Power regeneration device for working machine and working machine

Abstract

In a lowering operation of a boom, the amount of operation of a control lever is detected by a pressure sensor and input to a controller. Based on the input operation amount, the controller obtains a target flow rate Q.sub.0 of return oil discharged from a boom cylinder, calculates a deviation Q between the target flow rate Q.sub.0 and an actual flow rate Q obtained from an actual rotation speed N of an electric motor acquired by a rotation speed sensor, calculates a signal Sm for controlling the opening area of a proportional solenoid valve in a manner allowing hydraulic fluid to flow to a control valve in just as much as Q, and controls an operation pilot pressure of the control valve supplied from a sub-pump in accordance with the signal Sm so that the hydraulic fluid will flow to the control valve exactly in the amount of Q.

Claims

1. A power regeneration device for a working machine equipped with a hydraulic actuator for driving a work device, a control valve for operating and controlling the hydraulic actuator, and a control lever device with a control lever for operating the control valve to activate the hydraulic actuator, the power regeneration device comprising: a hydraulic motor driven by return oil from the hydraulic actuator; an electric motor connected mechanically to the hydraulic motor and driven thereby to generate electric power; an inverter which controls the rotation speed of the electric motor, and an electric storage device which stores the electric power generated by the electric motor; wherein the return oil discharged from the hydraulic actuator is branched and distributed to the side of the control valve and that of the hydraulic motor, the power regeneration device further comprising: a rotation speed detector which detects an actual rotation speed of the electric motor; an operation amount detector which detects the amount of operation of the control lever; a proportional solenoid valve which adjusts the opening area of the control valve, and a control device to which the rotation speed detected by the rotation speed detector and the operation amount detected by the operation amount detector are input; wherein the control device obtains a target flow rate of the return oil discharged from the hydraulic actuator and a target rotation speed of the electric motor based on the operation amount to control the rotation speed of the electric motor via the inverter in a manner attaining the target rotation speed of the electric motor, and obtains a deviation between the target flow rate and the actual flow rate of hydraulic fluid passing through the electric motor based on the target flow rate and on the actual rotation speed of the electric motor detected by the rotation speed detector, and controls the proportional solenoid valve based on the deviation obtained.

2. The power regeneration device for a working machine according to claim 1, wherein the control device includes: a target flow rate calculation unit which receives the operation amount and obtains the target flow rate based on the received operation amount; a target rotation speed calculation unit which obtains the target rotation speed from the target flow rate obtained; an electric motor command value calculation unit which obtains an inverter control signal for the inverter from the target rotation speed obtained; an actual flow rate calculation unit which receives the actual rotation speed and obtains the actual flow rate based on the received actual rotation speed; a control valve target flow rate calculation unit (35) which obtains the deviation from the actual flow rate and the target flow rate and provides the deviation obtained as a target flow rate for the control valve; and a proportional solenoid valve command value calculation unit which obtains a control signal for the proportional solenoid valve from the control valve target flow rate obtained.

3. The power regeneration device for a working machine according to claim 1, wherein the control device includes: a target flow rate calculation unit which receives the operation amount and obtains the target flow rate based on the received operation amount; a target rotation speed calculation unit which obtains the target rotation speed from the target flow rate obtained; an electric motor command value calculation unit which obtains an inverter control signal for the inverter from the target rotation speed obtained; a control valve target flow rate calculation unit which receives the actual rotation speed, obtains a deviation between the target flow rate and the actual flow rate from a deviation between the target rotation speed obtained by the target rotation speed calculation unit and the actual rotation speed, and provides the deviation obtained as a target flow rate for the control valve; and a proportional solenoid valve command value calculation unit which obtains a control signal for the proportional solenoid valve from the control valve target flow rate obtained.

4. The power regeneration device for a working machine according to claim 1, further comprising an on-off valve which is connected in parallel with the control valve, and interposed between the hydraulic pump and the hydraulic fluid supply side of the hydraulic actuator and which is switched to the opened position when the control lever of the control lever device is operated.

5. A working machine furnished with a power regeneration device for a working machine according to claim 1.

6. The power regeneration device for a working machine according to claim 2, further comprising an on-off valve which is connected in parallel with the control valve, and interposed between the hydraulic pump and the hydraulic fluid supply side of the hydraulic actuator and which is switched to the opened position when the control lever of the control lever device is operated.

