any conduit. If a component is described as being “directly connected” between two points or elements, that component is directly connected to each such point or element. 
         [0017]    With initial reference to  FIG. 1 , a hydraulic system  10  has three hydraulic functions  11 ,  12  and  13 , although a greater or lesser number of hydraulic functions may be used in other hydraulic systems that practice the present invention. Each hydraulic function  11 ,  12  and  13  respectively comprises a valve unit  14 ,  15  or  16  and a hydraulic actuator  21 ,  22  or  23 , such as a piston-cylinder arrangement, however, other types of actuators can be used. The three valve units  14 ,  15  and  16  combine to form a control valve assembly  17 . The valve units may be in physically separate assemblies or in a single monolithic assembly. The first valve unit  14  has a first control valve  24 , the second valve unit  15  has a second control valve  25 , and the third valve unit  16  has a third control valve  26 . Each control valve  24 ,  25  and  26  controls the flow of fluid between the associated hydraulic actuator  21 ,  22  or  23  and both a variable-displacement pump  20  and a tank  18 . The pump  20  furnishes pressurized fluid to a supply conduit  28  and is of a type such that the output pressure is equal to a pressure applied to a control input port  19  plus a fixed predefined amount referred to as the “pump margin”. The pump  20  increases or decreases its displacement in order to maintain the pump margin”. As an example, if the difference between the outlet pressure and control input port pressure is less than the pump margin, the pump will increase the displacement. If the difference between the outlet pressure and control input port pressure is greater than the pump margin, then pump displacement is reduced. It is commonly known that flow through an orifice can be represented as being proportional to the flow area and the square root of differential pressure. Since this pump control method provides a constant differential pressure of “pump margin”, the flow out of the pump  20  will be linearly proportional to the flow area between the pump outlet and control input port  19 . Fluid also flows into the tank  18  through a return conduit  30 . The supply conduit  28  and return conduit  30  extend to each of the valve units  14 - 16 . 
         [0018]    Each of the control valves  24 ,  25  and  26  is an open-center, three-position, valve and may be a spool type valve, for example. Although in the exemplary hydraulic system  10 , the control valves  24 - 26  are indicated as being solenoid operated, one or more of them could be operated by a pilot pressure or a mechanical lever or linkage. 
         [0019]    The first control valve  24  will be described in detail with the understanding that the description applies to the other two control valves  25  and  26  as well. The first control valve  24  has a supply port  32  that is connected to the supply conduit  28  from the pump  20 . A variable flow source orifice  34  within the control valve provides fluid communication between the supply port  32  and a flow outlet  36 . To facilitate understanding a subsequent operational description of the hydraulic system  10 , the variable flow source orifices for each of the control valves  24 ,  25  and  26  are identified with numerals  34   a ,  34   b  and  34   c , respectively. The flow outlet  36  of the first control valve is directly connected to a conduit that is connected to the flow outlet in all the valve units  14 - 16  and forms a flow summation node  44 . Thus, each variable flow source orifice  34   a, b , and  c  within a control valve is directly connected between the supply conduit  28  and the flow summation node  44  and provides a separate variable fluid path there between. 
         [0020]    The flow outlet  36  is connected by a conventional load check valve  38  to a metering orifice inlet  40  of the control valve, so that fluid cannot flow from the metering orifice inlet back into the supply conduit when a large load acts on the associated hydraulic actuator  21 . A variable metering orifice  45  within the first control valve  24  connects the flow outlet  36  to one of two workports  46  and  48  depending upon the direction that the first control valve is moved from the center, neutral position. The two workports  46  and  48  connect to different ports on the first hydraulic actuator  21  in the respective first hydraulic function  11 . The control valve  24  is normally biased into the center position in which both workports  46  and  48  are closed. 
         [0021]    The first control valve  24  also has a bypass orifice  50   a  that is directly connected between a bypass inlet  51  and a bypass outlet  52  of that control valve. The bypass orifices for each of the other control valves  25  and  26  are identified by numerals  50   b  and  50   c , respectively. The bypass orifices  50   a ,  50   b  and  50   c  are connected in series to provide fluid communication between the summation node  44  and the return conduit  30 . Specifically for the exemplary hydraulic system  10 , the bypass inlet  51  of the third control valve  26  is directly connected to the summation node  44 . The bypass outlet  52  of that control valve  26  is directly connected to the bypass inlet  51  of the second control valve  25  whose bypass outlet is directly connected to the bypass inlet  51  of the first control valve  24 . The bypass outlet  52  of the first control valve  24  is connected directly to the return conduit  30 . Thus the series of the bypass orifices  50   a ,  50   b  and  50   c  is directly connected between the summation node  44  and the return conduit  30 . 
