Abstract:
To reduce the cost by reducing the number of parts and simplify the control of regeneration in the hydraulic actuator even though the meter-in control and the meter-out control can be performed separately while the supply and the discharge of hydraulic fluid is controlled. A meter-in valve that controls supply flow from a hydraulic pump into a hydraulic cylinder is provided, and meter-out switching valve that switches the direction of supply and discharge of the hydraulic oil into the hydraulic cylinder and controls the discharge flow from the hydraulic cylinder to the oil tank is installed on the down stream side of the meter-in valve, and further, a regeneration control valve is installed on the down stream side of the meter-out switching valve.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to the technical field of a hydraulic actuator control circuit for controlling supply and discharge of oil to and from a hydraulic actuator provided in a construction machine such as an excavator. 
       BACKGROUND ART 
       [0002]    In general, various hydraulic actuators are provided in construction machines such as excavators. As a control circuit for controlling supply and discharge of oil to and from such a hydraulic actuator, a control circuit has been known which is configured to simultaneously perform, using one spool valve, directional control that switches between the direction of supply of hydraulic oil to the hydraulic actuator and the direction of discharge of hydraulic oil from the hydraulic actuator, meter-in control that controls a supply flow rate from a hydraulic pump to the hydraulic actuator, and meter-out control that controls a discharge flow rate from the hydraulic actuator to an oil tank. However, when the meter-in control and the meter-out control are thus performed using one spool valve, a relation between a meter-in opening area and a meter-out opening area with respect to a moving position of the spool valve is uniquely determined. This causes a problem in that the relation between the meter-in and the meter-out cannot be changed in accordance with the contents of work and an operator. 
         [0003]    Thus, a technique has been known in which the control of supply and discharge of oil to and from the hydraulic actuator is performed by a bridge circuit formed using four metering valves including a head-side meter-in valve and a rod-side meter-in valve (a head-end supply valve and a rod-end supply valve) that control supply flow rates from the hydraulic pump to a head-side oil chamber of a hydraulic cylinder and to a rod-side oil chamber of the hydraulic cylinder, respectively, and a head-side meter-out valve and a rod-side meter-out valve (a head-end drain valve and a rod-end drain valve) that control discharge flow rates from the head-side oil chamber and from the rod-side oil chamber to the oil tank, respectively (see, for example, PTL 1). In this technique, the four metering valves are individually actuated based on commands from a controller. Thus, the relation between the meter-in and the meter-out can be easily changed in accordance with the contents of work and the operator. 
         [0004]    Moreover, the bridge circuit using the four metering valves enables recycling such that discharged oil from one of the oil chambers of the hydraulic actuator is supplied to the other oil chamber. When the recycling is performed, both the head-side and rod-side meter-in valves are opened to merge the discharged oil from one of the oil chambers with pump discharge oil via one of the meter-in valves, and the merged hydraulic oil is then supplied to the other oil chamber via the other mere-in valve. 
         [0005]    On the other hand, as another technique that individually performs the meter-in control and the meter-out control on the hydraulic actuator, a technique is also known in which a meter-in selector valve that switches a port of the hydraulic actuator connected to the hydraulic pump while controlling a supply flow rate to the port and a meter-out selector valve that switches a port of the hydraulic actuator connected to the oil tank while controlling a discharge flow rate from the port are provided, and the meter-in selector valve and the meter-out selector valve are independently controlled (see, for example, PTL 2). 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [PTL 1] Japanese Patent No. 5214450 
         [PTL 2] Japanese Patent Application Laid-open No. H11-303814 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    However, in the configuration in which the control of supply and discharge of oil to and from the hydraulic actuator is performed using the four metering valves as in PTL 1, in addition to four spools (or poppets) forming the respective four metering valves, four actuators (in PTL 1, solenoids) allowing the spools to be moved are needed, disadvantageously leading to a large number of components and increased costs. Moreover, in PTL 1, when both meter-in valves are opened to perform the recycling as described above, recycling oil passes through the two meter-in valves, making the control of a recycling flow rate difficult to control. Furthermore, the total flow rate of the recycling flow rate and the pump flow rate passes through the other meter-in valve, resulting in the need for a larger opening and in an increased valve size. 
