Patent Publication Number: US-10760246-B2

Title: Work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a work machine such as a hydraulic excavator, and more particularly to a work machine capable of regenerating a return hydraulic fluid from a hydraulic actuator. 
     BACKGROUND ART 
     For example, Patent Document 1 discloses a conventional technology of a work machine capable of regenerating a return hydraulic fluid from a hydraulic actuator. 
     Patent Document 1 discloses a hydraulic fluid energy regeneration device for a work machine. The hydraulic fluid energy regeneration device includes a regeneration hydraulic motor, a hydraulic pump, and an electric motor. The regeneration hydraulic motor is driven by a return hydraulic fluid discharged by a hydraulic actuator. The hydraulic pump is mechanically coupled to the regeneration hydraulic motor. With this hydraulic fluid energy regeneration device, the hydraulic pump mechanically coupled to the regeneration hydraulic motor can be directly driven by recovered energy. This eliminates losses that result from temporary energy storage. This, as a result, makes it possible to reduce energy conversion losses, leading to efficient use of energy. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: WO2015/173963 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, there are problems in the hydraulic fluid energy regeneration device for the work machine disclosed in Patent Document 1. In the hydraulic fluid energy regeneration device, the revolution speed of the electric motor is controlled according to a target flow rate of the return hydraulic fluid or a constant revolution speed command. Therefore, when the revolution speed of the electric motor becomes excessive relative to the power regenerated by the regeneration hydraulic motor (regeneration power) or the power of the hydraulic pump (pump power), a drag loss of the regeneration hydraulic pump and the hydraulic pump increases. When the revolution speed of the electric motor becomes insufficient relative to the regeneration power or the pump power, the regeneration efficiency of the regeneration hydraulic motor decreases. 
     The present invention has been made in view of the problems described above. It is an object of the present invention to provide a work machine capable of regenerating a return hydraulic fluid from a hydraulic actuator while preventing a drag loss of a regeneration hydraulic motor and a hydraulic pump from increasing and preventing the regeneration efficiency of the regeneration hydraulic motor from decreasing. 
     Means for Solving the Problems 
     In order to achieve the object described above, the present invention provides a work machine including: a first hydraulic actuator; a second hydraulic actuator; a regeneration hydraulic motor that is driven by a return hydraulic fluid discharged from the first hydraulic actuator; a first hydraulic pump mechanically coupled to the regeneration hydraulic motor; an electric motor mechanically coupled to the regeneration hydraulic motor; a second hydraulic pump that delivers a hydraulic fluid for driving the first hydraulic actuator or the second hydraulic actuator; a junction line that allows a hydraulic fluid delivered by the first hydraulic pump to join a hydraulic fluid delivered by the second hydraulic pump; a first operation device that directs an operation of the first hydraulic actuator; a first operation amount sensor that detects an operation amount of the first operation device; a second operation device that directs an operation of the second hydraulic actuator; a second operation amount sensor that detects an operation amount of the second operation device; a first pressure sensor that detects a pressure in the first hydraulic actuator; a second pressure sensor that detects a pressure of the second hydraulic pump; and a controller configured to receive signals of the first operation amount sensor, the second operation amount sensor, the first pressure sensor, and the second pressure sensor and output a control command to the electric motor, the controller being configured to: compute a regeneration flow rate and a regeneration power of the regeneration hydraulic motor from the operation amount of the first operation device and the pressure in the first hydraulic actuator; compute a pump power of the second hydraulic pump from the operation amount of the second operation device and the pressure of the second hydraulic pump and set a smaller one of the regeneration power and the pump power as an assist power of the first hydraulic pump; and compute a target assist flow rate from the assist power and the pressure of the second hydraulic pump, in which the controller is configured to: compute a required regeneration hydraulic motor revolution speed from a regeneration hydraulic motor displacement and the regeneration flow rate, the required regeneration hydraulic motor revolution speed being a required revolution speed of the regeneration hydraulic motor, the regeneration hydraulic motor displacement being a displacement of the regeneration hydraulic motor; compute a required first hydraulic pump revolution speed from a first hydraulic pump displacement and the target assist flow rate, the required first hydraulic pump revolution speed being a required revolution speed of the first hydraulic pump, the first hydraulic pump displacement being a displacement of the first hydraulic pump; and select a greater one of the required regeneration hydraulic motor revolution speed and the required first hydraulic pump revolution speed as a target electric motor revolution speed, the target electric motor revolution speed being a target revolution speed of the electric motor. 
     According to the present invention configured as described above, a greater one of the required revolution speed of the regeneration hydraulic motor and the required revolution speed of the first hydraulic pump is selected as the target revolution speed of the electric motor. This configuration can prevent a drag loss of the regeneration hydraulic motor and the first hydraulic pump from increasing due to excessive revolution speed of the electric motor and prevent the regeneration efficiency of the regeneration hydraulic motor from decreasing due to insufficient revolution speed of the electric motor. 
     Advantages of the Invention 
     According to the present invention, it is possible to prevent a drag loss of a regeneration hydraulic motor and an auxiliary hydraulic pump from increasing and prevent the regeneration efficiency of the regeneration hydraulic motor from decreasing in a work machine capable of regenerating a return hydraulic fluid from a hydraulic actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a hydraulic excavator as an example of a work machine according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of a drive control system mounted in the hydraulic excavator illustrated in  FIG. 1 . 
