Work machine

Abstract

This work machine comprises: a regeneration valve that controls a regeneration flow rate, which is the flow rate of hydraulic oil flowing from a bottom chamber of a hydraulic cylinder to a pressure accumulation device; a boost valve that controls a boost flow rate, which is the flow rate of the hydraulic oil flowing from the bottom chamber to a rod chamber; a discharge valve that controls a return flow rate, which is the flow rate of hydraulic oil flowing from the bottom chamber to a tank; and a control device. The control device calculates a target bottom outflow flow rate and a target boost flow rate on the basis of the operation amount detected by a cylinder operation amount sensor, controls the boost valve so that the boost flow rate gradually increases with the passage of time and becomes equal to the target boost flow rate, and controls the regeneration valve or the regeneration valve and the discharge valve so that the total of the regeneration flow rate, the boost flow rate, and the return flow rate becomes equal to the target bottom outflow flow rate.

Description

Work machinery

[0001] The present invention relates to a work machine.

[0002] BACKGROUND ART Some working machines, such as hydraulic excavators, are equipped with an engine power assist system in which pressurized oil flowing out of a boom cylinder when the boom is lowered is stored in an accumulator and used to assist engine power.

[0003] Patent Document 1 describes that when pressurized oil from the head side of the boom cylinder is stored in an accumulator by a pressure storage circuit, hydraulic oil from the head side of the boom cylinder is regenerated to the boom cylinder via a main control valve in a regeneration circuit. Patent Document 1 also describes that hydraulic oil supplied under pressure from the main pump is supplied to the rod side of the boom cylinder while the head side of the boom cylinder is connected to a tank by a bleed-off valve in a bleed-off circuit, thereby improving the initial speed when the boom cylinder is retracted.

[0004] Japanese Patent Application Laid-Open No. 2016-205494

[0005] However, in the work machine described in Patent Document 1, if a sudden, large boom lowering operation is performed and the main control valve connecting the bottom side (head side) and rod side of the boom cylinder opens wide immediately after the operation, a shock may occur in the boom cylinder due to the pressure difference between the bottom side and the rod side. This shock deteriorates operability when lowering the boom.

[0006] The present invention aims to prevent the occurrence of shock in a hydraulic cylinder while ensuring the speed of the hydraulic cylinder according to the amount of operation in a work machine that can supply hydraulic oil flowing out from the bottom chamber of the hydraulic cylinder to a rod chamber and a pressure accumulator.

[0007] A work machine according to one aspect of the present invention comprises a prime mover, a hydraulic pump driven by the prime mover, a hydraulic cylinder having a bottom chamber and a rod chamber and driven by hydraulic oil discharged from the hydraulic pump, a pressure accumulator that accumulates hydraulic oil supplied from the bottom chamber, a regenerative valve that controls the flow rate of hydraulic oil flowing from the bottom chamber to the pressure accumulator, a boost valve that controls the flow rate of hydraulic oil flowing from the bottom chamber to the rod chamber, a discharge valve that controls the flow rate of hydraulic oil flowing from the bottom chamber to a tank, a cylinder operating device that operates the hydraulic cylinder, a cylinder operation amount sensor that detects the operation amount of the cylinder operating device, and a control device that controls at least the regenerative valve and the boost valve based on the operation amount detected by the cylinder operation amount sensor. The control device calculates a target bottom outflow flow rate, which is a target value for the flow rate of hydraulic oil flowing out of the bottom chamber of the hydraulic cylinder, and a target boost flow rate, which is a target value for the flow rate of hydraulic oil supplied from the bottom chamber to the rod chamber through the boost valve, based on the operation amount detected by the cylinder operation amount sensor, and controls the boost valve so that the boost flow rate, which is the flow rate of hydraulic oil supplied from the bottom chamber to the rod chamber through the boost valve, gradually increases over time at a predetermined time change rate to become equal to the target boost flow rate, and controls the regenerative valve, or the regenerative valve and the discharge valve, so that the sum of a regenerative flow rate, which is the flow rate of hydraulic oil supplied from the bottom chamber to the pressure accumulator device through the regenerative valve, the boost flow rate, and a return flow rate, which is the flow rate of hydraulic oil discharged from the bottom chamber to the tank through the discharge valve, becomes equal to the target bottom outflow flow rate, in each of a transient state before the boost flow rate reaches the target boost flow rate and a steady state after the boost flow rate has reached the target boost flow rate.

[0008] According to the present invention, in a work machine capable of supplying hydraulic oil flowing out from the bottom chamber of a hydraulic cylinder to a rod chamber and a pressure accumulator, it is possible to prevent shocks in the hydraulic cylinder while ensuring the speed of the hydraulic cylinder according to the amount of operation.

[0009] FIG. 1 is a perspective view of a hydraulic excavator shown as an example of a work machine according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a hydraulic drive unit mounted on the hydraulic excavator. FIG. 3 is a diagram showing a drive control system mounted on the hydraulic excavator. FIG. 4 is a diagram showing the change over time in the flow rate of hydraulic oil passing through a flow control valve when a sudden maximum boom lowering operation is performed. FIG. 5 is a schematic diagram showing the distribution of a target bottom outflow flow rate Qst in a steady state. FIG. 6 is a schematic diagram showing the distribution of a target bottom outflow flow rate Qst in a transient state. FIG. 7 is a block diagram showing the functions of a control device.

[0010] A work machine according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view of a hydraulic excavator 1 shown as an example of a work machine according to an embodiment of the present invention. As shown in FIG. 1, the hydraulic excavator 1 includes a vehicle body 52 and an articulated work device 51 attached to the vehicle body 52. ​​The work device 51 has a boom 1a, an arm 1b, and a bucket 1c. The work device 51 also includes a boom cylinder 3a that drives the boom 1a, an arm cylinder 3b that drives the arm 1b, and a bucket cylinder 3c that drives the bucket 1c. The vehicle body 52 includes a lower traveling body 1e and an upper rotating body 1d attached to the lower traveling body 1e. The upper rotating body 1d rotates relative to the lower traveling body 1e by a swing motor 4 (see FIG. 2). The lower traveling body 1e travels by left and right traveling motors 3e.

[0011] The boom 1a is rotatably attached to the upper rotating body 1d and driven by a boom cylinder 3a. The arm 1b is rotatably attached to the boom 1a and driven by an arm cylinder 3b. The bucket 1c is rotatably attached to the arm 1b and driven by a bucket cylinder 3c. The bucket 1c is a work tool used for excavation work and for loading excavated soil and other materials into a dump truck or the like.

[0012] FIG. 2 is a schematic diagram of a hydraulic drive system mounted on the hydraulic excavator 1. As shown in FIG. 2, the hydraulic drive system of the hydraulic excavator 1 includes an engine 5, a first hydraulic pump 6, a second hydraulic pump 7, multiple hydraulic actuators (3a, 3b, 3c, 4), multiple flow control valves (8, 13 to 22, 34), and an accumulator 9. The multiple hydraulic actuators include a boom cylinder 3a, an arm cylinder 3b, a bucket cylinder 3c, and a swing motor 4. The boom cylinder 3a, the arm cylinder 3b, and the bucket cylinder 3c are hydraulic cylinders each having a cylinder tube, a disc-shaped piston that divides the interior of the cylindrical cylinder tube into a bottom chamber 301 and a rod chamber 302, and a piston rod connected to the piston. The swing motor 4 is a hydraulic motor. The multiple flow control valves (8, 13 to 22, 34) are driven by commands from a control device 100 (see FIG. 3), which will be described later. The flow control valve (8, 13 to 22, 34) adjusts the area of ​​the opening by changing the position of the internal spool, thereby controlling the flow rate of the hydraulic oil passing through the opening.

[0013] The engine 5, which serves as a prime mover, is formed by an internal combustion engine such as a diesel engine. The first hydraulic pump 6 and the second hydraulic pump 7 are mechanically linked to the engine 5, driven by the power of the engine 5, and discharge hydraulic oil (pressurized oil). The first hydraulic pump 6 and the second hydraulic pump 7 are variable displacement hydraulic pumps whose capacity (displacement volume) can be changed. The first hydraulic pump 6 and the second hydraulic pump 7 are connected to multiple hydraulic actuators via pipelines, respectively. The multiple hydraulic actuators are driven by the pressurized oil discharged from the first hydraulic pump 6 and the second hydraulic pump 7. The accumulator 9 is a pressure accumulation device that accumulates pressurized oil supplied from the bottom chamber 301 of the boom cylinder 3a, which contracts due to the weight of the working device 51 during a boom lowering operation.

