Patent Publication Number: US-9903094-B2

Title: Work machine driving device

Description:
TECHNICAL FIELD 
     The present invention relates to a driving device used in a work machine such as a hydraulic excavator and more particularly to a driving device which drives a hydraulic actuator through a hydraulic pump. 
     BACKGROUND ART 
     In the recent years, energy saving has been demanded from the viewpoint of environmental problems, etc., and in order to achieve energy saving in a work machine such as a hydraulic excavator or wheel loader, it is important to save energy in the entire hydraulic system for driving the work machine. From the viewpoint of energy saving, a hydraulic closed-circuit system has been developed in which a hydraulic pump is connected in a closed loop to a hydraulic actuator to control the hydraulic actuator directly by the hydraulic pump. 
     Since the hydraulic closed-circuit system does not need a control valve which controls the supply direction and flow rate of hydraulic oil discharged from the hydraulic pump, no pressure loss attributable to the control valve occurs and it is only necessary to discharge hydraulic oil at a required flow rate from the hydraulic pump and there is little flow rate loss. In addition, the potential energy of the hydraulic actuator to be driven and the kinetic energy during deceleration can be regenerated so that energy saving can be achieved. 
     On the other hand, in the hydraulic closed-circuit system, in order for the amount of hydraulic oil discharged from a single hydraulic pump to cover the required amount of hydraulic oil to drive each hydraulic actuator, a large hydraulic pump with a high discharge rate is needed for each hydraulic actuator. Therefore, Patent Literature 1 discloses a conventional technique which converges flows of hydraulic oil discharged from a plurality of hydraulic pumps and achieves the drive speed of a hydraulic actuator without increasing the size of the hydraulic pumps. In Patent Literature 1, a hydraulic pump is allocated to each hydraulic actuator according to a priority order map which determines priority connection relationships between hydraulic pumps and hydraulic actuators and switching valves are controlled depending on this allocation. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Examined Patent Publication No. 1987-25882 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional technique disclosed in Patent Literature 1, a hydraulic pump is always allocated to each hydraulic actuator according to the priority order map and switching of the switching valves is controlled depending on the allocation, so the following problems may arise. 
     For example, if during operation of one hydraulic actuator another hydraulic actuator is to be driven, namely if the number of hydraulic pumps to be allocated increases, it may happen that according to the priority order map the hydraulic pump connected to one hydraulic actuator is connected to the hydraulic actuator to be started according to its priority order and another hydraulic pump is reconnected to the one hydraulic actuator. Also, if during operation of a plurality of hydraulic actuators the flow rate to one hydraulic actuator is decreased for deceleration, namely if the number of hydraulic pumps to be allocated is decreased, the state in which a plurality of hydraulic pumps are allocated to the hydraulic actuator to which the flow rate is decreased is changed to the state in which a certain hydraulic pump among the allocated hydraulic pumps is not connected. Therefore, this certain hydraulic pump becomes unused. However, if the unused hydraulic pump has a higher priority for another hydraulic actuator than the hydraulic pump connected to it, it may happen that according to the priority order map, the hydraulic pump connected to the other hydraulic actuator is unallocated and the unused hydraulic pump is reconnected to it. As a consequence, when changing the connection of a hydraulic pump, it may happen that switching of switching valves is made more times than necessary (hereinafter sometimes called the number of switching times) to perform control. This may result in increased vehicle body vibration due to a shock caused by pressure variation during switching, worsened operability, and shortened life due to deterioration of components including the switching valves. 
     The present invention has been made in view of the above circumstances of the conventional technique and an object thereof is to provide a work machine driving device which can reduce unnecessary switching control of switching valves. 
     Solution to Problem 
     In order to achieve the above object, the present invention is characterized in that a driving device for a work machine includes a plurality of hydraulic actuators, a plurality of variable displacement hydraulic pumps to drive the hydraulic actuators, a plurality of switching valves connected between the hydraulic actuators and the hydraulic pumps, an operation part to operate the hydraulic actuators, and a controller to control the hydraulic pumps and the switching valves, in which at least two of the hydraulic pumps can be connected in a closed loop to any one of the hydraulic actuators through the switching valves, the controller includes a priority order calculating circuit which calculates allocation of the hydraulic pumps to the hydraulic actuators according to operation of the operation part and a priority order map determining priority connection relationships between the hydraulic pumps and the hydraulic actuators, and the priority order calculating circuit selects and allocates an unallocated one of the hydraulic pumps when the number of hydraulic pumps to be allocated increases. 
     In the present invention thus configured, when the priority order calculating circuit of the controller calculates the allocation of the hydraulic pumps to the hydraulic actuators according to operation of the operation part and the priority order map determining the priority connection relationships between the hydraulic pumps and the hydraulic actuators, if the number of hydraulic pumps to be allocated increases, it allocates an unallocated hydraulic pump just before the increase. As a consequence, if during operation of one hydraulic actuator, another hydraulic actuator is to be driven, namely even if the number of hydraulic pumps to be allocated increases, an unused hydraulic pump can be connected to the hydraulic actuator to be started without changing the connection of the hydraulic pump connected to the one hydraulic actuator, so unnecessary switching control of electromagnetic switching valves can be reduced. As a consequence, the frequency of shock caused by pressure variation during switching can be reduced, leading to reduction of vehicle body vibration, improved operability and improvement in the life of the components including the switching valves. 
     In addition, the present invention is characterized in that a driving device for a work machine includes a plurality of hydraulic actuators, a plurality of variable displacement hydraulic pumps to drive the hydraulic actuators, a plurality of switching valves connected between the hydraulic actuators and the hydraulic pumps, an operation part to operate the hydraulic actuators, and a controller to control the hydraulic pumps and the switching valves, in which at least two of the hydraulic pumps can be connected in a closed loop to any one of the hydraulic actuators through the switching valves, the controller includes a priority order calculating circuit which calculates allocation of the hydraulic pumps to the hydraulic actuators according to operation of the operation part and a priority order map determining priority connection relationships between the hydraulic pumps and the hydraulic actuators, and when the number of hydraulic pumps to be allocated to a given hydraulic actuator decreases, the priority order calculating circuit maintains allocation of the hydraulic pumps allocated to the other hydraulic actuators than the given hydraulic actuator. 
