Patent Publication Number: US-11377814-B2

Title: Energy recuperation system and method for construction equipment

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. § 371 national stage application of PCT International Application No. PCT/KR2017/012624 filed on Nov. 8, 2017, the disclosure and content of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to energy recuperation system and method for construction equipment. 
     BACKGROUND ART 
     Construction equipment, for example, an excavator generates large force using hydraulic pressure. The force enables a work unit of the excavator to excavate earth and sand/solid rock or dump excavated earth and sand/solid rock. 
     In order to use this hydraulic pressure, a hydraulic pump pumps up oil stored in an oil tank and supplies the oil as a pressurized oil to an actuator that actuates the work unit. An engine needs to be operated to drive the hydraulic pump and fuel needs to be consumed to operate the engine. 
     Energy recuperation technology has been used to increase fuel efficiency of construction equipment by reducing fuel consumption. The energy recuperation technology has a mechanism that charges an accumulator with a pressurized oil, which has been supplied to the actuator while the work unit freely drops, without discharging the pressurized oil to the oil tank and then supplies the charged oil to another hydraulic component. 
     According to the energy recuperation technology, an energy recuperation ratio may be low, depending on a pressure condition of the accumulator, or in order to increase an energy recuperation ratio, the response speed of an excavator may be reduced. Accordingly, energy is not efficiently recuperated. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     An object of the present invention is to provide energy recuperation system and method for construction equipment, the system and method being able to improve energy recuperation efficiency by maintaining a dischargeable lowest limit pressure of an accumulator at an optimum level in energy recuperation while construction equipment is in operation. 
     Another object of the present invention is to provide energy recuperation system and method for construction equipment, the system and method being able to increase not only a response speed of construction equipment, but also energy recuperation efficiency. 
     Solution to Problem 
     According to an exemplary embodiment of the present invention, there is provided an energy recuperation system for construction equipment including: an actuator driving upward and downward operations of a work unit; an accumulator connected to the actuator; and a controller determining a predicted downward mode associated with the downward operation of the work unit, regulating a dischargeable lowest limit pressure of the accumulator to a target pressure corresponding to the predicted downward mode, and charging the accumulator having the dischargeable lowest limit pressure regulated to the target pressure with pressurized oil discharged from the actuator during the downward operation of the work unit to recuperate energy. 
     The system may further include: a memory configured to store information associated with the predicted downward mode and the target pressure, and to be controlled by the controller, in which the predicted downward mode may include: a first predicted downward mode where the work unit has a first downward acceleration force at the downward operation; and a second predicted downward mode where the work unit has a second downward acceleration force at the downward operation, the second downward acceleration force being less than the first downward acceleration force, in which the target pressure may include: a first target pressure; and a second target pressure having a higher pressure level than the first target pressure, in which the controller may be configured to correspond the first predicted downward mode with the first target pressure, and to correspond the second predicted downward mode with the second target pressure. 
     The system may further include: a hydraulic pump configured to supply pressurized oil to the actuator; an assist motor configured to assist an engine to drive the hydraulic pump; an assist passage connecting the accumulator and the assist motor to each other; and an assist valve disposed in the assist passage and configured to control supply of the pressurized oil charged in the accumulator to the assist motor through the assist passage, in which the controller may control opening/closing of the assist valve so that the dischargeable lowest limit pressure of the accumulator reaches the first target pressure or the second target pressure. 
     The accumulator may include a plurality of sub-accumulators having different initial pressures, and the controller may charge a sub-accumulator, which has an initial pressure corresponding to the target pressure, of the sub-accumulators with the pressurized oil. 
     The system may further include: a charge passage connecting the accumulator and the actuator to each other; and a charge valve disposed in the charge passage, in which the controller may regulate pressure of the pressurized oil to be supplied into the accumulator by controlling the charge valve. 
     The system may further include a motion sensor configured to measure information about one of upward and downward operations of the work unit, in which the controller may acquire upward/downward operation pattern information by analyzing information measured by the motion sensor and may determine the predicted downward mode on the basis of the upward/downward operation pattern information. 
     The system may further include: a lower driving structure; an upper swing structure on which the work unit is mounted; and a swing module connecting the upper swing structure rotatably to the lower driving structure, in which the controller may additionally charge the accumulator, which has been charged with the pressurized oil during downward of the work unit under the second target pressure, with pressurized oil discharged from the swing module while the swing module stops a swing operation. 
     According to another exemplary embodiment of the present invention, there is provided an energy recuperation method for construction equipment including: determining a predicted downward mode of a work unit; regulating a dischargeable lowest limit pressure of an accumulator to a target pressure on the basis of the predicted downward mode; and charging the accumulator having the dischargeable lowest limit pressure regulated to the target pressure with pressurized oil discharged from the actuator actuating the work unit during downward of the work unit to recuperate energy. 
