Hydraulic actuator control circuit

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.

Claims

1. A hydraulic actuator control circuit for controlling supply and discharge of oil to and from a hydraulic actuator, the hydraulic actuator control circuit comprising: a meter-in valve for controlling 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 and configured to switch between a direction of supply of hydraulic oil to 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, the meter-out selector valve including a first port, a second port, and a third port, the first port of the meter-out selector valve being fluidly coupled to the hydraulic pump via the meter-in valve, the second port of the meter-out selector valve being fluidly coupled to the oil tank along a flow path that does not include the meter-in valve, and the third port of the meter-out selector valve being fluidly coupled to a first port of the hydraulic actuator along a flow path that does not include the oil tank, wherein a first position of the meter-out selector valve effects fluid communication between the first port and the third port along a first meter-out flow passage within the meter-out selector valve, wherein a first position of the meter-in valve effects fluid communication between the pump and the first port of the meter-out selector valve via a first meter-in flow passage within the meter-in valve, and wherein a full-open flow area of the first meter-out flow passage is larger than a full-open flow area of the first meter-in flow passage, such that the first meter-out flow passage does not change the supply flow rate through the meter-in valve.

2. The hydraulic actuator control circuit according to claim 1, further comprising a pressure compensation valve disposed upstream of the meter-in valve for maintaining constant a pressure difference between an upstream side and a downstream side of the meter-in valve, an inlet port of the pressure compensation valve being fluidly coupled to the hydraulic pump along a flow path that does not include the meter-in valve, and the inlet port of the pressure compensation valve being fluidly coupled to an inlet port of the meter-in valve via an outlet port of the pressure compensation valve.

3. The hydraulic actuator control circuit according to claim 1, further comprising a recycling control valve disposed downstream of the meter-out selector valve, the recycling control valve being configured to control a recycling flow rate from a first oil chamber of the hydraulic actuator to a second oil chamber of the hydraulic actuator via the recycling control valve.

4. The hydraulic actuator control circuit according to claim 1, wherein the meter-out selector valve further includes a fourth port, the fourth port of the meter-out selector valve being fluidly coupled to a second port of the hydraulic actuator, and the first port of the hydraulic actuator is fluidly isolated from the second port of the hydraulic actuator by a piston disposed within the hydraulic actuator.

5. The hydraulic actuator control circuit according to claim 4, wherein the first position of the meter-out selector valve further effects fluid communication between the second port of the meter-out selector valve and the fourth port of the meter-out selector valve via a second meter-out flow passage through the meter-out selector valve.

6. The hydraulic actuator control circuit according to claim 5, wherein a second position of the meter-out selector valve effects fluid communication between the first port of the meter-out selector valve and the fourth port of the meter-out selector valve via a third meter-out flow passage through the meter-out selector valve, and effects fluid communication between the second port of the meter-out selector valve and the third port of the meter-out selector valve via a fourth meter-out flow passage through the meter-out selector valve.

7. The hydraulic actuator control circuit according to claim 6, wherein a third position of the meter-out selector valve blocks fluid communication between the first port of the meter-out selector valve and each of the second port of the meter-out selector valve, the third port of the meter-out selector valve, and the fourth port of the meter-out selector valve, and blocks fluid communication between the second port of the meter-out selector valve and each of the third port of the meter-out selector valve and the fourth port of the meter-out selector valve.

8. The hydraulic actuator control circuit according to claim 2, wherein the pressure compensation valve has a first position and a second position, the first position of the pressure compensation valve effecting a first flow restriction between the inlet port and the outlet port of the pressure compensation valve, the second position of the pressure compensation valve effecting a second flow restriction between the inlet port and the outlet port of the pressure compensation valve, the first flow restriction being less than the second flow restriction, and wherein a pressure applied to a first actuation port of the pressure compensation valve biases the pressure compensation valve toward the first position.

9. The hydraulic actuator control circuit according to claim 8, wherein the first actuation port of the pressure compensation valve is fluidly coupled to a first outlet port of the meter-in valve, the first outlet port of the meter-in valve being fluidly coupled to the inlet port of the meter-in valve via a first meter-in flow passage through the meter-in valve.

10. The hydraulic actuator control circuit according to claim 9, wherein the outlet port of the pressure compensation valve is fluidly coupled to the inlet port of the meter-in valve via a first conduit, wherein a pressure applied to a second actuation port of the pressure compensation valve biases the pressure compensation valve toward the second position, and wherein the second actuation port of the pressure compensation valve is fluidly coupled to the first conduit.

11. The hydraulic actuator control circuit according to claim 10, wherein a resilient member of the pressure compensation valve biases the pressure compensation valve toward the first position.

12. The hydraulic actuator control circuit according to claim 9, wherein the meter-in valve includes a second outlet port, the second outlet port of the meter-in valve being in selective fluid communication with the inlet port of the meter-in valve via a second meter-in flow passage through the meter-in valve, a first position of the meter-in valve effects a first flow restriction of the second meter-in flow passage, a second position of the meter-in valve effects a second flow restriction of the second meter-in flow passage, the first flow restriction of the second meter-in flow passage being greater than the second flow restriction of the second meter-in flow passage.

13. The hydraulic actuator control circuit according to claim 12, wherein the first port of the meter-out selector valve is fluidly coupled to the second outlet of the meter-in valve along a flow path that does not include either the oil tank or the hydraulic actuator.

14. The hydraulic actuator control circuit according to claim 12, wherein the first port of the hydraulic actuator is fluidly isolated from the second port of the hydraulic actuator by a piston disposed within the hydraulic actuator.