     In the present invention thus configured, when the priority order calculating circuit of the controller calculates the number of hydraulic pumps to be allocated to the hydraulic actuators according to operation of the operation part and the priority order map determining the priority connection relationships between the hydraulic pumps and the hydraulic actuators, if the number of hydraulic pumps to be allocated to a given hydraulic actuator decreases, the allocation of the hydraulic pumps allocated to the hydraulic actuators other than the given hydraulic actuator is maintained. Consequently, if during operation of a plurality of hydraulic actuators the flow rate to one hydraulic actuator is decreased for deceleration, namely even if the number of hydraulic pumps to be allocated decreases, a hydraulic pump is unallocated from the hydraulic actuator to which the flow rate is decreased and the allocation of the hydraulic pumps to the other hydraulic actuators is maintained, so unnecessary switching control of the switching valves is reduced. As a consequence, the frequency of shock caused by pressure variation during switching can be reduced, leading to reduction of vehicle body vibration, improved operability and improvement in the life of the components including the switching valves. 
     Advantageous Effects of Invention 
     The present invention is configured so that when the priority order calculating circuit calculates the allocation of the hydraulic pumps to the hydraulic actuators according to operation of the operation part and the priority order map determining the priority connection relationships between the hydraulic pumps and the hydraulic actuators, if the number of hydraulic pumps to be allocated increases, it allocates an unallocated hydraulic pump just before the increase. Also the present invention is configured so that if the number of hydraulic pumps to be allocated to a given hydraulic actuator decreases, the allocation of the hydraulic pumps allocated to the other hydraulic actuators than the given hydraulic actuator is maintained. The present invention thus configured can reduce unnecessary switching control of the switching valves. The other issues, configuration and effects will become apparent from the following description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view which shows a hydraulic excavator including an embodiment of the work machine driving device according to the present invention. 
         FIG. 2  is a circuit configuration diagram which shows the essential part of the driving device provided in the hydraulic excavator shown in  FIG. 1 . 
         FIG. 3  is a block diagram which shows a controller in the driving device shown in  FIG. 2 . 
         FIG. 4  shows graphs which indicate calculations in the required pump number calculating circuit of the controller shown in  FIG. 3 , in which (a), (b), (c), and (d) are for the boom cylinder, arm cylinder, bucket cylinder, and swing motor, respectively. 
         FIG. 5  is a table which shows the priority order map of the controller shown in  FIG. 3 . 
         FIG. 6  is a time chart which shows the operation of the controller shown in  FIG. 3  when the number of pumps required increases, in which (a) indicates the lever manipulated variable, (b) indicates the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the switching valves before time t 1 , and (e) shows the state of the switching valves at time t 1  and after time t 1 . 
         FIG. 7  is a time chart which shows the operation of the controller shown in  FIG. 3  when the number of pumps required decreases, in which (a) indicates the lever manipulated variable, (b) indicates the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the switching valves from time t 1  before time t 2 , and (e) indicates the state of the switching valves at time t 2  and after time t 2 . 
         FIG. 8  is a flowchart showing allocation control of the hydraulic pumps by the controller shown in  FIG. 3 . 
         FIG. 9  is a time chart which shows the operation of a conventional hydraulic excavator when the number of pumps required increases, in which (a) indicates the lever manipulated variable, (b) indicates the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the switching valves before time t 1 , and (e) indicates the state of the switching valves at time t 1  and after time t 1 . 
         FIG. 10  is a time chart which shows the operation of the conventional hydraulic excavator when the number of pumps required decreases, in which (a) indicates the lever manipulated variable, (b) shows the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the switching valves from time t 1  before time t 2 , and (e) indicates the state of the switching valves at time t 2  and after time t 2 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, an embodiment of a work machine driving device according to the present invention will be described referring to drawings.  FIG. 1  is a side view which shows a hydraulic excavator  1  including an embodiment of the work machine driving device according to the present invention. 
     The hydraulic excavator  1  as an example of the work machine according to the embodiment of the present invention includes a travel base  101  and a revolving upperstructure  102  over the travel base  101 . The main body is comprised of the travel base  101  and the revolving upperstructure  102 . The travel base  101  has crawler belts on the left and right sides of the main body and traveling motors  10   a ,  10   b  which are hydraulic actuators to give traveling power to the left and right crawler belts. The revolving upperstructure  102  is rotatable with respect to the travel base  101  by means of a bearing mechanism (not shown) interposed between it and the travel base  101  and a swing motor  10   c  as a hydraulic actuator. In the revolving upperstructure  102 , a working device  103  is mounted in front of a main frame  105  and a counterweight  108  is mounted on the back and a cab  104  is mounted on the left front. An engine  106  as a motor and a drive system  107  to be driven by driving power from the engine  106  are housed in the front part of the counterweight  108 . 
     The working device  103  is a front work machine which has a structure comprised of a boom  111 , arm  112  and bucket  113  connected by a link mechanism and makes rotary movement around each link axis to perform excavation work or the like. The working device  103  has a boom cylinder  7   a , an arm cylinder  7   b , and a bucket cylinder  7   c  which rotate the boom  111 , arm  112 , and bucket  113 . 
       FIG. 2  is a circuit configuration diagram which shows the essential part of the driving device provided on the hydraulic excavator  1  shown in  FIG. 1 . In the description of the driving device, response time from an instruction to operation is not taken into consideration. 
