Patent Publication Number: US-9897120-B2

Title: Hydraulic system having energy recovery

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/171,047, filed Jun. 28, 2011. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to a hydraulic control system and, more particularly, to a hydraulic system having energy recovery. 
     BACKGROUND 
     Swing-type excavation machines, for example hydraulic excavators and front shovels, require significant hydraulic pressure and flow to transfer material from a dig location to a dump location. These machines direct the high-pressure fluid from an engine-driven pump through a swing motor to accelerate a loaded work tool at the start of each swing, and then restrict the flow of fluid exiting the motor at the end of each swing to slow and stop the work tool. 
     One problem associated with this type of hydraulic arrangement involves efficiency. In particular, the fluid exiting the swing motor at the end of each swing is under a relatively high due to deceleration of the loaded work tool. Unless recovered, energy associated with the high-pressure fluid may be wasted. In addition, restriction of this high-pressure fluid exiting the swing motor at the end of each swing can result in heating of the fluid, which must be accommodated with an increased cooling capacity of the machine. 
     One attempt to improve the efficiency of a swing-type machine is disclosed in U.S. Pat. No. 7,908,852 of Zhang et al. that issued on Mar. 22, 2011 (the &#39;852 patent). The &#39;852 patent discloses a hydraulic control system for a machine that includes an accumulator. The accumulator stores exit oil from a swing motor that has been pressurized by inertia torque applied on the moving swing motor by an upper structure of the machine. The pressurized oil in the accumulator is then selectively reused to accelerate the swing motor during a subsequent swing by supplying the accumulated oil back to the swing motor. 
     Although the hydraulic control system of the &#39;852 patent may help to improve efficiencies of a swing-type machine in some situations, it may still be less than optimal. In particular, during discharge of the accumulator described in the &#39;852 patent, some pressurized fluid exiting the swing motor may still have useful energy that is wasted. In addition, there may be situations during operation of the hydraulic control system of the &#39;852 patent, for example during deceleration and accumulator charging, when a pump output is unable to supply fluid at a rate sufficient to prevent cavitation in the swing motor. Further, the machine may operate differently under different conditions and in different situations, and the hydraulic control system of the &#39;852 patent is not configured to adapt control to these conditions and situations. 
     The disclosed hydraulic control system is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art. 
     SUMMARY 
     One aspect of the present disclosure is directed to a hydraulic system. The hydraulic system can include a pump configured to pressurize fluid and a motor driven by a flow of pressurized fluid from the pump. An accumulator is configured to receive fluid discharged from the motor and to discharge fluid to the motor. The system can include a first valve disposed between the accumulator and the motor. The first valve is movable between a first position and a second position in response to a pressure difference between a first conduit and a second conduit fluidly coupled to the motor. The first valve is movable to the first position when the first conduit has a higher pressure compared to the second conduit to connect the first conduit to the accumulator and disconnect the second conduit from the accumulator. The first valve is movable to the second position when the second conduit has a higher pressure compared to the first conduit to connect the second conduit to the accumulator and disconnect the first conduit from the accumulator. The system can include a second valve disposed between the accumulator and the first valve. The second valve is selectively movable to permit fluid discharged from the motor in deceleration to enter the accumulator. The system can include a third valve disposed between the accumulator and the first valve. The third valve is selectively movable to permit fluid discharged from the accumulator to enter the motor to assist acceleration of the motor. 
     Another aspect of the present disclosure is directed to a method of controlling a machine. The method may include pressurizing a fluid with a pump, and directing the pressurized fluid through a motor to move a work tool through a work cycle having a plurality of segments. The method may further include selectively accumulating fluid that has been discharged from the motor and discharging fluid to the motor during different combinations of the plurality of segments to implement a plurality of modes of operation. 
     Another aspect of the present disclosure is directed to a hydraulic system including a pump configured to pressurize fluid, a motor driven by a flow of pressurized fluid from the pump, and an accumulator configured to receive fluid discharged from the motor. The system can include a means for selectively accumulating fluid that has been discharged from the motor and discharging fluid to the motor during different combinations of motor deceleration and motor acceleration segments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic illustration of an exemplary disclosed machine operating at a worksite with a haul vehicle; 
         FIG. 2  is a schematic illustration of an exemplary disclosed hydraulic control system that may be used with the machine of  FIG. 1 ; and 
         FIG. 3  is an exemplary disclosed control map that may be used by the hydraulic control system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10  having multiple systems and components that cooperate to excavate and load earthen material onto a nearby haul vehicle  12 . In one example, machine  10  may embody a hydraulic excavator. It is contemplated, however, that machine  10  may embody another swing-type excavation or material handling machine such as a backhoe, a front shovel, a dragline excavator, or another similar machine. Machine  10  may include, among other things, an implement system  14  configured to move a work tool  16  between a dig location  18  within a trench or at a pile, and a dump location  20 , for example over haul vehicle  12 . Machine  10  may also include an operator station  22  for manual control of implement system  14 . It is contemplated that machine  10  may perform operations other than truck loading, if desired, such as craning, trenching, and material handling. 
