Abstract:
A fluid control system includes at least one double-acting cylinder and at least one fluid-driven motor. A pressurized fluid source supplies pressurized fluid flow to the at least one double-acting cylinder and the at least one fluid-driven motor, and a tank receives return fluid flow from the at least one double-acting cylinder and the at least one fluid-driven motor. A back pressure element is disposed between the tank and the motor and may influence a fluid backpressure condition of fluid discharged from the motor. A dedicated flow line may provide make-up fluid to the motor at a location between the motor and the back pressure element.

Description:
TECHNICAL FIELD  
         [0001]    The invention relates generally to a fluid control system and, more particularly, to a hydraulic control system with reduced cavitation effects  
         BACKGROUND  
         [0002]    Conventional hydraulic systems typically include one or more hydraulic cylinders and/or hydraulic motors for operating work implements, for example, buckets, shovels, and handlers. In such systems, cavitation may be generated in a hydraulic motor when the supply fluid flow to the motor is less than the return fluid flow from the motor. Motor cavitation can damage the hydraulic system and, in particular, the motor. In addition, motor cavitation may cause an unpleasant noise as the motor is stopped.  
           [0003]    One mechanism for reducing motor cavitation involves joining the fluid-return lines of all hydraulic cylinders and hydraulic motors in a hydraulic system to form a main return line. A back pressure check valve is installed at the main return line downstream of where the fluid return lines are joined. The pressurized fluid upstream of the back pressure check valve provides a make-up function to the return flow sides of the cylinders and motors. Although a high back pressure setting is necessary to prevent motor cavitation, such a high setting is not necessary to prevent cavitation by the hydraulic cylinders. In addition, when retracting a plurality of cylinders, the return flow increases. Thus, an unnecessary and excessive amount of back pressure occurs at the return line, and pressurized fluid flows across the back pressure check valve, resulting in an undesirable energy loss.  
           [0004]    Another typical mechanism for reducing motor cavitation, as shown in U.S. Pat. No. 5,673,605, includes providing a hydraulic system with a back pressure check valve at a return flow line of a hydraulic motor and allowing return fluid from the other motors and hydraulic cylinders to return directly to the tank. Also, a flow line is added upstream of the back pressure check valve to feed the return fluid of the motor back to the motor. However, in this situation, a sufficient make-up flow may not be achieved due, for example, to drain leakage of pressurized oil as a result of the high pressure generated at the motor return port when stopping rotation of the motor. Consequently, make-up flow becomes short and motor cavitation may occur.  
           [0005]    A fluid control system for effectively and efficiently providing make-up fluid flow to a hydraulic motor to reduce motor cavitation is desired. The present invention is directed to provide such a system while solving one or more of the problems set forth above.  
         SUMMARY OF THE INVENTION  
         [0006]    According to one aspect of the invention, a fluid control system may include at least one double-acting cylinder and at least one fluid-driven motor. A pressurized fluid source may supply pressurized fluid flow to the at least one double-acting cylinder and the at least one fluid-driven motor, and a tank may receive return fluid flow from the at least one double-acting cylinder and the at least one fluid-driven motor. A back pressure element may be disposed between the tank and the motor. The back pressure element may be configured to influence a back pressure condition of fluid discharged from the motor. A dedicated flow line may be configured to provide make-up fluid to the motor at a location between the motor and the back pressure element.  
           [0007]    According to another aspect of the invention, a method for controlling a hydraulic circuit may include supplying fluid to at least one motor and to at least one cylinder from a pressurized supply. The method may also include directing fluid away from the at least one cylinder and into a tank, and directing fluid away from the at least one motor, across a back pressure element, and into a tank. The method may further include supplying a dedicated make-up fluid supply to a valve arrangement at a location between the at least one motor and the back pressure element.  
           [0008]    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  
       [0009]    The accompanying drawing, which is incorporated in and constitutes a part of this specification, illustrates an exemplary embodiment of the invention and, together with the description, serves to explain the principles of the invention. In the drawing,  
         [0010]    [0010]FIG. 1 is a schematic illustration of a hydraulic circuit in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]    Reference will now be made in detail to embodiments of the invention, an example of which is illustrated in the accompanying drawings.  