7. The power regeneration device for a working machine according to claim 3, further comprising an on-off valve which is connected in parallel with the control valve, and interposed between the hydraulic pump and the hydraulic fluid supply side of the hydraulic actuator and which is switched to the opened position when the control lever of the control lever device is operated.

8. A working machine furnished with a power regeneration device for a working machine according to claim 2.

9. A working machine furnished with a power regeneration device for a working machine according to claim 3.

10. A working machine furnished with a power regeneration device for a working machine according to claim 4.

11. A working machine furnished with a power regeneration device for a working machine according to claim 5.

12. A working machine furnished with a power regeneration device for a working machine according to claim 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an external view of a hybrid hydraulic excavator embodying the present invention.

(2) FIG. 2 is a schematic view showing part of a drive control system of the hydraulic excavator as a first embodiment of the present invention.

(3) FIG. 3 is a block diagram showing a typical structure of a controller 9 associated with the first embodiment of the invention.

(4) FIG. 4 is an illustration depicting the relationship between a target flow rate Q.sub.0 and a target rotation speed N.sub.0, stored in a target rotation speed calculation unit 32 associated with the first embodiment of the invention.

(5) FIG. 5 is a block diagram showing an alternative structure of the controller 9 associated with the first embodiment of the invention.

(6) FIG. 6 is an illustration depicting the relationship between an actual flow rate Q and the target flow rate Q.sub.0, relative to an operation start time at which a control lever 4a starts to be operated on the first embodiment of the invention.

(7) FIG. 7 is a schematic view showing part of a drive control system of a hydraulic excavator as a second embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

First Embodiment

(8) The first embodiment of the present invention is described below using the accompanying drawings. FIG. 1 is an external view of a hydraulic excavator (working machine) on which the hydraulic system of the present invention is mounted.

(9) The hydraulic excavator is made up of a lower travel structure 100, an upper swing structure 101, and a front work implement 102.

(10) The lower travel structure 100 possesses left-hand and right-hand crawler type travel devices 103a and 103b driven by left-hand and right-hand travel motors 104a and 104b respectively. The upper swing structure 101 is mounted swingably on the lower travel structure 100 and driven swingably by a swing motor (not shown). The front work implement 102 is attached elevatably to the front of the upper swing structure 101. The upper swing structure 101 is equipped with an engine room 106 and a cabin (cab) 107. The engine room 106 accommodates an engine E (to be discussed later) and such hydraulic devices as a hydraulic pump 1 and a sub-pump 8 (see FIG. 2), and the cabin 107 holds a control lever device 4 (see FIG. 2) and others. The front work implement 102 has an articulated structure equipped with a boom 111, an arm 112, and a bucket 113. The boom 111 is turned up and down by extension and contraction of a boom cylinder 3, the arm 112 is turned up and down and back and forth by extension and contraction of an arm cylinder 114, and the bucket 113 is turned up and down as well as back and forth by extension and contraction of a bucket cylinder 115.

(11) FIG. 2 shows a hydraulic circuit portion for driving the boom cylinder 3 and a power regeneration device built in that hydraulic circuit portion as part of the drive control system of the hydraulic excavator embodying the present invention. The same components as those in the preceding drawing are designated by the same reference numerals, and their explanations are omitted (the same also applies to the subsequent drawings).

(12) In FIG. 2, the drive control system is made up of the hydraulic pump 1 and sub-pump 8 which are driven by the engine E, a control valve 2, the boom cylinder 3, the control lever device 4, make-up valves (supplementary valves) 22a and 22b, and a power regeneration device 19.

(13) The hydraulic pump 1 is a main pump that supplies hydraulic fluid to the boom cylinder 3. The hydraulic line connected to the hydraulic ump 1 is fitted with a relief valve, not shown, that releases the hydraulic fluid into a tank 18 to avoid an excess buildup of the pressure inside the hydraulic line if it rises inordinately. The control valve 2 is connected to a bottom-side hydraulic chamber 3a and a rod-side hydraulic chamber 3b of the boom cylinder 3 via lines 6a and 6b. The hydraulic fluid from the hydraulic pump 1 is supplied to the bottom-side hydraulic chamber 3a or rod-side hydraulic chamber 3b of the boom cylinder through the line 6a or 6b via the control valve 2. Also, the return oil from the rod-side hydraulic chamber 3b of the boom cylinder 3 is recirculated to the tank 18 via the line 6b and control valve 2. The return oil from the bottom-side hydraulic chamber 3a is recirculated to the tank 18 partly through the line 6a and control valve 2 and mostly via a regeneration circuit 21 of the power regeneration device 19. In the ensuing description, the line 6a will be referred to as the bottom-side line and the line 6b as the rod-side line.