     The determining of the predicted downward mode of the work unit may further include: referring to a memory storing information about the predicted downward mode and the target pressure wherein the predicted downward mode includes a first predicted downward mode and a second predicted downward mode and the target pressure includes a first target pressure and a second target pressure having a higher pressure level than the first target pressure, and the regulating of the dischargeable lowest limit pressure of the accumulator to the target pressure on the basis of the predicted downward mode may include setting the dischargeable lowest limit pressure to the first target pressure to correspond to the first predicted downward mode, or setting the dischargeable lowest limit pressure to the second target pressure to correspond to the second predicted downward mode. 
     The regulating of the dischargeable lowest limit pressure of the accumulator to the target pressure on the basis of the predicted downward mode may include regulating opening/closing of an assist valve such that the dischargeable lowest limit pressure reaches the first target pressure or the second target pressure while the pressurized oil charged in the accumulator is discharged to an assist motor. 
     The accumulator may include a plurality of sub-accumulators having different initial pressures, and the regulating of the dischargeable lowest limit pressure of the accumulator to the target pressure on the basis of the predicted downward mode may include selecting a sub-accumulator having an initial pressure corresponding to the first target pressure or the second target pressure from the sub-accumulators as an object to be charged with the pressurized oil discharged from the actuator. 
     The selecting of the sub-accumulator having the initial pressure corresponding to the first target pressure or the second target pressure from the sub-accumulators as an object to be charged with the pressurized oil discharged from the actuator may include making some of the sub-accumulators be objects to be charged with pressurized oil by selectively opening/closing selection valves disposed for the sub-accumulators, respectively. 
     The accumulator may include a plurality of sub-accumulators having different initial pressures, and the charging of the accumulator having the dischargeable lowest limit pressure regulated to the target pressure with pressurized oil discharged from the actuator actuating the work unit during downward of the work unit to recuperate energy may include sequentially charging the sub-accumulators with the pressurized oil discharged from the actuator such that the dischargeable lowest limit pressure of the accumulator reaches the first target pressure and the second target pressure with an interval. 
     The determining of the predicted downward mode may include selecting one of the first predicted downward mode and the second predicted downward mode as the predicted downward mode on the basis of work input from an operator through a work selector. 
     The first predicted downward mode may be a mode where the work unit is turned down with a first downward acceleration force and the second predicted downward mode may be a mode where the work unit is turned down with a second downward acceleration force less than the first downward acceleration force. 
     The method may further include acquiring upward/downward operation pattern information by analyzing one of upward and downward operations of the work unit, in which the determining of the predicted downward mode of the work unit may include determining the predicted downward mode as one of the first predicted downward mode and the second predicted downward mode on the basis of the upward/downward operation pattern information. 
     The method may further include acquiring upward/downward operation pattern information by analyzing one of upward and downward operations of the work unit, in which the acquiring of the upward/downward operation pattern information by analyzing one of upward and downward operations of the work unit may include acquiring upward operation pattern information by analyzing upward factor information associated with upward of the work unit. 
     The upward operation pattern information may include one of a stroke value of the actuator, an operation amount of an operation lever for the actuator, operation time of the operation lever, and an upward acceleration value of the work unit. 
     The determining of the predicted downward mode of the work unit may include determining the predicted downward mode as the first predicted downward mode if the stroke value is larger than a reference stroke value, or determining the predicted downward mode as the second predicted downward mode if the stroke value is smaller than the reference stroke value. 
     The method may further include determining a starting time to charge the accumulator on the basis of the upward/downward operation pattern information. 
     Advantageous Effects of Invention 
     According to the energy recuperation system and method of the present invention, since a dischargeable lowest limit pressure of an accumulator is regulated to a target pressure suitable for a downward operation of a work unit to recuperate energy while construction equipment is in operation, pressurized oil discharged from the work unit can be maximally supplied into the accumulator, so a high energy recuperation ratio can be achieved. 
     Further, when the accumulator is charged with the pressurized oil, the difference between the pressure of the pressurized oil discharged from the work unit and the dischargeable lowest limit pressure of the accumulator is reduced, so a loss of pressure (a loss of energy) due to the difference can also be reduced. 
     Further, since the dischargeable lowest limit pressure of the accumulator is regulated, the energy of the pressurized oil discharged from the work unit can be recuperated and the response speed of operations of the work unit can be improved. 
     Further, since the dischargeable lowest limit pressure of the accumulator is regulated by combining different initial pressures of a plurality of sub-accumulators, it is possible to provide various designs to the accumulator in terms of energy recuperation and response speed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing construction equipment  100  having an energy recuperation system for construction equipment according to an embodiment of the present invention. 
         FIG. 2  is a conceptual view showing the main configuration of the energy recuperation system for construction equipment shown in  FIG. 1 . 
         FIG. 3  is a control block diagram of the construction equipment  100  for illustrating additional components of the energy recuperation system shown in  FIG. 2 . 
         FIG. 4  is a flow chart illustrating an energy recuperation method for construction equipment according to another embodiment of the present invention. 