         [0009]    In contrast, the control circuit in PTL 2 has fewer components than the hydraulic actuator in PTL 1 but needs, besides two spools forming the respective two selector valves of the meter-in selector valve and the meter-out selector valve, a total of four actuators (in Patent Literature 2, electromagnetic proportional pressure control valves) allowing each of the spools to be moved in both directions. Thus, a further reduction in the number of components has been desired, and this is an object of the present invention. 
       Solution to Problem 
       [0010]    The present invention has been developed in view of the above-described circumstances. An invention in claim  1  is a hydraulic actuator control circuit for controlling supply and discharge of oil to and from a hydraulic actuator, the hydraulic actuator control circuit including a meter-in valve that controls a supply flow rate from a hydraulic pump to the hydraulic actuator, and a meter-out selector valve arranged downstream of the meter-in valve to switch between a direction of supply of hydraulic oil td the hydraulic actuator and a direction of discharge of hydraulic oil from the hydraulic actuator while controlling a discharge flow rate from the hydraulic actuator to an oil tank. 
         [0011]    An invention in claim  2  is the hydraulic actuator control circuit in claim  1  characterized in that a pressure compensation valve for keeping a pressure difference between an upstream side and a downstream side of the meter-in valve constant is provided upstream of the meter-in valve. 
         [0012]    An invention in claim  3  is the hydraulic actuator control circuit in claim  1  characterized in that a recycling control valve for controlling a recycling flow rate at which discharged oil from one of oil chambers of the hydraulic actuator is supplied to the other oil chamber is provided downstream of the meter-out selector valve. 
       Advantageous Effects of Invention 
       [0013]    The invention in claim  1  allows meter-in control and meter-out control to be individually performed, while enabling the number of components to be reduced to contribute to cost reduction. 
         [0014]    The invention in claim  2  allows flow rate control to be accurately performed using the meter-in valve. 
         [0015]    The invention in claim  3  facilitates control of the recycling flow rate, allowing accurate recycling flow rate control to be achieved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0016]      FIG. 1  is a hydraulic circuit diagram depicting a hydraulic actuator control circuit. 
           [0017]      FIG. 2  is a block diagram illustrating inputs to and outputs from a controller. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    An embodiment of the present invention will be described below based on the drawings.  FIG. 1  is a hydraulic circuit diagram depicting a hydraulic actuator control circuit for controlling supply and discharge of oil to and from a hydraulic actuator such as a boom cylinder or a stick cylinder which is provided in, for example, an excavator. In  FIG. 1, 1  denotes a hydraulic cylinder (in the present embodiment, a stick cylinder in the excavator) serving as a hydraulic actuator,  2  denotes a hydraulic pump serving as a hydraulic pressure supply source of the hydraulic cylinder  1 , and  3  denotes an oil tank. The hydraulic cylinder  1  is a double acting cylinder in which a rod-side oil chamber  1   b  and a head-side oil chamber  1   c  are formed on both sides of a piston  1   a.    
         [0019]    Moreover, in  FIG. 1, 4  denotes a discharge line for the hydraulic pump  2 . In an oil path along the discharge line  4  to the hydraulic cylinder  1 , a pressure compensation valve  5 , a check valve  6 , a meter-in valve  7 , and a meter-out selector valve  8  are sequentially disposed in this order from an upstream side. 
         [0020]    The pressure compensation valve  5  receives an inlet-side pressure and an outlet-side pressure on the meter-in valve  7  disposed downstream of the pressure compensation valve  5  to control the flow rate so as to keep a pressure difference between an upstream side and a downstream of the meter-in valve  7  constant. 
         [0021]    Furthermore, the check valve  6  is configured to permit a flow of oil from the pressure compensation valve  5  to the meter-in valve  7 , while inhibiting a flow in the opposite direction. 