(14) The control lever device 4 is furnished with the control lever 4a and pilot valves (reducing valves) 4b1 and 4b2. When the control lever 4a is tilted in the direction a in the drawing (boom raising operation), the pilot valve 4b1 outputs to a pilot hydraulic line 5a a pilot pressure (hydraulic signal of pressure Pa) corresponding to the amount of operation of the control lever 4a relative to the discharge pressure of the sub-pump 8 as the source pressure. When the control lever 4a is tilted in the direction b in the drawing (operation to lower the boom cylinder 3), the pilot valve 4b2 outputs to a pilot hydraulic line 5b a pilot pressure (hydraulic signal of pressure Pb) corresponding to the operation amount of the control lever 4a relative to the discharge pressure of the sub-pump 8 as the source pressure.

(15) The control valve 2 possesses operation ports 2a and 2b. The operation port 2a is connected to the pilot valve 4b1 via the pilot hydraulic line 5a, and the operation port 2b is connected to a proportional solenoid valve 7 (to be discussed later) via a pilot hydraulic line 5c. In response to the pilot pressure (hydraulic signal) output to the pilot hydraulic lines 5a and 5c, control operations are carried out to switch the spool position of the control valve 2, thereby controlling the direction and the flow rate of the hydraulic fluid supplied to the boom cylinder 3.

(16) The make-up valves 22a and 22b are provided to prevent the lines 6a and 6b from developing a negative pressure causing cavitation. When the pressure in the line 6a or 6b drops below the pressure in the tank 18, the make-up valve 22a or 22b opens to feed the hydraulic fluid to the line 6a or 6b. The make-up valve 22b also performs the role of supplying the rod-side hydraulic chamber 3b of the boom cylinder 3 with the hydraulic fluid from the tank 18 in the lowering operation of the boom 111.

(17) The power regeneration device 19 is made up of a line 6d, a pilot check valve 10, a fixed displacement hydraulic motor 11, an electric motor 12, an inverter 13, a chopper 14, an electric storage device (battery) 15, a pressure sensor 16, a rotation speed sensor 17, a proportional solenoid valve 7, and a controller (control device) 9.

(18) The line 6d branches from a branching portion 6c of the bottom-side line 6a. The hydraulic motor 11 is connected to the line 6d via the pilot check valve 10 to constitute the regeneration circuit 21. In the lowering operation of the boom 111, the return oil discharged from the bottom-side hydraulic chamber 3a of the boom cylinder 3 is led to the hydraulic motor 11 via the pilot check valve 10 to rotate the hydraulic motor 11, the return oil being recirculated thereafter to the tank 18.

(19) The pilot check valve 10 is provided to prevent unnecessary flow of the hydraulic fluid from the bottom-side line 6a to the regeneration circuit 21 (line 6d) (causing the boom to fall), such as by preventing leaks of the hydraulic pressure into the regeneration circuit 21. Usually, the pilot check valve 10 keeps the regeneration circuit 21 isolated. When the operator performs an operation to lower the boom 111 (by tilting the control lever 4a of the control lever device 4 to the b side in FIG. 2), the pilot pressure (hydraulic signal of hydraulic pressure Pb) output from the pilot valve 4b2 is led to the pilot check valve 10 via the pilot hydraulic line 5b. The pilot pressure opens the pilot check valve 10 that in turn opens the regeneration circuit 21.

(20) The electric motor 12 is coupled to the hydraulic motor 11 that generates electric power when the hydraulic motor 11 rotates. The generated electric power is stored into the electric storage device (battery) 15 via the inverter 13 and the chopper 14. The chopper 14 is a boost chopper.

(21) The rotation speed sensor 17 is attached to the shaft coupling the hydraulic motor 11 with the electric motor 12. The rotation speed sensor 17 detects the rotation speed N (actual rotation speed) of the hydraulic motor 11 and electric motor 12.