     As shown in  FIG. 2 , the drive system  107  as a driving device includes variable displacement closed-circuit hydraulic pumps  2   a  to  2   f  (hereinafter sometimes simply called hydraulic pumps), a hydraulic closed-circuit system in which the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c  and swing motor  10   c  are connected by piping without control valves, and a hydraulic open-circuit system in which variable displacement open-circuit hydraulic pumps  1   a ,  1   b  and the traveling motors  10   a ,  10   b  are connected by piping through a control valve  11  as a hydraulic controller for controlling the supply flow rate and the supply direction. 
     Although the hydraulic closed-circuit system and hydraulic open-circuit system are mixed in this embodiment, the invention is not limited thereto but depending on the application purpose of the work machine, it may be embodied in another form: for example, a hydraulic closed-circuit system is used for all hydraulic actuators. 
     Next, the above hydraulic closed-circuit system will be described. 
     The hydraulic closed-circuit system includes an engine  106 , a power transmission device  15  comprised of a gear mechanism, etc., for example, a total of six closed-circuit hydraulic pumps  2   a  to  2   f  to which driving power as torque and revolving speed is supplied by the engine  106  through the power transmission device  15 , and hydraulic regulators  3   a  to  3   f  as discharge rate varying devices which can vary the discharge rates of the closed-circuit hydraulic pumps  2   a  to  2   f . The hydraulic closed-circuit system includes: the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c  and swing motor  10   c ; electromagnetic valves  12  as connecting devices for enabling hydraulic closed-loop connection of at least one of the closed-circuit hydraulic pumps  2   a  to  2   f  to the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c  and swing motor  10   c ; an operation device  17  which generates a lever manipulated variable as an operation signal to the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c  and swing motor  10   c ; and a controller  16  as a control device which controls the hydraulic regulators  3   a  to  3   f  and the electromagnetic valves  12  depending on the lever manipulated variable of the operation device  17 . 
     The closed-circuit hydraulic pumps  2   a  to  2   f  are two-way discharge mechanisms which can discharge hydraulic oil (pressure oil) from two connection ports of the closed-circuit hydraulic pumps  2   a  to  2   f  in order to give the drive direction and the discharge rate of the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c  and swing motor  10   c . The two-way discharge mechanisms are controlled by the hydraulic regulators  3   a  to  3   f.    
     When hydraulic oil is discharged from one of the two connection ports of the closed-circuit hydraulic pump  2   a  to  2   f  by the two-way discharge mechanisms, connection is made to one of the two connection ports of any hydraulic actuator among the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c  and swing motor  10   c  through the electromagnetic switching valve  12  and the hydraulic oil returned from the other connection port of the two connection ports of the hydraulic actuator is returned to the other connection port of the two connection ports of the closed-circuit hydraulic pump  2   a  to  2   f  through the electromagnetic valves  12 . In short, the hydraulic oil circulates between the closed-circuit hydraulic pump  2   a  to  2   f  and the hydraulic actuator without returning to a hydraulic oil tank  9 , thereby making up a hydraulic closed circuit. 
     In the hydraulic closed-circuit system, the potential energy of the boom  111  and arm  112  which is generated when the boom  111  and arm  112  move down in the direction of gravitational force or when the rotary motion of the revolving upperstructure  102  is stopped and the kinetic energy of the revolving upperstructure  102  are turned into regenerative energy which is transmitted to the return hydraulic oil and conveyed to one of the closed-circuit hydraulic pumps  2   a  to  2   f . At this time, the closed-circuit hydraulic pumps  2   a  to  2   f  perform regenerative operation by the regenerative energy. The regenerative energy is conveyed as driving power to another one of the closed-circuit hydraulic pumps  2   a  to  2   f  which drives another hydraulic actuator through the power transmission device  15 . As a consequence, for the engine  106 , energy is saved by the amount equivalent to the regenerative energy. 
     Although omitted in  FIG. 2 , the hydraulic closed-circuit system includes: a charge pump which increases the circuit pressure to prevent cavitation; a make-up check valve; a flushing valve which absorbs the flow rate difference between the head side and rod side of the hydraulic actuator as a single-rod hydraulic cylinder and changes the hydraulic oil in the closed circuit; and a relief valve which relieves the hydraulic oil when the hydraulic oil pressure reaches a prescribed value or more. 
     The electromagnetic valves  12  include a total of eighteen valves including “BM” switching valves, “AM” switching valves, “BK” switching valves and “SW” switching valves for connecting two or more ones of the closed-circuit hydraulic pumps  2   a  to  2   f  to one of the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c  and swing motor  10   c.    
     Among the electromagnetic switching valves  12 , the “BM” switching valve is a switching valve for connection to the boom cylinder  7   a  which enables connection of at most all the closed-circuit hydraulic pumps  2   a  to  2   f  located upstream of the electromagnetic switching valves  12 . The “AM” switching valve is a switching valve for connection to the arm cylinder  7   b  which enables connection of at most the closed-circuit hydraulic pumps  2   a  to  2   d  among the closed-circuit hydraulic pumps  2   a  to  2   f  located upstream of the electromagnetic switching valves  12 . The “BK” switching valve is a switching valve for connection to the bucket cylinder  7   c  which enables connection of at most all the closed-circuit hydraulic pumps  2   a  to  2   f  located upstream of the electromagnetic switching valves  12 . The “SW” switching valve is a switching valve for connection to the swing motor  10   c  which enables connection of at most the two closed-circuit hydraulic pumps  2   e  and  2   f  among the closed-circuit hydraulic pumps  2   a  to  2   f  located upstream of the electromagnetic switching valves  12 . 
     The form of connection of the electromagnetic switching valves  12  is not limited to the above but another form of connection may be adopted depending on the application purpose of the work machine. 
     The cab  104  where an operator boards is equipped with the operation device  17  to give an operation instruction to each hydraulic actuator. The operation device  17  has operation levers  17   a ,  17   b  which can tilt back and forth and left and right and a detector (not shown) which electrically detects the amount of tilt of the operation lever  17   a ,  17   b  as an operation signal, namely a lever manipulated variable, and outputs the lever manipulated variable detected by the detector as a lever manipulated variable signal to the controller  16  through an electric wire. 