         [0022]    Furthermore, the meter-in valve  7  is a pilot valve that is pilot-operated by an electromagnetic proportional valve for the meter-in valve  20  (not depicted in  FIG. 1 ) actuated based on control signals output from a controller  9 . While the controller  9  is outputting no actuating control signal to the electromagnetic proportional valve for the meter-in valve  20 , the meter-in valve  7  is located in a neutral position N where discharged oil supplied from the hydraulic pump  2  via the pressure compensation valve  5  and the check valve  6  is not passed to the meter-out selector valve  8  arranged downstream of the meter-in valve  7 . When the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the meter-in valve  20 , the meter-in valve  7  is switched to an actuation position X where the discharged oil from the hydraulic pump  2  is supplied to the hydraulic cylinder  1  via the meter-out selector valve  8 . An opening area of the meter-in valve  7  in the actuation position X is controllably increased or reduced based on control signals output from the controller  9  to the electromagnetic proportional valve for the meter-in valve  20 . The controllable increase or reduction in the opening area of the meter-in valve  7  enables a controllable increase or reduction in a supply flow rate from the hydraulic pump  2  to the hydraulic cylinder  1 . In this case, the pressure compensation valve  5  is actuated to keep the pressure difference between the upstream side and the downstream of the meter-in valve  7  constant. Thus, the flow rate control can be accurately achieved without being affected by a fluctuation in the pressure of the hydraulic pump  2 . 
         [0023]    Furthermore, the meter-out selector valve  8  is a pilot selector valve that is pilot-operated by a contraction-side electromagnetic proportional valve for the meter-out selector valve  21   a  and an extension-side electromagnetic proportional valve for the meter-out selector valve  21   b  (not depicted in  FIG. 1 ) actuated based on control signals output from the controller  9 . While the controller  9  is outputting no actuating control signal to the electromagnetic proportional valves  21   a  and  21   b , the meter-out selector valve  8  is located in a neutral position N where oil supplied from the meter-in valve  7  is not supplied to the rod-side oil chamber  1   b  or the head-side oil chamber  1   c  of the hydraulic cylinder  1  and where discharged oil from the rod-side oil chamber  1   b  and the head-side oil chamber  1   c  is not passed to the oil tank  3 . When the controller  9  outputs the actuating control signal to the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   a , the meter-out selector valve  8  is switched to a contraction-side actuation position X where a pump flow rate of oil supplied from the meter-in valve  7  is supplied to the rod-side oil chamber  1   b  of the hydraulic cylinder  1  via a rod-side oil path  10  and where oil discharged from the head-side oil chamber  1   c  to a head-side oil path  11  is passed to the oil tank  3 . Furthermore, when the controller  9  outputs the actuating control signal to the extension-side electromagnetic proportional valve for the meter-out selector valve  21   b , the meter-out selector valve  8  is switched to an extension-side actuation position Y where a pump flow rate of oil supplied through the meter-in valve  7  is supplied to the head-side oil chamber  1   c  of the hydraulic cylinder  1  via the head-side oil path  11  and where oil discharged from the rod-side oil chamber  1   b  to the rod-side oil path  10  is passed to the oil tank  3 . The opening amounts of the meter-out selector valve  8  in the contraction- and extension-side actuation positions X and Y are such that the opening areas of a contraction-side supply path  8   c  and an extension-side supply path  8   d  through which a pump flow rate of oil is supplied from the meter-out selector valve  8  to the rod-side oil chamber  1   b  of the hydraulic cylinder  1  and to the head-side oil chamber  1   c  of the hydraulic cylinder  1  are set sufficiently larger than the opening area of the meter-in valve  7 , whereby the supply flow rate of oil controlled by the meter-in valve  7  is supplied to the rod-side oil chamber  1   b  and the head-side oil chamber  1   c  without being changed. On the other hand, the opening areas of a contraction-side discharge path  8   e  and an extension-side discharge path  8   f  through which discharged oil from the head-side oil chamber  1   c  of the hydraulic cylinder  1  and from the rod-side oil chamber  1   b  of the hydraulic cylinder  1  is passed to the oil tank  3  are controllably increased or reduced based on control signals output from the controller  9  to the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   a  and the extension-side electromagnetic proportional valve for the meter-out selector valve  21   b . The controllable increase or reduction in the opening area of the meter-out selector valve  8  allows a controllable increase or reduction in the discharge flow rates from the head-side oil chamber  1   c  to the oil tank  3  and from the rod-side oil chamber  1   b  to the oil tank  3 . The rod-side oil path  10  is an oil path that couples the meter-out selector valve  8  to the rod-side oil chamber  1   b  of the hydraulic cylinder  1 . The head-side oil path  11  is an oil path that couples the meter-out selector valve  8  to the head-side oil chamber  1   c  of the hydraulic cylinder  1 . 