(22) The pressure sensor 16 is connected to the pilot hydraulic line 5b and detects the pilot pressure Pb output from the pilot valve 4b2 to the line 5b in the lowering operation of the boom 111. The pressure sensor 16 and rotation speed sensor 17 are connected to the controller 9, and convert the detected pilot pressure Pb and rotation speed N into electric signals that are input to the controller 9. Alternatively, the pressure sensor 16 may be replaced with a position sensor that detects the position of the control lever 4a.

(23) The controller 9 accepts detection signals from the pressure sensor 16 and rotation speed sensor 17 to perform predetermined calculations, and outputs control signals accordingly to the proportional solenoid valve 7 and inverter 13.

(24) The proportional solenoid valve 7 is activated by a control signal from the controller 9. Relative to the delivery pressure of the sub-pump 8 as the source pressure, the proportional solenoid valve 7 generates a pilot pressure designated by the control signal in question and outputs the generated pilot pressure to the pilot hydraulic line 5c. The pilot pressure output to the pilot hydraulic line 5c is led to the operation port 2b of the control valve 2. The opening area of the control valve 2 is adjusted in response to the pilot pressure.

(25) The control functions provided by the controller 9 are explained below with reference to FIG. 3. FIG. 3 is a block diagram depicting the control functions of the controller 9.

(26) As shown in FIG. 3, the controller 9 has the functions represented by a target flow rate calculation unit 31, a target rotation speed calculation unit 32, an electric motor command value calculation unit 33, an actual flow rate calculation unit 34, a control valve target flow rate calculation unit 35, and a proportional solenoid valve command value calculation unit 36.

(27) The target flow rate calculation unit 31 is a part that calculates a target flow rate Q.sub.0 of the return oil discharged from the bottom-side hydraulic chamber 3a of the boom cylinder 3 based on the operation amount (magnitude of pilot pressure Pb) in the boom lowering direction of the control lever 4a (b side in FIG. 2). Generally, the operation amount of the control lever 4a in the boom lowering direction (b side in FIG. 2) designates the target speed of lowering of the boom 111. Given the target speed of lowering of the boom 111, the target flow rate calculation unit 31 obtains the target flow rate Q.sub.0 of the return oil discharged from the bottom-side hydraulic chamber 3a of the boom cylinder 3. The target flow rate Q.sub.0 calculated by the target flow rate calculation unit 31 is output to the target rotation speed calculation unit 32 and control valve target flow rate calculation unit 35.

(28) The target rotation speed calculation unit 32 is a part that obtains as a target rotation speed N.sub.0 the rotation speed of the hydraulic motor 11 in effect when the entire target flow rate Q.sub.0 calculated by the target flow rate calculation unit 31 passes through the hydraulic motor 11. In this case, Q.sub.0 is related to N.sub.0 in such a manner that Q.sub.0=qN.sub.0, where q denotes the delivery capacity of the hydraulic motor 11. Since the hydraulic motor 11 is a fixed displacement type, the capacity q is a known quantity. As shown in FIG. 4, Q.sub.0 and N.sub.0 are in a proportional relationship in which the target rotation speed N.sub.0 increases simply in proportion to the increasing target flow rate Q.sub.0. The target rotation speed N.sub.0 calculated by the target rotation speed calculation unit 32 is output to the electric motor command value calculation unit 33.

(29) The electric motor command value calculation unit 33 is a part that calculates a power generation control command value Sg for rotating the electric motor 12 in a manner that attains the target rotation speed N.sub.0 calculated by the target rotation speed calculation unit 32. The command value Sg in question is output to the inverter 13. Based on the input command value Sg, the inverter 13 controls the electric motor 12 in power generation so that the rotation speed of the electric motor 12 and hydraulic motor 11 reaches the target rotation speed N.sub.0.

(30) The actual flow rate calculation unit 34 is a part that calculates the actual flow rate (passing flow rate) Q through the hydraulic motor 11 from the actual rotation speed N of the electric motor 12 detected by the rotation speed sensor 17. As with the foregoing relation between Q.sub.0 and N.sub.0, Q is related to N so that Q=qN, where q is a known quantity. Thus when N is known, Q can be obtained. The actual flow rate Q calculated by the actual flow rate calculation unit 34 is output to the control valve target flow rate calculation unit 35.

