Work machine

Abstract

This work machine comprises a first flow rate control valve that controls the flow rate of hydraulic oil supplied from a pressure accumulation device to a first hydraulic actuator, a second flow rate control valve that controls the flow rate of hydraulic oil supplied from a hydraulic pump to a second hydraulic actuator, and a control device that controls the first flow rate control valve such that the first hydraulic actuator operates at a target speed corresponding to an operating amount of a first operating device, and controls the second flow rate control valve such that the second hydraulic actuator operates at a target speed corresponding to an operating amount of a second operating device, wherein work is performed using a work device including at least the first hydraulic actuator. The control device controls the second flow rate control valve such that the actual speed of the second hydraulic actuator decreases when the pressure of the pressure accumulation device decreases and the actual speed of the first hydraulic actuator decreases.

Description

Working Machine

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

[0002] In a working machine such as a hydraulic excavator that includes a working device having a boom, an arm, and a bucket, and a revolving superstructure to which the working device is attached, there is known a working machine equipped with a system that uses energy accumulated in an accumulator (pressure accumulator) for driving a hydraulic actuator. In this system, a flow control valve is provided between the accumulator and the hydraulic actuator. By opening the flow control valve and sending pressurized oil in the accumulator to the hydraulic actuator, the hydraulic actuator is driven. In the above system, when the pressure of the accumulator decreases over time and the differential pressure across the flow control valve disappears, hydraulic oil can no longer be supplied from the accumulator to the hydraulic actuator.

[0003] Patent Document 1 discloses, in a working machine that drives a hydraulic actuator with hydraulic oil from a hydraulic pump, a system that assists the driving of the hydraulic actuator with hydraulic oil from an accumulator. In the working machine described in Patent Document 1, by increasing the flow rate of hydraulic oil supplied from the hydraulic pump to the hydraulic actuator in accordance with a decrease in the pressure of the accumulator, a decrease in the speed of the hydraulic actuator is prevented.

[0004] Japanese Unexamined Patent Application Publication No. 2008-014467

[0005] However, with the technology described in Patent Document 1, for example, when the discharge flow rate of the hydraulic pump is at the maximum discharge flow rate, if the pressure of the accumulator decreases and the differential pressure across the flow control valve disappears, the displacement of the hydraulic pump cannot be further increased, and the supply flow rate of hydraulic oil to the hydraulic actuator decreases. As a result, the speed of the assisted hydraulic actuator, to which hydraulic oil was supplied from the accumulator, decreases.

[0006] When multiple hydraulic actuators are operating in combination, a decrease in the supply flow rate from the accumulator will reduce the speed of the hydraulic actuator being assisted, while the speed of the hydraulic actuator not being assisted will remain unchanged. For example, if an operator is raising a boom while rotating a slewing body, and the accumulator can no longer assist the boom cylinder (the hydraulic actuator being assisted), the boom speed will decrease even if the amount of operation does not change. On the other hand, the speed of the slewing motor (the hydraulic actuator not being assisted) will remain unchanged. In other words, if the assistance from the accumulator is interrupted, the ratio of the boom cylinder speed to the slewing motor speed (speed ratio) changes. As a result, a discrepancy will occur between the movement trajectory of the work device when the flow rate of hydraulic fluid supplied from the accumulator to the boom cylinder does not decrease and the movement trajectory of the work device when the flow rate of hydraulic fluid supplied from the accumulator to the boom cylinder decreases, which may worsen operability and reduce work efficiency.

[0007] The present invention aims to provide a work machine that can suppress the discrepancy between the movement trajectory of the work machine when the flow rate of hydraulic fluid supplied from the pressure accumulator to the hydraulic actuator does not decrease, and the movement trajectory of the work machine when the flow rate of hydraulic fluid supplied from the pressure accumulator to the hydraulic actuator decreases.

[0008] A working machine according to one aspect of the present invention comprises a prime mover, a hydraulic pump driven by the prime mover, a pressure accumulator for storing hydraulic fluid, a first hydraulic actuator driven by hydraulic fluid supplied from the pressure accumulator, a first flow control valve for controlling the flow rate of hydraulic fluid supplied from the pressure accumulator to the first hydraulic actuator, a second hydraulic actuator driven by hydraulic fluid supplied from the hydraulic pump, a second flow control valve for controlling the flow rate of hydraulic fluid supplied from the hydraulic pump to the second hydraulic actuator, a first operating device for operating the first hydraulic actuator, a second operating device for operating the second hydraulic actuator, and a control device that controls the first flow control valve so that the first hydraulic actuator operates at a target speed corresponding to the amount of operation of the first operating device, and controls the second flow control valve so that the second hydraulic actuator operates at a target speed corresponding to the amount of operation of the second operating device, wherein work is performed using a working machine having at least one of the first hydraulic actuator and the second hydraulic actuator. The control device controls the second flow control valve so that when the pressure in the accumulator decreases and the actual speed of the first hydraulic actuator decreases relative to the target speed of the first hydraulic actuator corresponding to the amount of operation of the first operating device, the actual speed of the second hydraulic actuator decreases relative to the target speed corresponding to the amount of operation of the second operating device.

[0009] According to the present invention, it is possible to provide a work machine that can suppress the discrepancy between the movement trajectory of the work machine when the flow rate of the hydraulic fluid supplied from the pressure accumulator to the hydraulic actuator does not decrease, and the movement trajectory of the work machine when the flow rate of the hydraulic fluid supplied from the pressure accumulator to the hydraulic actuator decreases.

[0010] Figure 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. Figure 2 is a schematic diagram of a hydraulic drive unit mounted on the hydraulic excavator. Figure 3 is a diagram showing a drive control system mounted on the hydraulic excavator. Figure 4 is a block diagram showing the functions of a control device according to the first embodiment. Figure 5A is a diagram showing the time changes of the manipulated amount, pressure, flow rate, position, and angle during a slewing boom raising operation, and shows the case where the accumulator pressure does not fall below the minimum operating pressure of the accumulator from the start to the end of the slewing boom raising operation. Figure 5B is a diagram showing the time changes of the manipulated amount, pressure, flow rate, position, and angle during a slewing boom raising operation, and shows the case in a comparative example where the accumulator pressure falls below the minimum operating pressure of the accumulator during the slewing boom raising operation. Figure 5C is a diagram showing the time changes of the manipulated amount, pressure, flow rate, position, and angle during a slewing boom raising operation, and shows the case in this embodiment where the accumulator pressure falls below the minimum operating pressure of the accumulator during the slewing boom raising operation. Figure 6 is a block diagram showing the functions of the control device according to the second embodiment. Figure 7 is a diagram showing a modified example of a sensor that detects information regarding the actual speed of the boom cylinder.

[0011] <First Embodiment> A working machine according to the first embodiment of the present invention will be described with reference to Figures 1 to 5C. Figure 1 is a perspective view of a hydraulic excavator 1 shown as an example of a working machine according to an embodiment of the present invention. As shown in Figure 1, the hydraulic excavator 1 comprises a body 52 and a multi-jointed working device 51 attached to the body 52. ​​The working device 51 has a boom 1a, an arm 1b, and a bucket 1c. The working device 51 also comprises 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 body 52 comprises a traveling body 1e and a slewing body 1d attached to the traveling body 1e. The slewing body 1d rotates relative to the traveling body 1e by a slewing motor 4 (see Figure 2). The traveling body 1e is driven by left and right traveling motors 3e.

[0012] The boom 1a is rotatably attached to the slewing body 1d and driven by the boom cylinder 3a. The arm 1b is rotatably attached to the boom 1a and driven by the arm cylinder 3b. The bucket 1c is rotatably attached to the arm 1b and driven by the bucket cylinder 3c. The bucket 1c is a work tool used for excavation work and for loading excavated soil and other materials into dump trucks or the like.

[0013] Figure 2 is a schematic diagram of the hydraulic drive system mounted on the hydraulic excavator 1. As shown in Figure 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) that drive multiple drive target members (1a, 1b, 1c, 1d), multiple flow control valves (8, 13-22), and an accumulator 9. The multiple hydraulic actuators include a boom cylinder 3a, an arm cylinder 3b, a bucket cylinder 3c, and a slewing motor 4. The multiple drive target members (driven members) include a boom 1a, an arm 1b, a bucket 1c, and a slewing body 1d.

[0014] The boom cylinder 3a, arm cylinder 3b, and bucket cylinder 3c are hydraulic cylinders having a cylinder tube, a disc-shaped piston that divides the inside of the cylindrical cylinder tube into a bottom chamber 301 and a rod chamber 302, and a piston rod connected to the piston. The slewing motor 4 is a hydraulic motor. Multiple flow control valves (8, 13-22) are driven by commands from a control device 100 (see Figure 3), which will be described later. The opening area of ​​the multiple flow control valves (8, 13-22) is adjusted by changing the position of the internal spool (valve body). By adjusting the opening area, the flow rate of hydraulic fluid passing through the opening is controlled.

[0015] The prime mover, engine 5, is composed of an internal combustion engine such as a diesel engine. The first hydraulic pump 6 and the second hydraulic pump 7 are mechanically connected to engine 5 and driven by the power of engine 5 to discharge hydraulic fluid (pressurized oil). The first hydraulic pump 6 and the second hydraulic pump 7 are variable displacement hydraulic pumps whose discharge capacity (displacement volume) can be changed. The first hydraulic pump 6 and the second hydraulic pump 7 are, for example, well-known inclined-shaft or swashplate type hydraulic pumps, and their discharge capacity (volume) is adjusted by changing the tilt angle. The volume (tilt angle) of the first hydraulic pump 6 is controlled by a regulator 23 attached to the first hydraulic pump 6. The volume (tilt angle) of the second hydraulic pump 7 is controlled by a regulator 24 attached to the second hydraulic pump 7.

[0016] The first hydraulic pump 6 and the second hydraulic pump 7 are connected by pipelines to a plurality of hydraulic actuators, respectively. The plurality of hydraulic actuators are driven by pressurized oil discharged from the first hydraulic pump 6 and the second hydraulic pump 7. The accumulator 9 is a pressure storage device that stores pressurized oil (hydraulic oil) supplied from the bottom chamber 301 of the boom cylinder 3a, which contracts due to the weight of the work device 51 during boom lowering operations. The accumulator 9 comprises a liquid chamber and a gas chamber separated by a diaphragm. Nitrogen gas is sealed in the gas chamber. Hydraulic oil is stored in the liquid chamber.

[0017] A first boom control valve 14, which is a flow control valve (pump flow control valve) that controls the flow rate of hydraulic fluid supplied from the first hydraulic pump 6 to the boom cylinder 3a, is provided in the pipeline connecting the first hydraulic pump 6 and the boom cylinder 3a. The first boom control valve 14 also controls the flow rate of hydraulic fluid discharged from the boom cylinder 3a to the tank 12. A second boom control valve 15, which is a flow control valve (pump flow control valve) that controls the flow rate of hydraulic fluid supplied from the second hydraulic pump 7 to the boom cylinder 3a, is provided in the pipeline connecting the second hydraulic pump 7 and the boom cylinder 3a. The second boom control valve 15 also controls the flow rate of hydraulic fluid discharged from the boom cylinder 3a to the tank 12.