     Implement system  14  may include a linkage structure acted on by fluid actuators to move work tool  16 . Specifically, implement system  14  may include a boom  24  that is vertically pivotal relative to a work surface  26  by a pair of adjacent, double-acting, hydraulic cylinders  28  (only one shown in  FIG. 1 ) Implement system  14  may also include a stick  30  that is vertically pivotal about a horizontal pivot axis  32  relative to boom  24  by a single, double-acting, hydraulic cylinder  36 . Implement system  14  may further include a single, double-acting, hydraulic cylinder  38  that is operatively connected to work tool  16  to tilt work tool  16  vertically about a horizontal pivot axis  40  relative to stick  30 . Boom  24  may be pivotally connected to a frame  42  of machine  10 , while frame  42  may be pivotally connected to an undercarriage member  44  and swung about a vertical axis  46  by a swing motor  49 . Stick  30  may pivotally connect work tool  16  to boom  24  by way of pivot axes  32  and  40 . It is contemplated that a greater or lesser number of fluid actuators may be included within implement system  14  and connected in a manner other than described above, if desired. 
     Numerous different work tools  16  may be attachable to a single machine  10  and controllable via operator station  22 . Work tool  16  may include any device used to perform a particular task such as, for example, a bucket, a fork arrangement, a blade, a shovel, or any other task-performing device known in the art. Although connected in the embodiment of  FIG. 1  to lift, swing, and tilt relative to machine  10 , work tool  16  may alternatively or additionally rotate, slide, extend, or move in another manner known in the art. 
     Operator station  22  may be configured to receive input from a machine operator indicative of a desired work tool movement. Specifically, operator station  22  may include one or more input devices  48  embodied, for example, as single or multi-axis joysticks located proximal an operator seat (not shown). Input devices  48  may be proportional-type controllers configured to position and/or orient work tool  16  by producing a work tool position signal that is indicative of a desired work tool speed and/or force in a particular direction. The position signal may be used to actuate any one or more of hydraulic cylinders  28 ,  36 ,  38  and/or swing motor  49 . It is contemplated that different input devices may alternatively or additionally be included within operator station  22  such as, for example, wheels, knobs, push-pull devices, switches, pedals, and other operator input devices known in the art. 
     As illustrated in  FIG. 2 , machine  10  may include a hydraulic control system  50  having a plurality of fluid components that cooperate to move implement system  14  (referring to  FIG. 1 ). In particular, hydraulic control system  50  may include a first circuit  52  associated with swing motor  49 , and at least a second circuit  54  associated with hydraulic cylinders  28 ,  36 , and  38 . First circuit  52  may include, among other things, a swing control valve  56  connected to regulate a flow of pressurized fluid from a pump  58  to swing motor  49  and from swing motor  49  to a low-pressure tank  60  to cause a swinging movement of work tool  16  about axis  46  (referring to  FIG. 1 ) in accordance with an operator request received via input device  48 . Second circuit  54  may include similar control valves, for example a boom control valve (not shown), a stick control valve (not shown), a tool control valve (not shown), a travel control valve (not shown), and/or an auxiliary control valve connected in parallel to receive pressurized fluid from pump  58  and to discharge waste fluid to tank  60 , thereby regulating the corresponding actuators (e.g., hydraulic cylinders  28 ,  36 , and  38 ). 
     Swing motor  49  may include a housing  62  at least partially forming a first and a second chamber (not shown) located to either side of an impeller  64 . When the first chamber is connected to an output of pump  58  (e.g., via a first chamber passage  66  formed within housing  62 ) and the second chamber is connected to tank  60  (e.g., via a second chamber passage  68  formed within housing  62 ), impeller  64  may be driven to rotate in a first direction (shown in  FIG. 2 ). Conversely, when the first chamber is connected to tank  60  via first chamber passage  66  and the second chamber is connected to pump  58  via second chamber passage  68 , impeller  64  may be driven to rotate in an opposite direction (not shown). The flow rate of fluid through impeller  64  may relate to a rotational speed of swing motor  49 , while a pressure differential across impeller  64  may relate to an output torque thereof. 
     Swing motor  49  may include built-in makeup and relief functionality. In particular, a makeup passage  70  and a relief passage  72  may be formed within housing  62 , between first chamber passage  66  and second chamber passage  68 . A pair of opposing check valves  74  and a pair of opposing relief valves  76  may be disposed within makeup and relief passages  70 ,  72 , respectively. A low-pressure passage  78  may be connected to each of makeup and relief passages  70 ,  72  at locations between check valves  74  and between relief valves  76 . Based on a pressure differential between low-pressure passage  78  and first and second chamber passages  66 ,  68 , one of check valves  74  may open to allow fluid from low-pressure passage  78  into the lower-pressure one of the first and second chambers. Similarly, based on a pressure differential between first and second chamber passages  66 ,  68  and low-pressure passage  78 , one of relief valves  76  may open to allow fluid from the higher-pressure one of the first and second chambers into low-pressure passage  78 . A significant pressure differential may generally exist between the first and second chambers during a swinging movement of implement system  14 . 