(31) The control valve target flow rate calculation unit 35 is a part that obtains a deviation Q between the target flow rate Q.sub.0 calculated by the target flow rate calculation unit 31 and the actual flow rate Q calculated by the actual flow rate calculation unit 34. The deviation Q represents an insufficient rate of flow which falls short of the target flow rate Q0 and which fails to reach the side of the hydraulic motor 11. As such, the deviation Q is a meter-out flow rate (control valve target flow rate) that should flow through the control valve 2. The flow rate deviation Q calculated by the control valve target flow rate calculation unit 35 is output to the proportional solenoid valve command value calculation unit 36 as the control valve target flow rate Q.

(32) The proportional solenoid valve command value calculation unit 36 is a part that calculates a command value Sm for controlling the opening area of the proportional solenoid valve 7 to introduce the pilot pressure into the operation portion 2b of the control valve 2 in such a manner that the hydraulic fluid is allowed to flow through the control valve 2 in just as much as the control valve target flow rate Q calculated by the control valve target flow rate calculation unit 35. The command value Sm in question is output to the proportional solenoid valve 7.

(33) Incidentally, there may be provided beforehand a table that defines the relationship between the operation amount of the control lever 4a and the target flow rate Q.sub.0, the relationship between the target flow rate Q.sub.0 and the target rotation speed N.sub.0, the relationship between the target rotation speed N.sub.0 and the power generation control command value Sg, the relationship between the actual rotation speed N and the actual flow rate Q, and the relationship between the control valve target flow rate Q and the opening area of the control valve 2, the values being calculated by the respective calculation units.

(34) In FIG. 3, the target flow rate calculation unit 31 obtains the target flow rate Q.sub.0 of the hydraulic motor 11; the actual flow rate calculation unit 34 obtains the actual flow rate Q of the hydraulic motor 11; and the control valve target flow rate calculation unit 35 calculates the deviation Q between the target flow rate Q.sub.0 and the actual flow rate Q and uses the calculated deviation as the control valve target flow rate Q. Alternatively, the control valve target flow rate Q may be obtained from N.sub.0 acquired by the target rotation speed calculation unit 32 and from N detected by the rotation speed sensor 17.

(35) This alternative example is shown in FIG. 5. The target rotation speed N.sub.0 calculated by the target rotation speed calculation unit 32 is output to the electric motor command value calculation unit 33 and to a control valve target flow rate calculation unit 35A. From the target rotation speed N.sub.0 and from the actual rotation speed N of the electric motor 12 detected by the rotation speed sensor 17, the control valve target flow rate calculation unit 35A calculates Q=q(N.sub.0N) to obtain the flow rate deviation Q. The control valve target flow rate calculation unit 35A outputs this flow rate deviation Q to the proportional solenoid valve command value calculation unit 36 as the control valve target flow rate.

(36) The movements of this embodiment are explained next.

(37) The raising operation of the boom 111 (extension of the boom cylinder 3) is explained first.

(38) When the control lever 4a is operated toward the a side in FIG. 2, the pilot pressure Pa is transmitted from the pilot valve 4b1 to the operation port 2a of the control valve 2 via the pilot hydraulic line 5a. This switches the control valve 2 to feed the hydraulic fluid from the hydraulic pump 1 to the bottom-side hydraulic chamber 3a of the boom cylinder 3 via the bottom-side line 6a so that the boom cylinder 3 is extended (the boom 111 is turned upward). At the same time, the return oil discharged from the rod-side hydraulic chamber 3b of the boom cylinder 3 is recirculated to the tank 18 via the rod-side line 6b and control valve 2. At this point, no operating pilot pressure is led to the pilot check valve 10 so that the regeneration circuit 21 of the power regeneration device 19 attached to the bottom-side line 6a is in an isolated state and does not perform regeneration operation.

(39) The lowering operation of the boom 111 (contraction of the boom cylinder 3) is explained next.

(40) When the control lever 4a is operated toward the b side in FIG. 2, the pilot pressure Pb from the pilot valve 4b2 is led to the pilot check valve 10 via the pilot hydraulic line 5b, causing the pilot check valve 10 to open.

(41) At this point, the deadweight of the front work implement 102 including the boom 111 pushes the boom cylinder 3 to discharge the hydraulic fluid within the bottom-side hydraulic chamber 3a of the boom cylinder 3 into the line 6a. Because the pilot check valve 10 is currently open, the regeneration circuit 21 of the power regeneration device 19 is held open. The discharged hydraulic fluid is evacuated into the tank 18 via the line 6d and pilot check valve 10 past the hydraulic motor 11.