         [0024]    Moreover,  12  is a recycling oil path located downstream of the meter-out selector valve  8  to allow the rod-side oil path  10  and the head-side oil path  11  to communicate with each other. A recycling control valve  13  is disposed in the recycling oil path  12 . The recycling control valve  13  is a pilot selector valve that is pilot-operated by an electromagnetic proportional valve for the recycling control valve  22  (not depicted in  FIG. 1 ) actuated based on control signals output from the controller  9 . While the controller  9  is outputting no actuating control signal to the electromagnetic proportional valve for the recycling control valve  22 , the recycling control valve  13  is located in a closed position N where the recycling oil path  12  is closed. When the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the recycling control valve  22 , the recycling control valve  13  is switched to an open position X where the recycling oil path  12  is opened to pass discharged oil from the rod-side oil chamber  1   b  of the hydraulic cylinder  1  to the head-side oil chamber  1   c . The opening area of the recycling control valve  13  in the open position X is controllably increased or reduced based on control signals output from the controller  9  to the electromagnetic proportional valve for the recycling control valve  22 . The controllable increase or reduction in the opening area of the recycling control valve  13  allows a controllable increase or reduction in a recycling flow rate from the rod-side oil chamber  1   b  to the head-side oil chamber  1   c  of the hydraulic cylinder  1 . 
         [0025]    Additionally,  14  is a bypass oil path branching from a discharge line  4  from the hydraulic pump  2  and extending to the oil tank  3 . A bypass valve  15  is disposed in the bypass path  14 . The bypass valve  15  is a pilot selector valve that is pilot-operated by an electromagnetic proportional valve for the bypass valve  23  (not depicted in  FIG. 1 ) based on control signals output from the controller  9 . While the controller  9  is outputting no actuating control signal to the electromagnetic proportional valve for the bypass valve  23 , the bypass valve  15  opens the bypass path  14  with the maximum opening area. When the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the bypass valve  23 , the bypass valve  15  closes the bypass path  14 . The opening area of the bypass valve  15  is controllably increased or reduced based on control signals output from the controller  9  to the electromagnetic proportional valve for the bypass valve  23 . The controllable increase or reduction in the opening area of the bypass valve  15  allows a controllable increase or reduction in the flow rate of the bypass path  14 . 
         [0026]    On the other hand, as depicted in a block diagram in  FIG. 2 , the controller  9  receives signals from, for example, operation detection means  16  for detecting the operating direction and the amount of operation of a hydraulic-cylinder operation unit (not depicted in the drawings), a rod-side pressure sensor  17  connected to the rod-side oil path  10  to detect a rod-side pressure in the hydraulic cylinder  1 , and a head-side pressure sensor  18  connected to the head-side oil path  11  to detect a head-side pressure in the hydraulic cylinder  1 . Based on these input signals, the controller  9  outputs control signals to the electromagnetic proportional valve for the meter-in valve  20 , the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   a , the extension-side electromagnetic proportional valve for the meter-out selector valve  21   b , the electromagnetic proportional valve for the recycling control valve  22 , and the electromagnetic proportional valve for the bypass valve  23  to control the meter-in valve  7 , the meter-out selector valve  8 , the recycling control valve  13 , and the bypass valve  15 . 