[0014] A first boom control valve 14, which is a flow control valve that controls the flow rate of hydraulic oil supplied from the first hydraulic pump 6 to the boom cylinder 3a and the flow rate of hydraulic oil discharged from the boom cylinder 3a to the tank 12, is provided in the pipe connecting the first hydraulic pump 6 and the boom cylinder 3a. A second boom control valve 15, which is a flow control valve that controls the flow rate of hydraulic oil supplied from the second hydraulic pump 7 to the boom cylinder 3a and the flow rate of hydraulic oil discharged from the boom cylinder 3a to the tank 12, is provided in the pipe connecting the second hydraulic pump 7 and the boom cylinder 3a.

[0015] Second boom control valve 15 is installed on pipe 35 that connects bottom chamber 301 of boom cylinder 3a and tank 12. Second boom control valve 15 functions as a discharge valve that controls the flow rate of hydraulic oil that flows from the bottom chamber 301 side of boom cylinder 3a to the tank 12 side by adjusting the opening area.

[0016] A first arm control valve 16, which is a flow control valve that controls the flow rate of hydraulic oil supplied from the first hydraulic pump 6 to the arm cylinder 3b and the flow rate of hydraulic oil discharged from the arm cylinder 3b to the tank 12, is provided in the pipe connecting the first hydraulic pump 6 and the arm cylinder 3b. A second arm control valve 17, which is a flow control valve that controls the flow rate of hydraulic oil supplied from the second hydraulic pump 7 to the arm cylinder 3b and the flow rate of hydraulic oil discharged from the arm cylinder 3b to the tank 12, is provided in the pipe connecting the second hydraulic pump 7 and the arm cylinder 3b.

[0017] A bucket control valve 18, which is a flow control valve, is provided in the pipe connecting the first hydraulic pump 6 and the bucket cylinder 3c, and controls the flow rate of hydraulic oil supplied from the first hydraulic pump 6 to the bucket cylinder 3c and the flow rate of hydraulic oil discharged from the bucket cylinder 3c to the tank 12. A swing control valve 13, which is a flow control valve, is provided in the pipe connecting the second hydraulic pump 7 and the swing motor 4, and controls the flow rate of hydraulic oil supplied from the second hydraulic pump 7 to the swing motor 4 and the flow rate of hydraulic oil discharged from the swing motor 4 to the tank 12.

[0018] The above-mentioned flow control valves (13 to 18) are center bypass type control valves. The flow control valves (14, 16, 18) have a center bypass passage that communicates between the first hydraulic pump 6 and the tank 12 when in the neutral position, and discharge hydraulic oil discharged from the first hydraulic pump 6 to the tank 12 through the center bypass passage when in the neutral position. The flow control valves (13, 15, 17) have a center bypass passage that communicates between the second hydraulic pump 7 and the tank 12 when in the neutral position, and discharge hydraulic oil discharged from the second hydraulic pump 7 to the tank 12 through the center bypass passage when in the neutral position.

[0019] A make-up device is provided between the conduit 10 connecting the swing control valve 13 and the right swing port of the swing motor 4, and the conduit 11 connecting the swing control valve 13 and the left swing port of the swing motor 4. The make-up device has mutually opposite check valves 25, 26 and relief valves 27, 28. A conduit 29 is provided between these check valves 25, 26 and relief valves 27, 28 to return oil discharged from the swing motor 4 to the tank 12. The conduit 29 functions as a make-up conduit that can replenish hydraulic oil to the swing motor 4.

[0020] A conduit 38 is connected to the accumulator 9, and pressure oil discharged from the accumulator 9 is supplied to each hydraulic actuator through the conduit 38. The bottom chamber 301 of the boom cylinder 3a and the conduit 38 are connected by a conduit (hereinafter also referred to as the bottom-side conduit) 32. The bottom-side conduit 32 is provided with a regenerative valve 8 that regenerates pressure oil in the bottom chamber 301 of the boom cylinder 3a to the accumulator 9 when the boom is lowered. The regenerative valve 8 is a flow control valve that controls the flow rate of hydraulic oil flowing from the bottom chamber 301 side of the boom cylinder 3a to the accumulator 9 side by adjusting the opening area.

[0021] A conduit (hereinafter also referred to as rod-side conduit) 33 that branches off from the bottom-side conduit 32 and is connected to the rod chamber 302 of the boom cylinder 3a is provided in the bottom-side conduit 32 between the bottom chamber 301 of the boom cylinder 3a and the regenerative valve 8. A pressure boost valve 34 is provided in the rod-side conduit 33 to raise the pressure of the hydraulic oil in the bottom chamber 301 of the boom cylinder 3a by connecting the bottom chamber 301 and the rod chamber 302 when the boom is lowered. The pressure boost valve 34 is a flow control valve that controls the flow rate of hydraulic oil flowing from the bottom chamber 301 side to the rod chamber 302 side of the boom cylinder 3a by adjusting the opening area.

[0022] A plurality of branch pipelines 39, 40, 41, and 42 are connected to pipeline 38. Branch pipeline 39 is connected to bottom-side pipeline 32 between bottom chamber 301 of boom cylinder 3a and regenerative valve 8. Branch pipeline 39 is provided with boom assist valve 19, which is a flow control valve that controls the flow rate of hydraulic oil supplied from accumulator 9 to boom cylinder 3a.

[0023] The branch pipe 40 is provided with an arm assist valve 20, which is a flow control valve that controls the flow rate of hydraulic oil supplied from the accumulator 9 to the arm cylinder 3b. The branch pipe 41 is provided with a bucket assist valve 21, which is a flow control valve that controls the flow rate of hydraulic oil supplied from the accumulator 9 to the bucket cylinder 3c. The branch pipe 42 is provided with a swing assist valve 22, which is a flow control valve that controls the flow rate of hydraulic oil supplied from the accumulator 9 to the swing motor 4.

[0024] In this way, the accumulator 9 is connected to each hydraulic actuator via a respective pipe line. The energy stored in the accumulator 9 is used to drive each hydraulic actuator.

[0025] Figure 3 is a diagram showing a drive control system mounted on the hydraulic excavator 1. Note that the operator's cab of the hydraulic excavator 1 is provided with actuator operating devices for operating each hydraulic actuator, but Figure 3 shows only a boom operating lever (cylinder operating device) 30 for operating the boom cylinder 3a and a bucket operating lever (cylinder operating device) 44 for operating the bucket cylinder 3c, which is a hydraulic actuator different from the boom cylinder 3a, and omits illustration of the other actuator operating devices and actuator operation amount sensors that detect the operation amounts of the other actuator operating devices. Also omitted from Figure 3 are the arm cylinder 3b, the swing motor 4, and the flow control valves (13, 16, 17, 20, 22) used to control these hydraulic actuators.

[0026] 3 , the drive control system includes a plurality of sensors that detect the state of the hydraulic drive unit, and a control device 100 that controls at least one of a plurality of flow control valves of the hydraulic drive unit based on the detection results of the plurality of sensors. The plurality of sensors include a plurality of pressure sensors and a plurality of actuator operation amount sensors. The plurality of pressure sensors include a bottom pressure sensor 23, a rod pressure sensor 24, and a pressure accumulation sensor 43. The plurality of actuator operation amount sensors include a boom operation amount sensor 31 and a bucket operation amount sensor 45.

[0027] The bottom pressure sensor 23 detects the pressure of the hydraulic oil in the bottom chamber 301 of the boom cylinder 3a (hereinafter also referred to as the bottom pressure). The rod pressure sensor 24 detects the pressure of the hydraulic oil in the rod chamber 302 of the boom cylinder 3a (hereinafter also referred to as the rod pressure). The pressure accumulator sensor 43 is provided in the pipe 38 and detects the pressure of the hydraulic oil in the accumulator 9 (hereinafter also referred to as the accumulator pressure). Each pressure sensor (23, 24, 43) outputs a signal representing the detected pressure to the control device 100.

[0028] The boom operation amount sensor 31 is a cylinder operation amount sensor that detects the operation amount of the boom operation lever 30 (hereinafter also referred to as the boom operation amount) and outputs a signal representing the detected operation amount to the control device 100. The bucket operation amount sensor 45 is an actuator operation amount sensor that detects the operation amount of the bucket operation lever 44 (hereinafter also referred to as the bucket operation amount) and outputs a signal representing the detected operation amount to the control device 100. The boom operation amount detected by the boom operation amount sensor 31 includes a boom-raising operation amount for operating the boom 1a in the raising direction and a boom-lowering operation amount for operating the boom 1a in the lowering direction. The bucket operation amount detected by the bucket operation amount sensor 45 includes a bucket crowding operation amount for operating the bucket 1c in the crowding direction and a bucket dumping operation amount for operating the bucket 1c in the dumping direction.