         FIG. 3  is a block diagram of a controller illustrated in  FIG. 2 . 
         FIG. 4  is a characteristic diagram for describing a second function generation section of the controller illustrated in  FIG. 3 . 
         FIG. 5  is a block diagram for describing how the controller controls a flow rate of a hydraulic pump. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Hereinafter, a hydraulic excavator will be described as an example of a work machine according to an embodiment of the present invention with reference to the drawings. It is noted that like reference characters designate identical or corresponding components in each figure, and redundant description will be omitted as appropriate. 
       FIG. 1  is a perspective view of a hydraulic excavator according to the present embodiment.  FIG. 2  is a schematic diagram of a drive control system mounted in the hydraulic excavator illustrated in  FIG. 1 . 
     In  FIG. 1 , a hydraulic excavator  1  includes an articulated work device  1 A and a machine body  1 B. The work device  1 A includes a boom  1   a , an arm  1   b , and a bucket  1   c . The machine body  1 B includes an upper swing structure  1   d  and a lower track structure  1   e . The boom  1   a  is turnably supported by the upper swing structure  1   d  and is driven by a boom cylinder (hydraulic cylinder)  3   a . The boom cylinder  3   a  acts as a first hydraulic actuator. The upper swing structure  1   d  is swingably provided on the lower track structure  1   e . The upper swing structure  1   d  is driven to be swung by a swing motor  3   d  (illustrated in  FIG. 2 ). 
     The arm  1   b  is turnably supported by the boom  1   a  and is driven by an arm cylinder (hydraulic cylinder)  3   b . The bucket  1   c  is turnably supported by the arm  1   b  and is driven by a bucket cylinder (hydraulic cylinder)  3   c . The lower track structure  1   e  is driven by right and left track motors (not illustrated). The driving of the boom cylinder  3   a , the arm cylinder  3   b , and the bucket cylinder  3   c  is controlled by operation devices  4  and  24  (see  FIG. 2 ) that output respective hydraulic signals. The operation devices  4  and  24  are installed in a cabin (cab) of the upper swing structure  1   d.    
     The drive control system illustrated in  FIG. 2  includes a power regeneration device  70 , the operation devices  4  and  24 , a control valve  5 , a check valve  6 , a selector valve  7 , a solenoid selector valve  8 , an inverter  9 A, a chopper  9 B, an electric storage device  9 C, and a controller  100 . The control valve  5  includes a plurality of spool-type directional control valves. The controller  100  acts as a control device. 
     A variable displacement hydraulic pump  10 , a pilot hydraulic pump  11 , and a tank  12  are included as hydraulic fluid source devices. The hydraulic pump  10  acts as a second hydraulic pump. The pilot hydraulic pump  11  supplies a pilot hydraulic fluid. The hydraulic pump  10  and the pilot hydraulic pump  11  are driven by an engine  50  coupled thereto by a drive shaft. The hydraulic pump  10  includes a regulator  10 A. The regulator  10 A adjusts a delivery flow rate of the hydraulic pump  10  by controlling the swash plate tilting angle of the hydraulic pump  10  by a control pressure outputted from a solenoid proportional valve  74  described later. 
     An auxiliary hydraulic line  31 , the control valve  5 , and a pressure sensor  40  are provided in a hydraulic line  30 . The hydraulic line  30  supplies a hydraulic fluid from the hydraulic pump  10  to the boom cylinder  3   a  to the swing motor  3   d . The auxiliary hydraulic line  31  acts as a junction line and is coupled to the hydraulic line  30  via the check valve  6  described later. The control valve  5  includes the plurality of spool-type directional control valves that control the direction and flow rate of the hydraulic fluid to be supplied to each actuator. The pressure sensor  40  acts as a second pressure sensor and detects a delivery pressure of the hydraulic pump  10 . With the pilot hydraulic fluid supplied to pilot pressure receiving sections of the control valve  5 , the control valve  5  switches the spool position of each directional control valve and supplies the hydraulic fluid from the hydraulic pump  10  to each hydraulic actuator to drive the arm  1   b  and the like. The pressure sensor  40  outputs the detected delivery pressure of the hydraulic pump  10  to the controller  100  described later. 
     The spool position of each directional control valve of the control valve  5  is switched by the operations of operation levers or the like of the operation devices  4  and  24 . When the operation levers or the like are operated, the operation devices  4  and  24  supply pilot primary hydraulic fluids, which are supplied from the pilot hydraulic pump  11  via pilot primary-side hydraulic lines, not illustrated, to the pilot pressure receiving sections of the control valve  5  via pilot secondary-side hydraulic lines. Here, the operation device  4  is a first operation device that directs the operation of the boom cylinder  3   a  (first hydraulic actuator). The operation device  24  acts as a second operation device and collectively represents devices that direct the operation of the actuators (second hydraulic actuators) other than the boom cylinder  3   a.    