[0018] A first arm control valve 16, which is a flow control valve (pump flow control valve) that controls the flow rate of hydraulic fluid supplied from the first hydraulic pump 6 to the arm cylinder 3b, is provided in the pipeline connecting the first hydraulic pump 6 and the arm cylinder 3b. The first arm control valve 16 also controls the flow rate of hydraulic fluid discharged from the arm cylinder 3b to the tank 12. A second arm control valve 17, which is a flow control valve (pump flow control valve) that controls the flow rate of hydraulic fluid supplied from the second hydraulic pump 7 to the arm cylinder 3b, is provided in the pipeline connecting the second hydraulic pump 7 and the arm cylinder 3b. The second arm control valve 17 also controls the flow rate of hydraulic fluid discharged from the arm cylinder 3b to the tank 12.

[0019] A bucket control valve 18, which is a flow control valve (pump flow control valve) that controls the flow rate of hydraulic fluid supplied from the first hydraulic pump 6 to the bucket cylinder 3c, is provided in the pipeline connecting the first hydraulic pump 6 and the bucket cylinder 3c. The bucket control valve 18 also controls the flow rate of hydraulic fluid discharged from the bucket cylinder 3c to the tank 12. A slewing control valve 13, which is a flow control valve (pump flow control valve) that controls the flow rate of hydraulic fluid supplied from the second hydraulic pump 7 to the slewing motor 4, is provided in the pipeline connecting the second hydraulic pump 7 and the slewing motor 4. The slewing control valve 13 also controls the flow rate of hydraulic fluid discharged from the slewing motor 4 to the tank 12.

[0020] The flow control valves (13-18) described above are center bypass type control valves. The flow control valves (14, 16, 18) have a center bypass passage that connects the first hydraulic pump 6 and the tank 12 when in the neutral position, and discharge the hydraulic fluid 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 connects the second hydraulic pump 7 and the tank 12 when in the neutral position, and discharge the hydraulic fluid discharged from the second hydraulic pump 7 to the tank 12 through the center bypass passage when in the neutral position.

[0021] The center bypass line 36 of the first hydraulic pump 6 is provided with a first center bypass flow control valve 53 that controls the flow rate of hydraulic fluid discharged from the first hydraulic pump 6 to the tank 12 through each flow control valve (14, 16, 18). The center bypass line 37 of the second hydraulic pump 7 is provided with a second center bypass flow control valve 54 that controls the flow rate of hydraulic fluid discharged from the second hydraulic pump 7 to the tank 12 through each flow control valve (13, 15, 17).

[0022] The first center bypass flow control valve 53 is a two-position normally open directional control valve that switches the spool position in response to an electromagnetic command from the control device 100. When an electromagnetic command (shut-off command) from the control device 100 is input to the first center bypass flow control valve 53, the flow path from the first hydraulic pump 6 to the tank 12 is shut off. As a result, the discharge pressure of the first hydraulic pump 6 increases.

[0023] The second center bypass flow control valve 54 is a two-position normally open directional control valve that switches the spool position in response to an electromagnetic command from the control device 100. When an electromagnetic command (shut-off command) from the control device 100 is input to the second center bypass flow control valve 54, the flow path from the second hydraulic pump 7 to the tank 12 is shut off. As a result, the discharge pressure of the second hydraulic pump 7 increases.

[0024] A makeup device is provided between the conduit 10 connecting the swivel control valve 13 and the right-hand swivel port of the swivel motor 4, and the conduit 11 connecting the swivel control valve 13 and the left-hand swivel port of the swivel motor 4. The makeup device has check valves 25, 26 and relief valves 27, 28 that are facing in opposite directions. A conduit 29 is provided between these check valves 25, 26 and relief valves 27, 28 to return the oil discharged from the swivel motor 4 to the tank 12. Conduit 29 functions as a makeup conduit that can replenish the swivel motor 4 with hydraulic fluid. The check valves 25, 26 have a makeup function that replenishes the swivel motor 4 with hydraulic fluid from the tank 12 when the conduits 10, 11 become negative pressure. The check valves 25, 26 suppress the generation of cavitation within the swivel motor 4. The relief valves 27 and 28 open when the pressure in the pipelines 10 and 11 exceeds the set pressure of the relief valves 27 and 28, and discharge the hydraulic fluid to the tank 12 through the pipeline 29.

[0025] A pipeline 38 is connected to the accumulator 9, and the pressurized oil discharged from the accumulator 9 is supplied to each hydraulic actuator through the pipeline 38. The bottom chamber 301 of the boom cylinder 3a and the pipeline 38 are connected by a pipeline (hereinafter also referred to as the bottom-side pipeline) 32. The bottom-side pipeline 32 is equipped with a regenerative valve 8 that regenerates the pressurized oil from the bottom chamber 301 of the boom cylinder 3a to the accumulator 9 when the boom is lowered. The regenerative valve 8 is a regenerative flow control valve (flow control valve) that controls the regenerative flow rate, which is the flow rate of the hydraulic fluid that flows from the bottom chamber 301 side of the boom cylinder 3a to the accumulator 9 side, by adjusting the opening area.

[0026] Between the bottom chamber 301 of the boom cylinder 3a and the regenerative valve 8 in the bottom-side pipeline 32, a pipeline (hereinafter also referred to as the rod-side pipeline) 33 is provided that branches off from the bottom-side pipeline 32 and connects to the rod chamber 302 of the boom cylinder 3a. The rod-side pipeline 33 is equipped with a pressure boosting valve 34 that increases the pressure of the hydraulic fluid 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 boosting valve 34 is a flow control valve that controls the flow rate of hydraulic fluid from the bottom chamber 301 side to the rod chamber 302 side of the boom cylinder 3a by adjusting the opening area.

[0027] Multiple branch lines 39, 40, 41, and 42 are connected to the main pipeline 38. Branch line 39 is connected to the boom cylinder 3a. Branch line 40 is connected to the arm cylinder 3b. Branch line 41 is connected to the bucket cylinder 3c. Branch line 42 is connected to the slewing motor 4.

[0028] The branch pipeline 39 is equipped with a boom assist valve 19, which is a flow control valve (assist flow control valve) that controls the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a. The branch pipeline 40 is equipped with an arm assist valve 20, which is a flow control valve (assist flow control valve) that controls the flow rate of hydraulic fluid supplied from the accumulator 9 to the arm cylinder 3b. The branch pipeline 41 is equipped with a bucket assist valve 21, which is a flow control valve (assist flow control valve) that controls the flow rate of hydraulic fluid supplied from the accumulator 9 to the bucket cylinder 3c. The branch pipeline 42 is equipped with a slewing assist valve 22, which is a flow control valve (assist flow control valve) that controls the flow rate of hydraulic fluid supplied from the accumulator 9 to the slewing motor 4.

[0029] Thus, the accumulator 9 is connected to each hydraulic actuator by a pipeline. The energy stored in the accumulator 9 is used to drive each hydraulic actuator.

[0030] Figure 3 shows the drive control system installed on the hydraulic excavator 1. The operator's cab of the hydraulic excavator 1 is equipped with actuator operating devices (lever operating devices) for operating each hydraulic actuator. The actuator operating devices include a boom operating device 30, an arm operating device 45, a bucket operating device 47, and a slewing operating device 49. The boom operating device 30 includes a boom operating lever for operating the boom cylinder 3a and a boom operating amount sensor 31 for detecting the amount of movement of the boom operating lever. The arm operating device 45 includes an arm operating lever for operating the arm cylinder 3b and an arm operating amount sensor 46 for detecting the amount of movement of the arm operating lever. The bucket operating device 47 includes a bucket operating lever for operating the bucket cylinder 3c and a bucket operating amount sensor 48 for detecting the amount of movement of the bucket operating lever. The slewing operating device 49 includes a slewing operating lever for operating the slewing motor 4 and a slewing operating amount sensor 50 for detecting the amount of movement of the slewing operating lever. In Figure 3, the arm cylinder 3b, the bucket cylinder 3c, and the flow control valves (16, 17, 18, 20, 21) used to control these hydraulic actuators are omitted from the illustration. The swivel assist valve 22 is also omitted from the illustration.

[0031] As shown in Figure 3, the drive control system includes a plurality of sensors for detecting the state of the hydraulic drive unit, and a control device 100 for controlling each flow control valve of the hydraulic drive unit based on the detection results of the plurality of sensors. The plurality of sensors include an accumulator sensor 43, a bottom pressure sensor 44, a plurality of manipulator sensors (31, 46, 48, 50), and a stroke sensor 56. The accumulator sensor 43, which is a pressure sensor, is installed in the pipeline 38 and detects the pressure of the hydraulic fluid in the accumulator 9 (hereinafter also referred to as accumulator pressure). The bottom pressure sensor 44, which is a pressure sensor, detects the pressure of the hydraulic fluid in the bottom chamber 301 of the boom cylinder 3a (hereinafter also referred to as bottom pressure). The accumulator sensor 43 and the bottom pressure sensor 44 output signals representing the detected pressure to the control device 100.

[0032] The boom operation amount sensor 31 detects the amount of operation of the boom operation lever (hereinafter also referred to as boom operation amount) and outputs a signal representing the detected boom operation amount to the control device 100. The arm operation amount sensor 46 detects the amount of operation of the arm operation lever (hereinafter also referred to as arm operation amount) and outputs a signal representing the detected arm operation amount to the control device 100. The bucket operation amount sensor 48 detects the amount of operation of the bucket operation lever (hereinafter also referred to as bucket operation amount) and outputs a signal representing the detected bucket operation amount to the control device 100. The slewing operation amount sensor 50 detects the amount of operation of the slewing operation lever (hereinafter also referred to as slewing operation amount) and outputs a signal representing the detected slewing operation amount to the control device 100.

[0033] The boom operation range includes a boom raising operation range, which operates boom 1a in the upward direction, and a boom lowering operation range, which operates boom 1a in the downward direction. The arm operation range includes an arm cloud operation range, which operates arm 1b in the cloud direction, and an arm dump operation range, which operates arm 1b in the dump direction. The bucket operation range includes a bucket cloud operation range, which operates bucket 1c in the cloud direction, and a bucket dump operation range, which operates bucket 1c in the dump direction. The slewing operation range includes a left slewing operation range, which operates the slewing body 1d in the left slewing direction, and a right slewing operation range, which operates the slewing body 1d in the right slewing direction.

[0034] The stroke sensor 56 detects the stroke (extension position) of the boom cylinder 3a and outputs a signal representing the detected stroke to the control device 100. The stroke sensor 56 repeatedly outputs a signal representing the stroke to the control device 100 at a predetermined period. Therefore, the stroke sensor 56 functions as a displacement sensor that detects the amount of stroke of the boom cylinder 3a, that is, the displacement of the piston rod (amount of change in extension position).