[0029] The first boom control valve 14 is a three-position closed center directional control valve and is provided at both ends with operating terminals 14a, 14b that receive electromagnetic commands from the control device 100. The first boom control valve 14 switches its spool position in response to the electromagnetic command from the control device 100 sent to its operating terminal. When a boom-raising operation is performed, an electromagnetic command is sent to the operating terminal 14b, and pressure oil from the first hydraulic pump 6 is supplied to the bottom chamber 301 of the boom cylinder 3a through the first boom control valve 14, and hydraulic oil in the rod chamber 302 of the boom cylinder 3a is discharged to the tank 12 through the first boom control valve 14. When a boom-lowering operation is performed, an electromagnetic command is sent to the operating terminal 14a, and the first hydraulic pump 6 and the rod chamber 302 of the boom cylinder 3a are connected via the first boom control valve 14. When the boom is raised, the rod chamber 302 of the boom cylinder 3a and the tank 12 are connected via the first boom control valve 14, whereas when the boom is lowered, the bottom chamber 301 of the boom cylinder 3a and the tank 12 are not connected via the first boom control valve 14. Therefore, when the boom is lowered, hydraulic oil is not discharged from the bottom chamber 301 of the boom cylinder 3a to the tank 12 via the first boom control valve 14.

[0030] Second boom control valve 15 is a three-position closed center directional control valve and is provided at both ends with operating terminals 15a, 15b that receive electromagnetic commands from control device 100. Second boom control valve 15 switches its spool position in response to electromagnetic commands from control device 100 sent to its operating terminals. When a boom-raising operation is performed, an electromagnetic command is sent to operating terminal 15b, and pressure oil from second hydraulic pump 7 is supplied to bottom chamber 301 of boom cylinder 3a through second boom control valve 15, and hydraulic oil in rod chamber 302 of boom cylinder 3a is discharged to tank 12 through second boom control valve 15. When a boom-lowering operation is performed, an electromagnetic command is sent to operating terminal 15a, and second hydraulic pump 7 and rod chamber 302 of boom cylinder 3a are connected via second boom control valve 15. In addition, when the boom lowering operation is performed and certain conditions are met, the hydraulic oil in the bottom chamber 301 of the boom cylinder 3a is discharged into the tank 12 through the second boom control valve 15 in response to an electromagnetic command from the control device 100 described below.

[0031] The bucket control valve 18 is a three-position closed-center directional control valve, and is provided at both ends with operating terminals 18a, 18b that receive electromagnetic commands from the control device 100. The bucket control valve 18 switches its spool position in response to electromagnetic commands from the control device 100 sent to its operating terminals. When a bucket dump operation is performed, an electromagnetic command is sent to the operating terminal 18a, and pressure oil from the first hydraulic pump 6 is supplied to the rod chamber 302 of the bucket cylinder 3c through the bucket control valve 18, and hydraulic oil in the bottom chamber 301 of the bucket cylinder 3c is discharged to the tank 12 through the bucket control valve 18. When a bucket crowd operation is performed, an electromagnetic command is sent to the operating terminal 18b, and pressure oil from the first hydraulic pump 6 is supplied to the bottom chamber 301 of the bucket cylinder 3c through the bucket control valve 18, and hydraulic oil in the rod chamber 302 of the bucket cylinder 3c is discharged to the tank 12 through the bucket control valve 18.

[0032] Although omitted in FIG. 3, the driving of the arm cylinder 3b by the first hydraulic pump 6 and the second hydraulic pump 7 and the driving of the swing motor 4 by the second hydraulic pump 7 are similar to the driving of the bucket cylinder 3c described above.

[0033] The hydraulic drive system of the hydraulic excavator 1 according to this embodiment is equipped with a boost regeneration circuit including the regeneration valve 8 and the boost valve 34. By performing regeneration while communicating the bottom chamber 301 and rod chamber 302 of the boom cylinder 3a, the pressure in the accumulator 9 can be efficiently increased, thereby increasing the number of hydraulic actuators that can provide assistance and the opportunities for assistance.

[0034] However, for example, if the boom control lever 30 is suddenly operated from the neutral operation position to the maximum boom lowering operation position, immediately opening the boost valve 34 in response to that operation could cause a shock in the boom cylinder 3a due to the pressure difference between the bottom chamber 301 and the rod chamber 302 of the boom cylinder 3a. This shock could deteriorate operability during boom lowering, leading to reduced work efficiency. Therefore, in this embodiment, when a sudden maximum boom lowering operation is performed, the control device 100 controls the boost valve 34 to gradually open over time. By gradually opening the boost valve 34, the occurrence of the above-mentioned shock can be prevented.

[0035] However, if the boost valve 34 is opened slowly, the amount of hydraulic oil supplied from the bottom chamber 301 to the rod chamber 302 of the boom cylinder 3a is suppressed. This reduces the retraction speed of the boom cylinder 3a (i.e., the boom lowering speed), which may result in reduced work efficiency. Therefore, the control device 100 according to this embodiment adjusts the opening area of ​​the second boom control valve 15 and the opening area of ​​the regenerative valve 8 in accordance with changes in the opening area of ​​the boost valve 34 so that the boom cylinder 3a operates at a speed corresponding to the amount of boom lowering operation. This will be explained in detail below.

[0036] The control device 100 is composed of a computer equipped with a processing device 100a such as a CPU (Central Processing Unit), MPU (Micro Processing Unit), or DSP (Digital Signal Processor), a non-volatile memory 100b such as a ROM (Read Only Memory), flash memory, or hard disk drive, a volatile memory 100c called RAM (Random Access Memory), an input / output interface, and other peripheral circuits. These pieces of hardware work together to run software and realize multiple functions. The control device 100 may be composed of a single computer or multiple computers.

[0037] The nonvolatile memory 100b stores programs capable of executing various calculations. In other words, the nonvolatile memory 100b is a storage medium (storage device) from which the programs that realize the functions of this embodiment can be read. The volatile memory 100c is a storage medium (storage device) that temporarily stores the results of calculations performed by the processing device 100a and signals input from the input / output interface. The processing device 100a is a device that loads the programs stored in the nonvolatile memory 100b into the volatile memory 100c and executes the calculations, and performs predetermined calculations on data taken from the input / output interface, the nonvolatile memory 100b, and the volatile memory 100c in accordance with the programs.

[0038] The input section of the input / output interface converts signals input from various devices (sensors, etc.) into data that can be calculated by the processing device 100a. The output section of the input / output interface generates an output signal according to the calculation result in the processing device 100a and outputs the signal to various devices (flow control valves, etc.).

[0039] An overview of flow rate control during a boom lowering operation by the control device 100 according to this embodiment will be described with reference to Fig. 4. Fig. 4 is a diagram showing the change over time in the flow rate of hydraulic oil passing through the flow control valves when a sudden maximum boom lowering operation is performed. The horizontal axis of Fig. 4 represents elapsed time. The vertical axis of Fig. 4(a) represents the amount of boom lowering operation, and the vertical axis of Fig. 4(b) represents the flow rate of hydraulic oil passing through the flow control valves (8, 15, 34). The flow rate passing through each flow control valve is represented by different hatching for each flow control valve.

[0040] The control device 100 controls the boost valve 34 and also controls the regenerative valve 8, or the regenerative valve 8 and the second boom control valve 15, based on the boom lowering operation amount Lb detected by the boom operation amount sensor 31. In this embodiment, the control of each flow control valve when only the boom cylinder 3a of the multiple hydraulic actuators is operated (when an individual boom lowering operation is performed) will be described. When an individual boom lowering operation is performed, the flow rates of the first hydraulic pump 6 and the second hydraulic pump 7 are controlled to 0. In other words, when a boom lowering operation is performed, the boom 1a moves in the lowering direction due to the weight of the working device 51. The movement speed of the boom 1a in the lowering direction is determined by the flow rate of hydraulic oil flowing out of the boom cylinder 3a.

[0041] The control device 100 calculates a target bottom outflow flow rate Qst, which is a target value for the flow rate of hydraulic oil flowing out from the bottom chamber 301 of the boom cylinder 3a, based on the boom lowering operation amount Lb detected by the boom operation amount sensor 31. The target bottom outflow flow rate Qst is determined according to the boom lowering operation amount Lb, and corresponds to the standard flow rate of hydraulic oil discharged from the bottom chamber 301 of the boom cylinder 3a to the tank 12 through the second boom control valve 15 in a standard hydraulic drive system not equipped with a boost regeneration circuit, that is, in a hydraulic drive system configured to send return oil from the boom cylinder 3a only to the tank 12.