     A pilot valve  4 A is provided inside the operation device  4 . The operation device  4  is connected via pilot lines to the pressure receiving section of the corresponding spool-type directional control valve of the control valve  5  that controls driving of the boom cylinder  3   a . The pilot valve  4 A outputs a hydraulic signal to the corresponding pilot pressure receiving section of the control valve  5  according to the inclination direction and the operation amount of the operation lever of the operation device  4 . The spool-type directional control valve that controls driving of the boom cylinder  3   a  is switched in position according to the hydraulic signal inputted from the operation device, and controls the flow of the hydraulic fluid delivered from the hydraulic pump  10  according to the switching position. In this manner, the spool-type directional control valve controls driving of the boom cylinder  3   a . Here, a pressure sensor  75  is mounted in the pilot line through which a hydraulic signal (boom raising operation signal Pu) passes. The hydraulic signal (boom raising operation signal Pu) is for driving the boom cylinder  3   a  such that the boom  1   a  operates in the raising direction. The pressure sensor  75  outputs the detected boom raising operation signal Pu to the controller  100  described later. Further, a pressure sensor  41  acts as a first operation amount sensor and is mounted in the pilot line through which a hydraulic signal (boom lowering operation signal Pd) passes. The hydraulic signal (boom lowering operation signal Pd) is for driving the boom cylinder  3   a  such that the boom  1   a  operates in the lowering direction. The pressure sensor  41  outputs the detected boom lowering operation signal Pd to the controller  100  described later. 
     A pilot valve  24 A is provided inside the operation device  24 . The operation device  24  is connected via pilot lines to the pressure receiving sections of the respective spool-type directional control valves of the control valve  5  that control driving of the actuators other than the boom cylinder  3   a . The pilot valve  24 A outputs a hydraulic signal to the corresponding pilot pressure receiving section of the control valve  5  according to the inclination direction and the operation amount of the operation lever of the operation device  24 . The spool-type directional control valve that controls driving of a corresponding one of the actuators is switched in position according to the hydraulic signal inputted from the operation device, and controls the flow of the hydraulic fluid delivered from the hydraulic pump  10  according to the switching position. In this manner, the spool-type directional control valve controls driving of the corresponding actuator. 
     Pressure sensors  42  and  43  are provided in the two systems of the pilot lines connecting the pilot valve  24 A of the operation device  24  and the pressure receiving sections of the control valve  5 . The pressure sensors  42  and  43  act as second operation amount sensors and detect the respective pilot pressures. Each of the pressure sensors  42  and  43  outputs a detected operation amount signal of the operation device  24  to the controller  100  described later. 
     Each of the raising-side pilot pressure Pu and the lowering-side pilot pressure Pd outputted from the pilot valve  4 A inside the operation device  4  is inputted into a high pressure selection valve  71 , and one of the pressures that is higher is selected. Each of the pilot pressures outputted from the pilot valve  24 A inside the operation device  24  is inputted into a high pressure selection valve  73 , and one of the pressures that is higher is selected. The pressures selected by the high pressure selection valves  71  and  73  are inputted into a high pressure selection valve  72 , and one of the inputted pressures that is higher is selected. In other words, the highest pressure among the pressures outputted from the pilot valves  4 A and  24 A is selected by the high pressure selection valves  71 ,  72 , and  73  and is inputted into the solenoid proportional valve  74 . 
     The solenoid proportional valve  74  reduces the inputted pressure to a desired pressure according to a command from the controller  100 , and outputs the pressure to the regulator  10 A of the hydraulic pump  10 . The regulator  10 A controls the hydraulic pump  10  such that the displacement volume is proportional to the inputted pressure. 
     Next, the power regeneration device  70  will be described. The power regeneration device  70  includes a bottom-side hydraulic line  32 , a regeneration circuit  33 , the selector valve  7 , the solenoid selector valve  8 , the inverter  9 A, the chopper  9 B, the electric storage device  9   c , a variable displacement hydraulic motor  13 , an electric motor  14 , a variable displacement hydraulic pump  15 , and the controller  100 . The variable displacement hydraulic motor  13  acts as a regeneration hydraulic motor. The variable displacement hydraulic pump  15  acts as an auxiliary hydraulic pump (first hydraulic pump). The regeneration hydraulic motor  13  includes a regulator  13 A. The regulator  13 A controls the swash plate tilting angle of the hydraulic motor  13  according to a command from the controller  100  described later. 
     The bottom-side hydraulic line  32  is a hydraulic line through which a hydraulic fluid (return hydraulic fluid) returning to the tank  12  flows at the time of contraction of the boom cylinder  3   a . One end side of the bottom-side hydraulic line  32  is connected to a bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a , while the other end side of the bottom-side hydraulic line  32  is connected to a connection port of the control valve  5 . In the bottom-side hydraulic line  32 , a pressure sensor  44  and the selector valve  7  are provided. The pressure sensor  44  acts as a first pressure sensor and detects the pressure in the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a . The selector valve  7  switches whether to discharge the return hydraulic fluid from the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  to the tank  12  via the control valve  5 . The pressure sensor  44  outputs the detected pressure in the bottom-side hydraulic chamber  3   a   1  to the controller  100  described later. 