         [0027]    Now, control performed by the controller  9  will be described. First, when the hydraulic-cylinder operation unit has not been operated, the controller  9  outputs no actuating control signal to the electromagnetic proportional valve for the meter-in valve  20 , the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   a , the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   b , the electromagnetic proportional valve for the recycling control valve  22 , or the electromagnetic proportional valve for the bypass valve  23 . Thus, the meter-in valve  7  is controllably placed in the neutral position N where the discharged oil from the hydraulic pump  2  is not passed to the meter-out selector valve  8 . The meter-out selector valve  8  is controllably placed in the neutral position N where the oil supplied through the meter-in valve  7  is not passed to the hydraulic cylinder  1  and where the discharged oil from the hydraulic cylinder  1  is not passed to the oil tank  3 . The recycling control valve  13  is controllably placed in the closed position N where the recycling oil path  12  is closed. Thus, oil is not supplied from the hydraulic pump  2  to the hydraulic cylinder  1  or discharged from the hydraulic cylinder  1  to the oil tank  3 , the recycling is also not performed, and the hydraulic cylinder  1  is not contracted or extended. Furthermore, the bypass valve bypass valve  15  is controlled to open the bypass path  14  with the maximum opening area, and thus, the hydraulic pump  2  exerts a low discharge pressure. 
         [0028]    On the other hand, when the hydraulic-cylinder operation unit is operated to the contraction side, the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the meter-in valve  20  to controllably place the meter-in valve  7  in the actuation position X. In this case, the opening area of the meter-in valve  7  is controllably increased or reduced based on control signals from the controller  9 . Thus, a pump flow rate of oil controlled by the meter-in valve  7  is supplied from the meter-in valve  7  to the meter-out selector valve  8 . As described above, the pressure compensation valve  5  is actuated to keep the pressure difference between the upstream side and the downstream side of the meter-in valve  7  constant, allowing for accurate flow rate control. 
         [0029]    Moreover, when the hydraulic-cylinder operation unit is operated to the contraction side, the controller  9  outputs the actuating control signal to the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   a  to controllably place the meter-out selector valve  8  in the contraction-side actuation position X. In this case, since the opening area of the contraction-side supply path  8   c  is set sufficiently larger than the opening area of the meter-in valve  7  as described above, a pump flow rate of oil controlled by the meter-in valve  7  is supplied to the rod-side oil chamber  1   b  of the hydraulic cylinder  1  through the contraction-side supply path  8   c . On the other hand, the oil in the head-side oil chamber  1   c  is discharged to the oil tank  3  through the contraction-side discharge path  8   e  in the meter-out selector valve  8 . The opening area of the contraction-side discharge path  8   e  is controllably increased or reduced based on control signals from the controller  9 . Thus, a discharge flow rate of oil controlled by the meter-out selector valve  8  flows from the head-side oil chamber  1   c  to the oil tank  3 . 
         [0030]    Moreover, when the hydraulic-cylinder operation unit is operated to the contraction side, the controller  9  outputs no actuating control signal to the electromagnetic proportional valve for the recycling control valve  22 . Consequently, the recycling control valve  13  is controllably placed in the closed position N where the recycling oil path  12  is closed. 
         [0031]    When the hydraulic-cylinder operation unit is operated to the contraction side, a pump flow rate of oil controlled by the meter-in valve  7  is supplied to the rod-side oil chamber  1   b  of the hydraulic cylinder  1 . On the other hand, the discharged oil from the head-side oil chamber  1   c  is subjected to flow rate control by the meter-out selector valve  8  and then flows to the oil tank  3 . Thus, the hydraulic cylinder  1  is contracted. In this case, the controller  9  controllably increases or reduces the opening areas of the meter-in valve  7  and the meter-out selector valve  8  so that the pump flow rate and the discharge flow rate correspond to the amount of operation of the hydraulic-cylinder operation unit. This allows the hydraulic cylinder  1  to be contracted at a speed corresponding to the amount of operation of the hydraulic-cylinder operation unit. 