         [0012]    In accordance with the present invention, a fluid control system is provided. Referring to FIG. 1, a fluid control system, for example, hydraulic circuit  100 , may include a plurality of flow control valve arrangements such as independent metering valve arrangements  102 ,  104 ,  106 ,  108 . As shown in FIG. 1, the hydraulic circuit  100  may include a pressurized fluid source, for example, a pump  112 . The circuit  100  may also include a tank  114 . The pump  112  may comprise, for example, a variable output, high pressure pump or a constant output, high pressure pump. The hydraulic circuit  100  may further include an engine  162  or other motive force for providing a drive force to power the pump  112 . The drive force may be provided, for example, by way of a drive shaft  164  or other known mechanical linkage.  
         [0013]    Each independent metering valve arrangement  102 ,  104 ,  106 ,  108  may include a plurality of independently-operated, electronically-controlled metering valves. For example, independent metering valve arrangement  102  may include a plurality of metering valves  120 ,  122 ,  124 ,  126 . The metering valves  120 ,  122 ,  124 ,  126  control fluid flow between a double acting cylinder, for example, hydraulic cylinder  128 , the pump  112 , and the tank  114 . The metering valves may be spool valves, poppet valves, or any other conventional type of metering valve that would be appropriate. The hydraulic cylinder  128  includes a head end  127  and a rod end  129 . Thus, the metering valves may be referred to individually as a cylinder-to-tank head end (CTHE) metering valve  120 , a pump-to-cylinder head end (PCHE) metering valve  122 , a pump-to-cylinder rod end (PCRE) metering valve  124 , and a cylinder-to-tank rod end (CTRE) metering valve  126 .  
         [0014]    Similarly, independent metering valve arrangement  104  may include a CTHE metering valve  130 , a PCHE metering valve  132 , a PCRE metering valve  134 , and a CTRE metering valve  136  for controlling fluid flow between a hydraulic cylinder  138 , the pump  112 , and the tank  114 .  
         [0015]    The pump-to-cylinder metering valves  122 ,  124 ,  132 ,  134 , often referred to generally as meter-in valves, are fed in parallel with pressurized fluid from the pump via cylinder supply line  190 . The cylinder-to-tank metering valves  120 ,  126 ,  130 ,  136 , often referred to generally as meter-out valves, allow pressurized fluid to exit from the respective cylinder  128 ,  138  to the tank  114  via cylinder return line  192 .  
         [0016]    Independent metering valve arrangement  106  may include metering valves  140 ,  142 ,  144 ,  146 . In independent metering valve arrangement  106 , the metering valves  140 ,  142 ,  144 ,  146  control fluid flow between a fluid motor, for example, reversible hydraulic motor  148 , the pump  112 , and the tank  114 . Since the reversible hydraulic motor  148  does not have a head end and a rod end, the cylinder-to-tank metering valves  140 ,  146  may be referred to generally as meter-out valves and the pump-to-cylinder metering valves  142 ,  144  may be referred to generally as meter-in valves.  
         [0017]    Similarly, independent metering valve arrangement  108  may include cylinder-to-tank metering, or meter-out, valves  150 ,  156  and pump-to-cylinder metering, or meter-in, valves  152 ,  154  for controlling fluid flow between a reversible hydraulic motor  158 , the pump  112 , and the tank  114 .  
         [0018]    Pressurized fluid may be fed from the pump  112  to the meter-in valves  142 ,  144 ,  152 ,  154  via motor supply line  194 . The meter-in valves  142 ,  144 ,  152 ,  154  are fed parallel with one another and parallel with the pump-to-cylinder metering valves  122 ,  124 ,  132 ,  134 . The meter-out valves  140 ,  146 ,  150 ,  156  allow pressurized fluid to exit from the respective motor  148 ,  158  to the tank  114  via motor return line  196 . In addition, the hydraulic motors  148 ,  158  are in communication with the tank  114  via drain flow line  197  so that any fluid leakage during stoppage of the motors  148 ,  158  may be drained.  
         [0019]    A back pressure element, for example, a back pressure check valve  160 , may be disposed on the motor return line  196  between the meter-out valves  140 ,  146 ,  150 ,  156  and the tank  114 . The back pressure check valve  160  acts to create a supply of pressurized fluid upstream of the check valve  160 . The supply of fluid may be pressurized at or above the pressure setting of the check valve  160 .  
         [0020]    The hydraulic circuit  100  may also include a combination main relief and bypass valve  166 . The combination valve  166  may include an electronically-operated solenoid  168 . The combination valve  166  may be configured such that when the solenoid  168  is energized with predetermined current, the valve  166  functions as a main relief valve, and when the solenoid  168  is energized with current varying gradually from zero, the valve  166  functions as a bypass valve.  