     The operation device  17  has a mechanism which electrically detects the lever manipulated variable but instead it may have another type of mechanism such as a hydraulic mechanism. In the case of a hydraulic mechanism, typically it is a mechanism which has a pilot hydraulic pump separately and reduces the discharge pressure of the hydraulic pump depending on the lever manipulated variable. 
     The controller  16  performs prescribed control calculations and outputs an opening instruction signal to the hydraulic regulators  3   a  to  3   f , outputs a switching valve connection instruction signal to the electromagnetic switching valves  12  to control them. In other words, the controller  16  controls the hydraulic regulators  3   a  to  3   f , the electromagnetic switching valves  12 , and the control valve  11  according to such information as the lever manipulated variable signal outputted from the operation device  17  and hydraulic oil pressure signals outputted from pressure sensors  18   a  to  18   h  connected to the connection ports of the hydraulic actuators. 
     In the hydraulic open-circuit system, as mentioned above, the control valve  11  to give the drive direction and discharge rate of the traveling motors  10   a ,  10   b  is located downstream. The open-circuit hydraulic pumps  1   a  and  1   b  are one-way discharge mechanisms with two connection ports in which one of the two connection ports is connected to the hydraulic oil tank  9  by piping as a suction port for suction from the hydraulic oil tank  9  for storing pressure oil temporarily. The other connection port of the open-circuit hydraulic pumps  1   a ,  1   b  is connected as a discharge port to the connection port of the control valve  11 . The discharge rate from the discharge port is controlled by the one-way discharge mechanism. The one-way discharge mechanism is controlled by the hydraulic regulators  3   g ,  3   h.    
     The return hydraulic oil from the traveling motors  10   a ,  10   b  goes back to the hydraulic oil tank  9  through the control valve  11 . The control valve  11  and the hydraulic regulators  3   g ,  3   h  are controlled depending on the lever manipulated variable generated by the operation device (not shown) provided in the cab  104 . The lever manipulated variable is outputted to the controller  16 . The controller  16  performs control calculations which are different from those for the hydraulic closed-circuit system, makes conversion into an output signal and outputs it to the control valve  11  and the hydraulic regulators  3   g ,  3   h  through an electric wire. 
     Next, the configuration of the controller  16  will be described referring to  FIG. 3 .  FIG. 3  is a block diagram which shows the controller in the driving device shown in  FIG. 2 .  FIG. 4  shows graphs which indicate calculations in the required pump number calculating circuit  30  of the controller  16  shown in  FIG. 3 , in which (a), (b), (c), and (d) are for the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c , and swing motor  10   c , respectively.  FIG. 5  is a table which shows the priority order map of the controller  16  shown in  FIG. 3 . 
     The controller  16  includes a required pump number calculating circuit  30 , a priority order calculating circuit  31 , and a priority order map  32 . The required pump number calculating circuit  30  calculates the number of pumps required to be connected to a hydraulic actuator according to the manipulated variable of the operation levers  17   a ,  17   b  of the operation device  17 , namely the lever manipulated variable. As indicated in  FIG. 4( a )  to  FIG. 4( d ) , the required pump number calculating circuit  30  calculates the number of hydraulic pumps required from the hydraulic oil amount required to drive the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c , and swing motor  10   c , according to the lever manipulated variable signal outputted from the operation device  17  upon operation of the operation levers  17   a ,  17   b .  FIG. 4( a )  to  FIG. 4( d )  show an example that the amount of hydraulic oil increases in proportion to the lever manipulated variable but instead a different specification may be adopted depending on the work machine. 
     The priority order map  32  determines the priority connection relationships between the closed-circuit hydraulic pumps  2   a  to  2   f  and the hydraulic actuators as the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c , and swing motor  10   c  and as shown in  FIG. 5 , the vertical axis represents the hydraulic actuators and the horizontal axis represents the closed-circuit hydraulic pumps  2   a  to  2   f , and for example, “1”, “2” . . . “7” are indicated as priorities in the boxes corresponding to the axes. “-” in a box expresses that the closed-circuit hydraulic pumps  2   a  to  2   f  and hydraulic actuators are not connected through the electromagnetic switching valves  12 . 
     For example, if the actuator to be operated is the boom cylinder  7   a  which requires a high flow rate, when the closed-circuit hydraulic pumps  2   a  to  2   f  are expressed by P 1  to P 6  respectively, the connectable hydraulic pumps are all P 1  to P 6  and the order of connection is P 1 , P 4 , P 2 , P 5 , P 6 , and P 3  in the order of mention. If the actuator to be operated is the arm cylinder  7   b , the connectable hydraulic pumps are P 1  to P 4  and the order of connection is P 2 , P 1 , P 3 , and P 4  in the order of mention. If the actuator to be operated is the bucket cylinder  7   c  which requires a high flow rate, the connectable hydraulic pumps are all P 1  to P 6  and the order of connection is P 3 , P 6 , P 5 , P 5 , P 2 , and P 1  in the order of mention. If the actuator to be operated is the swing motor  10   c  which only requires a low flow rate, the connectable hydraulic pumps are P 5  and P 6  and the order of connection is P 5  and P 6  in the order of mention. The numbers “1” to “7” in the priority order map  32  indicate the priority order in which higher priority is given to connect a given closed-circuit hydraulic pump  2   a  to  2   f  to a hydraulic actuator corresponding to a smaller number. 