[0042] The control device 100 calculates a target boost flow rate Qbrt, which is a target value for the flow rate of hydraulic oil to be supplied from the bottom chamber 301 of the boom cylinder 3a to the rod chamber 302 through the boost valve 34, based on the target bottom outflow flow rate Qst and the pressure-receiving area ratio Ar / Ab of the bottom chamber 301 and the rod chamber 302 of the boom cylinder 3a. As described above, the target bottom outflow flow rate Qst is determined by the boom lowering operation amount Lb, and therefore the target boost flow rate Qbrt is also determined by the boom lowering operation amount Lb.

[0043] The control device 100 controls the boost valve 34 so that the boost flow rate Qbr, which is the flow rate of hydraulic oil supplied from the bottom chamber 301 of the boom cylinder 3a to the rod chamber 302 through the boost valve 34, gradually increases at a predetermined time change rate over time t until it becomes equal to the target boost flow rate Qbrt.

[0044] The control device 100 controls the boost valve 34, the regenerative valve 8, and the second boom control valve 15 so that the target bottom outflow flow rate Qst corresponds to the sum of the boost flow rate Qbr, the regenerative flow rate Qba, and the return flow rate Qbt. The regenerative flow rate Qba is the flow rate of hydraulic oil supplied (regenerated) from the bottom chamber 301 of the boom cylinder 3a to the accumulator 9 through the regenerative valve 8. The return flow rate Qbt is the flow rate of hydraulic oil discharged from the bottom chamber 301 of the boom cylinder 3a to the tank 12 through the second boom control valve 15.

[0045] 4, when the boom lowering operation is performed, the boost flow rate Qbr increases over time t until it reaches the target boost flow rate Qbrt corresponding to the boom lowering operation amount Lb. Here, the state in which the boost flow rate Qbr reaches the target boost flow rate Qbrt is defined as a steady state, and the state from the start time t0 of the boom lowering operation to the time t1 when the steady state is reached is defined as a transient state.

[0046] In a transient state, the boost flow rate Qbr is smaller than in a steady state. To ensure a sufficient boom lowering speed even in a transient state, the control device 100 compensates for the flow rate deficiency with the regenerative flow rate Qba and the return flow rate Qbt. Note that the flow rate deficiency corresponds to the remainder of the bottom outflow flow rate required to operate the boom cylinder 3a at the target speed corresponding to the boom lowering operation amount Lb, and is therefore referred to as the residual flow rate Qr for convenience. The residual flow rate Qr corresponds to the value obtained by subtracting the boost flow rate from the target bottom outflow flow rate (Qr = Qst - Qbr). As a result, the regenerative flow rate Qba is greater in a transient state than in a steady state. Similarly, the return flow rate Qbt is greater in a transient state than in a steady state.

[0047] When the accumulator pressure is sufficiently low, it is preferable to ensure the shortfall in flow rate (i.e., the residual flow rate Qr) using only the regenerative flow rate Qba. Therefore, when the accumulator pressure Pa is less than a predetermined value Pa0, the control device 100 controls the regenerative valve 8 so that the regenerative flow rate Qba becomes equal to the residual flow rate Qr. On the other hand, when the accumulator pressure Pa is equal to or greater than the predetermined value Pa0, the control device 100 controls the regenerative valve 8 and the second boom control valve 15 so that the sum of the regenerative flow rate Qba and the return flow rate Qbt becomes equal to the residual flow rate Qr.

[0048] Fig. 5 is a schematic diagram showing the distribution of the target bottom outflow flow rate Qst in a steady state, and Fig. 6 is a schematic diagram showing the distribution of the target bottom outflow flow rate Qst in a transient state. The control device 100 controls the regenerative valve 8, or the regenerative valve 8 and the second boom control valve 15, so that the sum of the regenerative flow rate Qba, the boosting flow rate Qbr, and the return flow rate Qbt becomes equal to the target bottom outflow flow rate Qst in each of the transient state before the boosting flow rate Qbrt reaches the target boosting flow rate Qbrt and the steady state after the boosting flow rate Qbrt has reached the target boosting flow rate Qbrt.

[0049] 5 and 6, in both the steady state and the transient state, when the accumulator pressure Pa is less than a predetermined value Pa0, the insufficient flow rate (residual flow rate Qr) is compensated for by the regenerative flow rate Qba. When the accumulator pressure Pa is less than the predetermined value Pa0, the pressure difference ΔPba across the regenerative valve 8 is large, so the opening of the regenerative valve 8 is narrowed.

[0050] When the accumulator pressure Pa is equal to or higher than a predetermined value Pa0, the differential pressure ΔPba across the regenerative valve 8 becomes small, and even if the regenerative valve 8 is fully opened, it becomes impossible to ensure the residual flow rate Qr with only the regenerative flow rate Qba. When the accumulator pressure Pa is equal to or higher than the predetermined value Pa0, the insufficient flow rate (residual flow rate Qr) is ensured by the regenerative flow rate Qba and the return flow rate Qbt.

[0051] In this way, the control device 100 according to this embodiment increases the regenerative flow rate Qba to the accumulator 9 as much as possible during boom lowering, thereby improving the regenerative efficiency of the accumulator 9 during boom lowering.

[0052] FIG. 7 is a block diagram showing the functions of the control device 100. The processing performed by the control device 100 will be described in detail with reference to FIG. 7. As shown in FIG. 7, the control device 100 includes a target bottom outflow rate calculation unit 101, a boost flow rate calculation unit 102, a gradient limiting unit 103, a first differential pressure calculation unit 104, a boost valve opening calculation unit 105, a residual flow rate calculation unit 106, a second differential pressure calculation unit 107, a temporary opening calculation unit 108, a regenerative valve opening calculation unit 109, a regenerative flow rate calculation unit 110, a return flow rate calculation unit 111, a second boom control valve opening calculation unit 112, a boost valve command unit 113, a regenerative valve command unit 114, and a second boom control valve command unit 115. The control device 100 achieves these functions by executing programs stored in the nonvolatile memory 100b.

[0053] As shown in FIG. 7 , the target bottom outflow flow rate calculation unit 101 receives the boom lowering operation amount Lb detected by the boom operation amount sensor 31. The target bottom outflow flow rate calculation unit 101 references an outflow flow rate table and calculates a target bottom outflow flow rate Qst based on the boom lowering operation amount Lb. The outflow flow rate table is a data table (characteristic data) that defines the relationship between the target bottom outflow flow rate Qst and the boom lowering operation amount Lb. The outflow flow rate table is determined in advance through experiments or the like and stored in the non-volatile memory 100b. The outflow flow rate table corresponds to a return flow rate table through the second boom control valve 15 in a standard hydraulic drive system without a boost regeneration circuit. Furthermore, the characteristic data of the target bottom outflow flow rate Qst according to the boom lowering operation amount Lb is not limited to a table format and may be defined in a functional format (mathematical formula). The target bottom outflow flow rate Qst calculated by the target bottom outflow flow rate calculation unit 101 is output to the boost flow rate calculation unit 102 and the remaining flow rate calculation unit 106.

[0054] The boost flow rate calculation unit 102 inputs the target bottom outflow rate Qst and multiplies the target bottom outflow rate Qst by the pressure-receiving area ratio Ar / Ab of the boom cylinder 3a to calculate a target flow rate Qbrt for boosting the flow rate in a steady state (hereinafter also referred to as the target boost flow rate) (Qbrt=Qst×(Ar / Ab)). Here, Ar is the pressure-receiving area of ​​the rod chamber 302 of the boom cylinder 3a, and Ab is the pressure-receiving area of ​​the bottom chamber 301 of the boom cylinder 3a. The target boost flow rate Qbrt calculated by the boost flow rate calculation unit 102 is output to the gradient restriction unit 103.

[0055] The slope limiting unit 103 inputs the target pressurization flow rate Qbrt and calculates a value by limiting the time rate of change of the rise of the pressurization flow rate Qbr. For example, the slope limiting unit 103 multiplies the target pressurization flow rate Qbrt by a correction coefficient c corresponding to the elapsed time t from the start time t0 of the boom lowering operation to calculate a corrected target pressurization flow rate Qbr* corresponding to the elapsed time t (Qbr* = c × Qbrt). The time rate of change (slope) of the correction coefficient c is stored in advance in the non-volatile memory 100b. The correction coefficient c is 0 at the start time t0 of the boom lowering operation and increases to 1 at a predetermined time rate as the time t passes. The time rate of change of the correction coefficient c, i.e., the time rate of change of the increasing corrected target pressurization flow rate Qbr*, is set to a value that does not cause a shock due to the communication between the bottom chamber 301 and the rod chamber 302. The corrected target boost flow rate Qbr* calculated by the gradient limiting section 103 is output to the boost valve opening calculating section 105 and the residual flow rate calculating section 106 .