         [0021]    In one embodiment, pressurized fluid flowing across the combination valve  166  may be brought in communication with the motor return line  196  at a location upstream of the back pressure check valve  160  via by-pass and relief return line  198 . Thus, the pressurized fluid supplied via by-pass and relief return line  198  may contribute to the pressurized fluid in the motor return line  196  upstream of the check valve  160 . Alternatively, the pressurized fluid flowing across the combination valve  166  may be emptied directly to the tank  114  via direct return line (not shown).  
         [0022]    The hydraulic circuit  100  may further include a pilot pump  170 . The pilot pump  170  provides pressurized fluid to the circuit  100  to perform work other than powering the hydraulic cylinders  128 .  138  and motors  148 ,  158 , such as controlling movement of valves and the like in a well known manner. For example, the pilot pump  170  may supply pressurized fluid used to shift valves between multiple positions.  
         [0023]    A pilot flow line  172  may supply pressurized fluid from the pilot pump  170  to the motor return line  196  at a location upstream of the back pressure check valve. Thus, the pressurized fluid supplied via pilot flow line  172  may contribute to the pressurized fluid in the motor return line  196  upstream of the check valve  160 . Alternatively, the pilot flow line  172  providing fluid communication with the motor return line  196  may be eliminated.  
         [0024]    A pilot relief valve  174  may be disposed at the pilot flow line  172 . Thus, the pressurized fluid supplied by the pilot pump  170  may flow across the pilot relief valve  174  and to the motor return line  196  when the relief valve  174  is opened. Otherwise, the pilot pump  170  may supply pressurized fluid to any pilot-operated elements of the hydraulic circuit  100 , for example, valves shifted by pressurized fluid from the pilot pump  170 .  
         [0025]    As shown in FIG. 1, the hydraulic circuit  100  may include the bypass and relief return line  198  from the combination main relief and bypass valve  166  and the pilot flow line  172  from the pilot pump  170  both communicating with the motor return line  196  at a location upstream of the back pressure check valve  160 . Alternatively, the circuit  100  may include one of the by-pass and relief return line  198  from the combination main relief and bypass valve  166  and the pilot flow line  172  from the pilot pump  170  in communication with the motor return line  196  at a location upstream of the back pressure check valve  160 .  
       INDUSTRIAL APPLICABILITY  
       [0026]    In use, the metering valves  120 ,  126 ,  130 ,  136  control cylinder-to-tank fluid flow while the metering valves  122 ,  124 ,  132 ,  134  control pump-to-cylinder fluid flow. Conventional extension of the hydraulic cylinders  128 ,  138  is achieved by selective, operator-controlled actuation of the metering valves  122 ,  126 ,  132 ,  136 , and retraction is achieved by simultaneous operator controlled actuation of the metering valves  120 ,  124 ,  130 ,  134 .  
         [0027]    Similarly, metering valves  140 ,  146 ,  150 ,  156  control motor-to-tank fluid flow while metering valves  142 ,  144 ,  152 ,  154  control pump-to-motor fluid flow. Conventional operation of the bi-directional motors  148 ,  158  is achieved by selective, operator-controlled actuation of the metering valves  142 ,  146 ,  152 ,  156  for a first direction and the metering valves  140 ,  144 ,  150 ,  154  for a second direction.  
         [0028]    Referring to FIG. 1, the cylinder return line  192  may be connected directly to the tank  114 . Thus, pressurized fluid being returned from the hydraulic cylinders  128 ,  138  will not pass through the back pressure check valve  160 . As a result, energy loss will not occur even though a significant amount of fluid flow from the cylinder return line  192  may be generated when retracting the cylinders  128 ,  138 .  
         [0029]    When at least one of the hydraulic motors  148 ,  158  is rotating, either motor  148 ,  158  may be stopped, for example, by returning an operational lever to a neutral position. When stopping the motors  148 ,  158 , the appropriate, associated meter-in valves  142 ,  144 ,  152 ,  154  are closed, shutting off the supply of pressurized fluid to the motors  148 ,  158 . Due to their momentum, the motors  148 ,  158  do not stop instantaneously. Thus, some amount of fluid continues to be returned to the appropriate, associated meter-out valves  140 ,  146 ,  150 ,  156  even after the meter-in valves  142 ,  144 ,  152 ,  154  are closed. In addition, some amount of pressurized fluid may leak from the motors  148 ,  158  and return to the tank  114  via drain flow line  197 .  