[0035] The first boom control valve 14 is a directional control valve with three open centers and is equipped with operating ends 14a and 14b at both ends that receive electromagnetic commands from the control device 100. The first boom control valve 14 switches the spool position in response to electromagnetic commands from the control device 100 to its operating ends. When the boom is raised, an electromagnetic command is sent to the operating end 14b, and pressurized 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 the hydraulic fluid in the rod chamber 302 of the boom cylinder 3a is discharged to the tank 12 through the first boom control valve 14. When the boom is lowered, an electromagnetic command is sent to the operating end 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, no hydraulic fluid is discharged from the bottom chamber 301 of the boom cylinder 3a into the tank 12 through the first boom control valve 14.

[0036] The second boom control valve 15 is a directional control valve with three open centers and is equipped with operating ends 15a and 15b at both ends that receive electromagnetic commands from the control device 100. The second boom control valve 15 switches the spool position in response to electromagnetic commands from the control device 100 to its operating ends. When the boom is raised, an electromagnetic command is sent to the operating end 15b, and pressurized oil from the second hydraulic pump 7 is supplied to the bottom chamber 301 of the boom cylinder 3a through the second boom control valve 15, and the hydraulic fluid from the rod chamber 302 of the boom cylinder 3a is discharged to the tank 12 through the second boom control valve 15. When the boom is lowered, an electromagnetic command is sent to the operating end 15a. As a result, the second hydraulic pump 7 and the rod chamber 302 of the boom cylinder 3a communicate via the second boom control valve 15, and the bottom chamber 301 of the boom cylinder 3a communicates with the tank 12 via the second boom control valve 15.

[0037] The swivel control valve 13 is a directional control valve with a three-position open center, and is equipped with operating ends 13a and 13b at both ends that receive electromagnetic commands from the control device 100. The swivel control valve 13 switches the spool position in response to electromagnetic commands from the control device 100 to its operating ends. When a left swivel operation is performed, an electromagnetic command is sent to the operating end 13a, and pressurized oil from the second hydraulic pump 7 is supplied to the left swivel port of the swivel motor 4 through the swivel control valve 13, and hydraulic fluid from the right swivel port of the swivel motor 4 is discharged to the tank 12 through the swivel control valve 13. When a right swivel operation is performed, an electromagnetic command is sent to the operating end 13b, and pressurized oil from the second hydraulic pump 7 is supplied to the right swivel port of the swivel motor 4 through the swivel control valve 13, and hydraulic fluid from the left swivel port of the swivel motor 4 is discharged to the tank 12 through the swivel control valve 13.

[0038] The regenerative valve 8 is a two-position normally closed directional control valve that switches the spool position in response to an electromagnetic command from the control device 100. When an electromagnetic command (communication command) from the control device 100 is input to the regenerative valve 8, the bottom chamber 301 of the boom cylinder 3a and the accumulator 9 are connected. This allows a portion of the pressurized oil discharged from the boom cylinder 3a to be regenerated by the accumulator 9.

[0039] The boost valve 34 is a two-position normally closed directional control valve that switches the spool position in response to an electromagnetic command from the control device 100. When an electromagnetic command (communication command) from the control device 100 is input to the boost valve 34, the bottom chamber 301 and the rod chamber 302 of the boom cylinder 3a are connected. As a result, a portion of the pressurized oil discharged from the bottom chamber 301 of the boom cylinder 3a is regenerated into the rod chamber 302. This allows the pressurized oil discharged from the bottom chamber 301 to be increased in pressure by the ratio of the pressure-receiving areas of the bottom chamber 301 and the rod chamber 302 of the boom cylinder 3a, thereby allowing the accumulator 9 to accumulate pressure at a higher level.

[0040] For example, if the pressure of the pressurized oil discharged from the bottom chamber 301 when the boom is lowered is 10 MPa, and the pressure receiving area ratio of the boom cylinder 3a is 1:2, then by opening the pressure boosting valve 34 when the boom is lowered, the pressure of the pressurized oil discharged from the bottom chamber 301 will eventually be increased to 20 MPa. This allows the accumulator 9 to accumulate pressure to a higher level, thus increasing the opportunities for the accumulator 9 to assist each hydraulic actuator.

[0041] The boom assist valve 19 is a two-position normally closed directional control valve that switches the spool position by an electromagnetic command from the control device 100. When the boom is raised, an electromagnetic command (communication command) from the control device 100 is input to the boom assist valve 19, and the boom assist valve 19 opens. As a result, the accumulator 9 and the bottom chamber 301 of the boom cylinder 3a are connected, and the pressurized oil regenerated by the accumulator 9 can be sent to the bottom chamber 301 of the boom cylinder 3a. In addition to the pressurized oil supplied from the first hydraulic pump 6 and the second hydraulic pump 7, the boom cylinder 3a is also supplied with pressurized oil from the accumulator 9, which increases the boom raising speed.

[0042] The control device 100's non-volatile memory 100b has a preset minimum operating pressure Pamin for the accumulator 9. When the pressure of the accumulator 9 falls below the minimum operating pressure Pamin, the supply of hydraulic fluid from the accumulator 9 to the assisted hydraulic actuator is interrupted, causing the speed of the assisted hydraulic actuator to decrease. The minimum operating pressure Pamin is determined based on the minimum pressure at which the control device 100 can supply the required flow rate from the accumulator 9 to the assisted hydraulic actuator by controlling the assist flow control valve. In other words, the minimum operating pressure Pamin is set to a value equal to or greater than the drive pressure of the assisted hydraulic actuator. For example, if the bottom pressure of the boom cylinder 3a is 15 [MPa] when the boom is raised, the minimum operating pressure Pamin is set to 16 [MPa].

[0043] Although not shown in Figure 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 bucket cylinder 3c by the first hydraulic pump 6 are the same as the driving of the boom cylinder 3a described above. Although not shown in Figure 3, the assistance of the arm cylinder 3b, bucket cylinder 3c, and swing motor 4 by the accumulator 9 is the same as the assistance of the boom cylinder 3a described above. Although not shown in Figure 3, the arm cylinder 3b, bucket cylinder 3c, and swing motor 4 are also provided with pressure sensors to detect the pressure of the hydraulic actuators, similar to the boom cylinder 3a, and signals representing the pressure of each hydraulic actuator detected by each pressure sensor are input to the control device 100. Although not shown in Figure 3, the arm cylinder 3b and bucket cylinder 3c are also provided with stroke sensors capable of detecting the cylinder stroke, similar to the boom cylinder 3a, and signals representing the stroke of each hydraulic cylinder detected by each stroke sensor are input to the control device 100. Although not shown in Figure 3, the slewing motor 4 is equipped with a slewing angle sensor capable of detecting the slewing angle of the slewing motor 4 (slewing body 1d), and a signal representing the slewing angle of the slewing motor 4 detected by the slewing angle sensor is input to the control device 100.

[0044] As described above, the hydraulic drive system of the hydraulic excavator 1 according to the present embodiment includes an assist system capable of driving a hydraulic actuator by using energy from the accumulator 9. In such a hydraulic drive system, the pressure of the accumulator 9 may drop, which may cause a decrease in the assist flow rate, which is the flow rate of hydraulic oil supplied from the accumulator 9 to the hydraulic actuator through the assist flow control valve. In a combined operation where a plurality of hydraulic actuators operate simultaneously, if the speed of the assisted hydraulic actuator decreases due to a pressure drop in the accumulator 9, and the speed of other hydraulic actuators is maintained, the speed ratio between the plurality of hydraulic actuators will change, resulting in an operation that differs from the operator's intention. Therefore, when the speed of the assisted hydraulic actuator decreases due to a pressure drop in the accumulator 9 during combined operation, the control device 100 according to the present embodiment controls the flow control valve so as to suppress a change in the speed ratio of the plurality of operating hydraulic actuators. A detailed description will be given below.

[0045] The control device 100 is constituted by a computer including a processor (processing unit) 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 that is a so-called RAM (Random Access Memory), an input interface, an output interface, and other peripheral circuits. These pieces of hardware cooperate to run software and realize a plurality of functions. The control device 100 may be constituted by a single computer or a plurality of computers.

[0046] A program capable of executing various calculations is stored in the non-volatile memory 100b. That is, the non-volatile memory 100b is a readable storage medium (storage device) storing a program that implements the functions of the present embodiment. The volatile memory 100c is a storage medium (storage device) that temporarily stores calculation results obtained by the processor 100a and signals input from the input interface. The processor 100a is a device that loads the program stored in the non-volatile memory 100b into the volatile memory 100c and executes calculation, and performs predetermined calculation processing on data acquired from the input interface, the non-volatile memory 100b and the volatile memory 100c in accordance with the program.

[0047] The input interface converts signals input from various devices (such as sensors) into data that can be calculated by the processor 100a. Further, the output interface generates output signals corresponding to the calculation results obtained by the processor 100a, and outputs the signals to various devices (such as flow control valves, regulators 23 and 24).

[0048] The control device 100 according to the present embodiment outputs a pump control command to the regulator 23, and controls the displacement volume of the first hydraulic pump 6 via the regulator 23. Further, the pump control command for the regulator 23 is fed back to the control device 100. Similarly, the control device 100 outputs a pump control command to the regulator 24, and controls the displacement volume of the second hydraulic pump 7 via the regulator 24. The pump control command for the regulator 24 is fed back to the control device 100.

[0049] The control device 100 outputs an electromagnetic command for driving at least one of the first boom control valve 14, the second boom control valve 15, the regeneration valve 8 and the boom assist valve 19 to the operating end of the flow control valve to be driven, based on the boom operation amount detected by the boom operation amount sensor 31.

[0050] For example, when the pressure in the accumulator 9 is above a first threshold and a boom raising operation is performed, the control device 100 controls the first boom control valve 14, the second boom control valve 15, and the boom assist valve 19 to have opening areas corresponding to the amount of boom raising operation. As a result, hydraulic fluid discharged from the first hydraulic pump 6 and the second hydraulic pump 7, as well as hydraulic fluid discharged from the accumulator 9, are supplied to the bottom chamber 301 of the boom cylinder 3a, and hydraulic fluid is discharged from the rod chamber 302 of the boom cylinder 3a to the tank 12, driving the boom cylinder 3a in the extension direction. This causes the boom 1a to move upward.

[0051] Furthermore, for example, if a boom lowering operation is performed when the pressure of the accumulator 9 is below the second threshold, the control device 100 controls the second boom control valve 15, the regenerative valve 8, and the boost valve 34 to have opening areas corresponding to the amount of boom lowering operation. As a result, hydraulic fluid from the bottom chamber 301 of the boom cylinder 3a, which contracts due to the weight of the working device 51, is supplied to the accumulator 9, and the accumulator 9 is pressurized.

[0052] The control device 100 outputs an electromagnetic command to the operating end of the flow control valve to be driven, based on the amount of rotation detected by the rotation amount sensor 50, to drive at least one of the rotation control valve 13 and the rotation assist valve 22.