     Pump  58  may be configured to draw fluid from tank  60  via an inlet passage  80 , pressurize the fluid to a desired level, and discharge the fluid to first and second circuits  52 ,  54  via a discharge passage  82 . A check valve  83  may be disposed within discharge passage  82 , if desired, to provide for a unidirectional flow of pressurized fluid from pump  58  into first and second circuits  52 ,  54 . Pump  58  may embody, for example, a variable displacement pump (shown in  FIG. 1 ), a fixed displacement pump, or another source known in the art. Pump  58  may be drivably connected to a power source (not shown) of machine  10  by, for example, a countershaft (not shown), a belt (not shown), an electrical circuit (not shown), or in another suitable manner. Alternatively, pump  58  may be indirectly connected to the power source of machine  10  via a torque converter, a reduction gear box, an electrical circuit, or in any other suitable manner. Pump  58  may produce a stream of pressurized fluid having a pressure level and/or a flow rate determined, at least in part, by demands of the actuators within first and second circuits  52 ,  54  that correspond with operator requested movements. Discharge passage  82  may be connected within first circuit  52  to first and second chamber passages  66 ,  68  via swing control valve  56  and first and second chamber conduits  84 ,  86 , respectively, which extend between swing control valve  56  and swing motor  49 . 
     Tank  60  may constitute a reservoir configured to hold a low-pressure supply of fluid. The fluid may include, for example, a dedicated hydraulic oil, an engine lubrication oil, a transmission lubrication oil, or any other fluid known in the art. One or more hydraulic systems within machine  10  may draw fluid from and return fluid to tank  60 . It is contemplated that hydraulic control system  50  may be connected to multiple separate fluid tanks or to a single tank, as desired. Tank  60  may be fluidly connected to swing control valve  56  via a drain passage  88 , and to first and second chamber passages  66 ,  68  via swing control valve  56  and first and second chamber conduits  84 ,  86 , respectively. Tank  60  may also be connected to low-pressure passage  78 . A check valve  90  may be disposed within drain passage  88 , if desired, to promote a unidirectional flow of fluid into tank  60 . 
     Swing control valve  56  may have elements that are movable to control the rotation of swing motor  49  and corresponding swinging motion of implement system  14 . Specifically, swing control valve  56  may include a first chamber supply element  92 , a first chamber drain element  94 , a second chamber supply element  96 , and a second chamber drain element  98  all disposed within a common block or housing  97 . The first and second chamber supply elements  92 ,  96  may be connected in parallel with discharge passage  82  to regulate filling of their respective chambers with fluid from pump  58 , while the first and second chamber drain elements  94 ,  98  may be connected in parallel with drain passage  88  to regulate draining of the respective chambers of fluid. A makeup valve  99 , for example a check valve, may be disposed between an outlet of first chamber drain element  94  and first chamber conduit  84  and between an outlet of second chamber drain element  98  and second chamber conduit  86 . 
     To drive swing motor  49  to rotate in a first direction (shown in  FIG. 2 ), first chamber supply element  92  may be shifted to allow pressurized fluid from pump  58  to enter the first chamber of swing motor  49  via discharge passage  82  and first chamber conduit  84 , while second chamber drain element  98  may be shifted to allow fluid from the second chamber of swing motor  49  to drain to tank  60  via second chamber conduit  86  and drain passage  88 . To drive swing motor  49  to rotate in the opposite direction, second chamber supply element  96  may be shifted to communicate the second chamber of swing motor  49  with pressurized fluid from pump  58 , while first chamber drain element  94  may be shifted to allow draining of fluid from the first chamber of swing motor  49  to tank  60 . It is contemplated that both the supply and drain functions of swing control valve  56  (i.e., of the four different supply and drain elements) may alternatively be performed by a single valve element associated with the first chamber and a single valve element associated with the second chamber or by a single valve element associated with both the first and second chambers, if desired. 
     Supply and drain elements  92 - 98  of swing control valve  56  may be solenoid-movable against a spring bias in response to a flow rate command issued by a controller  100 . In particular, swing motor  49  may rotate at a velocity that corresponds with the flow rate of fluid into and out of the first and second chambers. Accordingly, to achieve an operator-desired swing velocity, a command based on an assumed or measured pressure may be sent to the solenoids (not shown) of supply and drain elements  92 - 98  that causes them to open an amount corresponding to the necessary flow rate through swing motor  49 . This command may be in the form of a flow rate command or a valve element position command that is issued by controller  100 . 