     The selector valve  7  includes a spring  7   b  on one end side thereof and a pilot pressure receiving section  7   a  on the other end side thereof. By switching the spool position depending on whether the pilot hydraulic fluid is supplied to the pilot pressure receiving section  7   a , the selector valve  7  controls communication/interruption of the return hydraulic fluid flowing from the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  into the control valve  5 . The pilot hydraulic fluid is supplied from the pilot hydraulic pump  11  to the pilot pressure receiving section  7   a  via the solenoid selector valve  8  described later. 
     The hydraulic fluid outputted from the pilot hydraulic pump  11  is inputted into an input port of the solenoid selector valve  8 . By contrast, a command signal outputted from the controller  100  is inputted into an operation section of the solenoid selector valve  8 . According to this command signal, the solenoid selector valve  8  controls supply/interruption of the pilot hydraulic fluid, which has been supplied from the pilot hydraulic pump  11 , to the pilot pressure receiving section  7   a  of the selector valve  7 . 
     One end of the regeneration circuit  33  is connected between the selector valve  7  in the bottom-side hydraulic line  32  and the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a , while the other end of the regeneration circuit  33  is connected to an inlet of the hydraulic motor  13 . With this arrangement, the return hydraulic fluid from the bottom-side hydraulic chamber  3   a   1  is guided to the tank  12  via the regeneration hydraulic motor  13 . 
     The regeneration hydraulic motor  13  is mechanically coupled to the auxiliary hydraulic pump  15 . The auxiliary hydraulic pump  15  is rotated by the driving force of the hydraulic motor  13 . 
     One end side of the auxiliary hydraulic line  31  is connected to a delivery port of the auxiliary hydraulic pump  15 , which acts as the first hydraulic pump, while the other end side of the auxiliary hydraulic line  31  is connected to the hydraulic line  30 . The check valve  6  is provided in the auxiliary hydraulic line  31 . The check valve  6  allows the hydraulic fluid from the auxiliary hydraulic pump  15  to flow into the hydraulic line  30  while preventing the hydraulic fluid from the hydraulic line  30  to flow into the auxiliary hydraulic pump  15 . 
     The auxiliary hydraulic pump  15  includes a regulator  15 A. The regulator  15 A adjusts a delivery flow rate of the auxiliary hydraulic pump  15  by controlling the swash plate tilting angle of the auxiliary hydraulic pump  15  by a command from the controller  100  described later. 
     The hydraulic motor  13  is further mechanically coupled to the electric motor  14 . Electric power is generated by the driving force of the hydraulic motor  13 . The electric motor  14  is electrically connected to the inverter  9 A, the chopper  9 B, and the electric storage device  9 C. The inverter  9 A controls the revolution speed. The chopper  9 B boots voltage. The electric storage device  9 C stores generated electric energy. 
     The controller  100  receives the raising-side pilot pressure signal Pu of the pilot valve  4 A of the operation device  4  detected by the pressure sensor  75 , the lowering-side pilot pressure signal Pd of the pilot valve  4 A of the operation device  4  detected by the pressure sensor  41 , pilot pressure signals of the pilot valve  24 A of the operation device  24  detected by the pressure sensors  42  and  43 , and a pressure signal of the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  detected by the pressure sensor  44 . The controller  100  performs calculation based on these inputted values and outputs respective control commands to the solenoid selector valve  8 , the inverter  9 A, the solenoid proportional valve  74 , the regulator  13 A of the regeneration hydraulic motor  13 , and the regulator  15 A of the auxiliary hydraulic pump  15 . 
     The solenoid selector valve  8  is switched by a command signal from the controller  100  and supplies the hydraulic fluid from the pilot hydraulic pump  11  to the selector valve  7 . The inverter  9 A is controlled to a desired revolution speed by a signal from the controller  100 . The solenoid proportional valve  74  controls the displacement of the hydraulic pump  10  by outputting a pressure based on a command from the controller  100 . The regeneration hydraulic motor  13  is controlled to a desired displacement by a command from the controller. The auxiliary hydraulic pump  15  is controlled to a desired displacement by a signal from the controller  100 . 
     Next, the operation of the hydraulic excavator  1  according to the present embodiment described above will be described. 
     First, when the operation lever of the operation device  4  illustrated in  FIG. 2  is operated in the boom lowering direction, the pilot pressure Pd is transmitted from the pilot valve  4 A to the corresponding pilot pressure receiving section of the control valve  5 , causing the spool-type directional control valve of the control valve  5  that controls driving of the boom cylinder  3   a  to perform the switching operation. This causes the hydraulic fluid from the hydraulic pump  10  to flow into a rod-side hydraulic chamber  3   a   2  of the boom cylinder  3   a  via the control valve  5 . This, as a result, causes a piston rod of the boom cylinder  3   a  to perform the contraction operation. Accordingly, the return hydraulic fluid discharged from the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  is guided to the tank  12  through the selector valve  7  and the control valve  5  that are in communication with the bottom-side hydraulic line  32 . 
     At this point, the controller  100  receives the delivery pressure signal of the hydraulic pump  10  detected by the pressure sensor  40 , the pressure signal of the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  detected by the pressure sensor  44 , the raising-side pilot pressure signal Pu of the pilot valve  4 A detected by the pressure sensor  75 , and the lowering-side pilot pressure signal Pd of the pilot valve  4 A detected by the pressure sensor  41 . 