[0053] For example, when a rightward rotation is performed by the slewing control device 49, a signal representing the rightward rotation amount detected by the slewing amount sensor 50 is input to the control device 100. Based on the rightward rotation amount, the control device 100 outputs an electromagnetic command to the operating end 13b of the slewing control valve 13. The slewing control valve 13 is switched by the electromagnetic command from the control device 100, and the pressurized oil from the second hydraulic pump 7 is supplied to the rightward rotation port of the slewing motor 4 through the center bypass passage of the second boom control valve 15 and the slewing control valve 13, driving the slewing motor 4 and causing the slewing body 1d to rotate to the right. The hydraulic fluid used to drive the slewing motor 4 is discharged from the leftward rotation port of the slewing motor 4 through the slewing control valve 13 to the tank 12. When a leftward rotation operation is performed, an electromagnetic command is output from the control device 100 to the operating end 13a of the rotation control valve 13, and, as with the rightward rotation described above, hydraulic fluid is supplied to the rotation motor 4, causing the rotation body 1d to rotate to the left.

[0054] Figure 4 is a block diagram showing the functions of the control device 100 according to the first embodiment. The control device 100 controls each flow control valve (13 to 22) so that the hydraulic actuator operates at a target speed corresponding to the amount of operation of the actuator operating devices (30, 45, 47, 49).

[0055] Figure 4 shows the various calculation processes for controlling the slewing control valve 13 when the slewing boom raising operation is being performed. The slewing boom raising operation refers to a combined operation in which the slewing operation and the boom raising operation are performed simultaneously. Although not shown in the figures, the various calculation processes when a combined operation of slewing operation and arm operation, and a combined operation of slewing operation and bucket operation are performed are the same as the various calculation processes when a combined operation of slewing operation and boom operation is performed as shown in Figure 4, so an explanation is omitted.

[0056] As shown in Figure 4, the control device 100 includes a pump request flow rate calculation unit 101, an assist request flow rate calculation unit 102, a target flow rate calculation unit 103, a target speed calculation unit 104, an actual speed calculation unit 105, a speed difference calculation unit 106, an opening amount calculation unit 107, a correction unit 108, and a valve command unit 109. The control device 100 realizes these functions by executing a program stored in the non-volatile memory 100b.

[0057] In this embodiment, the control device 100 controls the swing control valve 13 so that the actual speed of the swing motor 4 decreases relative to the target speed corresponding to the amount of operation of the swing control device 49 when the pressure of the accumulator 9 decreases, causing a decrease in the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a through the boom assist valve 19, resulting in the actual speed of the boom cylinder 3a decreasing relative to the target speed of the boom cylinder 3a corresponding to the amount of operation of the boom operating device 30. The functions for realizing this control will be described in detail below.

[0058] The pump request flow rate calculation unit 101 calculates the pump request flow rate Qpreq, which is the requested flow rate of hydraulic fluid supplied from the first hydraulic pump 6 and the second hydraulic pump 7 to the boom cylinder 3a. The pump request flow rate calculation unit 101 calculates the pump request flow rate Qpreq based, for example, on the control command for the first hydraulic pump 6 fed back from the regulator 23, the control command for the second hydraulic pump 7 fed back from the regulator 24, the boom operating amount detected by the boom operating amount sensor 31, the arm operating amount detected by the arm operating amount sensor 46, the bucket operating amount detected by the bucket operating amount sensor 48, and the slewing operating amount detected by the slewing operating amount sensor 50.

[0059] For example, the pump request flow rate calculation unit 101 calculates the first flow rate Qbp1 of the hydraulic fluid supplied from the first hydraulic pump 6 to the boom cylinder 3a through the first boom control valve 14, based on the following formula: the discharge flow rate Qp1 of the first hydraulic pump 6 in response to the control command of the first hydraulic pump 6 fed back from the regulator 23; the opening area A11 of the first boom control valve 14 in response to the boom operation amount; the opening area A12 of the first arm control valve 16 in response to the arm operation amount; and the opening area A13 of the bucket control valve 18 in response to the bucket operation amount. Qbp1 = Qp1 × A11 / (A11 + A12 + A13) The discharge flow rate Qp1 is calculated by multiplying the displaced volume q1 corresponding to the control command of the first hydraulic pump 6 by the pump volumetric efficiency η1 of the first hydraulic pump 6, the engine rotational speed Nc detected by the engine rotational speed sensor, and the reduction ratio z1 of the reduction mechanism connecting the engine 5 and the first hydraulic pump 6 (Qp1 = q1・η1・Nc・z1).

[0060] Furthermore, the pump request flow rate calculation unit 101 calculates the second flow rate Qbp2 of the hydraulic fluid supplied from the second hydraulic pump 7 to the boom cylinder 3a through the second boom control valve 15, based on the following formula: the discharge flow rate Qp2 of the second hydraulic pump 7 in response to the control command of the second hydraulic pump 7 fed back from the regulator 24; the opening area A21 of the second boom control valve 15 in response to the boom operation amount; the opening area A22 of the second arm control valve 17 in response to the arm operation amount; and the opening area A23 of the slewing control valve 13 in response to the slewing operation amount. Qbp2 = Qp2 × A21 / (A21 + A22 + A23) The discharge flow rate Qp2 is calculated by multiplying the displaced volume q2 corresponding to the control command of the second hydraulic pump 7 by the pump volumetric efficiency η2 of the second hydraulic pump 7, the engine rotational speed Nc detected by the engine rotational speed sensor, and the reduction ratio z2 of the reduction mechanism connecting the engine 5 and the second hydraulic pump 7 (Qp2 = q2・η2・Nc・z2).

[0061] The pump request flow rate calculation unit 101 calculates the pump request flow rate Qpreq by adding together the first flow rate Qbp1, which is the flow rate of hydraulic fluid supplied from the first hydraulic pump 6 to the boom cylinder 3a, and the second flow rate Qbp2, which is the flow rate of hydraulic fluid supplied from the second hydraulic pump 7 to the boom cylinder 3a (Qpreq = Qbp1 + Qbp2).

[0062] The assist request flow rate calculation unit 102 calculates the assist request flow rate Qareq, which is the required flow rate of the hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a. The assist request flow rate calculation unit 102 calculates the assist request flow rate Qareq based on the assist request flow rate characteristic data, the pump request flow rate Qpreq calculated by the pump request flow rate calculation unit 101, and the boom lifting amount (also referred to as boom lifting amount) Lb detected by the boom operating amount sensor 31.

[0063] The assist request flow rate characteristic data defines the characteristic that the assist request flow rate Qareq increases as the boom lifting operation amount Lb increases, and the assist request flow rate Qareq increases as the pump request flow rate Qpreq increases. The assist request flow rate characteristic data is determined in advance through experiments, etc., and stored in the non-volatile memory 100b. For example, the assist request flow rate characteristic data includes multiple request flow rate characteristics C(Qpreq) corresponding to the pump request flow rate Qpreq. The assist request flow rate calculation unit 102 selects a request flow rate characteristic C(Qpreq) corresponding to the pump request flow rate Qpreq, and calculates the assist request flow rate Qareq based on the boom lifting operation amount Lb by referring to the selected request flow rate characteristic C(Qpreq). The assist request flow rate characteristic data may be in data table format or function format (mathematical formula).

[0064] The target flow rate calculation unit (addition unit) 103 calculates the target flow rate Qbt, which is the target value of the flow rate supplied to the boom cylinder 3a, by adding the assist request flow rate Qareq to the pump request flow rate Qpreq calculated by the pump request flow rate calculation unit 101 (Qbt = Qpreq + Qareq).

[0065] The target speed calculation unit 104 calculates the target speed Vbit of the boom cylinder 3a by dividing the target flow rate Qbit calculated by the target flow rate calculation unit 103 by the cross-sectional area of ​​the boom cylinder 3a (pressure-receiving area of ​​the bottom chamber). The pressure-receiving area of ​​the bottom chamber or the inner diameter of the boom cylinder 3a necessary for calculating the pressure-receiving area is predetermined according to the specifications of the boom cylinder 3a.

[0066] The actual speed calculation unit 105 calculates the actual speed Vba of the boom cylinder 3a, which is the time rate of change of the extension / retraction position of the boom cylinder 3a, based on the stroke of the boom cylinder 3a detected by the stroke sensor 56. In other words, the actual speed calculation unit 105 functions as a differentiator, calculating the actual speed Vba of the boom cylinder 3a by differentiating the displacement of the boom cylinder 3a with respect to time. Preferably, the actual speed calculation unit 105 has a function to perform filtering to attenuate the amplitude of noise included in the result calculated as a differentiator.

[0067] The speed difference calculation unit 106 calculates the speed difference ΔV and the speed ratio Rv between the target speed Vbat calculated by the target speed calculation unit 104 and the actual speed Vba calculated by the actual speed calculation unit 105. The speed difference ΔV is calculated by subtracting the actual speed Vba from the target speed Vbat (ΔV = Vbat - Vba). The speed ratio Rv is calculated by dividing the actual speed Vba by the target speed Vbat (Rv = Vba / Vbat).

[0068] The opening amount calculation unit 107 refers to the opening characteristic data and calculates the target opening area Ast, which is the target value of the opening area of ​​the swivel control valve 13, based on the swivel operation amount Ls. The opening characteristic data defines a characteristic in which the target opening area Ast increases as the swivel operation amount Ls increases. The opening characteristic data is determined in advance through experiments or the like and stored in the non-volatile memory 100b. The opening characteristic data may be in data table format or function format (mathematical formula).

[0069] In this embodiment, the target opening area Ast is the target value for the cross-sectional area of ​​the flow path of the hydraulic fluid that flows from the second hydraulic pump 7 to the swivel motor 4 in the swivel control valve 13.

[0070] The correction unit 108 calculates the corrected target opening area Asts by multiplying the target opening area Ast of the swivel control valve 13, calculated by the opening amount calculation unit 107, by the speed ratio Rv, calculated by the speed difference calculation unit 106 (Asts = Ast・Rv).

[0071] The valve command unit 109 calculates a control command value (for example, the current value of the solenoid of the swivel control valve 13) corresponding to the correction target opening area Asts of the swivel control valve 13, and outputs a control command (control current) corresponding to the calculated control command value to the swivel control valve 13. As a result, the swivel control valve 13 is controlled so that its actual opening area becomes equal to the correction target opening area Asts of the swivel control valve 13.

[0072] As described above, the control device 100 calculates the actual speed of the boom cylinder 3a based on the information detected by the stroke sensor 56. The control device 100 calculates the target speed of the boom cylinder 3a based on the amount of operation of the boom operating device 30. The control device 100 corrects the target opening area of ​​the slewing control valve 13 so that the speed ratio between the actual speed of the slewing motor 4 and the target speed is equal to the calculated speed ratio Rv between the actual speed Vba of the boom cylinder 3a and the target speed Vbt.

[0073] Referring to Figures 5A to 5C, the operation of the hydraulic excavator 1 according to this embodiment will be explained in comparison with the operation of a hydraulic excavator according to a comparative example of this embodiment. The control device of the hydraulic excavator according to the comparative example of this embodiment does not have the function of a correction unit 108, and the valve command unit 109 outputs a control command to the slewing control valve 13 according to the target opening area As calculated by the opening amount calculation unit 107.