         FIG. 5  is a flow chart illustrating in detail determining a predicted downward mode shown in  FIG. 4  (S 1 ). 
         FIG. 6  is a flow chart illustrating in detail regulating dischargeable lowest limit pressure of an accumulator shown in  FIG. 4  (S 3 ). 
         FIG. 7  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a first predicted downward mode. 
         FIG. 8  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a second predicted downward mode. 
         FIG. 9  is a conceptual view showing the main configuration of an energy recuperation system for construction equipment according to another embodiment of the present invention. 
         FIG. 10  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a first predicted downward mode in the construction equipment  200  shown in  FIG. 9 . 
         FIG. 11  is a conceptual view showing the main configuration of an energy recuperation system for construction equipment according to still another embodiment of the present invention. 
         FIG. 12  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a second predicted downward mode in the construction equipment  300  shown in  FIG. 11 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, energy recuperation system and method for construction equipment according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings. The same and like reference numerals are used for the same and like components herein even in different embodiments and the latter description refers to the earlier description. 
       FIG. 1  is a perspective view showing construction equipment  100  having an energy recuperation system for construction equipment according to an embodiment of the present invention. 
     Referring to the figure, the construction equipment  100  will be described by exemplifying an excavator. The excavator is given reference number ‘ 100 ’ hereafter, the same as the construction equipment  100 . However, the construction equipment  100  is not limited to an excavator. The construction equipment  100  may include a back-hoe and a dragline as long as they have a work unit that is hydraulically turned up and down such as a boom or an arm. 
     The excavator  100  may include a lower driving structure  10 , an upper swing structure  20 , work units  31 ,  33 , and  35 , actuators  41 ,  43 , and  45 , and a swing module  47 . 
     The lower driving structure  10  is disposed at a lower portion in the excavator  100  and is in charge of moving the excavator  100 . The lower driving structure  10 , in detail, includes a frame  11  and crawlers  16 . The frame  11  has a substantially rectangular top. The crawlers  16  are coupled to both sides of the frame  11  and protrude up further than the frame  11 . The crawlers  16  are rotated by power from an engine or an electric motor so that the excavator  100  can move. Unlike the crawler excavator, wheels and covers that cover the wheels may be employed instead of the crawlers  16  in a wheel excavator. 
     The upper swing structure  20  is disposed at an upper portion in the excavator  100  and is in direct charge of work by the excavator  100 . To this end, the boom  31  of the work units  31 ,  33 , and  35  is rotatably mounted on the upper swing structure  20 . Further, the upper swing structure  20  may have a cab  21  and a machine room  26 . An operator controls the work units  31 ,  33 , and  35  by operating an operation lever  96  (see  FIG. 3 ) in the cab  21 . Hydraulic machines such as a hydraulic pump  53  (see  FIG. 2 ) are disposed in the machine room  26  and drive the actuators  41 ,  43 , and  45  using hydraulic power. 
     The work units  31 ,  33 , and  35  are components that directly perform various works on earth and sand, or solid rocks, for example, digging and grading, using hydraulic power. The work units  31 ,  33 , and  35 , in detail, may include a boom  31 , an arm  33 , and a bucket  35 . The boom  31  is rotatably connected to the upper swing structure  20  and the free end of the boom  31  can be moved along an arc-shaped path. The arm  33  is also rotatably connected to the free end of the boom  31 . The arm  33  may be shorter than the boom  31 . The bucket  35  is rotatably connected to the free end of the arm  33  and has a structure that can load earth and sand therein. Instead of the bucket  35 , a ripper or a crusher may be coupled to the arm  33 . 
     The actuators  41 ,  43 , and  45  actuate the work units  31 ,  33 , and  35  by supplying hydraulic power to the work units  31 ,  33 , and  35 . The actuators  41 ,  43 , and  45 , in detail, may include a boom cylinder  41 , an arm cylinder  43 , and a bucket cylinder  45 . The boom cylinder  41  connects the upper swing structure  20  and the arm cylinder  43  to turn up and down the boom  31  by stretching and contracting. The arm cylinder  43  connects the boom  31  and the arm  33  to each other to turn up and down the arm  33 . Similarly, the bucket cylinder  45  connects the bucket  35  and the arm  33  to each other to turn up and down the bucket  35 . 
     The swing module  47  connects the lower driving structure  10  and the upper swing structure  20  to each other. Further, the swing module  170  includes parts such as a swing bearing that enables the upper swing structure  20  to swing with respect the lower driving structure  10  and a swing motor that generates hydraulic force for a swing operation. 
     The energy recuperation system and method according to the present invention are described with a focus on the boom  31  of the work units  31 ,  33 , and  35 . Accordingly, the actuators  41 ,  43 , and  45  are described with a focus on the boom cylinder  41  associated with the boom  31 . Even though described with a focus on the boom  31  and the boom cylinder  41 , the energy recuperation system and method can be equivalently applied to the arm  33  and the arm cylinder  43 , etc. 