[0056] The first differential pressure calculation unit 104 receives the bottom pressure Pb detected by the bottom pressure sensor 23 and the rod pressure Pr detected by the rod pressure sensor 24. The first differential pressure calculation unit 104 subtracts the rod pressure Pr from the bottom pressure Pb to calculate a differential pressure ΔPbr across the boost valve 34 (ΔPbr=Pb−Pr). The differential pressure ΔPbr across the boost valve 34 calculated by the first differential pressure calculation unit 104 is output to the boost valve opening calculation unit 105.

[0057] The boost valve opening calculation unit 105 receives a corrected target boost flow rate Qbr*, which is a target value of the flow rate passing through the boost valve 34, and a pressure difference ΔPbr across the boost valve 34. The boost valve opening calculation unit 105 references a boost valve opening table corresponding to the pressure difference ΔPbr, and calculates a target opening area Abr of the boost valve 34 based on the corrected target boost flow rate Qbr*. The boost valve opening table is a data table (characteristic data) that defines the relationship between the corrected target boost flow rate Qbr* and the target opening area Abr of the boost valve 34. The non-volatile memory 100b stores a plurality of boost valve opening tables corresponding to the pressure difference ΔPbr across the boost valve 34. Note that the characteristic data of the target opening area Abr of the boost valve 34 corresponding to the corrected target boost flow rate Qbr* and the pressure difference ΔPbr is not limited to a table format, and may be defined in a functional format (e.g., an orifice equation). The target opening area Abr of the boost valve 34 calculated by the boost valve opening calculation unit 105 is output to the boost valve command unit 113 .

[0058] The boost valve command unit 113 receives the target opening area Abr of the boost valve 34, calculates a control command value (e.g., a current value of a solenoid) to the boost valve 34 according to the target opening area Abr, and outputs an electromagnetic command according to the calculated control command value to the boost valve 34. In this way, the boost valve 34 is controlled so that the actual opening area of ​​the boost valve 34 becomes equal to the target opening area Abr of the boost valve.

[0059] The residual flow rate calculation unit 106 receives the corrected target boost flow rate Qbr* and the target bottom outflow flow rate Qst. The residual flow rate calculation unit 106 subtracts the corrected target boost flow rate Qbr* from the target bottom outflow flow rate Qst to calculate the residual flow rate Qr (Qr = Qst - Qbr*). The residual flow rate Qr calculated by the residual flow rate calculation unit 106 is output to the temporary opening calculation unit 108 and the return flow rate calculation unit 111. The residual flow rate Qr is a flow rate (a remaining flow rate other than the boost flow rate) required to ensure a boom speed corresponding to the boom lowering operation amount Lb, and is provided by only the regenerative flow rate Qba, or by the regenerative flow rate Qba and the return flow rate Qbt.

[0060] The second differential pressure calculation unit 107 receives the bottom pressure Pb detected by the bottom pressure sensor 23 and the accumulator pressure Pa detected by the pressure accumulation sensor 43. The second differential pressure calculation unit 107 subtracts the accumulator pressure Pa from the bottom pressure Pb to calculate a differential pressure ΔPba across the regenerative valve 8 (ΔPba = Pb - Pa). The differential pressure ΔPba across the regenerative valve 8 calculated by the second differential pressure calculation unit 107 is output to the temporary opening calculation unit 108 and the regenerative flow rate calculation unit 110.

[0061] The temporary opening calculation unit 108 receives the residual flow rate Qr (the sum of the flow rate through the regenerative valve 8 and the flow rate through the second boom control valve 15) and the front-to-rear differential pressure ΔPba of the regenerative valve 8. The temporary opening calculation unit 108 calculates, as a temporary target opening area, the opening area when the flow rate of hydraulic oil passing through the regenerative valve 8 is equal to the residual flow rate Qr. The temporary opening calculation unit 108 references a valve opening table corresponding to the front-to-rear differential pressure ΔPba and calculates a temporary target opening area Aba0 of the regenerative valve 8 based on the residual flow rate Qr. The valve opening table is a data table (characteristic data) that defines the relationship between the residual flow rate Qr and the temporary target opening area Aba0 of the regenerative valve 8. The non-volatile memory 100b stores a plurality of valve opening tables corresponding to the front-to-rear differential pressure ΔPba of the regenerative valve 8. The characteristic data of the provisional target opening area Aba0 of the regenerative valve 8 according to the regenerative flow rate Qba and the upstream / downstream differential pressure ΔPba is not limited to a table format, and may be specified in a functional format (e.g., an orifice equation). The provisional target opening area Aba0 of the regenerative valve 8 calculated by the provisional opening calculation unit 108 is output to the regenerative valve opening calculation unit 109.

[0062] The provisional target opening area Aba0 of the regenerative valve 8 calculated by the provisional opening calculation unit 108 may exceed the maximum opening area Abamax of the regenerative valve 8. In other words, the characteristic data of the provisional target opening area Aba0 used in the calculation of the provisional target opening area Aba0 is defined as data that can be calculated up to a value sufficiently larger than the maximum opening area Abamax of the regenerative valve 8, assuming that there is no upper limit to the opening area of ​​the regenerative valve 8.

[0063] The regenerative valve opening calculation unit 109 receives a provisional target opening area Aba0 of the regenerative valve 8 and calculates a target opening area Aba of the regenerative valve 8 based on the provisional target opening area Aba0 and a maximum opening area Abamax of the regenerative valve 8. Specifically, the regenerative valve opening calculation unit 109 first compares the maximum opening area Abamax of the regenerative valve 8 with the provisional target opening area Aba0. The maximum opening area Abamax of the regenerative valve 8 is stored in the non-volatile memory 100b. If the provisional target opening area Aba0 is equal to or smaller than the maximum opening area Abamax, the regenerative valve opening calculation unit 109 calculates the provisional target opening area Aba0 as the target opening area Aba of the regenerative valve 8 (Aba = Aba0, Aba0 ≦ Abamax). If the provisional target opening area Aba0 is larger than the maximum opening area Abamax, the regenerative valve opening calculation unit 109 calculates the maximum opening area Abamax as the target opening area Aba of the regenerative valve 8 (Aba = Abamax, Aba0 > Abamax). The target opening area Aba of the regenerative valve 8 calculated by the regenerative valve opening calculation unit 109 is output to the regenerative valve command unit 114 and the regenerative flow rate calculation unit 110.

[0064] The regenerative valve command unit 114 receives the target opening area Aba of the regenerative valve 8, calculates a control command value (e.g., a current value of a solenoid) to the regenerative valve 8 according to the target opening area Aba, and outputs an electromagnetic command according to the calculated control command value to the regenerative valve 8. In this way, the regenerative valve 8 is controlled so that the actual opening area of ​​the regenerative valve 8 becomes equal to the target opening area Aba of the regenerative valve 8.

[0065] The regenerative flow rate calculation unit 110 receives as input a target opening area Aba of the regenerative valve 8 and a differential pressure ΔPba across the regenerative valve 8. The regenerative flow rate calculation unit 110 references a regenerative flow rate table corresponding to the differential pressure ΔPba across the regenerative valve 8 and calculates an estimated regenerative flow rate Qba*, which is an estimated value of the regenerative flow rate, based on the target opening area Aba of the regenerative valve 8. The regenerative flow rate table is a data table (characteristic data) that defines the relationship between the target opening area Aba of the regenerative valve 8 and the estimated regenerative flow rate Qba*. The non-volatile memory 100b stores a plurality of regenerative flow rate tables corresponding to the differential pressure ΔPba across the regenerative valve 8. Note that the characteristic data of the estimated regenerative flow rate Qba* corresponding to the target opening area Aba and the differential pressure ΔPba across the regenerative valve 8 is not limited to a table format and may be defined in a functional format (e.g., an orifice equation). The estimated regenerative flow rate Qba* calculated by the regenerative flow rate calculation unit 110 is output to the return flow rate calculation unit 111 .

[0066] The return flow rate calculation unit 111 receives an estimated regenerative flow rate (estimated value of the regenerative flow rate) Qba* and a residual flow rate (target value of the sum of the regenerative flow rate and the return flow rate) Qr. The return flow rate calculation unit 111 subtracts the estimated regenerative flow rate Qba* from the residual flow rate Qr to calculate a target return flow rate (target value of the return flow rate) Qbt* (Qbt* = Qr - Qba*). The target return flow rate Qbt* calculated by the return flow rate calculation unit 111 is output to the second boom control valve opening calculation unit 112.