     In this state, when the operation lever of the operation device  4  is operated equal to or greater than a specified value by an operator in the boom lowering direction, the controller  100  outputs a switching command to the solenoid selector valve  8 , a revolution speed command to the inverter  9 A, displacement commands to the regulator  13 A of the regeneration hydraulic motor  13  and the regulator  15 A of the auxiliary hydraulic pump  15 , and a control command to the solenoid proportional valve  74 . 
     As a result, the selector valve  7  is switched to the interruption position, causing the hydraulic line to the control valve  5  to be interrupted. Therefore, the return hydraulic fluid from the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  flows into the regeneration circuit  33  and drives the hydraulic motor  13 . After that, the return hydraulic fluid is discharged to the tank  12 . At this point, the flow rate (bottom-side flow rate) discharged from the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  is the flow rate (regeneration flow rate) regenerated by the regeneration hydraulic motor  13 . 
     The auxiliary hydraulic pump  15  is rotated by the driving force of the regeneration hydraulic motor  13 . The hydraulic fluid delivered from the auxiliary hydraulic pump  15  joins the hydraulic fluid delivered from the hydraulic pump  10  via the auxiliary hydraulic line  31  and the check valve  6 . The controller  100  outputs a displacement command to the regulator  15 A of the auxiliary hydraulic pump  15  such that the power of the hydraulic pump  10  is assisted. The controller  100  outputs a control command to the solenoid proportional valve  74  such that the displacement of the hydraulic pump  10  is reduced by the flow rate of the hydraulic fluid supplied from the auxiliary hydraulic pump  15 . 
     Of the hydraulic energy inputted into the regeneration hydraulic motor  13 , excess energy that has not been consumed by the auxiliary hydraulic pump  15  is used to drive the electric motor  14  to generate electric power. The electric energy generated by the electric motor  14  is stored in the electric storage device  9 C. 
     In the present embodiment, the energy of the hydraulic fluid discharged from the boom cylinder  3   a  is recovered by the regeneration hydraulic motor  13 , and then used to assist the power of the hydraulic pump  10  as the driving force of the auxiliary hydraulic pump  15 . Further, excess power is stored in the electric storage device  9 C via the electric motor  14 . In this manner, effective use of energy and reduction in fuel consumption are achieved. 
     Next, the control of the controller  100  will be described with reference to  FIGS. 3, 4, and 5 .  FIG. 3  is a block diagram of the controller  100 . 
     As illustrated in  FIG. 3 , the controller  100  includes a first function generation section  101 , a second function generation section  102 , a first subtraction section  103 , a first multiplication section  104 , a second multiplication section  105 , a first output conversion section  106 , a third function generation section  107 , a minimum value selection section  108 , a first division section  109 , a fourth function generation section  111 , a second subtraction section  112 , a second output conversion section  113 , a minimum flow rate command section  114 , a second division section  121 , a third division section  122 , a maximum value selection section  123 , a fourth division section  124 , a fifth division section  125 , a third output conversion section  126 , a fourth output conversion section  127 , and a fifth output conversion section  128 . 
     The first function generation section  101  receives, as a lever operation signal  141 , the lowering-side pilot pressure Pd of the pilot valve  4 A of the operation device  4  detected by the pressure sensor  41 . A switching start point for the lever operation signal  141  is stored in a table of the first function generation section  101  in advance. 
     When the lever operation signal  141  is equal to or smaller than the switching start point, the first function generation section  101  outputs an OFF signal to the first output conversion section  106 . When the lever operation signal  141  exceeds the switching start point, the first function generation section  101  outputs an ON signal to the first output conversion section  106 . The first output conversion section  106  converts the inputted signal into a control signal for the solenoid selector valve  8  and outputs the control signal to the solenoid selector valve  8  as a solenoid valve command signal  208 . This causes the solenoid selector valve  8  to operate. This, in turn, causes the selector valve  7  to be switched and the hydraulic fluid in the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  to flow into the regeneration circuit  33 . 
     The lowering-side pilot pressure Pd is inputted into one input end of the second function generation section  102  as the lever operation signal  141 . The pressure in the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a  detected by the pressure sensor  44  is inputted into another input end of the second function generation section  102  as a pressure signal  144 . Based on these inputted signals, the second function generation section  102  computes a target bottom flow rate signal  102 A of the boom cylinder  3   a.    
     The calculation of the second function generation section  102  will be described in detail with reference to FIG.  4 .  FIG. 4  is a characteristic diagram for describing the second function generation section  102 . In  FIG. 4 , the horizontal axis represents the operation amount of the lever operation signal  141 , while the vertical axis represents a target bottom flow rate (a target flow rate of the return hydraulic fluid flowing out of the bottom-side hydraulic chamber  3   a   1  of the boom cylinder  3   a ). In  FIG. 4 , a basic characteristic line a indicated by a solid line is set in order to obtain a characteristic equivalent to conventional control of the return hydraulic fluid by the control valve  5 . A characteristic line b indicated by an upper broken line and a characteristic line c indicated by a lower broken line represent a case where the characteristic line a is corrected by the pressure signal  144  of the bottom-side hydraulic chamber  3   a   1 . 