[0074] Figures 5A to 5C show the time changes in the maneuvering amount, pressure, flow rate, position, and angle during the slewing boom raising operation. Figure 5A shows the case where the accumulator pressure does not fall below the minimum operating pressure Pamin of the accumulator 9 from the start to the end of the slewing boom raising operation. Figure 5B shows the case in a comparative example where the accumulator pressure falls below the minimum operating pressure Pamin of the accumulator 9 during the slewing boom raising operation. Figure 5C shows the case in this embodiment where the accumulator pressure falls below the minimum operating pressure Pamin of the accumulator 9 during the slewing boom raising operation.

[0075] In Figures 5A to 5C, the horizontal axis of each graph represents elapsed time. The vertical axis of graph (a) represents the amount of operation, the vertical axis of graph (b) represents pressure, the vertical axis of graph (c) represents flow rate, and the vertical axis of graph (d) represents position and angle. Graph (a) shows the time change of the boom lifting operation amount (solid line) and the slewing operation amount (dashed line). Graph (b) shows the time change of the bottom pressure (solid line) and accumulator pressure (dashed line) of the boom cylinder 3a. Graph (c) shows the inflow flow rate (solid line) of the bottom chamber 301 of the boom cylinder 3a and the inflow flow rate (dashed line) to the slewing motor 4. Graph (d) shows the stroke (solid line) of the boom cylinder 3a and the slewing angle (dashed line) of the slewing body 1d.

[0076] Referring to Figure 5A, the behavior when the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is not interrupted during the slewing boom raising operation will be explained. The behavior shown in Figure 5A is common to the hydraulic excavator 1 according to this embodiment and to the hydraulic excavator according to the comparative example. As shown in Figure 5A, when the slewing operation and the boom raising operation are started simultaneously at time ts, the hydraulic fluid discharged from the accumulator 9, the first hydraulic pump 6, and the second hydraulic pump 7 is supplied to the bottom chamber 301 of the boom cylinder 3a, and the boom cylinder 3a extends. In addition, the hydraulic fluid discharged from the second hydraulic pump 7 is supplied to the slewing motor 4, and the slewing motor 4 rotates. In this embodiment, the only object assisted in the slewing boom raising operation is the boom cylinder 3a. Therefore, the flow rate of hydraulic fluid supplied from the accumulator 9 to the slewing motor 4 through the slewing assist valve 22 is 0.

[0077] The accumulator pressure decreases over time from time ts. However, the accumulator pressure does not fall below the minimum operating pressure Pamin until time te, when the slewing and boom raising operations are completed simultaneously. Furthermore, the accumulator pressure does not fall below the bottom pressure of the boom cylinder 3a. Therefore, in the example shown in Figure 5A, the supply flow rate from the accumulator 9 to the boom cylinder 3a is not interrupted during the slewing boom raising operation. Since the boom cylinder 3a is supplied with a constant flow rate of hydraulic fluid from the hydraulic pumps (6, 7) and a constant flow rate of hydraulic fluid from the accumulator 9, the boom cylinder 3a can be moved at a constant speed by maintaining a constant boom operating amount. In addition, the slewing motor 4 can also be moved at a constant speed, resulting in good operability.

[0078] Referring to Figure 5B, the behavior in a comparative example of this embodiment when the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is interrupted during the slewing boom raising operation will be explained. Note that only the boom cylinder 3a is assisted during the slewing boom raising operation. Therefore, the flow rate of hydraulic fluid supplied from the accumulator 9 to the slewing motor 4 through the slewing assist valve 22 is 0.

[0079] As shown in Figure 5B, when the slewing operation and boom raising operation are started simultaneously at time ts, the boom cylinder 3a is supplied with a constant flow rate of hydraulic fluid from the hydraulic pumps (6, 7) and also from the accumulator 9 until time t1. Therefore, the boom cylinder 3a can be moved at a constant speed from time t0 to time t1. The slewing motor 4 can also be moved at a constant speed.

[0080] However, the pressure in the accumulator 9 decreases over time, falling below the minimum operating pressure Pamin just before time t1, and dropping to the bottom pressure of the boom cylinder 3a at time t1. As a result, the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a decreases to almost zero. Due to the interruption of the hydraulic fluid supply, the only hydraulic fluid flow rate supplied to the boom cylinder 3a is the flow rate supplied by the hydraulic pumps (6, 7). Consequently, even if the boom operation amount is kept constant, the boom raising speed will decrease. On the other hand, the slewing motor 4 is supplied with a constant flow rate of hydraulic fluid before and after time t1. In other words, the slewing motor 4 rotates at a constant target speed corresponding to a constant slewing operation amount. As a result, the speed ratio between the speed of the boom cylinder 3a and the speed of the slewing motor 4 changes before and after time t1. Consequently, the movement trajectory of the work device 51 (for example, the movement trajectory of the tip of the bucket 1c) differs from the example shown in Figure 5A.

[0081] Thus, in the comparative example, the difference between the pressure of the accumulator 9 and the bottom pressure of the boom cylinder 3a disappears, which can cause a change in the speed ratio between the boom cylinder 3a and the slewing motor 4, potentially resulting in the working device 51 moving in a trajectory unintended by the operator. For example, when performing a slewing boom-raising operation to move the bucket 1c toward the excavation position above the dump truck's vessel, if only the boom-raising speed decreases, the bucket 1c may not be moved upward as intended by the operator, and there is a risk that the bucket 1c may interfere with the side of the dump truck. Therefore, the operator needs to interrupt the slewing operation (time te1) before the bucket 1c interferes with the dump truck and continue only the boom-raising operation. At time te2, after completing the boom-raising operation (not shown), the operator resumes the slewing operation and moves the bucket 1c to the excavation position on the dump truck's vessel.

[0082] Therefore, in this embodiment, the control device 100 controls the slewing control valve 13 to decelerate the slewing motor 4 when the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is interrupted, so that the speed ratio between the boom cylinder 3a and the slewing motor 4 is kept constant before and after the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is interrupted.

[0083] Referring to Figure 5C, the behavior in this embodiment when the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is interrupted during the slewing boom raising operation will be described. The behavior from time ts to time t1 is the same as in the comparative example (Figure 5B).

[0084] When the pressure of the accumulator 9 falls below the minimum operating pressure Pamin, and furthermore, at time t1, the difference between the pressure of the accumulator 9 and the bottom pressure of the boom cylinder 3a disappears, the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is interrupted, and the actual speed of the boom cylinder 3a decreases. In other words, the actual speed Vba of the boom cylinder 3a deviates from the target speed Vbt to the lower side. When the actual speed of the boom cylinder 3a decreases due to a decrease in the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a through the boom assist valve 19, the control device 100 controls the slewing control valve 13 to reduce the speed of the slewing motor 4.

[0085] The control device 100 reduces the opening area of ​​the flow path connecting the second hydraulic pump 7 and the slewing motor 4 in the slewing control valve 13. As a result, the speed of the slewing motor 4 decreases, and changes in the speed ratio between the boom cylinder 3a and the slewing motor 4 are suppressed.

[0086] For example, if the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3 is interrupted during a right-swing boom raising operation, the control device 100 outputs an electromagnetic command to the operating end 13b of the swing control valve 13 so that the swing control valve 13 moves towards the neutral position. In other words, it reduces the current value of the control current supplied to the operating end 13b. As the swing control valve 13 moves towards the neutral position, the opening area of ​​the passage in the swing control valve 13 that connects the second hydraulic pump 7 and the right-swing port of the swing motor 4 becomes smaller. Also, the opening area of ​​the passage in the swing control valve 13 that connects the left-swing port of the swing motor 4 and the tank 12 becomes smaller. Furthermore, the opening area of ​​the passage in the swing control valve 13 that connects the second hydraulic pump 7 and the tank 12 becomes larger. As a result, the pressure in the right-swing port of the swing motor 4 decreases and the pressure in the left-swing port increases, causing the swing operation to decelerate.

[0087] In this embodiment, as described above, the control device 100 calculates the target opening area Ast of the slewing control valve 13 based on the amount of operation of the slewing operation device 49. The control device 100 corrects the target opening area of ​​the slewing control valve 13 so that the speed ratio between the actual speed of the slewing motor 4 and the target speed matches the speed ratio between the actual speed of the boom cylinder 3a and the target speed, and calculates the corrected target opening area Asts. The control device 100 controls the slewing control valve 13 so that the actual opening area of ​​the slewing control valve 13 becomes the corrected target opening area Asts.

[0088] This allows the slewing control valve 13 to be controlled so that the speed ratio between the speed of the boom cylinder 3a and the speed of the slewing motor 4 after the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is interrupted is the same as the speed ratio between the speed of the boom cylinder 3a and the speed of the slewing motor 4 before the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a was interrupted.

[0089] As described above, when the supply of hydraulic fluid (assistance) from the accumulator 9 to the boom cylinder 3a is interrupted during the slewing boom raising operation, causing the boom raising speed to decrease, the speed of the slewing motor 4 is reduced, thereby maintaining the speed ratio between the boom raising speed and the slewing speed before and after the interruption of assistance from the accumulator 9. As a result, in this embodiment, the movement trajectory of the work device 51 (for example, the movement trajectory of the tip of the bucket 1c) can be made similar to the movement trajectory of the work device 51 in the example shown in Figure 5A.

[0090] According to the above-described embodiment, the following effects are achieved.

[0091] (1) The work machine according to this embodiment is a hydraulic excavator 1 that performs work using a work device 51 having at least a boom cylinder (first hydraulic actuator) 3a. The hydraulic excavator 1 comprises an engine (prime mover) 5, hydraulic pumps (first hydraulic pump 6 and second hydraulic pump 7) driven by the engine 5, an accumulator (pressure accumulator) 9 for storing hydraulic fluid, a boom cylinder (first hydraulic actuator) 3a driven by hydraulic fluid supplied from at least the accumulator (pressure accumulator) 9, and a swing motor (second hydraulic actuator) 4 driven by hydraulic fluid supplied from the second hydraulic pump 7. The hydraulic excavator 1 includes a boom assist valve (first flow control valve) 19 that controls the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a, a first boom control valve (third flow control valve) 14 that controls the flow rate of hydraulic fluid supplied from the first hydraulic pump 6 to the boom cylinder 3a, a second boom control valve (third flow control valve) 15 that controls the flow rate of hydraulic fluid supplied from the second hydraulic pump 7 to the boom cylinder 3a, and a slewing control valve (second flow control valve) 13 that controls the flow rate of hydraulic fluid supplied from the second hydraulic pump 7 to the slewing motor 4. The boom cylinder 3a is driven by hydraulic fluid supplied from the accumulator 9 through the boom assist valve (first flow control valve) 19, hydraulic fluid supplied from the first hydraulic pump 6 through the first boom control valve 14, and hydraulic fluid supplied from the second hydraulic pump 7 through the second boom control valve 15. The hydraulic excavator 1 includes a boom operating device (first operating device) 30 for operating the boom cylinder 3a, a slewing operating device (second operating device) 49 for operating the slewing motor 4, and a control device 100.