         [0032]    Now, a case where the hydraulic-cylinder operation unit is operated to the extension side will be described. Control varies between a case where the head-side pressure in the hydraulic cylinder  1  is higher than the rod-side pressure in the hydraulic cylinder  1 , precluding the recycling from the rod-side oil chamber  1   b  to the head-side oil chamber  1   c , and a case where the rod-side pressure is higher than the head-side pressure and where the recycling from the rod-side oil chamber  1   b  to the head-side oil chamber  1 C is performed. Thus, first, the case where the head-side pressure is higher than the rod-side pressure will be described. 
         [0033]    If the hydraulic-cylinder operation unit is operated to the extension side when the head-side pressure in the hydraulic cylinder  1  is higher than the rod-side pressure in the hydraulic cylinder  1 , the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the meter-in valve  20  to controllably place the meter-in valve  7  in the actuation position X. In this case, the opening area of the meter-in valve  7  is controllably increased or reduced based on control signals from the controller  9 . Thus, a pump flow rate of oil controlled by the meter-in valve  7  is supplied from the meter-in valve  7  to the meter-out selector valve  8 . 
         [0034]    Moreover, if the hydraulic-cylinder operation unit is operated to the extension side when the head-side pressure is higher than the rod-side pressure, the controller  9  outputs the actuating control signal to the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   b . Consequently, the meter-out selector valve  8  is controllably placed in an extension-side actuation position Y. In this case, since the opening area of the extension-side supply path  8   d  is set sufficiently larger than the opening area of the meter-in valve  7  as described above, a pump flow rate of oil controlled by the meter-in valve  7  is supplied to the head-side oil chamber  1   c  of the hydraulic cylinder  1  through the extension-side supply path  8   d . On the other hand, the oil in the rod-side oil chamber  1   b  is discharged to the oil tank  3  through the extension-side discharge path  8   f . The opening area of the extension-side discharge path  8   f  is controllably increased or reduced based on control signals from the controller  9 . Thus, the discharge flow rate controlled by the meter-out selector valve  8  flows from the rod-side oil chamber  1   b  to the oil tank  3 . 
         [0035]    If the hydraulic-cylinder operation unit is operated to the extension side when the head-side pressure is higher than the rod-side pressure, the controller  9  outputs no actuating control signal to the electromagnetic proportional valve for the recycling control valve  22 . Thus, the recycling control valve  13  is controllably placed in the closed position N where the recycling oil path  12  is closed. 
         [0036]    If the hydraulic-cylinder operation unit is operated to the extension side when the head-side pressure is higher than the rod-side pressure (when the recycling from the rod-side oil chamber  1   b  to the head-side oil chamber  1   c  cannot be preformed), a pump flow rate of oil controlled by the meter-in valve  7  is supplied to the head-side oil chamber  1   c  of the hydraulic cylinder  1 . On the other hand, the discharged oil from the rod-side oil chamber  1   b  is subjected to flow rate control by the meter-out selector valve  8  and then flows to the oil tank  3 . Consequently, the hydraulic cylinder  1  is extended. In this case, the controller  9  controllably increases or reduces the opening areas of the meter-in valve  7  and the meter-out selector valve  8  so that the pump flow rate and the discharge flow rate correspond to the amount of operation of the hydraulic-cylinder operation unit. Thus, the hydraulic cylinder  1  can be extended at a speed corresponding to the amount of operation of the hydraulic-cylinder operation unit. 
         [0037]    On the other hand, if the hydraulic-cylinder operation unit is operated to the extension side when the rod-side pressure in the hydraulic cylinder  1  is higher than the head-side pressure in the hydraulic cylinder  1 , the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the meter-in valve  20  and the contraction-side electromagnetic proportional valve for the meter-out selector valve  21   b  as in the case where the head-side pressure is higher than the rod-side pressure as described above. Consequently, the meter-in valve  7  is controllably placed in the actuation position X, whereas the meter-out selector valve  8  is controllably placed in the extension-side actuation position Y. Thus, a pump flow rate of oil controlled by the meter-in valve  7  is supplied to the head-side oil chamber  1   c  of the hydraulic cylinder  1  through the meter-out selector valve  8  in the extension-side actuation position Y. On the other hand, the oil in the rod-side oil chamber  1   b  is discharged to the oil tank  3  through the meter-out selector valve  8  in the extension-side actuation position Y. In this case, the opening areas of the meter-in valve  7  and the meter-out selector valve  8  are controllably increased or reduced based on control signals from the controller  9 . 