         [0030]    To provide a make-up fluid flow to the appropriate meter-out valves  140 ,  146 ,  150 ,  156  which is able to allow reverse flow from motor return line  196  to respective motor circuit to avoid motor cavitation, the back pressure check valve  160  is disposed at the motor return line  196 . In addition, pressurized fluid flow passing through the combination main relief and by-pass valve  166  and/or pressurized fluid from the pilot flow line  172  are joined to the motor return line  196  at a location upstream of the back pressure check valve  160 . Thus, when at least one of the hydraulic motors  148 ,  158  is stopped, for example, by placing an operational lever at a neutral position, a proper back pressure will occur upstream of the back pressure check valve  160 .  
         [0031]    When the hydraulic cylinders  128 ,  138  are in a standby mode, for example, by placing an operational lever at a neutral position, the combination main relief and by-pass valve  166  may be opened. As a result, pressurized fluid passing through the combination valve  166  provides fluid flow to the motor return line  196  at a location upstream of the back pressure check valve  160 . The fluid flow passing through the combination valve  166  may generate the necessary back pressure upstream of the back pressure check valve  160  to provide make-up flow to the motors  148 ,  158 , thus reducing motor cavitation and its associated noise. An upstream back pressure greater than atmospheric pressure provides a quicker and more complete make-up function, for example, by causing a make-up spool to lift and allow the flow of make-up fluid.  
         [0032]    In a hydraulic control system including pressurized fluid flow passing through the combination main relief and by-pass valve  166  and pressurized fluid from the pilot flow line  172 , both joined to the motor return line  196  at a location upstream of the back pressure check valve  160 , the fluid from the pilot flow line  172  may generate or contribute to the generation of the necessary back pressure upstream of the back pressure check valve  160  that provides make-up flow to the motors  148 ,  158 . In a hydraulic control system where the motor return line  196  does not receive pressurized fluid flow passing through the combination main relief and by-pass valve  166 , the fluid from the pilot flow line  172  may generate the necessary back pressure upstream of the back pressure check valve  160  that provides make-up flow to the motors  148 ,  158 .  
         [0033]    Referring again to FIG. 1, when one or more of the hydraulic cylinders  128 ,  138  is being operated and at least one of the motors  148 ,  158  is stopped, for example, by returning an operation lever to a neutral position, the combination main relief and by-pass valve  166  may be closed. Therefore, a significant fluid flow across the combination valve  166  cannot be expected. However, pressurized fluid from the pilot flow line  172 , which is joined to the motor return line  196  at a location upstream of the back pressure check valve  160 , may generate the necessary back pressure upstream of the back pressure check valve  160  that provides make-up flow to the motors  148 ,  158 . Thus, motor cavitation and its associated noise may be reduced.  
         [0034]    It should be appreciated that the hydraulic circuit  100  may include any number of hydraulic cylinders  128 ,  138  and/or any number of hydraulic motors  148 ,  158  and/or other additional hydraulically-operated actuators. Also, it should be appreciated that the circuit  100  may include more than one pump  112 . If more than one pump  112  is provided, the circuit  100  may include more than one combination main relief and by-pass valve  166  and/or one or more flow combiners, as is readily known in the art.  
         [0035]    Thus, the present invention may provide a hydraulic control system that may minimize motor cavitation when stopping a motor. Since return flow from a motor is nearly equal to an inlet supply flow from a pump to the motor, only a relatively small amount of additional fluid is needed to provide a make-up function to the motor when the motor is stopped to supplement the amount of drain flow from the motor. This amount of additional fluid will not reach the magnitude of return flow from a cylinder head end when retracting the cylinder. A back pressure check valve disposed at the motor return line and pressurized fluid provided from at least one of a by-pass and relief return line and a pilot flow line generate sufficient back pressure to provide a make-up fluid flow to a motor and reduce motor cavitation. Separation of the cylinder return line from the motor return line and connection of the cylinder return line to the tank avoids a large power loss that would otherwise occur at the back pressure check valve. Therefore, when properly implemented, the hydraulic control system of the present invention may minimize cavitation in an effective and efficient manner and without undesirable energy loss.  
         [0036]    It will be apparent to those skilled in the art that various modifications and variations can be made in the hydraulic control system 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.