       FIG. 2  is a conceptual view showing the main configuration of the energy recuperation system for construction equipment shown in  FIG. 1 . 
     Referring to  FIG. 2 , the energy recuperation system may include a pressurized oil production module, a pressurized oil guide module, a pressurized flow control module, and a pressurized oil storage module. 
     The pressurized oil production module produces a pressurized oil at high pressure, for example, a pressurized oil having pressure required for the boom cylinder  41  from oil at the atmospheric pressure. The pressurized oil production module may include an engine  51 , a hydraulic pump  53 , an assist motor  55 , and an oil tank  57 . The engine  51  generates mechanical torque by burning fuel such as diesel. The hydraulic pump  53  is rotated by the torque from the engine  51 , thereby pumping the oil in the oil tank  57  as the pressurized oil. The assist motor  55  is disposed between the engine  51  and the hydraulic pump  53  and assists the engine  51  to rotate the hydraulic pump  53 . The assist motor  55  is a hydraulic motor that is operated by hydraulic pressure. 
     The pressurized oil guide module has passages for guiding the pressurized oil discharged from the hydraulic pump  53  to the boom cylinder  41 , the assist motor  55 , the oil tank  57 , or an accumulator  81 . The passages, in detail, may include an output passage  61 , a supply passage A  63 , a supply passage B  64 , an assist passage  65 , and a charge passage  67 . The output passage  61  means a passage through which the pressurized oil is discharged from the hydraulic pump  53 . The supply passage A  63  is connected to the output passage  61  and to a chamber A  41   a  of the boom cylinder  41 . The supply passage B  64  is connected to the output passage  61  and to a chamber B  41   b  of the boom cylinder  41 . The assist passage  65  connects the assist motor  55  and the accumulator  81  to each other. The charge passage  67  connects the supply passage A  63  and the accumulator  81  to each other. Further, there may be provided a bridge passage  68  connecting the supply passage A  63  and the supply passage B  64  to each other and a return passage  69  connecting the supply passage A  63  and the supply passage B  64  to the oil tank  57 . 
     The pressurized flow control module controls flow of the pressurized oil in the passages by opening/closing the passages. The pressurized flow control module may include a supply valve A  71 , a supply valve B  72 , a return valve A  73 , a return valve  74 , a bridge valve  75 , an assist valve  77 , and a charge valve  79 . The supply valve A  71  is disposed in the supply passage A  63  and controls the pressurized oil that is supplied to the chamber A  41   a  of the boom cylinder  41  through the output passage  61  and the supply passage A  63 . The supply valve B  72  is disposed in the supply passage B  64  and controls the pressurized oil that is supplied to the chamber B  41   b  of the boom cylinder  41  through the output passage  61  and the supply passage B  64 . The return valve A  73  and the return valve B  74  open/close the passage for returning the pressurized oil from the chamber A  41   a /chamber B  41   b  of the boom cylinder  41  to the oil tank  57 . The bridge valve  75  is disposed in the bridge passage  68  and controls the pressurized oil that is supplied from one of the chamber A  41   a  and the chamber B  41   b  to the other one. The assist valve  77  controls the pressurized oil that is supplied to the assist module  55  from the accumulator  81 . The charge valve  79  is disposed in the charge passage  67  and opened/closed so that the pressurized oil discharged from the chamber  41   a  is supplied into the accumulator  81  or stops being supplied. 
       FIG. 3  is a control block diagram of the construction equipment  100  for illustrating additional components of the energy recuperation system shown in  FIG. 2 . 
     Referring to this figure, the excavator  100 , in addition to the engine  51  and valves  71 ,  72 ,  73 ,  74 ,  75 ,  77 , and  79 , may further include a controller  91 , a motion sensor  93 , a work selector  95 , an operation lever  96 , and a memory  97 . 
     The controller  91  is electrically connected to the engine  51 , the valves  71 ,  72 ,  73 ,  74 ,  75 ,  77 , and  79 , the motion sensor  93 , and the like, thereby controlling them or receiving information from them. The controller  91  controls the assist valve  77 , the charge valve  79  etc. to recuperate energy from the pressurized oil discharged from the boom cylinder  41 , which will be described with reference to  FIG. 4  etc. 
     Referring back to  FIG. 3 , the motion sensor  93  acquires information about an upward operation and a downward operation of the boom  31 . To this end, a sensor that measures upward angle/upward acceleration/stroke of the boom  31  or a sensor that measures operation amount/operation time of the operation lever  96  may be employed as the motion sensor  93 . 
     The work selector  95  is provided to select next works to be performed by an operator. The controller  91  can predict information about an upward operation and a downward operation of the boom  31  during working from a selected work. The work selector  95  may be a manual button for selecting exemplary works or a touch button on a control screen. 
     The operation lever  96  produces instructions for an upward operation and a downward operation of the boom  31  when being operated by the operator, and inputs the instructions to the controller  91 . 