15. The hydraulic actuator control circuit of claim 1, further comprising a controller operatively coupled to the meter-in valve and the meter-out selector valve, wherein the meter-out selector valve further includes a fourth port, the fourth port of the meter-out selector valve being fluidly coupled to a second port of the hydraulic actuator, wherein the controller is configured to: actuate the meter-in valve to the first position of the meter-in valve to effect a target flow rate of the hydraulic oil through the first meter-in flow passage, actuate the meter-out selector valve to the first position of the meter-out selector valve while maintaining the meter-in valve in the first position to effect fluid communication between the meter-in valve and the hydraulic actuator without changing the target flow rate of the hydraulic oil through the meter-out selector valve, and effect fluid communication between the second port of the meter-out selector valve and the fourth port of the meter-out selector valve via a second meter-out flow passage within the meter-out selector valve, and actuate the meter-out selector valve to a second position while maintaining the meter-in valve in the first position to effect fluid communication between the meter-in valve and the hydraulic actuator without changing the target flow rate of the hydraulic oil through the meter-out selector valve, and effect fluid communication between the second port of the meter-out selector valve and the fourth port of the meter-out selector valve via the second meter-out flow passage, wherein a fluid restriction of the second meter-out flow passage in the second position of the meter-out selector valve is greater than a fluid restriction of the second meter-out flow passage in the first position of the meter-out selector valve.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a hydraulic circuit diagram depicting a hydraulic actuator control circuit.

(2) FIG. 2 is a block diagram illustrating inputs to and outputs from a controller.

DESCRIPTION OF EMBODIMENTS

(3) 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 1b and a head-side oil chamber 1c are formed on both sides of a piston 1a.

(4) 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.

(5) 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.

(6) 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.

(7) 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.

(8) 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 21a and an extension-side electromagnetic proportional valve for the meter-out selector valve 21b (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 21a and 21b, 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 1b or the head-side oil chamber 1c of the hydraulic cylinder 1 and where discharged oil from the rod-side oil chamber 1b and the head-side oil chamber 1c 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 21a, 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 1b of the hydraulic cylinder 1 via a rod-side oil path 10 and where oil discharged from the head-side oil chamber 1c 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 21b, 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 1c of the hydraulic cylinder 1 via the head-side oil path 11 and where oil discharged from the rod-side oil chamber 1b 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 8c and an extension-side supply path 8d through which a pump flow rate of oil is supplied from the meter-out selector valve 8 to the rod-side oil chamber 1b of the hydraulic cylinder 1 and to the head-side oil chamber 1c 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 1b and the head-side oil chamber 1c without being changed. On the other hand, the opening areas of a contraction-side discharge path 8e and an extension-side discharge path 8f through which discharged oil from the head-side oil chamber 1c of the hydraulic cylinder 1 and from the rod-side oil chamber 1b 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 21a and the extension-side electromagnetic proportional valve for the meter-out selector valve 21b. 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 1c to the oil tank 3 and from the rod-side oil chamber 1b 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 1b 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 1c of the hydraulic cylinder 1.

(9) 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 1b of the hydraulic cylinder 1 to the head-side oil chamber 1c. 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 1b to the head-side oil chamber 1c of the hydraulic cylinder 1.

(10) 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.

(11) 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 21a, the extension-side electromagnetic proportional valve for the meter-out selector valve 21b, 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.

(12) 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 21a, the contraction-side electromagnetic proportional valve for the meter-out selector valve 21b, 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.

(13) 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.

(14) 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 21a 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 8c 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 1b of the hydraulic cylinder 1 through the contraction-side supply path 8c. On the other hand, the oil in the head-side oil chamber 1c is discharged to the oil tank 3 through the contraction-side discharge path 8e in the meter-out selector valve 8. The opening area of the contraction-side discharge path 8e 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 1c to the oil tank 3.

(15) 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.

(16) 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 1b of the hydraulic cylinder 1. On the other hand, the discharged oil from the head-side oil chamber 1c 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.

(17) 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 1b to the head-side oil chamber 1c, 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 1b to the head-side oil chamber 1C is performed. Thus, first, the case where the head-side pressure is higher than the rod-side pressure will be described.

(18) 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.

(19) 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 21b. 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 8d 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 1c of the hydraulic cylinder 1 through the extension-side supply path 8d. On the other hand, the oil in the rod-side oil chamber 1b is discharged to the oil tank 3 through the extension-side discharge path 8f. The opening area of the extension-side discharge path 8f 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 1b to the oil tank 3.

(20) 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.

(21) 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 1b to the head-side oil chamber 1c cannot be preformed), a pump flow rate of oil controlled by the meter-in valve 7 is supplied to the head-side oil chamber 1c of the hydraulic cylinder 1. On the other hand, the discharged oil from the rod-side oil chamber 1b 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.

(22) 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 21b 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 1c 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 1b 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.

(23) 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 1b to the head-side oil chamber 1c of the hydraulic cylinder 1.

(24) 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 1c 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 1b is supplied to the head-side oil chamber 1c 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 1c, 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 1b, the controller 9 controls the opening area of the extension-side discharge path 8f 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.

(25) 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.

(26) 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.

(27) 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.

(28) 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.

(29) 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 1b) of the hydraulic cylinder 1 is supplied to the other oil chamber (in the present embodiment, the head-side oil chamber 1c). 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.

(30) 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.

(31) 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 1c 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.

(32) 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

(33) 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.