[0067] The second boom control valve opening calculation unit 112 receives the target return flow rate Qbt* and the bottom pressure Pb detected by the bottom pressure sensor 23. The bottom pressure Pb is a gauge pressure and corresponds to the differential pressure between the bottom pressure (absolute pressure) and the tank pressure (absolute pressure), i.e., the front-to-rear differential pressure ΔPbt of the second boom control valve 15 (ΔPbt = Pb). The second boom control valve opening calculation unit 112 references a second boom control valve opening table corresponding to the front-to-rear differential pressure ΔPbt (bottom pressure Pb) and calculates a target opening area Abt of the second boom control valve 15 based on the target return flow rate Qbt*. The second boom control valve opening table is a data table (characteristic data) that defines the relationship between the target return flow rate Qbt* and the target opening area Abt of the second boom control valve 15. The non-volatile memory 100b stores a plurality of second boom control valve opening tables corresponding to the front-to-rear differential pressure ΔPbt (bottom pressure Pb) of the second boom control valve 15. The characteristic data of the target opening area Abt of the second boom control valve 15 according to the target return flow rate Qbt* and the front-to-rear differential pressure ΔPbt is not limited to a table format, and may be specified in a functional format (for example, an orifice equation). The target opening area Abt of the second boom control valve 15 calculated by the second boom control valve opening calculation unit 112 is output to the second boom control valve command unit 115.

[0068] The second boom control valve command unit 115 receives the target opening area Abt of the second boom control valve 15, calculates a control command value (e.g., a current value of a solenoid) to the second boom control valve 15 according to the target opening area Abt, and outputs an electromagnetic command according to the calculated control command value to the second boom control valve 15. In this way, the second boom control valve 15 is controlled so that the actual opening area of ​​the second boom control valve 15 becomes equal to the target opening area Abt of the second boom control valve 15.

[0069] As a result of the above, the boost valve 34 is controlled so that the boost flow rate Qbr (actual flow rate through the boost valve 34) is equal to the corrected target boost flow rate Qbr*, the regenerative valve 8 is controlled so that the regenerative flow rate Qba (actual flow rate through the regenerative valve 8) is equal to the estimated regenerative flow rate Qba*, and the second boom control valve 15 is controlled so that the return flow rate (actual flow rate through the second boom control valve 15) is equal to the target return flow rate Qbt*.

[0070] According to the above-described embodiment, the following advantageous effects are achieved.

[0071] (1) The hydraulic excavator (work machine) 1 includes a vehicle body 52 and a working device 51 attached to the vehicle body 52. ​​The working device 51 includes a boom 1a attached to the vehicle body 52, an arm 1b attached to the boom 1a, and a bucket (work tool) 1c attached to the arm 1b. The hydraulic excavator 1 also includes an engine (prime mover) 5, hydraulic pumps (a first hydraulic pump 6 and a second hydraulic pump 7) driven by the engine 5, and a plurality of hydraulic cylinders driven by hydraulic oil discharged from the hydraulic pumps (6, 7). The plurality of hydraulic cylinders are hydraulic actuators used to drive the working device 51, and each have a bottom chamber 301 and a rod chamber 302. The plurality of hydraulic cylinders includes a boom cylinder 3a that drives the boom 1a. The hydraulic excavator 1 includes an accumulator (pressure accumulator) 9 that accumulates hydraulic oil supplied from the bottom chamber 301 of the boom cylinder 3a, a regenerative valve 8 that controls the flow rate of hydraulic oil flowing from the bottom chamber 301 to the accumulator 9, a boost valve 34 that controls the flow rate of hydraulic oil flowing from the bottom chamber 301 to the rod chamber 302, a second boom control valve (discharge valve) 15 that controls the flow rate of hydraulic oil flowing from the bottom chamber 301 to the tank 12, a boom operation lever (cylinder operation device) 30 that operates the boom cylinder 3a, a boom operation amount sensor (cylinder operation amount sensor) 31 that detects the operation amount of the boom operation lever 30, and a control device 100 that controls at least the regenerative valve 8 and the boost valve 34 based on the operation amount detected by the boom operation amount sensor 31. The operation amount detected by the boom operation amount sensor 31 includes a boom lowering operation amount Lb that operates the boom 1a in the lowering direction.

[0072] In this configuration, when the control device 100 opens the regenerative valve 8 during a boom lowering operation, the accumulator 9 is charged with pressure oil from the bottom chamber 301 of the boom cylinder 3a, which contracts due to the weight of the working device 51. When the control device 100 opens the boost valve 34 during a boom lowering operation, the pressure in the rod chamber 302 is increased by pressure oil from the bottom chamber 301 of the boom cylinder 3a, which contracts due to the weight of the working device 51. As a result, the bottom chamber 301 is further pushed in by the piston, and the pressure in the bottom chamber 301 is increased. In this manner, in this embodiment, when the boom is lowered, regeneration can be performed in the accumulator 9 while the bottom chamber 301 is being pressurized. As a result, a high pressure can be charged in the accumulator 9. Therefore, this embodiment can increase the timing at which the accumulator 9 can assist various hydraulic actuators, such as those for boom raising, arm dumping, bucket dumping, and swinging.

[0073] (2) The control device 100 calculates a target bottom outflow flow rate Qst, which is a target value for the flow rate of hydraulic oil flowing out from the bottom chamber 301 of the boom cylinder 3a, and a target boost flow rate Qbrt, which is a target value for the flow rate of hydraulic oil supplied from the bottom chamber 301 to the rod chamber 302 through the boost valve 34, based on the boom lowering operation amount Lb detected by the boom operation amount sensor 31. The control device 100 controls the boost valve 34 so that the boost flow rate Qbr, which is the flow rate of hydraulic oil supplied from the bottom chamber 301 to the rod chamber 302 through the boost valve 34, gradually increases over time t until it becomes equal to the target boost flow rate Qbrt. The control device 100 increases the boost flow rate Qbr at a predetermined time change rate (time change characteristic). The control device 100 controls the regenerative valve 8, or the regenerative valve 8 and the second boom control valve 15, so that the sum of the regenerative flow rate Qba, which is the flow rate of hydraulic oil supplied from the bottom chamber 301 to the accumulator 9 through the regenerative valve 8, the boost flow rate Qbr, which is the flow rate of hydraulic oil supplied from the bottom chamber 301 to the rod chamber 302 through the boost valve 34, and the return flow rate Qbt, which is the flow rate of hydraulic oil discharged from the bottom chamber 301 to the tank 12 through the second boom control valve 15, is equal to the target bottom outflow flow rate Qst, both in the transient state before the boost flow rate Qbr reaches the target boost flow rate Qbrt, and in the steady state after the boost flow rate Qbrt has reached the target boost flow rate Qbrt.

[0074] With this configuration, the control device 100 gradually opens the boost valve 34, thereby preventing shock from occurring in the boom cylinder 3a when the bottom side and rod side of the boom cylinder 3a are connected. Furthermore, any shortfall in flow rate when the boost valve 34 is gradually opened can be compensated for by the regenerative flow rate Qba, or by the regenerative flow rate Qba and the return flow rate Qbt. Therefore, according to this embodiment, the boom cylinder 3a can be driven at a working speed equivalent to that of a hydraulic drive system that sends boom return oil only to the tank 12 without boom regeneration, i.e., a hydraulic drive system that does not include a boost regenerative circuit. In other words, the speed of the boom cylinder 3a can be appropriately ensured according to the boom lowering operation amount Lb. Thus, according to this embodiment, the operability of the boom cylinder 3a can be maintained while the accumulator 9 is pressurized to a high pressure using return oil from the boom cylinder 3a, thereby improving the efficiency of the hydraulic excavator as a whole.

[0075] (3) When the accumulator pressure (pressure of the accumulator 9) Pa is less than a predetermined value Pa0, the control device 100 controls the regenerative valve 8 so that the sum of the regenerative flow rate Qba and the boost flow rate Qbr becomes equal to the target bottom outflow flow rate Qst. When the accumulator pressure Pa is equal to or greater than the predetermined value Pa0, the control device 100 controls the regenerative valve 8 and the second boom control valve 15 so that the sum of the regenerative flow rate Qba, the boost flow rate Qbr, and the return flow rate Qbt becomes equal to the target bottom outflow flow rate Qst.

[0076] In this configuration, the regenerative valve 8 is preferentially controlled out of the regenerative valve 8 and the second boom control valve 15, thereby increasing the regenerative flow rate Qba. By supplementing the remaining flow rate (residual flow rate Qr) obtained by subtracting the boost flow rate Qbr from the target bottom outflow flow rate Qst with the regenerative flow rate Qba as much as possible, the flow rate (return flow rate Qbt) discharged from the bottom chamber 301 to the tank 12 through the second boom control valve 15 can be reduced as much as possible. Therefore, according to this embodiment, the regenerative efficiency of the accumulator 9 can be increased as much as possible.