     Controller  100  may be in communication with the different components of hydraulic control system  50  to regulate operations of machine  10 . For example, controller  100  may be in communication with the elements of swing control valve  56  in first circuit  52  and with the elements of control valves (not shown) associated with second circuit  54 . Based on various operator input and monitored parameters, as will be described in more detail below, controller  100  may be configured to selectively activate the different control valves in a coordinated manner to efficiently carry out operator requested movements of implement system  14 . 
     Controller  100  may include a memory, a secondary storage device, a clock, and one or more processors that cooperate to accomplish a task consistent with the present disclosure. Numerous commercially available microprocessors can be configured to perform the functions of controller  100 . It should be appreciated that controller  100  could readily embody a general machine controller capable of controlling numerous other functions of machine  10 . Various known circuits may be associated with controller  100 , including signal-conditioning circuitry, communication circuitry, and other appropriate circuitry. It should also be appreciated that controller  100  may include one or more of an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a computer system, and a logic circuit configured to allow controller  100  to function in accordance with the present disclosure. 
     The operational parameters monitored by controller  100 , in one embodiment, may include a pressure of fluid within first and/or second circuits  52 ,  54 . For example, one or more pressure sensors  102  may be strategically located within first chamber and/or second chamber conduits  84 ,  86  to sense a pressure of the respective passages and generate a corresponding signal indicative of the pressure directed to controller  100 . It is contemplated that any number of pressure sensors  102  may be placed in any location within first and/or second circuits  52 ,  54 , as desired. It is further contemplated that other operational parameters such as, for example, speeds, temperatures, viscosities, densities, etc. may also or alternatively be monitored and used to regulate operation of swing energy recovery system  50 , if desired. 
     Hydraulic control system  50  may be fitted with an energy recovery arrangement  104  that is in communication with at least first circuit  52  and configured to selectively extract and recover energy from waste fluid that is discharged from swing motor  49 . Energy recovery arrangement (ERA)  104  may include, among other things, a recovery valve block (RVB)  106  that is fluidly connectable between pump  58  and swing motor  49 , a first accumulator  108  configured to selectively communicate with swing motor  49  via RVB  106 , and a second accumulator  110  also configured to selectively communicate with swing motor  49 . In the disclosed embodiment, RVB  106  may be fixedly and mechanically connectable to one or both of swing control valve  56  and swing motor  49 , for example directly to housing  62  and/or directly to housing  97 . RVB  106  may include an internal first passage  112  fluidly connectable to first chamber conduit  84 , and an internal second passage  114  fluidly connectable to second chamber conduit  86 . First accumulator  108  may be fluidly connected to RVB  106  via a conduit  116 , while second accumulator  110  may be fluidly connectable to drain passages  78  and  88 , in parallel with tank  60 , via a conduit  118 . 
     RVB  106  may house a selector valve  120 , a charge valve  122  associated with first accumulator  108 , and a discharge valve  124  associated with first accumulator  108  and disposed in parallel with charge valve  122 . Selector valve  120  may selectively fluidly communicate one of first and second passages  112 ,  114  with charge and discharge valves  122 ,  124  based on a pressure of first and second passages  112 ,  114 . Charge and discharge valves  122 ,  124  may be movable in response to commands from controller  100  to selectively fluidly communicate first accumulator  108  with selector valve  120  for fluid charging and discharging purposes. 
     Selector valve  120  may be a pilot-operated, 2-position, 3-way valve that is movable in response to fluid pressure in first and second passages  112 ,  114  (i.e., in response to a fluid pressure within the first and second chambers of swing motor  49 ). In particular, selector valve  120  may include a valve element  126  that is movable from a first position (shown in  FIG. 2 ) at which first passage  112  is fluidly connected to charge and discharge valves  122 ,  124  via an internal passage  128 , toward a second position (not shown) at which second passage  114  is fluid connected to charge and discharge valves  122 ,  124  via passage  128 . When first passage  112  is fluidly connected to charge and discharge valves  122 ,  124  via passage  128 , fluid flow through second passage  114  may be inhibited by selector valve  120  and vice versa. First and second pilot passages  130 ,  132  may communicate fluid from first and second passages  112 ,  114  to opposing ends of valve element  126  such that a higher-pressure one of first or second passages  112 ,  114  may cause valve element  126  to move and fluidly connect the corresponding passage with charge and discharge valves  122 ,  124  via passage  128 . 