     Specifically, when the pressure signal  144  of the bottom-side hydraulic chamber  3   a   1  increases, the inclination of the basic characteristic line a increases and is corrected in the direction of the characteristic line b, which leads to a continuous change in the characteristic. Conversely, when the pressure signal  144  decreases, the inclination of the basic characteristic line a decreases and is corrected in the direction of the characteristic line c, which leads to a continuous change in the characteristic. In this manner, the second function generation section  102  computes a target bottom flow rate signal serving as a base according to the lever operation signal  141  and corrects the target bottom flow rate signal serving as a base according to the change in the pressure signal  144  of the bottom-side hydraulic chamber  3   a   1 , thereby computing the final target bottom flow rate signal  102 A. 
     Returning to  FIG. 3 , the second function generation section  102  outputs the target bottom flow rate signal  102 A to the fourth division section  124  and the first multiplication section  104 . 
     The pressure signal  144  is inputted into the third function generation section  107 . The third function generation section  107  computes a required displacement of the regeneration hydraulic motor  13  according to the pressure signal  144 . The characteristic of the third function generation section  107  is such that the third function generation section  107  lowers the displacement as the bottom pressure increases. This is because, since the maximum torque is set for the electric motor  14 , controlling the regeneration hydraulic motor  13  to a large displacement with high pressure may result in overspeed exceeding the maximum torque of the electric motor  14 . For this reason, the displacement of the regeneration hydraulic motor  13  is controlled such that the displacement is lowered and the torque borne by the electric motor  14  is lowered at the time of high pressure. Another reason is to attain a large displacement as much as possible when the pressure is not high. This is because it is generally more efficient to control a hydraulic motor with a large displacement. 
     The required displacement from the third function generation section  107  and the target bottom flow rate signal  102 A are inputted into the second division section  121 . The second division section  121  computes a required regeneration hydraulic motor revolution speed by dividing the target bottom flow rate signal  102 A by the required flow rate and outputs the required regeneration hydraulic motor revolution speed to the maximum value selection section  123 . 
     The first subtraction section  103  receives a minimum flow rate signal from the minimum flow rate command section  114  and a required pump flow rate signal  120 , and computes a deviation therebetween as a required pump flow rate signal  103 A. The first subtraction section  103  outputs the required pump flow rate signal  103 A to the second multiplication section  105  and the second subtraction section  112 . Here, a method for computing the required pump flow rate signal  120  will be described with reference to  FIG. 5 .  FIG. 5  is a block diagram for describing how the controller  100  controls the flow rate of the hydraulic pump. 
     With reference to  FIG. 5 , the pressures of individual pilot valves are detected by the pressure sensors  41 ,  75 ,  42 , and  43  and are outputted to the controller  100  as lever operation signals  141 ,  175 ,  142 , and  143 , respectively. 
     In the controller  100 , function generation sections  145 ,  146 ,  147 , and  148  corresponding to individual lever operation signals compute respective required pump flow rates such that the required pump flow rate signal  120  based on each lever operation signal is obtained. The required pump flow rates computed by the respective function generation sections are summed by addition sections  149 ,  150 , and  151 . This is a calculation for securing a necessary hydraulic pump flow rate when a combined operation is performed. Then, a function generation section  152  cuts off the total value of the required pump flow rates outputted from the addition section  151  at an upper limit. This is because there is an upper limit on the flow rate that can be delivered by the hydraulic pump  10 . The upper limit in the function generation section  152  is a value that is obtained from the maximum displacement of the hydraulic pump  10 . 
     In this manner, this control logic computes, without excess or deficiency, the flow rate based on each lever operation signal. At the time of a combined operation, the control logic figures as much flow rate as necessary and computes the required pump flow rate signal  120  without exceeding the upper limit of the flow rate that can be delivered by the hydraulic pump  10 . 
     Returning to  FIG. 3 , the first multiplication section  104  receives the target bottom flow rate signal  102 A from the second function generation section  102  and the pressure signal  144  of the bottom-side hydraulic chamber  3   a   1 . The first multiplication section  104  computes a multiplication value of these signals as a regeneration power signal  104 A and outputs the regeneration power signal  104 A to the minimum value selection section  108 . 
     One input end of the second multiplication section  105  receives the delivery pressure of the hydraulic pump  10  detected by the pressure sensor  40  as a pressure signal  140 . Another input end of the second multiplication section  105  receives the required pump flow rate signal  103 A computed by the first subtraction section  103 . The second multiplication section  105  computes a multiplication value of these signals as a required pump power signal  105 A and outputs the required pump power signal  105 A to the minimum value selection section  108 . 
     The minimum value selection section  108  receives the regeneration power signal  104 A from the first multiplication section  104  and the required pump power signal  105 A from the second multiplication section  105 , and selects a smaller one of these signals as a target assist power signal  108 A of the auxiliary hydraulic pump  15 . The minimum value selection section  108  outputs the target assist power signal  108 A to the first division section  109 . 