[0077] (4) The control device 100 calculates the target boost flow rate Qbrt based on the target bottom outflow flow rate Qst and the pressure-receiving area ratio (Ar / Ab) between the bottom chamber 301 and the rod chamber 302 of the boom cylinder 3a. The control device 100 calculates the corrected target boost flow rate Qbr* according to the elapsed time t by multiplying the target boost flow rate Qbrt by a correction coefficient c that increases from 0 to 1 according to the elapsed time t from the start time t0 of the operation of the boom control lever 30. This calculates the corrected target boost flow rate Qbr* that increases at a predetermined time change rate. Note that in this embodiment, the correction process of multiplying the target boost flow rate Qbrt by the correction coefficient c=1 is continued even after the corrected target boost flow rate Qbr* becomes equal to the target boost flow rate Qbrt. However, the correction process may be omitted after the corrected target boost flow rate Qbr* becomes equal to the target boost flow rate Qbrt. In this case, the target boost flow rate Qbrt is input as is to the boost valve opening calculation unit 105 and is used to calculate the target opening area Abr of the boost valve 34 .

[0078] The control device 100 calculates the residual flow rate Qr by subtracting the corrected target boost flow rate Qbr from the target bottom outflow flow rate Qst. The control device 100 calculates the opening area when the flow rate of hydraulic oil passing through the regenerative valve 8 is equal to the residual flow rate Qr as a provisional target opening area Aba0. The control device 100 compares the provisional target opening area Aba0 with the maximum opening area Abamax of the regenerative valve 8. If the provisional target opening area Aba0 is equal to or smaller than the maximum opening area Abamax of the regenerative valve 8, the control device 100 calculates the provisional target opening area Aba0 as the target opening area Aba of the regenerative valve 8. If the provisional target opening area Aba0 is greater than the maximum opening area Abamax of the regenerative valve 8, the control device 100 calculates the maximum opening area Abamax as the target opening area Aba of the regenerative valve 8.

[0079] The control device 100 calculates an estimated regenerative flow rate Qba*, which is an estimated value of the regenerative flow rate Qba, based on the target opening area Aba of the regenerative valve 8. The control device 100 calculates a target return flow rate Qbt* by subtracting the estimated regenerative flow rate Qba* from the residual flow rate Qr. The control device 100 controls the boost valve 34 so that the boost flow rate Qbr becomes equal to the corrected target boost flow rate Qbr*. The control device 100 controls the regenerative valve 8 so that the opening area of ​​the regenerative valve 8 becomes equal to the target opening area Aba of the regenerative valve 8. The control device 100 controls the second boom control valve 15 so that the return flow rate Qbt becomes equal to the target return flow rate Qbt*.

[0080] With this configuration, there is no need to compare the accumulator pressure Pa with a predetermined value Pa0 and change the calculation method depending on the magnitude relationship between the accumulator pressure Pa and the predetermined value Pa0. When the accumulator pressure Pa becomes equal to or greater than the predetermined value Pa0 and it becomes impossible to ensure the residual flow rate Qr with only the regenerative flow rate Qba, the target return flow rate Qbt* is calculated as a value greater than 0. This opens the second boom control valve 15, making it possible to ensure the residual flow rate Qr with the regenerative flow rate Qba and the return flow rate Qbt.

[0081] (5) The hydraulic excavator 1 is equipped with a pressure accumulation sensor 43 that detects the accumulator pressure (pressure in the accumulator 9) Pa, a bottom pressure sensor 23 that detects the bottom pressure (pressure in the bottom chamber 301) Pb of the boom cylinder 3a, and a rod pressure sensor 24 that detects the rod pressure (pressure in the rod chamber 302) Pr of the boom cylinder 3a.

[0082] The control device 100 calculates a target opening area Abr of the boost valve 34 based on the bottom pressure Pb detected by the bottom pressure sensor 23, the rod pressure Pr detected by the rod pressure sensor 24, and the corrected target boost flow rate Qbr*. The control device 100 controls the boost valve 34 so that the opening area of ​​the boost valve 34 becomes equal to the target opening area Abr of the boost valve 34.

[0083] The control device 100 calculates a provisional target opening area Aba0 based on the accumulator pressure Pa detected by the pressure accumulation sensor 43, the bottom pressure Pb detected by the bottom pressure sensor 23, and the residual flow rate Qr. The control device 100 calculates a target opening area Aba of the regenerative valve 8 based on the provisional target opening area Aba0 and the maximum opening area Abamax of the regenerative valve 8. The control device 100 controls the regenerative valve 8 so that the opening area of ​​the regenerative valve 8 becomes equal to the target opening area Aba of the regenerative valve 8.

[0084] The control device 100 calculates an estimated regenerative flow rate Qba* based on the accumulator pressure Pa detected by the pressure accumulation sensor 43, the bottom pressure Pb detected by the bottom pressure sensor 23, and the target opening area Aba of the regenerative valve 8. The control device 100 calculates a target opening area Abt of the second boom control valve 15 based on the bottom pressure Pb detected by the bottom pressure sensor 23 and the target return flow rate Qbt*. The control device 100 controls the second boom control valve 15 so that the opening area of ​​the second boom control valve 15 is equal to the target opening area Abt of the second boom control valve 15.

[0085] In this configuration, the target opening area of ​​each flow control valve can be appropriately calculated based on the differential pressure across each flow control valve (boost valve 34, regenerative valve 8, and second boom control valve 15). In other words, according to this embodiment, each flow control valve can be controlled with high precision.

[0086] (6) The hydraulic excavator 1 is equipped with an arm cylinder 3b, a bucket cylinder 3c, and a swing motor 4 as hydraulic actuators different from the boom cylinder 3a. The hydraulic excavator 1 is equipped with an actuator operation device (e.g., bucket operation lever 44) that operates these hydraulic actuators, an actuator operation amount sensor (e.g., bucket operation amount sensor 45) that detects the operation amount of the actuator operation device, and a supply valve (e.g., bucket assist valve 21) that controls the flow rate of hydraulic oil supplied from the accumulator 9 to the hydraulic actuator. The control device 100 controls the supply valve (e.g., bucket assist valve 21) based on at least the operation amount detected by the actuator operation amount sensor (e.g., bucket operation amount sensor 45). Note that the control device 100 may control the supply valve (e.g., bucket assist valve 21) taking into account not only the operation amount (e.g., bucket operation amount) but also the accumulator pressure Pa detected by the pressure accumulation sensor 43. For example, when the bucket operation amount is less than a predetermined value or when the accumulator pressure Pa is less than a predetermined value, the control device 100 keeps the bucket assist valve 21 closed, and when the bucket operation amount is equal to or greater than the predetermined value and the accumulator pressure Pa is equal to or greater than the predetermined value, the control device 100 opens the bucket assist valve 21.

[0087] According to this configuration, the energy stored by the return oil of the boom cylinder 3a can be used to drive another hydraulic actuator, thereby improving energy efficiency (i.e., improving fuel efficiency).

[0088] The following modifications are also within the scope of the present invention, and one or more of the modifications may be combined with the above-described embodiment.

[0089] (Variation 1) For example, in the above-described embodiment, the flow rate through the regenerative valve 8 is estimated from the differential pressure ΔPba across the regenerative valve 8 and the target opening area Aba of the regenerative valve 8. However, a flow meter may be installed to measure the flow rate through the regenerative valve 8. That is, the estimated regenerative flow rate Qba* in the above-described embodiment may be replaced with a value measured by a flow meter, and various calculations may be performed. In this case, the regenerative valve 8 and the second boom control valve 15 can be controlled with greater accuracy.

[0090] (Variation 2) The work machine is not limited to the crawler-type hydraulic excavator 1. The present invention may be applied to a wheel-type hydraulic excavator. Furthermore, the work implement may be a grapple, a breaker, a lifting magnet, or the like, instead of a bucket.

[0091] (Modification 3) Two or more accumulators may be provided. The hydraulic pump that supplies hydraulic oil to the hydraulic actuator may be configured to be provided with only one or three or more.

[0092] (Modification 4) The prime mover that drives the hydraulic pumps (6, 7) is not limited to the engine 5. The prime mover may be an electric motor.

[0093] Although the embodiments of the present invention have been described above, the above embodiments merely illustrate some of the application examples of the present invention, and it is not intended that the technical scope of the present invention be limited to the specific configurations of the above embodiments.