     Charge valve  122  may be a solenoid-operated, variable position, 2-way valve that is movable in response to a command from controller  100  to allow fluid from passage  128  to enter first accumulator  108 . In particular, charge valve  122  may include a valve element  134  that is movable from a first position (shown in  FIG. 2 ) at which fluid flow from passage  128  into first accumulator  108  is inhibited, toward a second position (not shown) at which passage  128  is fluidly connected to first accumulator  108 . When valve element  134  is away from the first position (i.e., in the second position or in another position between the first and second positions) and a fluid pressure within passage  128  exceeds a fluid pressure within first accumulator  108 , fluid from passage  128  may fill (i.e., charge) first accumulator  108 . Valve element  134  may be spring-biased toward the first position and movable in response to a command from controller  100  to any position between the first and second positions to thereby vary a flow rate of fluid from passage  128  into first accumulator  108 . A check valve  136  may be disposed between charge valve  122  and first accumulator  108  to provide for a unidirectional flow of fluid into accumulator  108  via charge valve  122 . 
     Discharge valve  124  may be substantially identical to charge valve  122  in composition, and movable in response to a command from controller  100  to allow fluid from first accumulator  108  to enter passage  128  (i.e., to discharge). In particular, discharge valve  124  may include a valve element  138  that is movable from a first position (not shown) at which fluid flow from first accumulator  108  into passage  128  is inhibited, toward a second position (shown in  FIG. 2 ) at which first accumulator  108  is fluidly connected to passage  128 . When valve element  138  is away from the first position (i.e., in the second position or in another position between the first and second positions) and a fluid pressure within first accumulator  108  exceeds a fluid pressure within passage  128 , fluid from first accumulator  108  may flow into passage  128 . Valve element  138  may be spring-biased toward the first position and movable in response to a command from controller  100  to any position between the first and second positions to thereby vary a flow rate of fluid from first accumulator  108  into passage  128 . A check valve  140  may be disposed between first accumulator  108  and discharge valve  124  to provide for a unidirectional flow of fluid from accumulator  108  into passage  128  via discharge valve  124 . 
     An additional pressure sensor  102  may be associated with first accumulator  108  and configured to generate signals indicative of a pressure of fluid within first accumulator  108 , if desired. In the disclosed embodiment, the additional pressure sensor  102  may be disposed between first accumulator  108  and discharge valve  124 . It is contemplated, however, that the additional pressure sensor  102  may alternatively be disposed between first accumulator  108  and charge valve  122  or directly connected to first accumulator  108 , if desired. Signals from the additional pressure sensor  102  may be directed to controller  100  for use in regulating operation of charge and/or discharge valves  122 ,  124 . 
     First and second accumulators  108 ,  110  may each embody pressure vessels filled with a compressible gas that are configured to store pressurized fluid for future use by swing motor  49 . The compressible gas may include, for example, nitrogen, argon, helium, or another appropriate compressible gas. As fluid in communication with first and second accumulators  108 ,  110  exceeds predetermined pressures of first and second accumulators  108 ,  110 , the fluid may flow into accumulators  108 ,  110 . Because the gas therein is compressible, it may act like a spring and compress as the fluid flows into first and second accumulators  108 ,  110 . When the pressure of the fluid within conduits  116 ,  118  drops below the predetermined pressures of first and second accumulators  108 ,  110 , the compressed gas may expand and urge the fluid from within first and second accumulators  108 ,  110  to exit. It is contemplated that first and second accumulators  108 ,  110  may alternatively embody membrane/spring-biased or bladder types of accumulators, if desired. 
     In the disclosed embodiment, first accumulator  108  may be a larger (i.e., about 5-20 times larger) and higher-pressure (i.e., about 5-60 times higher-pressure) accumulator, as compared to second accumulator  110 . Specifically, first accumulator  108  may be configured to accumulate up to about 50-100 L of fluid having a pressure in the range of about 260-300 bar, while second accumulator  110  may be configured to accumulate up to about 10 L of fluid having a pressure in the range of about 5-30 bar. In this configuration, first accumulator  108  may be used primarily to assist the motion of swing motor  49  and to improve machine efficiencies, while second accumulator may be used primarily as a makeup accumulator to help reduce a likelihood of voiding at swing motor  49 . It is contemplated, however, that other volumes and pressures may be accommodated by first and/or second accumulators  108 ,  110 , if desired. 
     Controller  100  may be configured to selectively cause first accumulator  108  to charge and discharge, thereby improving performance of machine  10 . In particular, a typical swinging motion of implement system  14  instituted by swing motor  49  may consist of segments of time during which swing motor  49  is accelerating a swinging movement of implement system  14  and segments of time during which swing motor  49  is decelerating the swinging movement of implement system  14 . The acceleration segments may require significant energy from swing motor  49  that is conventionally realized by way of pressurized fluid supplied to swing motor  49  by pump  58 , while the deceleration segments may produce significant energy in the form of pressurized fluid that is conventionally wasted through discharge to tank  53 . Both the acceleration and the deceleration segments may require swing motor  49  to convert significant amounts of hydraulic energy to swing kinetic energy, and vice versa. After pressurized fluid passes through swing motor  49 , however, it still contains a large amount of energy. If the fluid passing through swing motor  49  is selectively collected within first accumulator  108  during the deceleration segments, this energy can then be returned to (i.e., discharged) and reused by swing motor  49  during the ensuing acceleration segments. Swing motor  49  can be assisted during the acceleration segments by selectively causing first accumulator  108  to discharge pressurized fluid into the higher-pressure chamber of swing motor  49  (via discharge valve  124 , passage  128 , selector valve  120 , and the appropriate one of first and second chamber conduits  84 ,  86 ), alone or together with high-pressure fluid from pump  58 , thereby propelling swing motor  49  at the same or greater rate with less pump power than otherwise possible via pump  58  alone. Swing motor  49  can be assisted during the deceleration segments by selectively causing first accumulator  108  to charge with fluid exiting swing motor  49 , thereby providing additional resistance to the motion of swing motor  49  and lowering a restriction and cooling requirement of the fluid exiting swing motor  49 . 