     The memory  97  stores information on a predicted downward mode associated with a downward operation of the boom  31  and a target pressure of the accumulator  81  (see  FIG. 2 ). The predicted downward mode is divided into a first predicted downward mode and a second predicted downward mode on the basis of the downward acceleration force of the boom  31 . The downward acceleration force is larger in the first predicted downward mode than in the second predicted downward mode. The target pressure is a target pressure value for setting the dischargeable lowest limit pressure of the accumulator  81 . The dischargeable lowest limit pressure means the lower limit of pressure that the accumulator  81  can have in consideration of the efficiency of recuperating energy from the boom cylinder  41  under the assumption that a pressurized oil is maximally discharged and sent to the assist motor  55  from the accumulator  81 . 
     The target pressure may be divided into a first target pressure and a second target pressure higher than the first target pressure. A control program may be stored in the memory  97 . The control program may contain instructions to correspond the first target pressure with the first predicted downward mode and the second target pressure with the second predicted downward mode. 
     The energy recuperation method is described hereafter with reference to  FIGS. 4 to 6  on the basis of the above description. 
       FIG. 4  is a flow chart illustrating an energy recuperation method for construction equipment according to another embodiment of the present invention. 
     Referring to this figure (and  FIGS. 1 to 3 ), the energy recuperation method may include determining a predicted downward mode (S 1 ), regulating a dischargeable lowest limit pressure of the accumulator (S 3 ), and charging the accumulator (S 5 ). 
     First, in the determining of the predicted downward mode (S 1 ), the controller  91  predicts which mode the boom  31  is turned down in after an upward operation. As described above, the controller  91  determines whether the predicted downward mode is the first predicted downward mode or the second predicted downward mode. The predicted downward modes are obtained by predicting actual downward operations of the boom  31 , but the actual downward operations may not follow the predicted downward modes. 
     In the regulating of the dischargeable lowest limit pressure of the accumulator (S 3 ), the controller  91  differently regulates the dischargeable lowest limit pressure of the accumulator  81 , depending on the predicted downward modes. In other words, the controller  91  should increase or decrease the dischargeable lowest limit pressure of the accumulator  81 . 
     In the charging of the accumulator (S 5 ), the controller  91  opens the charge valve  79  so that the accumulator  81  is charged with the pressurized oil in the chamber A  41   a  of the boom cylinder  41  through the charge passage  67 . The accumulator  81  is charged while the boom  31  is actually turned down. In detail, the pressurized oil in the chamber A  41   a  of the boom cylinder  41  is not discharged to the oil tank  57 , but supplied into the accumulator  81  to turn down the boom  31 , thereby recuperating the energy of the pressurized oil. Further, as the pressurized oil in the chamber A  41   a  is discharged to the accumulator  81 , the boom  31  is freely dropped by its own weight. 
     The controller  91  can regulate the flow rate of the pressurized oil to be supplied into the accumulator  81  by controlling the charge valve  79 . The control of a flow rate is in connection with the pressure of the pressurized oil that is supplied into the accumulator  81 . Accordingly, as the pressure of the pressurized oil is regulated, the speed of the pressurized oil discharged from the chamber A  41   a  can be regulated. This means that the downward speed of the boom  31  is regulated, so the response speed of the excavator  100  can be regulated. 
     The determining of the predicted downward mode (S 1 ) is described hereafter with reference to  FIG. 5 . 
       FIG. 5  is a flow chart illustrating in detail the determining of the predicted downward mode (S 1 ). 
     Referring to this figure (and  FIGS. 1 to 3 ), the controller  91  analyzes first (actual) upward and downward operations of the boom  31  to determine a predicted downward mode of the boom  31  (S 11 ). Operation information about one or several upward operations and downward operations of the boom  31  is stored in the memory  97  to analyze operations of the boom  31 . The operation information may include upward angle/acceleration force or the like when the boom  31  is turned up and downward angle/acceleration force when the boom  31  is turned down. The controller  91  can analyze the upward and downward operations for those operations with reference to the memory  97 . 
     Next, the controller  91  determines whether it is possible to acquire upward/downward operation pattern information through the operation analysis, and if possible, it can acquire the information (S 13  and S 15 ). The upward/downward operation pattern information is defined by finding predetermined patterns in the upward operations and the downward operations from the operation information. The upward/downward operation pattern information may include, for example, information that the boom  31  is quickly turned up and also quickly turned down. Further, the upward/downward operation pattern information may include information about the interval between the end of the upward operation and the beginning of the downward operation. Accordingly, the controller  91  can determine when to charge the accumulator  81  on the basis of the information about the interval. 