     The priority order calculating circuit  31  calculates the allocation of the closed-circuit hydraulic pumps  2   a  to  2   f  to the hydraulic actuators according to the number of pumps required calculated from the manipulated variable of the operation levers  17   a ,  17   b  by the required pump number calculating circuit  30  and the priority order map  32 . A switching valve connection instruction signal for controlling switching of a given electromagnetic switching valve  12  and a hydraulic pump connection instruction for connecting a given closed-circuit hydraulic pump  2   a  to  2   f  are outputted according to the result of calculation by the priority order calculating circuit  31  and switching of the given electromagnetic switching valve  12  is controlled according to the outputted switching valve connection instruction signal and hydraulic pump connection instruction and the closed-circuit hydraulic pumps  2   a  to  2   f  are thus connected to the hydraulic actuators. 
     Furthermore, the priority order calculating circuit  31  is configured so that when the number of pumps allocated to the closed-circuit hydraulic pumps  2   a  to  2   f  increases (in the case of increase in the number of pumps), the closed-circuit hydraulic pumps  2   a  to  2   f  which are not allocated just before the increase or unused are allocated, and also configured so that when the number of plural closed-circuit hydraulic pumps  2   a  to  2   f  allocated to a given hydraulic actuator decreases (in the case of decrease in the number of pumps), the allocation of the closed-circuit hydraulic pumps  2   a  to  2   f  to the other hydraulic actuators than the given hydraulic actuator is maintained. 
     Next, the operation of the controller  16  will be described in further detail referring to  FIGS. 6 to 8 . 
       FIG. 6  is a time chart which shows the operation of the controller  16  shown in  FIG. 3  when the number of pumps required increases, in which (a) indicates the lever manipulated variable, (b) indicates the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the electromagnetic switching valves  12  before time t 1 , and (e) indicates the state of the electromagnetic switching valves  12  at time t 1  and after time t 1 .  FIG. 7  is a time chart which shows the operation of the controller  16  shown in  FIG. 3  when the number of pumps required decreases, in which (a) indicates the lever manipulated variable, (b) indicates the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the electromagnetic switching valves  12  from time t 1  before time t 2 , and (e) indicates the state of the electromagnetic switching valves  12  at time t 2  and after time t 2 .  FIG. 8  is a flowchart which shows allocation control of the closed-circuit hydraulic pumps  2   a  to  2   f  by the controller  16  shown in  FIG. 3 . In  FIG. 6( a )  to  FIG. 6( c )  and  FIG. 7( a )  to  FIG. 7( c ) , the horizontal axis represents time, and the vertical axis represents the lever manipulated variable in  FIG. 6( a )  and  FIG. 7( a )  and the number of pumps required in  FIG. 6( b )  and  FIG. 7( b ) , and the number of pumps used in  FIG. 6( c )  and  FIG. 7( c ) . 
     For the lever manipulated variable at each time, the required pump number calculating circuit  30  calculates the number of pumps required for the hydraulic actuators as the boom cylinder  7   a , arm cylinder  7   b , bucket cylinder  7   c , and swing motor  10   c . The priority order calculating circuit  31  calculates the pump allocation of the closed-circuit hydraulic pumps  2   a  to  2   f  according to the number of pumps required at each time as calculated by the required pump number calculating circuit  30  and referring to the priority order map  32 . 
     (In the Case of Increase) 
     Let&#39;s assume, for example, that as shown in  FIG. 6( b ) , when before time t 1 , the number of pumps required for the boom cylinder  7   a  is “0”, the number of pumps required for the arm cylinder  7   b  is “2”, the number of pumps required for the bucket cylinder  7   c  is “0”, and the number of pumps required for the swing motor  10   c  is “0” (hereinafter expressed as “0, 2, 0, 0”), the operation levers  17   a ,  17   b  are operated at time t 1  and consequently the number of pumps required becomes “1, 2, 0, 0” or the number of pumps required increases by “1, 0, 0, 0”. 
     In this case, as shown in  FIG. 8 , the number of pumps required at time t 1  and after time t 1 , “1, 2, 0, 0” and the number of pumps used before time t 1  “0, 2, 0, 0” are entered in the priority order calculating circuit  31  (step S 1 ) and whether or not the number of pumps required NPr is the number of pumps used NPu or more, namely NPr≧NPu is decided (step S 2 ). In the flowchart shown in  FIG. 8 , control begins at START and upon arrival at RETURN, the sequence returns to START. This control is performed in a predetermined cycle by an internal timer (not shown) provided in the controller  16 . 
     If it is decided at step S 2  that the number of pumps required NPr is the number of pumps used NPu or more (Yes), whether or not the number of pumps required NPr is equal to the number of pumps used NPu, namely NPr=NPu, is decided (step S 3 ). 
     If it is decided at step S 3  that the number of pumps required NPr is equal to the number of pumps used NPu (Yes), calculation for allocation of the closed-circuit hydraulic pumps  2   a  to  2   f  is not newly performed. On the other hand, if it is decided at the step S 3  that the number of pumps required NPr is not equal to the number of pumps used NPu (No), whether or not there is an unused pump among the closed-circuit hydraulic pumps  2   a  to  2   f  is decided (step S 4 ). 
     If it is decided at the step S 4  that there is an unused pump among the closed-circuit hydraulic pumps  2   a  to  2   f  (Yes), the difference between the number of pumps required NPr and the number of pumps used NPu, namely NPr−NPu, is calculated and the unused hydraulic pump is allocated according to the difference (step S 5 ). On the other hand, if it is decided at the step S 4  that there is no unused hydraulic pump (No), a given closed-circuit hydraulic pump  2   a  to  2   f  is allocated to a given hydraulic actuator according to the priority order map  32  (step S 6 ). 