     Here, considering the efficiency of the equipment, using the recovered power in the auxiliary hydraulic pump  15  as much as possible can reduce losses and is therefore more efficient than causing the electric motor  14  to convert the recovered power into electric energy and store the electric energy in the electric storage device  9 C for reuse. For this reason, the minimum value selection section  108  selects a smaller one of the regeneration power signal  104 A and the required pump power signal  105 A. With this configuration, the regeneration power can be supplied to the auxiliary hydraulic pump  15  as much as possible without exceeding the required pump power signal  105 A. 
     The first division section  109  receives the target assist power signal  108 A from the minimum value selection section  108  and the pressure signal  140  of the delivery pressure of the hydraulic pump  10 . The first division section  109  computes a target assist flow rate signal  109 A by dividing the target assist power signal  108 A by the pressure signal  140 , and outputs the target assist flow rate signal  109 A to the third division section  122 , the second subtraction section  112 , and the fifth division section  125 . 
     The pressure signal  140  is inputted into the fourth function generation section  111 . The fourth function generation section  111  computes the required displacement of the auxiliary hydraulic pump  15  according to the pressure signal  140 . The characteristic of the fourth function generation section  111  is such that the fourth function generation section  111  lowers the displacement as the pump pressure increases. This is because, since the maximum torque is set for the electric motor  14 , controlling the auxiliary hydraulic pump  15  to a large displacement with high pressure may result in overspeed exceeding the maximum torque of the electric motor  14 . For this reason, the displacement of the auxiliary hydraulic pump  15  is controlled such that the displacement is lowered and the torque borne by the electric motor  14  is lowered at the time of high pressure. Another reason is to attain a large displacement as much as possible when the pressure is not high. This is because it is generally more efficient to control a hydraulic pump with a large displacement. 
     The required displacement from the fourth function generation section  111  and the target assist flow rate signal  109 A are inputted into the third division section  122 . The third division section  122  computes a required auxiliary hydraulic pump revolution speed by dividing the target assist flow rate signal  109 A by the required displacement and outputs the required auxiliary hydraulic pump revolution speed to the maximum value selection section  123 . 
     The maximum value selection section  123  selects a larger one of the inputted signals as a target electric motor revolution speed and inputs the larger one to the third output conversion section  126 , the fourth division section  124 , and the fifth division section  125 . The third output conversion section  126  converts the inputted target electric motor revolution speed into a command signal for the inverter  9 A and outputs the command signal to the inverter  9 A. 
     The fourth division section  124  computes a target displacement signal for the regeneration hydraulic motor  13  by dividing the target bottom flow rate signal  102 A from the second function generation section  102  by the target electric motor revolution speed from the maximum value selection section  123 . The target displacement signal for the regeneration hydraulic motor  13  is inputted into the fourth output conversion section  127 . The fourth output conversion section  127  converts the inputted target displacement signal for the regeneration hydraulic motor  13  into a command signal for the regulator  13 A and outputs the command signal to the regulator  13 A. 
     The fifth division section  125  computes a target displacement signal for the auxiliary hydraulic pump  15  by dividing the target assist flow rate signal  109 A from the first division section  109  by the target electric motor revolution speed from the maximum value selection section  123 . The target displacement signal for the auxiliary hydraulic pump  15  is inputted into the fifth output conversion section  128 . The fifth output conversion section  128  converts the inputted target displacement signal for the auxiliary hydraulic pump  15  into a command signal for the regulator  15 A and outputs the command signal to the regulator  15 A. 
     Since a greater one of the required revolution speed of the regeneration hydraulic motor  13  and the required revolution speed of the auxiliary hydraulic pump  15  is selected as the target electric motor revolution speed as a result of the calculation described above, the revolution speed of the regeneration hydraulic motor  13  or the auxiliary hydraulic pump  15  whose required revolution speed is smaller becomes greater than the required revolution speed. However, it is possible to regenerate or deliver the target flow rate by reducing the displacement of the regeneration hydraulic motor  13  or the auxiliary hydraulic pump  15  whose required revolution speed is smaller. 
     By controlling in this manner, moreover, when there is no regeneration power, the electric motor  14  does not rotate even when the required pump flow rate signal is inputted. Therefore, it is possible to suppress an unnecessary drag loss of the regeneration hydraulic motor  13  or the auxiliary hydraulic pump  15 . On the other hand, when there is regeneration power and the required pump flow rate signal is inputted (at the time of assisting the power of the hydraulic pump  10 ), the electric motor  14  is actively rotated. Therefore, it is possible to reuse the hydraulic energy as the driving force of the auxiliary hydraulic pump  15  without converting the hydraulic energy into electric energy. As a matter of course, when there is regeneration power and the required pump flow rate signal is not inputted (at the time of not assisting the power of the hydraulic pump  10 ), it is possible to store regeneration energy obtained by rotating the electric motor as electric energy. 
     The second subtraction section  112  receives the required pump flow rate signal  103 A from the first subtraction section  103 , the target assist flow rate signal  109 A from the first division section  109 , and the minimum flow rate signal from the minimum flow rate command section  114 . The second subtraction section  112  adds the required pump flow rate signal  103 A and the minimum flow rate signal to compute the required pump flow rate signal  120  inputted from a machine controller  200 . The second subtraction section  112  computes a deviation between the required pump flow rate signal  120  and the target assist flow rate signal  109 A as a target pump flow rate signal  112 A and outputs the target pump flow rate signal  112 A to the second output conversion section  113 . 