[0092] The control device 100 controls the boom assist valve 19, the first boom control valve 14, and the second boom control valve 15 so that the boom cylinder 3a operates at a target speed corresponding to the amount of operation of the boom operating device 30. The control device 100 also controls the slewing control valve 13 so that the slewing motor 4 operates at a target speed corresponding to the amount of operation of the slewing operating device 49. When the pressure in the accumulator 9 decreases and the flow rate of the hydraulic fluid passing through the boom assist valve 19 decreases, causing the actual speed Vba of the boom cylinder 3a to decrease relative to the target speed Vbt of the boom cylinder 3a corresponding to the amount of operation of the boom operating device 30, the control device 100 controls the slewing control valve 13 so that the actual speed of the slewing motor 4 decreases relative to the target speed corresponding to the amount of operation of the slewing operating device 49.

[0093] With this configuration, if the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a through the boom assist valve 19 decreases due to a decrease in the pressure of the accumulator 9, the speed of the slewing motor 4 can be reduced in accordance with the decrease in the speed of the boom cylinder 3a. Therefore, according to this embodiment, it is possible to suppress the discrepancy between the movement trajectory of the work device 51 (for example, the movement trajectory of the bucket 1c) when the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a does not decrease during the slewing boom raising operation, and the movement trajectory of the work device 51 when the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a decreases during the slewing boom raising operation.

[0094] Furthermore, as in the comparative example, if only the speed of the boom cylinder 3 decreases, the movement trajectory of the work device 51 may not be the movement trajectory intended by the operator. In other words, the operability of the work device 51 deteriorates. In contrast, in this embodiment, even if the supply (assistance) of hydraulic fluid from the accumulator 9 to the boom cylinder 3a is interrupted, deviations in the movement trajectory of the work device 51 are suppressed, and the work device 51 can be operated along the movement trajectory intended by the operator. In other words, according to this embodiment, the operability of the work device 51 is improved compared to the comparative example. As a result, work efficiency can be improved.

[0095] (2) In this embodiment, the boom cylinder 3a is the hydraulic actuator to be assisted (first hydraulic actuator), and the slewing motor 4 is the hydraulic actuator to be decelerated (second hydraulic actuator). With this configuration, for example, if the pressure of the accumulator 9 falls below the minimum operating pressure during the slewing boom raising operation in loading work onto a dump truck, the speed of the slewing motor 4 decreases along with the speed of the boom cylinder 3a. This prevents the operation of stopping the slewing movement of the slewing body 1d before operating the working device 51 before the working device 51 interferes with the dump truck (see Figure 5B). In other words, according to this embodiment, there is no need to change the slewing boom raising operation depending on whether the pressure of the accumulator 9 does not fall below the minimum operating pressure (when the flow rate of hydraulic fluid from the accumulator 9 to the boom cylinder 3a does not decrease) or whether the pressure of the accumulator 9 falls below the minimum operating pressure (when the flow rate of hydraulic fluid from the accumulator 9 to the boom cylinder 3a decreases).

[0096] (3) When the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a decreases, the control device 100 controls the swing control valve 13 so that the speed ratio R between the speed of the boom cylinder 3a and the speed of the swing motor 4 is the same as the speed ratio R before the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a decreased.

[0097] This configuration virtually eliminates the discrepancy between the movement trajectory of the work device 51 (for example, the movement trajectory of the bucket 1c) when the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a does not decrease during the slewing boom raising operation, and the movement trajectory of the work device 51 when the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a decreases during the slewing boom raising operation. In other words, this configuration can further improve operability.

[0098] (4) The control device 100 calculates the target opening area Ast of the slewing control valve 13 based on the amount of operation of the slewing control device 49. The control device 100 corrects the target opening area Ast of the slewing control valve 13 so that the speed ratio between the actual speed of the slewing motor 4 and the target speed is equal to the speed ratio Rv between the actual speed Vba of the boom cylinder 3a and the target speed Vbt. The control device 100 controls the slewing control valve 13 so that the actual opening area of ​​the slewing control valve 13 becomes the corrected target opening area (corrected target opening area) Asts.

[0099] In this embodiment, as described above, the speed of the slewing motor 4 is controlled such that the speed ratio of the slewing motor 4 before and after the assist is interrupted becomes equal to the speed ratio of the boom cylinder 3a before and after the assist is interrupted, by calculating the corrected target opening area Asts by multiplying the target opening area Ast of the slewing control valve 13 by the speed ratio Rv. With this configuration, by controlling the opening area of ​​the slewing control valve 13, even if the assist is interrupted during the slewing boom raising operation, the speed ratio between the speed of the boom cylinder 3a and the speed of the slewing motor 4 can be maintained at a constant value.

[0100] (5) The hydraulic excavator 1 is equipped with a stroke sensor (sensor) 56 that detects the displacement of the boom cylinder 3a as information regarding the actual speed of the boom cylinder 3a. The control device 100 calculates the actual speed Vba of the boom cylinder 3a based on the information (displacement) detected by the stroke sensor 56. The control device 100 calculates the actual speed of the boom cylinder 3a as the rate of change of the displacement of the boom cylinder 3a over time. That is, the control device 100 calculates the actual speed of the boom cylinder 3a by differentiating the displacement of the boom cylinder 3a with respect to time. With this configuration, the actual speed of the boom cylinder 3a can be determined with high accuracy.

[0101] (6) The control device 100 calculates the target speed Vbit of the boom cylinder 3a based on the amount of operation of the boom operating device 30. The control device 100 corrects the target opening area of ​​the slewing control valve 13 so that the speed ratio between the actual speed of the slewing motor 4 and the target speed is Rv, which is the speed ratio between the calculated actual speed Vba of the boom cylinder 3a and the calculated target speed Vbit of the boom cylinder 3a. With this configuration, the slewing control valve 13 is controlled using the accurately determined actual speed, so even if the assist is interrupted during the slewing boom raising operation, the speed ratio between the speed of the boom cylinder 3a and the speed of the slewing motor 4 can be maintained more appropriately.

[0102] <Second Embodiment> Referring to Figure 6, a work machine according to the second embodiment of the present invention will be described. Components that are the same as or equivalent to those described in the first embodiment will be given the same reference numerals, and the differences will be mainly explained.

[0103] Figure 6 is a block diagram showing the functions of the control device 200 according to the second embodiment. As shown in Figure 6, the control device 200 includes a pump request flow rate calculation unit 101, an assist request flow rate calculation unit 102, a target flow rate calculation unit 103, an assist actual flow rate calculation unit 205, a boom supply flow rate calculation unit 210, a flow rate difference calculation unit 206, an opening amount calculation unit 107, a correction unit 208, and a valve command unit 109. The control device 100 realizes these functions by executing a program stored in the non-volatile memory 100b.

[0104] The assist actual flow rate calculation unit 205 calculates the assist actual flow rate Qbacc, which is the actual flow rate of hydraulic fluid supplied from the accumulator 9 to the bottom chamber 301 of the boom cylinder 3a through the boom assist valve 19. In this embodiment, the assist actual flow rate calculation unit 205 calculates the assist actual flow rate Qbacc based on the pressure of the accumulator 9 detected by the pressure accumulation sensor 43, the boom operation amount detected by the boom operation amount sensor 31, and the pressure of the bottom chamber 301 of the boom cylinder 3a detected by the bottom pressure sensor 44.

[0105] The assist flow rate calculation unit 205 calculates the differential pressure ΔP across the boom assist valve 19 by subtracting the pressure in the bottom chamber 301 of the boom cylinder 3a, detected by the bottom pressure sensor 44, from the pressure in the accumulator 9, detected by the pressure accumulation sensor 43. Based on the boom operation amount detected by the boom operation amount sensor 31, the assist flow rate calculation unit 205 calculates the opening area Aac of the flow path connecting the accumulator 9 and the bottom chamber 301 of the boom cylinder 3a in the boom assist valve 19. Based on the differential pressure ΔP across the boom assist valve 19 and the opening area Aac of the boom assist valve 19, the assist flow rate Qbac, which is the flow rate passing through the boom assist valve 19, is calculated using the well-known orifice formula.

[0106] Furthermore, the calculation method for the assist flow rate Qbac is not limited to this. When calculating the flow rate through a valve using the orifice equation, the flow coefficient changes depending on the state of the hydraulic fluid flow (temperature, viscosity, etc.), the opening area changes due to fluid force, and parameters (valve opening area, hydraulic fluid density, differential pressure across the valve) change depending on various conditions. In addition, when the load on the boom cylinder 3a changes oscillatingly during operation, the calculated flow rate may be affected by this change. Furthermore, even if pressure sensors are installed before and after the valve, the calculated value of the flow rate through the valve may change due to pressure loss in the oil passage inside the valve and pressure loss in the joints connecting the pressure sensors. For this reason, in order to improve the accuracy of the calculated value of the flow rate through the valve calculated using the orifice equation, it is necessary to investigate the characteristics of each fluctuating parameter under various conditions, which requires a huge amount of development effort, i.e., a huge amount of development cost.

[0107] Therefore, for example, it is preferable that the assist actual flow rate calculation unit 205 calculates the assist actual flow rate Qbacc based on the fundamental thermodynamic equations for adiabatic changes, assuming that the pressure and volume of nitrogen gas in the accumulator 9 change in a reversible adiabatic change. According to this method, the assist actual flow rate calculation unit 205 can calculate the assist actual flow rate Qbacc, which is the flow rate of hydraulic fluid supplied from the accumulator 9 to the boom cylinder 3a through the boom assist valve 19, based on the amount of change per unit time of the accumulator pressure detected by the pressure sensor 43 (the amount of change per unit time of the volume of hydraulic fluid in the accumulator 9). In other words, the assist actual flow rate calculation unit 205 can calculate the assist actual flow rate Qbacc without using the bottom pressure of the boom cylinder 3a, which is affected by load fluctuations.

[0108] Specifically, the assist flow rate Qbacc is calculated by the following equation (1): Qbacc = d / dt(Va0 - Va0 * (Pa0 / Pa)^(1 / κ)) ... (1) The volume Va of nitrogen gas when pressurized oil flows into the accumulator 9 can be calculated by the following equation (2): Va = Va0 - ∫Qbac * dt ... (2) Equation (1) above can be derived by substituting the following basic thermodynamic equation (3) into equation (2) and differentiating the value of the volume of pressurized oil in the accumulator 9 with respect to time: Pa0 * Va0^κ = Pa * Va^κ ... (3) Here, Pa is the pressure of the accumulator 9 and is detected by the pressure sensor 43. Pa0 is the reference pressure of the accumulator 9 and Va0 is the reference volume of the accumulator 9.