         [0038]    Moreover, if the hydraulic-cylinder operation unit is operated to the extension side when the rod-side pressure is higher than the head-side pressure, the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the recycling control valve  22  to controllably position the recycling control valve  13  in the open position X. In this case, the opening area of the recycling control valve  13  is controllably increased or reduced based on control signals from the controller  9 . Thus, a recycling flow rate of oil controlled by the recycling control valve  13  is supplied from the rod-side oil chamber  1   b  to the head-side oil chamber  1   c  of the hydraulic cylinder  1 . 
         [0039]    If the hydraulic-cylinder operation unit is operated to the extension side when the rod-side pressure is higher than the head-side pressure, the head-side oil chamber  1   c  of the hydraulic cylinder  1  is supplied with a pump flow rate of oil controlled by the meter-in valve  7  and a recycling flow rate of oil controlled by the recycling control valve  13 . On the other hand, the discharged oil from the rod-side oil chamber  1   b  is supplied to the head-side oil chamber  1   c  as the recycling flow rate, and the remaining oil is subjected to flow rate control by the meter-out selector valve  8  and is then discharged to the oil tank  3 . Consequently, the hydraulic cylinder  1  is extended. In this case, the controller  9  controls the opening area of the recycling control valve  13  so as to provide the desired recycling flow rate according to the difference between the rod-side pressure and the head-side pressure and the amount of operation of the hydraulic-cylinder operation unit. Further, in order for a flow rate of oil corresponding to the amount of operation of the hydraulic-cylinder operation unit to be supplied to the head-side oil chamber  1   c , the controller  9  controls the opening area of the meter-in valve  7  so as to provide the pump flow rate equal to a pump flow rate obtained by subtracting the recycling flow rate from the corresponding flow rate. Furthermore, in order for a flow rate of oil corresponding to the amount of operation of the hydraulic-cylinder operation unit to be discharged from the rod-side oil chamber  1   b , the controller  9  controls the opening area of the extension-side discharge path  8   f  in the meter-out selector valve  8  so as to provide the discharge flow rate equal to a discharge flow rate obtained by subtracting the recycling flow rate from the corresponding flow rate. Thus, the hydraulic cylinder  1  can be extended at a speed corresponding to the amount of operation of the hydraulic-cylinder operation unit. 
         [0040]    When the hydraulic-cylinder operation unit is operated to the contraction side or the extension side, the controller  9  outputs the actuating control signal to the electromagnetic proportional valve for the bypass valve  23  to controllably increase or reduce the opening area of the bypass valve  15  in order to adjustably increase or reduce the pump flow rate in association with the opening area of the meter-in valve  7 . Thus, a proper amount of discharged oil can be supplied from the hydraulic pump  2  to the meter-in valve  7 . 
         [0041]    In the present embodiment configured as described above, the hydraulic actuator control circuit for controlling supply and discharge of oil to and from the hydraulic cylinder  1  includes the meter-in valve  7  that controls the supply flow rate from the hydraulic pump  2  to the hydraulic cylinder  1  and the meter-out selector valve  8  arranged downstream of the meter-in valve  7  to switch between the direction of supply of hydraulic oil to the hydraulic cylinder  1  and the direction of discharge of hydraulic oil from the hydraulic cylinder  1  while controlling the discharge flow rate from the hydraulic cylinder  1  to the oil tank  3 . The hydraulic oil supplied from the hydraulic pump  2  is subjected to flow rate control by the meter-in valve  7  and is then supplied to the hydraulic cylinder  1  via the meter-out selector valve  8  that switches between the supply direction and the discharge direction. On the other hand, the discharged oil from the hydraulic cylinder  1  is subjected to flow rate control by the meter-out selector valve  8  that switches between the supply direction and the discharge direction and is then discharged to the oil tank  3 . 