     In an alternative embodiment, controller  100  may be configured to selectively control charging of first accumulator  108  with fluid exiting pump  58 , as opposed to fluid exiting swing motor  49 . That is, during a peak-shaving or economy mode of operation, controller  100  may be configured to cause accumulator  108  to charge with fluid exiting pump  58  (e.g., via control valve  56 , the appropriate one of first and second chamber conduits  84 ,  86 , selector valve  126 , passage  128 , and charge valve  122 ) when pump  58  has excess capacity (i.e., a capacity greater than required by swing motor  49  to complete a current swing of work tool  16  requested by the operator). Then, during times when pump  58  has insufficient capacity to adequately power swing motor  49 , the high-pressure fluid previously collected from pump  58  within first accumulator  108  may be discharged in the manner described above to assist swing motor  49 . 
     Controller  100  may be configured to regulate the charging and discharging of first accumulator  108  based on a current or ongoing segment of the excavation work cycle of machine  10 . In particular, based on input received from one or more performance sensors  141 , controller  100  may be configured to partition a typical work cycle performed by machine  10  into a plurality of segments, for example, into a dig segment, a swing-to-dump acceleration segment, a swing-to-dump deceleration segment, a dump segment, a swing-to-dig acceleration segment, and a swing-to-dig deceleration segment, as will be described in more detail below. Based on the segment of the excavation work cycle currently being performed, controller  100  may selectively cause first accumulator  108  to charge or discharge, thereby assisting swing motor  49  during the acceleration and deceleration segments. 
     One or more maps relating signals from sensor(s)  141  to the different segments of the excavation work cycle may be stored within the memory of controller  100 . Each of these maps may include a collection of data in the form of tables, graphs, and/or equations. In one example, threshold speeds, cylinder pressures, and/or operator input (i.e., lever position) associated with the start and/or end of one or more of the segments may be stored within the maps. In another example, threshold forces and/or actuator positions associated with the start and/or end of one or more of the segments may be stored within the maps. Controller  100  may be configured to reference the signals from sensor(s)  141  with the maps stored in memory to determine the segment of the excavation work cycle currently being executed, and then regulate the charging and discharging of first accumulator  108  accordingly. Controller  100  may allow the operator of machine  10  to directly modify these maps and/or to select specific maps from available relationship maps stored in the memory of controller  100  to affect segment partitioning and accumulator control, as desired. It is contemplated that the maps may additionally or alternatively be automatically selectable based on modes of machine operation, if desired. 
     Sensor(s)  141  may be associated with the generally horizontal swinging motion of work tool  16  imparted by swing motor  49  (i.e., the motion of frame  42  relative to undercarriage member  44 ). For example, sensor  141  may embody a rotational position or speed sensor associated with the operation of swing motor  49 , an angular position or speed sensor associated with the pivot connection between frame  42  and undercarriage member  44 , a local or global coordinate position or speed sensor associated with any linkage member connecting work tool  16  to undercarriage member  44  or with work tool  16  itself, a displacement sensor associated with movement of operator input device  48 , or any other type of sensor known in the art that may generate a signal indicative of a swing position, speed, force, or other swing-related parameter of machine  10 . The signal generated by sensor(s)  141  may be sent to and recorded by controller  100  during each excavation work cycle. It is contemplated that controller  100  may derive a swing speed based on a position signal from sensor  141  and an elapsed period of time, if desired. 
     Alternatively or additionally, sensor(s)  141  may be associated with the vertical pivoting motion of work tool  16  imparted by hydraulic cylinders  28  (i.e., associated with the lifting and lowering motions of boom  24  relative to frame  42 ). Specifically, sensor  141  may be an angular position or speed sensor associated with a pivot joint between boom  24  and frame  42 , a displacement sensor associated with hydraulic cylinders  28 , a local or global coordinate position or speed sensor associated with any linkage member connecting work tool  16  to frame  42  or with work tool  16  itself, a displacement sensor associated with movement of operator input device  48 , or any other type of sensor known in the art that may generate a signal indicative of a pivoting position or speed of boom  24 . It is contemplated that controller  100  may derive a pivot speed based on a position signal from sensor  141  and an elapsed period of time, if desired. 