     Specifically, as shown in  FIG. 6( a ) , the lever manipulated variable for the boom cylinder  7   a  is entered at time t 1  in a manner to be combined with the lever manipulated variable entered before time t 1  for the arm cylinder  7   b . As a consequence, as shown in  FIG. 6( b ) , the number of pumps required NPr changes from “0, 2, 0, 0” before time t 1  to “1, 2, 0, 0”. Also, as shown in  FIG. 6( c ) , before time t 1  the pumps used are P 1  and P 2  and the number of pumps used NPu is “0, 2, 0, 0”. Next, the number of pumps required NPr “1, 2, 0, 0” and the number of pumps used NPu “0, 2, 0, 0” are entered in the priority order calculating circuit  31  (step S 1 ) and the number of pumps required NPr and the number of pumps used NPu are compared and it is decided that there is an increase of “0, 1, 0, 0”, namely the number of pumps required NPr is the number of pumps used NPu or more (step S 2 ). Furthermore, it is decided that the number of pumps required NPr is not equal to the number of pumps used NPu (step S 3 ) and the sequence goes to decision about whether or not there is an unused hydraulic pump. In this case, since there are unused hydraulic pumps P 3 , P 4 , P 5 , and P 6 , it is decided that there are unused hydraulic pumps, namely YES (step S 4 ), the hydraulic pump with the highest priority for the boom cylinder  7   a  among the unused hydraulic pumps in the priority order map  32  is allocated (step S 5 ). The priorities of the closed-circuit hydraulic pumps  2   a  to  2   f  for the boom cylinder  7   a  are “6” for P 3 , “2” for P 4 , “4” for P 5 , and “5” for P 6  as shown in  FIG. 5 , so P 4 , which has the highest priority “2”, is allocated. 
     On the other hand, regarding the electromagnetic switching valves  12 , as shown in  FIG. 6( d ) , before time t 1 , as P 1  and P 2  are connected to the arm cylinder  7   b , the “AM” switching valves located downstream of P 1  and P 2  are on. Furthermore, as shown in  FIG. 6( e ) , at time t 1 , as the allocated P 4  is connected to the boom cylinder  7   a , the “BM” switching valve located downstream of P 4  is on. 
     As a consequence, as shown in  FIG. 6( b )  and  FIG. 6( c ) , the number of pumps used NPu after time t 1  meets the number of pumps required NPr and the number of pumps used NPu is the same as the number of pumps required NPr, so the hydraulic actuator operation speed as required is achieved and also the number of switching times of the electromagnetic switching valves  12  is only one for the “BM” switching valve. 
     (In the Case of Decrease) 
     Furthermore, when at the step S 2  the number of pumps required NPr is not the number of pumps used NPu or more (No), namely the number of pumps required NPr has decreased, if the allocation of the closed-circuit hydraulic pumps  2   a  to  2   f  is returned to the original allocation according to the priority order map  32  shown in  FIG. 5 , unnecessary switching of the electromagnetic switching valves  12  would be required as described later; for this reason, the number of pumps as closed-circuit hydraulic pumps  2   a  to  2   f  used to drive the hydraulic actuator for which the number of pumps required decreases is decreased and the connection of the closed-circuit hydraulic pumps  2   a  to  2   f  to another hydraulic actuator than the hydraulic actuator concerned, namely the arm cylinder  7   b , that is the boom cylinder  7   a , is not changed (step S 7 ). 
     Specifically, as shown in  FIG. 7( a ) , at time t 2 , the lever manipulated variable of the arm cylinder  7   b , which is one of the lever manipulated variables of the boom cylinder  7   a  and arm cylinder  7   b  as entered before time t 2 , decreases. As a consequence, as shown in  FIG. 7( b ) , the number of pumps required NPr changes from “1, 2, 0, 0” to “1, 1, 0, 0”. Also, as shown in  FIG. 7( c ) , the pumps used before time t 2  are P 1 , P 2 , and P 4  and the number of pumps used NPu is “1, 2, 0, 0”. Next, as in the case of increase, the number of pumps required NPr “1, 1, 0, 0” and the number of pumps used NPu “1, 2, 0, 0” are entered in the priority order calculating circuit  31  (step S 1 ) and the number of pumps required NPr and the number of pumps used NPu are compared and it is decided that there is a decrease of “0, 1, 0, 0”, namely the number of pumps required NPr is smaller than the number of pumps used NPu (step S 2 ). Then, the number of pumps used as the hydraulic pumps connected to the arm cylinder  7   b  as the hydraulic actuator for which the number of required pump decreases is decreased (step S 7 ). As shown in  FIG. 5  and  FIG. 7( c ) , before time t 2 , the hydraulic pumps connected to the arm cylinder  7   b  are P 1  having priority “2” and P 2  having priority “1” and at time t 2  the decrease in the number of pumps required is “0, 1, 0, 0”, so, as shown in  FIG. 5 , P 1 , which has the lower priority “2”, is unused. 
     On the other hand, regarding the electromagnetic switching valves  12 , as shown in  FIG. 7( d ) , before time t 2 , as P 1  and P 2  are connected to the arm cylinder  7   b , the “AM” switching valves located downstream of P 1  and P 2  are on. Also, as shown in  FIG. 7( e ) , at time t 2 , as the number of pumps required NPr decreases, the “AM” switching valve located downstream of P 1  is off. 
     As a consequence, P 1 , having priority “1” for the boom cylinder  7   a , becomes unused or free but the connection of the closed-circuit hydraulic pumps  2   a  to  2   f  to the other hydraulic actuators than the arm cylinder  7   b  for which the number of pumps required NPr decreases is not changed and as shown in  FIG. 7( c ) , the boom cylinder  7   a  remains connected to P 4  and is not reconnected to P 1 . Also, as shown in  FIG. 7( e ) , regarding the electromagnetic switching valves  12 , after time t 2 , only the “AM” switching valve changes from the on state to the off state and the number of switching times is only one. 