     The controller  91  may refer to only upward operation pattern information as a part of the upward/downward operation pattern information (S 17 ). The upward operation pattern information means which pattern the upward/downward operation shows. For example, the upward/downward operation pattern information is information about whether the boom  31  has been turned up a little or a lot. Under a common work environment, when the boom  31  has been turned up a little, the boom  31  will be turned down at a low acceleration force, while when the boom  31  has been turned up a lot, the boom  31  will be turned down at a large acceleration force. The controller  91  can determine the predicted downward mode on the basis of this estimation. The upward operation pattern information can be acquired by analyzing upward factor information associated with the upward operation of the boom  31 . The upward factor information, for example, may be any one of an upward acceleration value of the boom  31 , an upward stroke value of the boom cylinder  41  driving the boom  31 , and the operation amount or operation time of the operation lever  96  driving the boom cylinder  41 . 
     Accordingly, it is possible to determine a predicted downward mode of the boom  31  as a first predicted downward mode and a second predicted downward mode on the basis of the information about the upward operations and the downward operations included in the upward/downward operation pattern information (S 19 ). For example, if the downward operation with large downward acceleration force is repeated after the upward operation, the controller  91  can determine the predicted downward mode as the first predicted downward mode. 
     Unlikely, the controller  91  can determine the predicted downward mode from an upward operation immediately before a downward operation of the boom  31  on the basis of the upward operation pattern information. For example, if the boom  31  is turned up a little in the upward operation (the upward stroke value is smaller than a reference stroke value), it is possible to determine the predicted downward mode as the second predicted downward mode by predicting that the downward acceleration force of the boom  31  would also be small while the boom  31  is turned down. Unlikely, if the upward stroke value is larger than the reference stroke value, it is possible to determine the predicted downward mode as the first predicted downward mode. 
     Further, the controller  91  can determine a predicted downward mode of the boom  31  as one of a first predicted downward mode and a second predicted downward mode in the above work on the basis of work inputted by an operator through the work selector  95 . For example, if an operate selects grading, the controller  91  can predict that the downward acceleration force of the boom  31  would be small on the basis of a grading pattern. Accordingly, the controller  91  can determine the predicted downward mode as the second predicted downward mode. 
       FIG. 6  is a flow chart illustrating in detail the regulating of the dischargeable lowest limit pressure of the accumulator shown in  FIG. 4  (S 3 ). 
     Referring to the figure (and  FIGS. 1 to 3 ), depending on whether the predicted downward mode is the first predicted downward mode (S 21 ), the controller  91  differently regulates corresponding dischargeable lowest limit pressure of the accumulator  81 . 
     In detail, when the predicted downward mode is a first predicted downward mode, the controller  91  determines the dischargeable lowest limit pressure of the accumulator  81  as the first target pressure (S 23  and S 27 ). Unlikely, when the predicted downward mode is a second predicted downward mode, the controller  91  determines the dischargeable lowest limit pressure of the accumulator  81  as the second target pressure (S 25  and S 29 ). 
     In order to finally set the dischargeable lowest limit pressure to the first target pressure, the controller  91  opens the assist valve  77  so that the pressurized oil charged in the accumulator  81  is supplied to the assist motor  55  (S 31 ). If the pressurized oil is being supplied to the assist motor  55 , the controller  91  can keep the pressurized oil being supplied for a predetermined time. Accordingly, the dischargeable lowest limit pressure of the accumulator  81  is reduced to the first target pressure. 
     On the contrary, in order to finally set the dischargeable lowest limit pressure to the second target pressure, the controller  91  closes the assist valve  77  so that the pressurized oil in the accumulator  81  is maintained therein. Accordingly, the pressurized oil is not supplied to the assist motor  55  (S 33 ). If the pressurized oil is being supplied to the assist motor  55 , the controller  91  can stop the supply to the assist motor  55 . Accordingly, the dischargeable lowest limit pressure of the accumulator  81  can reach the second target pressure higher than the first target pressure. 
     With the dischargeable lowest limit pressure of the accumulator  81  reaching the second target pressure, the accumulator  81  can be additionally charged with the pressurized oil in the swing module  47  after the accumulator  81  is charged with the pressurized oil from the boom cylinder  41 . This is a method of additionally recuperating energy from the pressurized oil that is discharged from the swing module  47  while the swing module  47  stops a swing operation. 
     Energy recuperation efficiency while the accumulator  81  is charged with the pressurized oil discharged from the boom cylinder  41  is described hereafter. 
       FIG. 7  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a first predicted downward mode. 
     Referring to this figure (and  FIGS. 1 to 3 ), when an operator rapidly operates the operation lever  96  to turn down the boom  31 , the controller  91  is supposed to quickly discharge pressurized oil in the chamber A  41   a  of the boom cylinder  41 . 
     The controller  91  has determined the predicted downward mode as the first predicted downward mode by predicting this situation in advance. The controller  91  has set the dischargeable lowest limit pressure of the accumulator  81  to the first target pressure before the boom  31  is actually turned down. 
     Accordingly, even if the pressurized oil in the chamber A  41   a  is quickly discharged and the pressure CP 1  in the chamber A  41   a  is greatly reduced, the minimum of the pressure of the pressurized oil discharged from the chamber A  41   a  can be regulated to be slightly larger than or equivalent to the first target pressure AP 1 . Therefore, most of the pressurized oil in the chamber A  41   a  can be supplied into the accumulator  81  without being discharged to the oil tank  57 . Further, as the accumulator  81  is charged with the pressurized oil, the size of the first target pressure AP 1  constructs a gradually increasing line. 