     In yet an additional embodiment, sensor(s)  141  may be associated with the tilting force of work tool  16  imparted by hydraulic cylinder  38 . Specifically, sensor  141  may be a pressure sensor associated with one or more chambers within hydraulic cylinder  38  or any other type of sensor known in the art that may generate a signal indicative of a tilting force of machine  10  generated during a dig and dump operation of work tool  16 . 
     With reference to  FIG. 3 , an exemplary curve  142  may represent a swing speed signal generated by sensor(s)  141  relative to time throughout each segment of the excavation work cycle, for example throughout a work cycle associated with 90° truck loading. During most of the dig segment, the swing speed may typically be about zero (i.e., machine  10  may generally not swing during a digging operation). At completion of a dig stroke, machine  10  may generally be controlled to swing work tool  16  toward the waiting haul vehicle  12  (referring to  FIG. 1 ). As such, the swing speed of machine  10  may begin to increase toward the end of the dig segment. As the swing-to-dump segment of the excavation work cycle progresses, the swing speed may accelerate to a maximum when work tool  16  is about midway between dig location  18  and dump location  20 , and then decelerate toward the end of the swing-to-dump segment. During most of the dump segment, the swing speed may typically be about zero (i.e., machine  10  may generally not swing during a dumping operation). When dumping is complete, machine  10  may generally be controlled to swing work tool  16  back toward dig location  18  (referring to  FIG. 1 ). As such, the swing speed of machine  10  may increase toward the end of the dump segment. As the swing-to-dig segment of the excavation cycle progresses, the swing speed may accelerate to a maximum in a direction opposite to the swing direction during the swing-to-dump segment of the excavation cycle. This maximum speed may generally be achieved when work tool  16  is about midway between dump location  20  and dig location  18 . The swing speed of work tool  16  may then decelerate toward the end of the swing-to-dig segment, as work tool  16  nears dig location  18 . Controller  100  may partition a current excavation work cycle into the six segments described above based on signals received from sensor(s)  141  and the maps stored in memory, based on swing speeds, tilt forces, and/or operator input recorded for a previous excavation work cycle, or in any other manner known in the art. 
     Controller  100  may selectively cause first accumulator  108  to charge and to discharge based on the current or ongoing segment of the excavation work cycle. For example, a chart portion  144  (i.e., the lower portion) of  FIG. 3  illustrates 6 different modes of operations during which the excavation cycle can be completed, together with an indication as to when first accumulator  108  is controlled to charge with pressurized fluid (represented by “C”) or to discharge pressurized fluid (represented by “D”) relative the segments of each excavation work cycle. First accumulator  108  can be controlled to charge with pressurized fluid by moving valve element  134  of charge valve to the second or flow-passing position when the pressure within passage  128  is greater than the pressure within first accumulator  108 . First accumulator  108  can be controlled to discharge pressurized fluid by moving valve element  138  to the second or flow-passing position when the pressure within first accumulator  108  is greater than the pressure within passage  128 . 
     Based on the chart of  FIG. 3 , some general observations may be made. First, it can be seen that controller  100  may inhibit first accumulator  108  from receiving or discharging fluid during the dig and dump segments of all of the modes of operation (i.e., controller  100  may maintain valve elements  134  and  138  in the flow-blocking first positions during the dig and dump segments). Controller  100  may inhibit charging and discharging during the dig and dump segments, as no or little swinging motion is required during completion of these portions of the excavation work cycle. Second, the number of segments during which controller  100  causes first accumulator  108  to receive fluid may be greater than the number of segments during which controller  100  causes first accumulator  108  to discharge fluid for a majority of the modes (e.g., for modes 2-6). Controller  100  may generally cause first accumulator  108  to charge more often than discharge, because the amount of charge energy available at a sufficiently high pressure (i.e., at a pressure greater than the threshold pressure of first accumulator  108 ) may be less than an amount of energy required during movement of implement system  14 . Third, the number of segments during which controller  100  causes first accumulator  108  to discharge fluid may never be greater than the number of segments during which controller  100  causes first accumulator  108  to receive fluid for all modes. Fourth, controller  100  may cause first accumulator  108  to discharge fluid during only a swing-to-dig or a swing-to-dump acceleration segment for all modes. Discharge during any other segment of the excavation cycle may only serve to reduce machine efficiency. Fifth, controller  100  may cause first accumulator  108  to receive fluid during only a swing-to-dig or swing-to-dump deceleration segment for a majority of the modes of operation (e.g., for modes 1-4). 