     (Conventional Drive System) 
     Here, operation of the conventional drive system is described referring to  FIGS. 9 and 10 .  FIG. 9  is a time chart which shows the operation of a conventional hydraulic excavator  1  when the number of pumps required increases, in which (a) indicates the lever manipulated variable, (b) indicates the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the electromagnetic switching valves  12  before time t 1 , and (e) indicates the state of the electromagnetic switching valves  12  at time t 1  and after time t 1 .  FIG. 10  is a time chart which shows the operation of the conventional hydraulic excavator  1  when the number of pumps required decreases, in which (a) indicates the lever manipulated variable, (b) indicates the number of pumps required, (c) indicates the number of pumps used, (d) indicates the state of the electromagnetic switching valves  12  from time t 1  before time t 2 , and (e) indicates the state of the electromagnetic switching valves  12  at time t 2  and after time t 2 . The horizontal and vertical axes of  FIG. 9( a )  to  FIG. 9( c )  and  FIG. 10( a )  to  FIG. 10( c )  are the same as those of  FIG. 6( a )  to  FIG. 6( c )  and  FIG. 7( a )  to  FIG. 7( c ) . 
     (In the Case of Increase) 
     As shown in  FIG. 9( b ) , before time t 1 , the number of pumps required for the boom cylinder  7   a  is “0”, the number of pumps required for the arm cylinder  7   b  is “2”, the number of pumps required for the bucket cylinder  7   c  is “0”, and the number of pumps required for the swing motor  10   c  is “0” and this is expressed as “0, 2, 0, 0”. In this case, since the number of pumps required for the arm cylinder  7   b  is “2” and the other hydraulic actuators require no pumps, referring to the priority order map  32  shown in  FIG. 5 , pumps with higher priorities, namely P 2  and P 1  in the order of mention, are allocated to the arm cylinder  7   b.    
     At time t 1 , as shown in  FIG. 9( a ) , the lever manipulated variable for the boom cylinder  7   a  is entered at time t 1  in a manner to be combined with the lever manipulated variable entered before time t 1  for the arm cylinder  7   b . As a consequence, as shown in  FIG. 9( b ) , the number of pumps required changes from “0, 2, 0, 0” to “1, 2, 0, 0”, which means an increase of “1, 0, 0, 0” or change in the number of pumps required for the boom cylinder  7   a  from “0” to “1”. In this case, referring to the priority order map  32  shown in  FIG. 5 , in the closed-circuit hydraulic pumps  2   a  to  2   f  to be allocated to the boom cylinder  7   a , the priority of P 1  is “1”, the priority of P 4  is “2”, and the priority of P 2  is “3” and in the closed-circuit hydraulic pumps  2   a  to  2   f  to be allocated to the arm cylinder  7   b , the priority of P 2  is “1”, the priority of P 1  is “2”, and the priority of P 3  is “3” and thus P 1  has priority “1” for the boom cylinder  7   a  and priority “2” for the arm cylinder  7   b . Therefore, as shown in  FIG. 9( c ) , P 1  is reallocated from the arm cylinder  7   b  to the boom cylinder  7   a  for which it has higher priority, and P 3 , which has priority “3” or next to P 1  in priority, is allocated to the arm cylinder  7   b.    
     In this case, as shown in  FIG. 9( c ) , the number of pumps used meets the number of pumps required and thus the operation speed to be achieved by the number of pumps required is achieved, but as shown in  FIG. 9( d )  and  FIG. 9( e ) , regarding the electromagnetic switching valves  12 , the “AM” switching valve located downstream of P 1  for connection to the arm cylinder  7   b  is changed from the on state to the off state, the “BM” switching valve located downstream of P 1  for connection to the boom cylinder  7   a  is changed from the off state to the on state and the “AM” switching valve located downstream of P 3  for connection to the arm cylinder  7   b  is changed from the off state to the on state, so three switching times of the electromagnetic switching valves  12  are required. 
     On the other hand, as shown in  FIG. 10( a )  and  FIG. 10( b ) , in the period from time t 1  to before time t 2 , P 4  is connected to the boom cylinder  7   a  and P 1  and P 2  are connected to the arm cylinder  7   b  and at time t 2 , the number of pumps required changes from “1, 2, 0, 0” to “1, 1, 0, 0”, which means a decrease of “0, 1, 0, 0” or change in the number of pumps required for the arm cylinder  7   b  from “2” to “1”. In this case, referring to the priority order map  32  shown in  FIG. 5 , P 1  has priority “1” for the boom cylinder  7   a  and priority “2” for the arm cylinder  7   b  and P 4  has priority “2” for the boom cylinder  7   a . Therefore, as shown in  FIG. 10( c ) , P 1  is reallocated from the arm cylinder  7   b  to the boom cylinder  7   a  for which it has higher priority, and P 4  becomes unused. 
     Regarding the electromagnetic switching valves  12 , as shown in  FIG. 10( d )  and  FIG. 10( e ) , the “AM” switching valve for connection of P 1  to the arm cylinder  7   b  is changed from the on state to the off state, the “BM” switching valve for connection of P 1  to the boom cylinder  7   a  is changed from the off state to the on state and the “BM” switching valve for connection of P 4  to the boom cylinder  7   a  is changed from the on state to the off state, so three switching times of the electromagnetic switching valves  12  are required. 
     On the other hand, in the drive system  107  according to the above embodiment of the present invention, for example, as shown in  FIG. 6( b ) , if at time t 1  the number of pumps required NPr is changed from “0, 2, 0, 0” to “1, 2, 0, 0”, or in the case of an increase of “1, 0, 0, 0”, and if there is an unused hydraulic pump, the unused hydraulic pump is first allocated as the hydraulic pump to be added when allocating given closed-circuit hydraulic pumps  2   a  to  2   f  to a given hydraulic actuator according to the priority order map  32 . In other words, as shown in  FIG. 5 , since the priorities of the closed-circuit hydraulic pumps  2   a  to  2   f  for the boom cylinder  7   a  are “6” for P 3 , “2” for P 4 , “4” for P 5 , and “5” for P 6  as shown in  FIG. 5 , P 4 , having the highest priority, is allocated. 