         [0022]      FIG. 2  is a schematic diagram of the hydraulic system  10  in which the variable flow source orifices  34   a, b  and  c  and the bypass orifices  50   a, b  and  c  are arranged in more functional groupings with those respective orifices shown outside the corresponding control valve  24 ,  25  and  26  in which they are actually located. This functional diagram shows that the three variable flow source orifices  34   a, b  and  c  are connected in parallel directly between the supply conduit  28  from the pump  20  and the flow summation node  44 . This parallel connection forms a variable flow section  56 . The three bypass orifices  50   a, b  and  c  are connected in series between the flow summation node  44  and the return conduit  30  to the tank  18  and form a bypass section  58  of the hydraulic system  10 . 
         [0023]    Assume initially that all the control valves  24 - 26  are in the center position in which both workports  46  and  48  are closed. In that state, the output from the pump  20 , applied to supply conduit  28 , passes through the variable flow source orifices  34   a - c , which are all now shrunk to a relatively small flow areas. Therefore, a relatively small amount of fluid flows from the pump  20  through the variable flow section  56  to the summation node  44 . At this time, all the bypass orifices  50   a - c  in the bypass section  58  are enlarged to provide relatively large flow areas, thereby allowing the fluid entering the summation node  44  to pass easily into the return conduit  30 . As a consequence, the pressure at the fluid summation node  44  is at a relatively low level, that is transmitted through a pump control conduit  60  to the control input port  19  of the variable displacement pump  20 . 
         [0024]    Alternatively when a control valve  24 ,  25  or  26  is in the center position, its variable flow source orifice  34   a, b  or  c  can be fully closed so that no fluid flows through that control valve between the supply conduit  28  and the flow summation node  44 . In this version of the system, a separate small, fixed orifice  35  may be added to connect the supply conduit  28  to the flow summation node  44  in the variable flow section  56 , so that some flow from the supply conduit enters the flow summation node when all the control valves are in the center position. 
         [0025]    Operation of the present control technique will be described in respect of the first hydraulic function  11  with the understanding that the other hydraulic functions  12  and  13  operate in the same manner. The opening movement of the first control valve  24  in either direction from the center position connects the metering orifice inlet  40  through the variable metering orifice  45  to one of the workports  46  or  48 , depending upon the direction of that motion. Opening the first control valve  24  also connects the other workport  48  or  46  to the outlet port  42  that leads to the return conduit  30 . At the same time, the variable flow source orifice  34   a  enlarges by an amount related to the distance that the control valve moves, thereby causing the pump to increase fluid flow from the supply conduit  28  to the flow summation node  44  in order to maintain the “pump margin,” as previously described. Simultaneously, the size of the bypass orifice  50   a  shrinks, causing pressure at the summation node  44  to increase. Thus as the first control valve  24  opens a path through which fluid is supplied to the first hydraulic actuator  21 , the flow through the variable flow section  56  into the summation node  44  increases, while the restriction, created by bypass orifice  50   a , to flow occurring out of that node to the tank return conduit  30  also increases thereby causing the pressure at the flow summation node  44  to increase. 
         [0026]    When the flow summation node pressure is sufficiently great to overcome the load force acting on the first actuator  21 , fluid begins to flow through the metering orifice  45  in the first control valve  24  to drive the first actuator. 
         [0027]    At the same time that the first control valve  24  is opening one or more of the other control valves  25  or  26  also may be open. Their respective variable flow source orifices  34   b  and  34   c  also will be conveying fluid from the supply conduit  28  into the flow summation node  44 . Because the three variable flow source orifices  34   a - 34   c  are connected in parallel, the same pressure differential is across each of those orifices. That pressure differential and the cross sectional area of each flow source orifice determines the amount of flow through that orifice. The total flow into the flow summation node is the aggregate of the individual flows through each variable flow source orifice  34   a - 34   c . As a result, the sum of the areas that each variable flow source orifice is open determines the aggregate flow into the flow summation node  44  and thus controls the output flow from the variable displacement pump  20 . The respective flow area of the metering orifice  45  in each control valve  24 ,  25 ,  26  and the respective load forces on actuators  21 ,  22 , and  23  determine the amount of flow each actuator receives from the flow summation node  44 . 
         [0028]    When the first hydraulic actuator  21  reaches the desired position, the first control valve  24  is returned to the center position by whatever apparatus controls that valve. In the center position, the two workports are closed again cutting off fluid flow from the flow summation node  44  to the first hydraulic actuator  21 . In addition, the variable flow source orifice  34   a  shrinks to a relatively small size which reduces the flow from the supply conduit  28  to the flow summation node  44 . Returning the first control valve  24  to the center position also enlarges the size of the bypass orifice  50   a . Now if the other control valves  25  and  26  also are in the center position, all their bypass orifice  50   a - c  are relatively large thereby relieving the flow summation node pressure into the return conduit  30 . 
         [0029]    Alternatively, a single relatively small fixed orifice could be employed in place of a variable bypass orifice  50   a - c  in each valve unit  11 - 13 . The size of that single fixed bypass orifice would be selected so as not to appreciably affect the pressure buildup at the flow summation node as one or more control valve  24 ,  25  or  26  opens, but still release the pressure at that node when all the control valves are closed. 
         [0030]    The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.