(42) Also, the hydraulic fluid is supplied from the tank 18 to the rod-side hydraulic chamber 3b of the boom cylinder 3 via the make-up valve 22b so as to prevent a negative pressure from developing inside the rod-side line 6b when the boom cylinder 3 is pushed by the deadweight of the front work implement 102.

(43) This causes the boom cylinder 3 to contract and the boom 111 to start being lowered.

(44) The hydraulic motor 11 is rotated by the return oil flowing thereto. The electric motor 12 coupled directly to the hydraulic Motor 11 is thus rotated to perform a power generation operation. The generated electric energy is stored into the battery 15, whereby the power regeneration operation is carried out.

(45) At the same time, an electric signal corresponding to the pilot pressure Pb is input to the controller 9. Based on the operation amount of the control lever 4a thus input, the target flow rate calculation unit 31 calculates the target flow rate Q.sub.0 of the hydraulic motor 11. The target rotation speed calculation unit 32 calculates the target rotation speed N.sub.0 of the electric motor 12 from the target flow rate Q.sub.0. The electric motor command value calculation unit 33 calculates the power generation control command value Sg to the inverter 13 from the target rotation speed N.sub.0. Given the input actual rotation speed N of the hydraulic motor 11, the actual flow rate calculation unit 34 calculates the actual flow rate Q flowing through the hydraulic motor 11. The control valve target flow rate calculation unit 35 calculates an insufficient flow rate Q from the target flow rate Q.sub.0 and actual flow rate Q. Thereafter, given the insufficient flow rate Q, the proportional solenoid valve command value calculation unit 36 calculates the command value Sm for controlling the opening area of the proportional solenoid valve 7.

(46) The control command value Sm is output to the proportional solenoid valve 7. Based on the input control command value Sm, the proportional solenoid valve 7 has its opening area adjusted to control the operation pilot pressure supplied from the sub-pump 8. Controlled as desired in this manner, the operation pilot pressure is led to the operation port 2b of the control valve 2 via the pilot hydraulic line 5c. The hydraulic fluid is controlled to flow to the control valve 2 just in the amount of Q. The hydraulic fluid in the amount of Q is therefore supplied from the hydraulic pump 1 to the rod-side hydraulic chamber 3b of the boom cylinder 3, and the hydraulic fluid in the amount of Q from the bottom-side hydraulic chamber 3a of the boom cylinder 3 is discharged into the tank 18 via the control valve 2.

(47) At the same time, the power generation control command value Sg is output to the inverter 13. Based on the input power generation control command value Sg, the inverter 13 controls the electric motor 12 in power generation in such a manner that the rotation speed of the electric motor 12 attains the target rotation speed N.sub.0, that the electric motor 12 and hydraulic motor 11 rotate at the target rotation speed N.sub.0, and that the flow rate of the hydraulic fluid flowing through the hydraulic motor 11 coincides with the target flow rate Q.sub.0, whereby the above-described power regeneration operation is carried out.

(48) FIG. 6 is an illustration depicting the relationship between the actual flow rate Q and the target flow rate Q.sub.0 relative to an operation start time at which the control lever 4a starts to be operated.

(49) It is assumed that a lowering operation of the boom 111 starts at time t.sub.0. In this case, as shown in FIG. 6, an attempt is made to control the amount of the hydraulic fluid discharged from the bottom-side hydraulic chamber 3a of the boom cylinder 3 to be the target flow rate Q.sub.0 (dotted line curve) corresponding to the target rotation speed N.sub.0. However, because the delivery capacity q of the hydraulic motor 11 is fixed, it takes time for the actual rotation speed N to coincide with the target rotation speed N.sub.0. When the boom cylinder 3 starts to move, the actual flow rate Q (solid line curve) flowing through the hydraulic motor 11 does not coincide with the target flow rate Q.sub.0, so that a flow rate difference Q develops between the target flow rate (Q.sub.0) and the actual flow rate (Q)(a deviation between Q.sub.0 and Q). For example, at a given time t.sub.2 relative to the start of the operation, the target flow rate that should flow through the hydraulic motor 11 is Q.sub.02 which does not coincide with an actual flow rate Q.sub.r2 flowing through the hydraulic motor 11. Whereas an ideal time is t.sub.3 required for the hydraulic motor 11 to rotate so that the amount of the hydraulic fluid discharged from the bottom-side hydraulic chamber 3a would attain the target flow rate Q.sub.0, the actual time required is t.sub.4.