     As a consequence, the number of pumps used NPu meets the number of pumps required NPr and the number of pumps used NPu is the same as the number of pumps required NPr and thus the hydraulic actuator operation speed as required is achieved and also as shown in  FIG. 6( e ) , regarding the electromagnetic switching valves  12 , at time t 1  only the “BM” switching valve for connection of P 4  to the boom cylinder  7   a  is changed from the off state to the on state, so the number of switching times of the electromagnetic switching valves  12  is only one. Therefore, when during operation of one hydraulic actuator another hydraulic actuator is started, namely even when the number of hydraulic pumps to be allocated increases, an unused hydraulic pump is connected to the hydraulic actuator to be started without changing the connection of the hydraulic pump connected to the one hydraulic actuator, so that whereas the number of electromagnetic valve switching times is 3 in the case of increase in the number of pumps required in the conventional drive system, the number of switching times of the electromagnetic switching valves  12  is 1 in the drive system  107  according to the embodiment of the present system, so unnecessary switching control of the electromagnetic switching valves  12  is reduced. 
     Furthermore, for example, as shown in  FIG. 7( b ) , when at time t 2  the number of pumps required NPr is changed from “1, 2, 0, 0” to “1, 1, 0, 0” or in the case of decrease of “0, 1, 0, 0”, if as shown in  FIG. 5  and  FIG. 7( c ) , P 1  having priority “2” and P 2  having priority “1” are used before time t 2 , P 1 , having the lower priority “2” for the arm cylinder  7   b  for which the number of pumps required NPr decreases, is unused. 
     As a consequence, P 1 , having priority “1” for the boom cylinder  7   a , becomes unused but the connection of the closed-circuit hydraulic pumps  2   a  to  2   f  to the other hydraulic actuators than the arm cylinder  7   b  for which the number of pumps required NPr decreases is not changed and the boom cylinder  7   a  remains connected (allocated) to P 4  and is not reconnected to P 1 . Also, as shown in  FIG. 7( e ) , regarding the electromagnetic switching valves  12 , at time t 2  only the “AM” switching valve for connection of P 1  to the arm cylinder  7   b  is changed from the on state to the off state and the number of switching times of the electromagnetic switching valves  12  is only one. 
     As a consequence, the number of pumps used NPu meets the number of pumps required NPr, so the hydraulic actuator operation speed as required is achieved and as shown in  FIG. 7( e ) , at time t 2  the number of switching times of the electromagnetic switching valves  12  is 1. Therefore, if during operation of a plurality of hydraulic actuators the flow rate to one hydraulic actuator is decreased for deceleration, namely even if the number of hydraulic pumps to be allocated decreases, a hydraulic pump is unallocated from the hydraulic actuator to which the flow rate is decreased and the allocation of the hydraulic pumps to the other hydraulic actuators is maintained, so whereas the number of electromagnetic valve switching times is 3 in the case of decrease in the number of pumps required in the conventional drive system, the number of switching times of the electromagnetic switching valves  12  is 1, so unnecessary switching control of the electromagnetic switching valves  12  is reduced. 
     As mentioned so far, in both the cases of increase and decrease in the number of pumps required, the number of switching times of the electromagnetic switching valves  12  can be decreased, so the frequency of shock caused by hydraulic oil pressure variation during switching of the electromagnetic switching valves  12  can be reduced, which leads to reduction of vehicle body vibration, improved operability and improvement in the life of the components including the electromagnetic switching valves  12 . In addition, thanks to the decrease in the number of switching times of the electromagnetic switching valves  12 , the power consumption for switching of the electromagnetic switching valves  12  can be reduced. 
     Other Embodiments 
     The present invention is not limited to the above embodiment but the invention includes various modified embodiments. For example, the above embodiment has been described for easy understanding of the invention and the invention is not limited to an embodiment which includes the whole configuration described above. 
     Also, in the above embodiment, an explanation has been made by taking a case that the drive system  107  is mounted on the hydraulic excavator  1  as an example; however the present invention is not limited thereto but the invention can be applied to work machines other than the hydraulic excavator  1 , such as hydraulic cranes and wheel loaders, if they have hydraulically drivable hydraulic actuators. 
     Furthermore, the closed-circuit hydraulic pumps  2   a  to  2   f  of the drive system  107  according to the above embodiment may have the same discharge capacity or may have different discharge capacities. 
     The above embodiment is configured so that the number of switching times of the electromagnetic switching valves  12  is decreased in both the cases of increase and decrease in the number of pumps required; however, instead a process according to the present invention may be performed so that the number of switching times of the electromagnetic switching valves  12  is decreased only in the case of increase in the number of pumps required or only in the case of decrease in the number of pumps required. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  . . . hydraulic excavator (work machine), 
               1   a ,  1   b  . . . open-circuit hydraulic pump, 
               2   a  to  2   f  . . . closed-circuit hydraulic pump (hydraulic pump), 
               3   a  to  3   g  . . . hydraulic regulator, 
               7   a  . . . boom cylinder (hydraulic actuator), 
               7   b  . . . arm cylinder (hydraulic actuator), 
               7   c  . . . bucket cylinder (hydraulic actuator), 
               9  . . . hydraulic oil tank, 
               10   a ,  10   b  . . . traveling motor, 
               10   c  . . . swing motor (hydraulic actuator), 
               11  . . . control valve, 
               12  . . . electromagnetic switching valve (switching valve), 
               15  . . . power transmission device, 
               16  . . . controller, 
               17  . . . operation device (operation part), 
               17   a ,  17   b  . . . operation lever, 
               18   a  to  18   h  . . . pressure sensor, 
               30  . . . required pump number calculating circuit, 
               31  . . . priority order calculating circuit, 
               32  . . . priority order map, 
               101  . . . travel base, 
               102  . . . revolving upperstructure, 
               103  . . . working device, 
               104  . . . cab, 
               105  . . . main frame, 
               106  . . . engine, 
               107  . . . drive system (driving device), 
               108  . . . counterweight, 
               111  . . . boom, 
               112  . . . arm, 
               113  . . . bucket