[0094] 1...hydraulic excavator (work machine), 1a...boom, 1b...arm, 1c...bucket, 1d...upper swing body, 1e...lower traveling body, 3a...boom cylinder (hydraulic cylinder), 3b...arm cylinder (hydraulic actuator), 3c...bucket cylinder (hydraulic actuator), 3e...travel motor (hydraulic actuator), 4...swing motor (hydraulic actuator), 5...engine (prime mover), 6...first hydraulic pump (hydraulic pump), 7...second hydraulic pump (hydraulic pump), 8...regenerative valve (flow control valve), 9...accumulator (pressure storage device), 12...tank, 13...swing control valve ( flow control valve), 14...first boom control valve (flow control valve), 15...second boom control valve (discharge valve, flow control valve), 16...first arm control valve (flow control valve), 17...second arm control valve (flow control valve), 18...bucket control valve (flow control valve), 19...boom assist valve (supply valve, flow control valve), 20...arm assist valve (supply valve, flow control valve), 21...bucket assist valve (supply valve, flow control valve), 22...swing assist valve (supply valve, flow control valve), 23...bottom pressure sensor (pressure sensor), 24...rod pressure sensor (pressure sensor), 30...boom operation lever (cylinder operation lever), operation device), 31... boom operation amount sensor (cylinder operation amount sensor), 34... boost valve (flow rate control valve), 43... pressure accumulation sensor (pressure sensor), 44... bucket operation lever (actuator operation device), 45... bucket operation amount sensor (actuator operation amount sensor), 51... work device, 52... vehicle body, 100... control device, 100a... processing device, 100b... non-volatile memory (storage device), 100c... volatile memory (storage device), 101... target bottom outflow flow rate calculation unit, 102... boost flow rate calculation unit, 103... tilt limiting unit, 104... first differential pressure calculation unit, 105... boost valve opening calculation unit, 1 06...residual flow rate calculation unit, 107...second differential pressure calculation unit, 108...opening calculation unit, 109...regenerative valve opening calculation unit, 110...regenerative flow rate calculation unit, 111...return flow rate calculation unit, 112...second boom control valve opening calculation unit, 113...pressure boost valve command unit, 114...regenerative valve command unit, 115...second boom control valve command unit, 301...bottom chamber, 302...rod chamber, Aba...target opening area of ​​regenerative valve, Aba0...provisional target opening area of ​​regenerative valve, Abr...target opening area of ​​pressure boost valve, Abt...target opening area of ​​second boom control valve (discharge valve), c...correction coefficient, Lb...operation amount, Pa...accumulator pressure, Pa0...predetermined value,Pb...bottom pressure, Pr...rod pressure, Qba...regenerative flow rate, Qba*...estimated regenerative flow rate, Qbr...boosting flow rate, Qbr*...corrected target boosting flow rate, Qbrt...target boosting flow rate, Qbt...return flow rate, Qbt*...target return flow rate, Qr...residual flow rate, Qst...target bottom outflow flow rate, ΔPba...differential pressure before and after regenerative valve, ΔPbr...differential pressure before and after boost valve, ΔPbt...differential pressure before and after second boom control valve (discharge valve),

Claims

1. A work machine comprising: a prime mover; a hydraulic pump driven by the prime mover; a hydraulic cylinder having a bottom chamber and a rod chamber and driven by hydraulic oil discharged from the hydraulic pump; a pressure accumulator that accumulates hydraulic oil supplied from the bottom chamber; a regenerative valve that controls the flow rate of hydraulic oil flowing from the bottom chamber to the pressure accumulator; a boost valve that controls the flow rate of hydraulic oil flowing from the bottom chamber to the rod chamber; a discharge valve that controls the flow rate of hydraulic oil flowing from the bottom chamber to a tank; a cylinder operating device that operates the hydraulic cylinder; a cylinder operation amount sensor that detects the operation amount of the cylinder operating device; and a control device that controls at least the regenerative valve and the boost valve based on the operation amount detected by the cylinder operation amount sensor, wherein the control device calculates a target bottom outflow flow rate, which is a target value for the flow rate of hydraulic oil flowing out of the bottom chamber of the hydraulic cylinder, and a target boost flow rate, which is a target value for the flow rate of hydraulic oil supplied from the bottom chamber to the rod chamber through the boost valve, based on the operation amount detected by the cylinder operation amount sensor, a working machine comprising: controlling the boost valve so that a boost flow rate, which is the flow rate of hydraulic oil supplied from the bottom chamber to the rod chamber through the boost valve, gradually increases over time at a predetermined time change rate to become equal to the target boost flow rate; and controlling the regenerative valve, or the regenerative valve and the discharge valve, so that a sum of a regenerative flow rate, which is the flow rate of hydraulic oil supplied from the bottom chamber to the pressure accumulator device through the regenerative valve, the boost flow rate, and a return flow rate, which is the flow rate of hydraulic oil discharged from the bottom chamber to the tank through the discharge valve, becomes equal to the target bottom outflow flow rate, in each of a transient state before the boost flow rate reaches the target boost flow rate and a steady state after the boost flow rate has reached the target boost flow rate.

2. A work machine as described in claim 1, wherein the control device controls the regenerative valve so that the sum of the regenerative flow rate and the boosted flow rate becomes equal to the target bottom outflow flow rate when the pressure of the pressure accumulator is below a predetermined value, and controls the regenerative valve and the discharge valve so that the sum of the regenerative flow rate, the boosted flow rate and the return flow rate becomes equal to the target bottom outflow flow rate when the pressure of the pressure accumulator is equal to or greater than the predetermined value.

3. In the work machine of claim 1, the control device: calculates the target boost flow rate based on the target bottom outflow flow rate and the pressure-receiving area ratio between the bottom chamber and the rod chamber of the hydraulic cylinder; calculates a corrected target boost flow rate according to the elapsed time from the start of operation of the cylinder operating device based on the target boost flow rate; calculates a residual flow rate by subtracting the corrected target boost flow rate from the target bottom outflow flow rate; calculates an opening area when the flow rate of hydraulic oil passing through the regenerative valve is equal to the residual flow rate as a provisional target opening area; compares the provisional target opening area with the maximum opening area of ​​the regenerative valve; and if the provisional target opening area is equal to or less than the maximum opening area of ​​the regenerative valve, calculates the provisional target opening area as the target opening area of ​​the regenerative valve; and if the provisional target opening area is greater than the maximum opening area of ​​the regenerative valve, calculates the maximum opening area as the target opening area of ​​the regenerative valve. a work machine comprising: calculating an estimated regenerative flow rate, which is an estimated value of the regenerative flow rate, based on a target opening area of ​​the regenerative valve; calculating a target return flow rate by subtracting the estimated regenerative flow rate from the residual flow rate; controlling the boost valve so that the boost flow rate becomes equal to the corrected target boost flow rate; controlling the regenerative valve so that the opening area of ​​the regenerative valve becomes equal to the target opening area of ​​the regenerative valve; and controlling the discharge valve so that the return flow rate becomes equal to the target return flow rate.

4. A working machine according to claim 3, comprising a pressure accumulator sensor that detects the pressure of the pressure accumulator device, a bottom pressure sensor that detects the pressure in the bottom chamber of the hydraulic cylinder, and a rod pressure sensor that detects the pressure in the rod chamber of the hydraulic cylinder, wherein the control device calculates a target opening area of ​​the boost valve based on the pressure in the bottom chamber detected by the bottom pressure sensor, the pressure in the rod chamber detected by the rod pressure sensor, and the corrected target boost flow rate, and controls the boost valve so that the opening area of ​​the boost valve becomes equal to the target opening area of ​​the boost valve, calculates the provisional target opening area based on the pressure of the accumulator device detected by the pressure accumulator sensor, the pressure in the bottom chamber detected by the bottom pressure sensor, and the residual flow rate, calculates the target opening area of ​​the regenerative valve based on the provisional target opening area and the maximum opening area of ​​the regenerative valve, and controls the regenerative valve so that the opening area of ​​the regenerative valve becomes equal to the target opening area of ​​the regenerative valve, a work machine comprising: a pressure sensor for detecting a pressure in the pressure accumulator; a bottom chamber pressure sensor for detecting a bottom pressure; a regenerative valve for detecting a target opening area; a discharge valve for detecting a target opening area; and a discharge valve for controlling the discharge valve so that the opening area of ​​the discharge valve is equal to the target opening area of ​​the discharge valve.

5. A work machine as claimed in claim 1, comprising: a vehicle body; and a working device attached to the vehicle body; the working device comprising a boom attached to the vehicle body, an arm attached to the boom, and a working implement attached to the arm; the hydraulic cylinder is a boom cylinder that drives the boom; and the operation amount detected by the cylinder operation amount sensor is an operation amount for operating the boom in a lowering direction.

6. A work machine as claimed in claim 1, comprising: a hydraulic actuator different from the hydraulic cylinder; an actuator operating device that operates the hydraulic actuator; an actuator operation amount sensor that detects the operation amount of the actuator operating device; and a supply valve that controls the flow rate of hydraulic oil supplied from the pressure accumulator to the hydraulic actuator, wherein the control device controls the supply valve based on at least the operation amount detected by the actuator operation amount sensor.

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