(50) Thus in order to get the boom 111 starting to move smoothly, it is necessary to control the opening area of the control valve 2 to let the hydraulic fluid flow therethrough in the amount of the flow rate difference Q so that the hydraulic fluid may be discharged from the bottom-side hydraulic chamber 3a into the tank 18 via the control valve 2.

(51) Thus based on the electric signal reflecting the input operation amount of the control lever 4a and on the actual rotation speed of the hydraulic motor 11, the controller 9 calculates the power generation control command value Sg to the inverter 13 and the command value Sm to the proportional solenoid valve 7. Upon receipt of the power generation control command value Sg thus calculated, the inverter 13 controls the electric motor 12 in power generation so that the motor rotation speed will attain the target rotation speed N.sub.0. On receiving the command value Sm, the proportional solenoid valve 7 adjusts its opening area to control the operation pilot pressure fed from the sub-pump 8 so that the hydraulic fluid will flow to the control value 2 in just as much as the amount of Q.

(52) As described, whereas it takes time t.sub.4 for the amount of the hydraulic fluid discharged from the bottom-side hydraulic chamber 3a to attain the target flow rate Q.sub.0 if the boom 111 is lowered by getting the hydraulic fluid to flow only to the power regeneration device 19 as in conventional cases, this embodiment allows the hydraulic fluid to be evacuated in the amount corresponding to Q from the bottom-side hydraulic chamber 3a of the boom cylinder 3 into the tank 18. As a result, it takes time t.sub.3 for the amount of the hydraulic fluid discharged from the bottom-side hydraulic chamber 3a to reach the target flow rate Q.sub.0, the time t3 being shorter.

(53) The boom cylinder 3 is thus moved smoothly in the contracting operation (the boom 111 is turned downward) in keeping with the operator's boom lowering operation.

(54) With the above-described structures and workings in effect, when the operator performs an operation to lower the boom 111, the amount of the return oil from the boom cylinder 3 is controlled to be the target flow rate. This guarantees the responsiveness of the boom cylinder 3 in response to the operator's operations and keeps the boom cylinder 3 starting to move smoothly. Because there is no need to let any more hydraulic fluid than is necessary flow to the control valve 2, the power regeneration device 19 is allowed to maintain its good power regeneration efficiency.

Second Embodiment

(55) A hybrid hydraulic excavator as the second embodiment of the present invention is explained below. FIG. 7 is similar to FIG. 2, showing a hydraulic circuit portion for driving the boom cylinder 3 and a power regeneration device built in that hydraulic circuit portion as part of the drive control system of the hydraulic excavator embodying the present invention.

(56) As with the drive control system in FIG. 2, the drive control system in FIG. 7 includes a hydraulic pump 1 and a sub-pump 8 which are driven by the engine E, a control valve 2, a boom cylinder 3, a control lever device 4, and a power regeneration device 19. The drive control system of this embodiment is further equipped with an on-off valve 23 interposed between the hydraulic pump 1 and the line 6b and connected in parallel with the control valve 2.

(57) The on-off valve 23 has an operation port 23a that is connected to a pilot valve 4b2 via pilot hydraulic lines 5d and 5b. The on-off valve 23 is usually in the closed position and switched to the opened position in response to the pilot pressure Pb output to the pilot hydraulic lines 5b and 5d. This allows the hydraulic pump 1 to supply the hydraulic fluid to the rod-side hydraulic chamber 3b of the boom cylinder 3 via the lines 6e and 6b.

(58) The movements of this embodiment are explained below.

(59) The raising operation of the boom 111 with this embodiment is substantially the same as with the first embodiment and thus will not be discussed further. Only the lowering operation of the boom 111 with this embodiment will be explained hereunder.

(60) When the control lever 4a is operated toward the b side in FIG. 7, the pilot pressure Pb is led from the pilot valve 4b2 to the pilot check valve 10 via the pilot hydraulic line 5b, causing the pilot check valve 10 to open.