     Accordingly, all of the pressurized oil discharged from the boom cylinder  41  is restored, so the energy recuperation efficiency can be maximized. Further, since the pressurized oil is quickly discharged from the boom cylinder  41 , the response speed for the downward operation of the boom  31  to operation for downward by an operator can be increased. 
     If the dischargeable lowest limit pressure of the accumulator  81  has reached the second target pressure, the controller  91  cannot rapidly discharge the pressurized oil in the chamber A  41   a  in order to increase the energy recuperation efficiency. This reduces the response speed for the downward operation of the boom  31 , which may cause complaint of the operator. 
       FIG. 8  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a second predicted downward mode. 
     Referring to this figure (and  FIGS. 1 to 3 ), when an operator smoothly pulls the operation lever  96  to turn down the boom  31 , the controller  91  is supposed to discharge only slightly the pressurized oil in the chamber A  41   a  of the boom cylinder  41 . 
     The controller  91  has determined the predicted downward mode as the second predicted downward mode by predicting this situation in advance. The controller  91  has set the dischargeable lowest limit pressure of the accumulator  81  to the second target pressure CP 2  before the boom  31  is actually turned down. 
     Accordingly, even if the pressurized oil in the chamber A  41   a  is slowly discharged and the pressure CP 2  in the chamber A  41   a  is slightly reduced, the minimum of the pressure of the pressurized oil discharged from the chamber A  41   a  can be regulated to be slightly larger than or equivalent to the second target pressure AP 2 . Therefore, most of the pressurized oil discharged from the chamber A  41   a  can be supplied into the accumulator  81  without being discharged to the oil tank  57 . Further, as the accumulator  81  is charged with the pressurized oil, the size of the second target pressure AP 2  constructs a gradually increasing line. 
     It can be seen that the pressure difference L 1  between the pressure CP 2  of the pressurized oil in the chamber A  41   a  and the second target pressure AP 2  is smaller than the pressure difference L 2  between the pressure CP 2  of the pressurized oil in the chamber A  41   a  and the first target pressure AP 1 . This means that it is possible to reduce a loss of energy due to a pressure difference by regulating the dischargeable lowest limit pressure of the accumulator  81  to the second target pressure AP 2  rather than the first target pressure AP 1 . 
       FIG. 9  is a conceptual view showing the main configuration of an energy recuperation system for construction equipment according to another embodiment of the present invention. 
     Referring to  FIG. 9 , construction equipment  200  is similar to the construction equipment  100  of the previous embodiment for the most part, but is different from the construction equipment  100  having only one accumulator  81  in that it has a plurality of sub-accumulators. 
     Three sub-accumulators  181 ,  183 , and  185  are exemplified as the plurality of sub-accumulators. The sub-accumulators  181 ,  183 , and  185  are connected in parallel to a charge passage  167 . The sub-accumulators  181 ,  183 , and  185  have different initial pressures. The initial pressures mean pre-charged gas pressure of the sub-accumulators  181 ,  183 , and  185 . For example, the initial pressures of a first sub-accumulator  181 , a second sub-accumulator  183 , and a third sub-accumulator  185  may be 80 bar, 150 bar, and 200 bar, respectively. In this configuration, the pressure of a boom cylinder  141  is higher than the initial pressure of the third sub-accumulator  185 , for example, may be 250 bar. 
     According to this configuration, the sub-accumulators  181 ,  183 , and  185  can be sequentially charged with pressurized oil that is discharged from a chamber A  141   a  through the charge passage  167  during a downward operation of the boom cylinder  141 . 
     The charging process is described hereafter in detail with reference to  FIG. 10 . 
       FIG. 10  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a first predicted downward mode in the construction equipment  200  shown in  FIG. 9 . 
     Referring to this figure (and  FIG. 9 ), when an operator rapidly pulls the operation lever  96  to turn down the boom  31 , the controller  91  is supposed to quickly discharge pressurized oil in the chamber A  141   a  of the boom cylinder  141 . 
     The controller  91  has determined the predicted downward mode as the first predicted downward mode by predicting this situation in advance. The controller  91  has set the dischargeable lowest limit pressure of the accumulator to the first target pressure before the boom  31  is actually turned down. As a detailed method for this purpose, the controller  91  opens a charge valve  179  so that one, which has an initial pressure corresponding to the first target pressure, of the sub-accumulators  181 ,  183 , and  185 , is selected and charged with pressurized oil in the chamber A  141   a . If the dischargeable lowest limit pressure of the accumulator has to be set to the second target pressure, the pressurized oil discharged from the chamber A  141   a  may be supplied into the sub-accumulator having an initial pressure corresponding to the second target pressure of the sub-accumulators  181 ,  183 , and  185 . This is because the pressurized oil cannot be supplied into sub-accumulators having an initial pressure lower than the second target pressure. 