[0109] The reference pressure Pa0 is the pressure of the nitrogen gas sealed in the accumulator 9 when nitrogen gas is sealed inside it. In other words, the reference pressure Pa0 is the pressure of the nitrogen gas when there is no oil in the accumulator 9. The reference volume Va0 is the volume of the nitrogen gas sealed inside the accumulator 9 when nitrogen gas is sealed inside it. In other words, the reference volume Va0 is the volume of the nitrogen gas when there is no oil in the accumulator 9. κ is the adiabatic index (polytropic index). The adiabatic index κ is a value that changes depending on the pressure of the accumulator 9 and the inflow and outflow time of the accumulator 9. Note that the usage time of the accumulator 9 used to assist the hydraulic actuator of the hydraulic excavator 1 is short (for example, at most about 6 to 7 seconds). Therefore, the effect of the inflow and outflow time can be ignored. The adiabatic index κ is stored in the non-volatile memory 100b as changing only in response to the pressure of the accumulator 9.

[0110] The boom supply flow rate calculation unit (addition unit) 210 calculates the flow rate Qball supplied to the bottom chamber 301 of the boom cylinder 3a by adding the assist actual flow rate Qbac calculated by the assist actual flow rate calculation unit 205 to the pump request flow rate Qpreq calculated by the pump request flow rate calculation unit 101. As described above, the assist actual flow rate Qbac decreases as the differential pressure across the boom assist valve 19 disappears. Here, we assume that the flow rate of the hydraulic fluid supplied from the hydraulic pump to the boom cylinder 3a does not change, and that the pump request flow rate and the pump actual flow rate always match. As a result, the flow rate Qball, which is the sum of the assist actual flow rate Qbac and the pump request flow rate Qpreq, can be considered as the flow rate of the hydraulic fluid actually supplied to the boom cylinder 3a (actual flow rate Qba) (Qba = Qball = Qpreq + Qbac).

[0111] The flow rate difference calculation unit 206 calculates the flow rate difference ΔQ and the flow rate ratio Rq between the target flow rate Qbt calculated by the target flow rate calculation unit 103 and the actual flow rate Qba calculated by the boom supply flow rate calculation unit 210. The flow rate difference ΔQ is calculated by subtracting the actual flow rate Qba from the target flow rate Qbt (ΔQ = Qbt - Qba). The flow rate ratio Rq is calculated by dividing the actual flow rate Qba by the target flow rate Qbt (Rq = Qba / Qbt). Since the flow rate is proportional to the speed, the flow rate ratio corresponds to the speed ratio Rv described in the first embodiment.

[0112] The speed V of the boom cylinder 3a corresponds to the flow rate Q supplied to the boom cylinder 3a divided by the pressure-receiving area Ar of the boom cylinder 3a (V = Q / Ar). In other words, the speed V of the boom cylinder 3a and the flow rate Q supplied to the boom cylinder 3a have a linear relationship with the pressure-receiving area (cross-sectional area) of the boom cylinder 3a as a coefficient. Therefore, in this control, the swing control valve 13 can be controlled using the boom flow rate without having to calculate the boom speed, similar to the first embodiment.

[0113] The correction unit 208 calculates the corrected target opening area Asts by multiplying the target opening area Ast of the swivel control valve 13, calculated by the opening amount calculation unit 107, by the flow rate ratio Rq (= Rv), calculated by the flow rate difference calculation unit 206 (Asts = Ast・Rq).

[0114] According to this second embodiment, in addition to the same effects and advantages as the first embodiment, the following effects and advantages can be obtained.

[0115] The hydraulic excavator 1 is equipped with a pressure sensor 43 that detects the pressure of the accumulator 9. The control device 100 calculates the actual flow rate Qba of the hydraulic fluid supplied to the boom cylinder 3a based on the pressure of the accumulator 9 detected by the pressure sensor 43. The control device 100 calculates the target flow rate Qbt of the boom cylinder 3a based on the amount of operation of the boom operating device 30. The control device 100 corrects the target opening area of ​​the swing control valve 13 so that the speed ratio between the actual speed and the target speed of the swing motor 4 is equal to the flow rate ratio between the calculated actual flow rate of the boom cylinder 3a and the calculated target flow rate of the boom cylinder 3a.

[0116] With this configuration, the control device 100 can quickly acquire the decrease in the speed of the boom cylinder 3a, and therefore can quickly reduce the speed of the slewing motor 4 when the speed of the boom cylinder 3a decreases. Furthermore, in this second embodiment, the target opening area of ​​the slewing control valve 13 is corrected based on the pressure of the accumulator 9, so there is no need to provide the stroke sensor 56 described in the first embodiment. Accordingly, this second embodiment can reduce the number of parts and costs.

[0117] The following modifications are also within the scope of the present invention, and it is possible to combine one or more of these modifications with the embodiments described above.

[0118] <Modification 1> In the above embodiment, an example was described in which the target opening area Ast is the target value of the cross-sectional area of ​​the flow path (meter-in passage) for the hydraulic fluid flowing from the second hydraulic pump 7 to the slewing motor 4 in the slewing control valve 13. However, the target opening area Ast is not limited to this. The target opening area Ast may be the target value of the cross-sectional area of ​​the flow path (meter-out passage) for the hydraulic fluid flowing from the slewing motor 4 to the tank 12 in the slewing control valve 13, or the target value of the cross-sectional area of ​​the flow path (bleed-off passage) for the hydraulic fluid flowing from the second hydraulic pump 7 to the tank 12. In other words, the target opening area Ast of the slewing control valve 13 may be at least one of the target opening area of ​​the flow path connecting the second hydraulic pump 7 and the slewing motor 4, the target opening area of ​​the flow path connecting the slewing motor 4 and the tank 12, and the target opening area of ​​the flow path connecting the second hydraulic pump 7 and the tank 12.

[0119] The corrected target opening area Asts of the meter-out passage of the swirl control valve 13 is obtained by multiplying the target opening area Ast by the velocity ratio Rv (or flow rate ratio Rq), similar to the embodiment described above. In contrast, the corrected target opening area Asts of the bleed-off passage of the swirl control valve 13 is obtained by multiplying the target opening area Ast by the reciprocal of the velocity ratio Rv (or flow rate ratio Rq).

[0120] <Modification 2> In the embodiment described above, an example was described in which the correction unit 108 always performs a correction process for the target opening area using the velocity ratio Rv (or flow rate ratio Rq). However, the correction process may be performed only when predetermined conditions are met. By defining predetermined conditions, the circumstances under which the correction process is performed can be limited.

[0121] <Modification 2-1> For example, in the first embodiment, the correction unit 108 may determine whether or not to perform the correction process based on the speed ratio Rv. In this case, the correction unit 108 determines whether or not the speed ratio Rv calculated by the speed difference calculation unit 106 is less than a predetermined threshold. If the speed ratio Rv is less than the predetermined threshold, the correction unit 108 performs a correction process for the target aperture area using the speed ratio Rv. If the speed ratio Rv is equal to or greater than the predetermined threshold, the correction unit 108 does not perform the correction process.

[0122] In this modified example, the control device 100 detects that the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a has been interrupted, that is, that the actual speed of the boom cylinder 3a has deviated from the target speed, when the speed ratio Rv (= Vba / Vbt) between the actual speed Vba of the boom cylinder 3a, calculated based on the stroke (displacement) detected by the stroke sensor 56, and the target speed Vbt, calculated based on the boom raising operation amount, falls below a predetermined threshold.

[0123] <Modification 2-2> In Modification 2-1 described above, the feasibility of performing the correction process is determined based on the speed ratio Rv. Therefore, when the boom cylinder 3a is operating at a low speed, there is a risk that the correction process may be performed even if the speed difference ΔV is small. As a result, when the boom cylinder 3a is operating at a low speed, the speed ratio Rv may fall below a predetermined threshold due to a detection error by the stroke sensor 56, and the correction process may be performed unintentionally.

[0124] Therefore, for example, in the first embodiment, the correction unit 108 may determine whether or not to perform the correction process based on the speed difference ΔV. In this case, the correction unit 108 determines whether or not the speed difference ΔV calculated by the speed difference calculation unit 106 is greater than or equal to a predetermined threshold. If the speed difference ΔV is greater than or equal to the predetermined threshold, the correction unit 108 performs a correction process for the target aperture area using the speed ratio Rv. If the speed difference ΔV is less than the predetermined threshold, the correction unit 108 does not perform the correction process.

[0125] In this modified example, the control device 100 detects that the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a has been interrupted, that is, that the actual speed of the boom cylinder 3a has deviated from the target speed, when the speed difference ΔV (= Vbt - Vba) between the actual speed Vba of the boom cylinder 3a, calculated based on the stroke (displacement) detected by the stroke sensor 56, and the target speed Vbt, calculated based on the boom raising operation amount, exceeds a predetermined threshold.

[0126] This configuration prevents the correction process from being performed more than necessary.

[0127] <Modification 2-3> In the second embodiment, the correction unit 208 may determine whether or not to perform the correction process based on the flow rate ratio Rq or the flow rate difference ΔQ. The determination of whether or not to perform the correction process based on the flow rate ratio Rq is the same as the determination of whether or not to perform the correction process based on the speed ratio Vq (Modification 2-1), and the determination of whether or not to perform the correction process based on the flow rate difference ΔQ is the same as the determination of whether or not to perform the correction process based on the speed difference ΔV (Modification 2-2), so a detailed explanation will be omitted.

[0128] <Modification 2-4> In the first embodiment, the correction unit 108 may determine whether or not to perform the correction process based on the accumulator pressure. In this case, if the accumulator pressure detected by the pressure sensor 43 is less than a predetermined threshold (for example, the minimum operating pressure), the correction unit 108 performs a correction process for the target opening area using the speed ratio Rv. If the accumulator pressure is above the predetermined threshold, the correction unit 108 does not perform the correction process.

[0129] In this modified example, the control device 100 detects when the pressure of the accumulator 9 detected by the pressure sensor 43 falls below a predetermined threshold that the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a has been interrupted, meaning that the actual speed of the boom cylinder 3a has deviated from the target speed.

[0130] <Modification 2-5> In the second embodiment, the correction unit 208 may determine whether or not to perform the correction process based on the accumulator pressure. In this case, if the accumulator pressure detected by the pressure sensor 43 is less than a predetermined threshold (for example, the minimum operating pressure), the correction unit 208 performs a correction process for the target opening area using the flow rate ratio Rq. If the accumulator pressure is above the predetermined threshold, the correction unit 208 does not perform the correction process.

[0131] In this modified example, the control device 200 detects that the supply of hydraulic fluid from the accumulator 9 to the boom cylinder 3a has been interrupted, i.e., that the actual speed of the boom cylinder 3a has deviated from the target speed, when the pressure of the accumulator 9 detected by the pressure sensor 43 falls below a predetermined threshold. According to this modified example 2-5, the correction process and the determination of whether or not to perform the correction process are performed based solely on the accumulator pressure, thus simplifying the control compared to modified example 2-4. Also, similar to the second embodiment described above, the stroke sensor 56 can be omitted, reducing the number of parts and costs.

[0132] <Modification 3> In the above embodiment, a hydraulic system was described in which the boom cylinder 3a is driven by hydraulic fluid discharged from hydraulic pumps (6, 7) and accumulator 9, but the present invention is not limited thereto. If the hydraulic excavator 1 is small, the boom cylinder 3a may be configured to be driven only by hydraulic fluid discharged from accumulator 9.