         [0042]    As a result, the control of the supply flow rate from the hydraulic pump  2  to the hydraulic cylinder  1  (meter-in control) is performed by the meter-in valve  7 , whereas the control of the discharge flow rate from the hydraulic cylinder  1  to the oil tank  3  (meter-out control) is performed by the meter-out selector valve  8 . Thus, the meter-in control and the meter-out control are performed by the individual valves, allowing the relation between the meter-in and the meter-out to be easily changed in association with the contents of work and an operator. Moreover, even though the meter-in control and the meter-out control can be individually performed, the meter-in control, the meter-out control, and the directional control can be performed on the hydraulic cylinder  1  using the two valves of the meter-in valve  7  and the meter-out selector valve  8 . Furthermore, the meter-in valve  7  performs only the meter-in control and does not execute the directional control. Consequently, the meter-in valve  7  need not be moved in both directions. Accordingly, movement of the meter-in valve  7  may be achieved exclusively using a single actuator (or a single set of actuators; in the present embodiment, the electromagnetic proportional valve for the meter-in valve  20  or a set of the electromagnetic proportional valve for the meter-in valves  20 ), enabling the number of components to be reduced to contribute to cost reduction. 
         [0043]    Furthermore, on the upstream side of the meter-in valve  7 , the pressure compensation valve  5  is provided which keeps the pressure difference between the upstream side and the downstream side of the meter-in valve  7  constant. Thus, the flow rate control can be accurately performed using the meter-in valve  7  without being affected by, for example, a fluctuation in the pressure of the hydraulic pump  2 . 
         [0044]    Moreover, on the downstream side of the meter-out selector valve  8 , the recycling control valve  13  is provided which controls the recycling flow rate at which the discharged oil from one of the oil chambers (in the present embodiment, the rod-side oil chamber  1   b ) of the hydraulic cylinder  1  is supplied to the other oil chamber (in the present embodiment, the head-side oil chamber  1   c ). This enables the pump flow rate to be reduced by an amount equal to the recycling flow rate from one of the oil chambers to the other oil chamber, contributing to energy saving. The recycling control valve  13  is provided independently of the meter-in valve  7  and the meter-out selector valve  8 , allowing the recycling flow rate to be easily and accurately controlled. 
         [0045]    The present invention is of course not limited to the above-described embodiment. For example, in the above-described embodiment, the meter-in valve, the meter-out selector valve, the recycling control valve, and the bypass valve are pilot selector valves, and the electromagnetic proportional valves are used as actuators that operate the pilot selector valves. However, the meter-in valve, the meter-out selector valve, the recycling control valve, and the bypass valve may be electromagnetic proportional valves directly driven by solenoids. 
         [0046]    Furthermore, in the above-described embodiment, the recycling is performed such that the discharged oil from the rod-side oil chamber is supplied to the head-side oil chamber  1   c  when the hydraulic cylinder is extended. However, depending on a load imposed on a working portion driven by the hydraulic cylinder, the orientation of the working portion, and the like, the recycling may be performed such that discharged oil from the head-side oil chamber is supplied to the rod-side oil chamber when the hydraulic cylinder is contracted. 
         [0047]    Moreover, in the above-described embodiment, the recycling is controlled so as to make the total flow rate of the recycling flow rate and the pump flow rate equal to the flow rate corresponding to the amount of operation of the hydraulic-cylinder operation unit. However, the recycling may be controlled so as to make the total flow rate of the recycling flow rate and the pump flow rate higher than the flow rate corresponding to the amount of operation of the hydraulic-cylinder operation unit, increasing an actuation speed of the hydraulic cylinder. Such a change in control may be optionally made by changing settings for the controller  9 . 
       INDUSTRIAL APPLICABILITY 
       [0048]    The present invention can be utilized for a hydraulic actuator control circuit for controlling supply and discharge of oil to and from a hydraulic actuator such as a hydraulic cylinder. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  Hydraulic cylinder 
           2  Hydraulic pump 
           3  Oil tank 
           5  Pressure compensation valve 
           7  Meter-in valve 
           8  Meter-out selector valve 
           13  Recycling control valve