     Accordingly, when the pressurized oil in the chamber A  141   a  is discharged, the pressure change of the accumulator does not follow the existing graph AP 1 , but follows a pressure change graph APC by combination of the three sub-accumulators  181 ,  183 , and  185 . In other words, the first sub-accumulator  181  having the lowest initial pressure to the third sub-accumulator  185  having the highest initial pressure can be sequentially charged with the pressurized oil. 
     When the pressure change of the accumulator follows the graph AP 1 , the dischargeable lowest limit pressure of the accumulator is higher than the pressure of the pressurized oil in the loss period G, so the pressurized oil cannot be supplied into the accumulator. Accordingly, the pressurized oil in the chamber A  141   a  has to be sent to the oil tank  157 , so energy cannot be recuperated from the pressurized oil. 
     Unlikely, when the pressure change of the accumulator follows a new graph APC, the pressure of the pressurized oil is higher than the dischargeable lowest limit pressure of the accumulator even in the loss period G, so the pressurized oil cannot be supplied into the accumulator. 
       FIG. 11  is a conceptual view showing the main configuration of an energy recuperation system for construction equipment according to another embodiment of the present invention. 
     Referring to  FIG. 11 , construction equipment  300  according to a new embodiment, similar to the construction equipment  200  of the previous embodiment, has a plurality of sub-accumulators  281 ,  283 , and  285  having different initial pressures as accumulators. The initial pressures of the sub-accumulators  281 ,  283 , and  285  may be 80 bar, 150 bar, and 200 bar, respectively, the same as in the previous embodiment. 
     The sub-accumulators  281 ,  283 , and  285  are connected to each other by inflow passages  269   a ,  269   b , and  269   c  in parallel with a charge passage  267 . Selection valves  279   a ,  279   b , and  279   c  are respectively disposed in the inflow passages  269   a ,  269   b , and  269   c . The controller  91  can make the sub-accumulators  281 ,  283 , and  285  be objects to be charged or not with pressurized oil by selectively opening/closing the selection valves  279   a ,  279   b , and  279   c.    
     Further, check valves  279   d  and  279   e  may be disposed between the inflow passages  269   a ,  269   b , and  269   c . The check valves  279   d  and  279   e  allow sub-accumulators having a higher initial pressure to be charged with the pressurized oil as the pressure of the pressurized oil increases after sub-accumulators having a lower initial pressure is charged with the pressurized oil, but does not allow for the opposite case. 
     A charge operation in the construction equipment  300  is described hereafter in detail with reference to  FIG. 12 . 
       FIG. 12  is a graph showing pressure changes of a boom cylinder and an accumulator while a work unit is turned down in a second predicted downward mode in the construction equipment  300  shown in  FIG. 11 . 
     Referring to this figure (and  FIG. 11 ), when an operator smoothly pulls the operation lever  96  to turn down the boom  31 , the controller  91  is supposed to only slightly discharge pressurized oil in the chamber A  241   a  of the boom cylinder  241 . 
     The controller  91  has determined the predicted downward mode as the second predicted downward mode by predicting this situation in advance. Accordingly, the controller  91  has set the dischargeable lowest limit pressure of an accumulator to the second target pressure before the boom  31  is actually turned down. 
     As a detailed method of setting the second target pressure, the controller  91  opens the charge valve  279  and opens only a second selection valve  279   b  of the selection valves  279   a ,  279   b , and  279   c . Accordingly, the pressurized oil in the boom cylinder  241  is supplied first into the second sub-accumulator  283 . Thereafter, when the pressure of the pressurized oil increases, the pressurized oil can be supplied a third sub-accumulator  285  through a second check valve  279   e . This process can be seen from a pressure change graph APS showing the actual charge process of an accumulator. Accordingly, the two sub-accumulators  283  and  285  are sequentially charged with the pressurized oil, but the initial pressures are different, so the dischargeable lowest limit pressures of the accumulators can reach the first target pressure and the second target pressure with an interval. 
     For reference, another pressure change graph APC of the accumulators show a pressure change when the first selection valve  279   a  is opened with the charge valve  279  by the controller  91  and the first sub-accumulator  281  to the third sub-accumulator  285  are sequentially charged with the pressurized oil. 
     As a result, since the controller  91  opens not the first selection valve  279   a , but the second selection valve  279   b , it is possible to set the dischargeable lowest limit pressure of the accumulator to the second target pressure. Accordingly, it is possible to prevent a decrease in energy recuperation ratio due to a loss of pressure while an accumulator is charged with the pressurized oil. 
     The energy recuperation systems and methods for construction equipment described above are not limited to the configurations and operation methods of the embodiments described above. The embodiments may be selectively partially or fully combined for various modifications. 
     INDUSTRIAL APPLICABILITY 
     The present invention has industrial applicability to an energy recuperation system and method for construction equipment.