(61) At this point, the boom cylinder 3 is pushed by the deadweight of the front work implement 102 including the boom 111 so that the hydraulic fluid in the bottom-side hydraulic chamber 3a of the boom cylinder 3 is discharged into the line 6a. Because the pilot check valve 10 is currently open, the regeneration circuit 21 of the power regeneration device 19 is held open. The discharged hydraulic fluid is evacuated into the tank 18 via the line 6d and pilot check valve 10 past the hydraulic motor 11. At the same time, the pilot pressure Pb from the pilot valve 4b2 is led to the operation port 23a of the on-off valve 23 via the pilot hydraulic line 5d. This switches the on-off valve 23 to the opened position, allowing the hydraulic fluid to be supplied from the hydraulic pump 1 to the rod-side hydraulic chamber 3b of the boom cylinder 3 via the hydraulic lines 6e and 6b. As a result, the rod-side hydraulic chamber 3b of the boom cylinder 3 is supplied positively with the hydraulic fluid from the hydraulic pump 1 via the on-off valve 23, which causes the boom cylinder 3 to contract rapidly and the boom 111 to start descending smoothly.

(62) The hydraulic motor 11 is rotated by the return oil discharged from the boom cylinder 3, causing the electric motor 12 directly coupled with the hydraulic motor 11 to perform a power generation operation. The generated electric power is stored into the battery 15, whereby the power regeneration operation is carried out.

(63) As with the first embodiment, a control signal from the controller 9 controls the opening area of the proportional solenoid valve 7 to switch the control valve 2.

(64) With this embodiment structured as described above, the flow rate of the return oil from boom cylinder 3 is controlled to be the target flow rate, and the on-off valve 23 is further provided interposingly between the hydraulic pump 1 and the line 6b. This allows the hydraulic fluid from the hydraulic pump 1 to be fed to the rod-side hydraulic chamber 3b of the boom cylinder 3, thereby providing better responsiveness of the boom cylinder 3 in the lowering operation in response to the operator's operations. Also with this embodiment, there is no need for feeding any more hydraulic fluid than is necessary to the control valve 2, which permits the power regeneration device 19 to maintain excellent efficiency in power regeneration.

(65) <Others>

(66) Whereas the above embodiments were explained by referring to cases where the boom cylinder is used as the hydraulic cylinder, this embodiment can also be applied to the arm cylinder or others. In the latter case, the same advantages offered by the above embodiments are also provided. Furthermore, although cases where the electric motor is driven as a generator were explained, the position of the electric motor may be occupied alternatively by a power generator that only performs power generation operation.

(67) In addition, although the hydraulic excavator was explained above as a typical working machine, the present invention is not limited to the hydraulic excavator serving as the working machine. This invention may also be applied to working machines equipped with hydraulic actuators driving a work implement, such as a forklift or a wheel loader. In these cases, too, the present invention provides advantages similar to those discussed above.

DESCRIPTION OF REFERENCE CHARACTERS

(68) 1 Hydraulic pump 2 Control valve 3 Boom cylinder 3a Bottom-side hydraulic chamber 3b Rod-side hydraulic chamber 4 Control lever device 4a Control lever 4b Pilot valve 5a, 5b, 5c Pilot hydraulic line 6a, 6b, 6e Hydraulic line 6c Branching portion 6d Branching line 7 Proportional solenoid valve 8 Sub-pump 9 Controller 10 Pilot check valve 11 Hydraulic motor 12 Electric motor 13 Inverter 14 Chopper 15 Electric storage device (battery) 16 Pressure sensor 17 Rotation speed sensor 18 Tank 19 Power regeneration device 21 Regeneration circuit 22a, 22b Make-up valve 23 On-off valve 23a Operation port 31 Target flow rate calculation unit 32 Target rotation speed calculation unit 33 Electric motor command value calculation unit 34 Actual flow rate calculation unit 35, 35A Control valve target flow rate calculation unit 36 Proportional solenoid valve command value calculation unit 100 Lower travel structure 101 Upper swing structure 102 Front work implement 103a Travel device 104a Travel motor 106 Engine room 107 Cab (cabin) 111 Boom 112 Arm 113 Bucket 114 Arm cylinder 115 Bucket cylinder E Engine N Actual rotation speed N.sub.0 Target rotation speed Q.sub.0 Target flow rate Q Insufficient flow rate