[0133] <Modification 4> In the first embodiment, an example was described in which the actual speed of the boom cylinder 3a is calculated based on the detection result of the stroke sensor 56, but the present invention is not limited thereto.

[0134] <Modification 4-1> For example, as shown in Figure 7, an acceleration sensor 71 may be provided on the hydraulic excavator 1 instead of the stroke sensor 56. The acceleration sensor 71 is provided, for example, on the piston rod of the boom cylinder 3a and detects the acceleration of the piston rod as information regarding the actual speed of the boom cylinder 3a. In this case, the actual speed calculation unit 105 calculates the actual speed of the boom cylinder 3a by integrating the acceleration (information) of the boom cylinder 3a detected by the acceleration sensor 71 with respect to time.

[0135] <Modification 4-2> Alternatively, as shown in Figure 7, for example, a posture sensor 72 may be provided on the hydraulic excavator 1 instead of the stroke sensor 56. The posture sensor 72 is, for example, a boom posture sensor attached to the boom 1a to detect the posture of the boom 1a. The posture of the boom 1a is, for example, the rotation angle of the boom (driven target member driven by the first hydraulic actuator) 1a with respect to the slewing body 1d (hereinafter, "boom angle"). The posture sensor 72 is, for example, an angle sensor such as a rotary potentiometer. In this case, the actual speed calculation unit 105 calculates the speed of the boom cylinder 3a based on the boom angle detected by the posture sensor (angle sensor) 72. The actual speed calculation unit 105 converts the boom angle to the position of the piston rod of the boom cylinder 3a using a conversion map, for example, and calculates the speed of the boom cylinder 3a based on the time rate of change of that position. The conversion map is stored in advance in the non-volatile memory 100b. The attitude sensor 72 may also be an IMU (Inertial Measurement Unit).

[0136] <Modification 5> In the above embodiment, an example was described in which the hydraulic actuator to be assisted (the first hydraulic actuator driven by hydraulic fluid supplied from the accumulator 9) is the boom cylinder 3a, and the hydraulic actuator to be subject to deceleration control when the assistance to the first hydraulic actuator is interrupted (the second hydraulic actuator driven by hydraulic fluid supplied from the hydraulic pump) is the slewing motor 4. However, the first hydraulic actuator and the second hydraulic actuator are not limited to the above example. For example, the first hydraulic actuator (the hydraulic actuator to be assisted) may be the slewing motor 4, and the second hydraulic actuator (the hydraulic actuator to be subject to deceleration control) may be the boom cylinder 3a. The hydraulic actuator to be assisted (the first hydraulic actuator) may be any of the boom cylinder 3a, arm cylinder 3b, bucket cylinder 3c, and slewing motor 4, and the hydraulic actuator to be subject to deceleration control (the second hydraulic actuator) may be any hydraulic actuator other than the hydraulic actuator to be assisted among the boom cylinder 3a, arm cylinder 3b, bucket cylinder 3c, and slewing motor 4.

[0137] <Modification 6> In the above embodiment, an example was described in which the flow control valves (13-18) are open-center directional control valves, but the present invention is not limited thereto. The present invention may also be applied to a hydraulic system (closed-center system) in which the flow control valves (13-18) are closed-center directional control valves.

[0138] <Modification 7> In the above embodiment, an example was described in which the return oil from the boom cylinder 3a is regenerated by the accumulator 9, but the return oil from other hydraulic actuators may also be regenerated by the accumulator 9.

[0139] <Modification 8> The work machine is not limited to a crawler-type hydraulic excavator 1. The present invention may also be applied to a wheeled hydraulic excavator. Furthermore, the work tool may be a grapple, breaker, lifting magnet, etc., instead of a bucket.

[0140] <Modification 9> More than one accumulator may be provided. The hydraulic pump that supplies hydraulic fluid to the hydraulic actuator may be configured to have only one unit, or to have three or more units.

[0141] <Modification 10> 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.

[0142] Although embodiments of the present invention have been described above, these embodiments only represent a part of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments.

[0143] 1... Hydraulic excavator (working machine), 1a... Boom (driven component), 1b... Arm (driven component), 1c... Bucket (driven component), 1d... Slewing body (driven component), 1e... Traveling body, 3... Boom cylinder (example of first hydraulic actuator), 3b... Arm cylinder (hydraulic actuator), 3c... Bucket cylinder (hydraulic actuator), 3e... Travel motor (hydraulic actuator), 4... Slewing motor (second 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 accumulation device), 12... Tank, 13... Swivel control valve (second flow control valve), 14... First boom control valve (third flow control valve), 15... Second boom control valve (third 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 (first flow control valve), 20... Arm assist valve (flow control valve), 21... Bucket assist valve (flow control valve), 22... Swivel assist valve (flow control valve), 23,24... Regulator, 30... Boom operating device (first operating device), 31... Boom operating amount sensor (operating amount sensor), 34... Pressure boosting valve, 43... Accumulative pressure sensor (pressure sensor), 44... Bottom pressure sensor (pressure sensor), 45... Arm operating device, 46... Operating amount sensor, 47... Bucket operating device, 48... Operating amount sensor, 49... Swivel operating device (second operating device), 50... Operating amount sensor, 51... Working device, 52... Vehicle body, 56... Stroke sensor (sensor), 71... Acceleration sensor (sensor), 72... Attitude sensor (angle sensor, sensor), 100... Control device, 100a... Processor (processing device), 100b... Non-volatile memo Ri (memory device), 100c...Volatile memory (memory device), 101...Pump request flow rate calculation unit, 102...Assist request flow rate calculation unit, 103...Target flow rate calculation unit (addition unit), 104...Target speed calculation unit, 105...Actual speed calculation unit, 106...Speed ​​difference calculation unit, 107...Opening amount calculation unit, 108...Correction unit, 109...Valve command unit, 200...Control device, 205...Assist actual flow rate calculation unit, 206...Flow rate difference calculation unit, 208...Correction unit, 210...Boom supply flow rate calculation unit (addition unit), 301...Bottom chamber, 302...Rod chamber, Ast...Target opening area, Asts...Corrected target opening area (corrected target opening area), Pamin...Minimum operating pressure,

Claims

1. A work machine that performs work using a work device comprising: a prime mover; a hydraulic pump driven by the prime mover; a pressure accumulator for storing hydraulic fluid; a first hydraulic actuator driven by hydraulic fluid supplied from the pressure accumulator; a first flow control valve for controlling the flow rate of hydraulic fluid supplied from the pressure accumulator to the first hydraulic actuator; a second hydraulic actuator driven by hydraulic fluid supplied from the hydraulic pump; a second flow control valve for controlling the flow rate of hydraulic fluid supplied from the hydraulic pump to the second hydraulic actuator; a first operating device for operating the first hydraulic actuator; a second operating device for operating the second hydraulic actuator; and a control device that controls the first flow control valve so that the first hydraulic actuator operates at a target speed corresponding to the amount of operation of the first operating device, and controls the second flow control valve so that the second hydraulic actuator operates at a target speed corresponding to the amount of operation of the second operating device, wherein the work machine performs work using a work device having at least one of the first hydraulic actuator and the second hydraulic actuator. The control device controls the second flow control valve so that when the pressure in the accumulator decreases and the actual speed of the first hydraulic actuator decreases relative to the target speed of the first hydraulic actuator corresponding to the amount of operation of the first operating device, the actual speed of the second hydraulic actuator decreases relative to the target speed corresponding to the amount of operation of the second operating device.

2. A work machine according to claim 1, comprising a slewing body to which the work device is attached, wherein the work device comprises a boom rotatably attached to the slewing body, an arm rotatably attached to the boom, and a work tool attached to the arm, the first hydraulic actuator being a boom cylinder for driving the boom, and the second hydraulic actuator being a slewing motor for driving the slewing body.

3. A work machine according to claim 1, comprising a third flow control valve for controlling the flow rate of hydraulic fluid supplied from the hydraulic pump to the first hydraulic actuator, wherein the first hydraulic actuator is driven by hydraulic fluid supplied from the pressure accumulator through the first flow control valve and hydraulic fluid supplied from the hydraulic pump through the third flow control valve, and the control device controls the first flow control valve and the third flow control valve so that the first hydraulic actuator operates at a target speed corresponding to the amount of operation of the first operating device.

4. A work machine according to claim 1, wherein the control device controls the second flow control valve such that, when the flow rate of hydraulic fluid supplied from the pressure accumulator to the first hydraulic actuator decreases, the speed ratio between the speed of the first hydraulic actuator and the speed of the second hydraulic actuator is the same as the speed ratio before the flow rate of hydraulic fluid supplied from the pressure accumulator to the first hydraulic actuator decreased.

5. The work machine according to claim 4, wherein the control device calculates a target opening area of ​​the second flow control valve based on the amount of operation of the second operating device, corrects the target opening area of ​​the second flow control valve so that the speed ratio of the actual speed to the target speed of the second hydraulic actuator becomes the speed ratio of the actual speed to the target speed of the first hydraulic actuator, and controls the second flow control valve so that the opening area of ​​the second flow control valve becomes the corrected target opening area.

6. The work machine according to claim 5, characterized in that the target opening area of ​​the second flow control valve is at least one of the target opening area of ​​the flow path connecting the hydraulic pump and the second hydraulic actuator, the target opening area of ​​the flow path connecting the second hydraulic actuator and the tank, and the target opening area of ​​the flow path connecting the hydraulic pump and the tank.

7. A work machine according to claim 5, comprising a sensor for detecting information relating to the actual speed of the first hydraulic actuator, wherein the control device calculates the actual speed of the first hydraulic actuator based on the information detected by the sensor, calculates the target speed of the first hydraulic actuator based on the amount of operation of the first operating device, and corrects the target opening area of ​​the second flow control valve such that the speed ratio between the actual speed and the target speed of the second hydraulic actuator is the speed ratio between the calculated actual speed of the first hydraulic actuator and the calculated target speed of the first hydraulic actuator.

8. The work machine according to claim 7, characterized in that the information relating to the actual speed of the first hydraulic actuator detected by the sensor is the displacement of the first hydraulic actuator.

9. The work machine according to claim 7, characterized in that the information relating to the actual speed of the first hydraulic actuator detected by the sensor is the acceleration of the first hydraulic actuator.

10. The work machine according to claim 7, wherein the information relating to the actual speed of the first hydraulic actuator detected by the sensor is the rotation angle of the driven member driven by the first hydraulic actuator.

11. A work machine according to claim 5, comprising a pressure sensor for detecting the pressure of the pressure accumulator, wherein the control device calculates the actual flow rate of hydraulic fluid supplied to the first hydraulic actuator based on the pressure of the pressure accumulator detected by the pressure sensor, calculates the target flow rate of the first hydraulic actuator based on the amount of operation of the first operating device, and corrects the target opening area of ​​the second flow control valve such that the speed ratio between the actual speed and the target speed of the second hydraulic actuator is the flow rate ratio between the calculated actual flow rate of the first hydraulic actuator and the calculated target flow rate of the first hydraulic actuator.

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