     The second output conversion section  113  converts the inputted target pump flow rate signal  112 A into, for example, the displacement of the hydraulic pump  10  and outputs a control pressure command signal  210 A to the solenoid proportional valve  74  such that a control pressure based on the displacement is attained. The solenoid proportional valve  74  reduces the pressure outputted from the high pressure selection valve  72  to attain the control pressure based on the command from the controller  100 , and outputs the control pressure to the regulator  10 A. The regulator  10 A controls the displacement of the hydraulic pump  10  according to the inputted control pressure. 
     With the hydraulic excavator  1  according to the present embodiment described above, the auxiliary hydraulic pump  15  mechanically coupled to the regeneration hydraulic motor  13  can be directly driven by regeneration energy. This eliminates losses that result from temporary energy storage. This, as a result, makes it possible to reduce energy conversion losses, leading to efficient use of energy. 
     Further, a greater one of the required revolution speed of the regeneration hydraulic motor  13  and the required revolution speed of the auxiliary hydraulic pump  15  is selected as the target revolution speed of the electric motor  14 . This configuration can prevent a drag loss of the regeneration hydraulic motor  13  and the auxiliary hydraulic pump  15  from increasing due to excessive revolution speed of the electric motor  14  and prevent the regeneration efficiency of the regeneration hydraulic motor  13  from decreasing due to insufficient revolution speed of the electric motor  14 . 
     Although the embodiment of the present invention has been described in detail hereinabove, the present invention is not limited to the embodiment described above but includes various modifications. For example, the embodiment has been described in detail to describe the present invention in a comprehensible manner, and is not necessarily limited to the one including all the configurations that have been described. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1 : Hydraulic excavator 
           1   a : Boom 
           3   a : Boom cylinder (first hydraulic actuator) 
           3   a   1 : Bottom-side hydraulic chamber 
           3   a   2 : Rod-side hydraulic chamber 
           3   b : Arm cylinder (second hydraulic actuator) 
           3   c : Bucket cylinder (second hydraulic actuator) 
           3   d : Swing motor (second hydraulic actuator) 
           4 : Operation device (first operation device) 
           4 A: Pilot valve 
           5 : Control valve 
           6 : Check valve 
           7 : Selector valve 
           8 : Solenoid selector valve 
           9 A: Inverter 
           9 B: Chopper 
           9 C: Electric storage device 
           10 : Hydraulic pump (second hydraulic pump) 
           10 A: Regulator 
           11 : Pilot hydraulic pump 
           12 : Tank 
           13 : Regeneration hydraulic motor 
           14 : Electric motor 
           15 : Auxiliary hydraulic pump (first hydraulic pump) 
           15 A: Regulator 
           16 : Bleed valve 
           17 : Solenoid proportional pressure reducing valve 
           24 : Operation device (second operation device) 
           24 A: Pilot valve 
           25 : Chopper 
           30 : Hydraulic line 
           31 : Auxiliary hydraulic line (junction line) 
           32 : Bottom-side hydraulic line 
           33 : Regeneration circuit 
           34 : Discharge hydraulic line 
           40 : Pressure sensor (second pressure sensor) 
           41 : Pressure sensor (first operation amount sensor) 
           42 : Pressure sensor (second operation amount sensor) 
           43 : Pressure sensor (second operation amount sensor) 
           44 : Pressure sensor (first pressure sensor) 
           50 : Engine 
           70 : Power regeneration device 
           71 : High pressure selection valve 
           72 : High pressure selection valve 
           73 : High pressure selection valve 
           74 : Solenoid proportional valve 
           75 : Pressure sensor 
           76 : Revolution speed sensor 
           77 : Pressure sensor 
           100 : Controller 
           101 : First function generation section 
           102 : Second function generation section 
           102 A: Target bottom flow rate signal 
           103 : First subtraction section 
           103 A: Required pump flow rate signal 
           104 : First multiplication section 
           104 A: Regeneration power signal 
           105 : Second multiplication section 
           105 A: Required pump power signal 
           106 : First output conversion section 
           107 : Third function generation section 
           108 : Minimum value selection section 
           108 A: Target assist power signal 
           109 : First division section 
           109 A: Target assist flow rate signal 
           111 : Fourth function generation section 
           112 : Second subtraction section 
           112 A: Target pump flow rate signal 
           113 : Second output conversion section 
           114 : Minimum flow rate command section 
           120 : Required pump flow rate signal 
           121 : Second division section 
           122 : Third division section 
           123 : Maximum value selection section 
           124 : Fourth division section 
           125 : Fifth division section 
           126 : Third output conversion section 
           127 : Fourth output conversion section 
           128 : Fifth output conversion section 
           141 : Lever operation signal 
           142 : Lever operation signal 
           143 : Lever operation signal 
           145 : Function generation section 
           146 : Function generation section 
           147 : Function generation section 
           148 : Function generation section 
           149 : Addition section 
           150 : Addition section 
           151 : Addition section 
           152 : Function generation section 
           175 : Lever operation signal 
           208 : Solenoid valve command signal 
           210 A: Control pressure command signal