     Mode 1 may correspond with a swing-intensive operation where a significant amount of swing energy is available for storage by first accumulator  108 . An exemplary swing-intensive operation may include a 150° (or greater) swing operation, such as the truck loading example shown in  FIG. 1 , material handling (e.g., using a grapple or magnet), hopper feeding from a nearby pile, or another operation where an operator of machine  10  typically requests harsh stop-and-go commands. When operating in mode 1, controller  100  may be configured to cause first accumulator  108  to discharge fluid to swing motor  49  during the swing-to-dump acceleration segment, receive fluid from swing motor  49  during the swing-to-dump deceleration segment, discharge fluid to swing motor  49  during the swing-to-dig acceleration segment, and receive fluid from swing motor  49  during the swing-to-dig deceleration segment. 
     Controller  100  may be instructed by the operator of machine  10  that the first mode of operation is currently in effect (e.g., that truck loading is being performed) or, alternatively, controller  100  may automatically recognize operation in the first mode based on performance of machine  10  monitored via sensor(s)  141 . For example, controller  100  could monitor swing angle of implement system  14  between stopping positions (i.e., between dig and dump locations  18 ,  20 ) and, when the swing angle is repeatedly greater than a threshold angle, for instance greater than about 150°, controller  100  may determine that the first mode of operation is in effect. In another example, manipulation of input device  48  could be monitored via sensor(s)  141  to detect “harsh” inputs indicative of mode 1 operation. In particular, if the input is repeatedly moved from below a low threshold (e.g., about 10% lever command) to above a high threshold level (e.g., about 100% lever command) within a short period of time (e.g., about 0.2 sec or less), input device  48  may be considered to be manipulated in a harsh manner, and controller  100  may responsively determine that the first mode of operation is in effect. In a final example, controller  100  may determine that the first mode of operation is in effect based on a cycle and/or value of pressures within accumulator  100 , for example when a threshold pressure is repetitively reached. In this final example, the threshold pressure may be about 75% of a maximum pressure. 
     Modes 2-4 may correspond generally with swing operations where only a limited amount of swing energy is available for storage by first accumulator  108 . Exemplary swing operations having a limited amount of energy may include 90° truck loading, 45° trenching, tamping, or slow and smooth craning. During these operations, fluid energy may need to be accumulated from two or more segments of the excavation work cycle before significant discharge of the accumulated energy is possible. It should be noted that, although mode 4 is shown as allowing two segments of discharge from first accumulator  108 , one segment (e.g., the swing-to-dump segment) may only allow for a partial discharge of accumulated energy. As with mode 1 described above, modes 2-4 may be triggered manually by an operator of machine  10  or, alternatively, automatically triggered based on performance of machine  10  as monitored via sensor(s)  141 . For example, when machine  10  is determined to be repeatedly swinging through an angle less than about 100°, controller  100  may determine that one of modes 2-4 is in effect. In another example, controller  100  may determine that modes 2-4 are in effect based on operator requested boom movement less than a threshold amount (e.g., less than about 80% lever command for mode 2 or 4), and/or work tool tilting less than a threshold amount (e.g., less than about 80% lever command for mode 3 or 4). 
     During mode 2, controller  100  may cause first accumulator  108  to discharge fluid to swing motor  49  during only the swing-to-dump acceleration segment, receive fluid from swing motor  49  during the swing-to-dump deceleration segment, and receive fluid from swing motor  49  during the swing-to-dig deceleration segment. During mode 3, controller  100  may cause first accumulator  108  to receive fluid from swing motor  49  during the swing-to-dump deceleration segment, discharge fluid to swing motor  49  during only the swing-to-dig acceleration segment, and receive fluid from swing motor  49  during the swing-to-dig deceleration segment. During mode 4, controller  100  may cause first accumulator  108  to discharge only a portion of previously-recovered fluid to swing motor  49  during the swing-to-dump acceleration segment, receive fluid from swing motor  49  during the swing-to-dump deceleration segment, discharge fluid to swing motor  49  during the swing-to-dig acceleration segment, and receive fluid from swing motor  49  during the swing-to-dig deceleration segment. 
     Modes 5 and 6 may be known as economy or peak-shaving modes, where excess fluid energy during one segment of the excavation work cycle is generated by pump  58  (fluid energy in excess of an amount required to adequately drive swing motor  49  according to operator requests) and stored for use during another segment when less than adequate fluid energy may be available for a desired swinging operation. During these modes of operation, controller  100  may cause first accumulator  108  to charge with pressurized fluid from pump  58  during a swing acceleration segment, for example during the swing-to-dump or swing-to-dig acceleration segments, when the excess fluid energy is available. Controller  100  may then cause first accumulator  108  to discharge the accumulated fluid during another acceleration segment when less than adequate energy is available. Specifically, during mode 5, controller  100  may cause first accumulator  108  to discharge fluid to swing motor  49  during only the swing-to-dump acceleration segment, receive fluid from swing motor  49  during the swing-to-dump deceleration segment, receive fluid from pump  58  during the swing-to-dig acceleration segment, and receive fluid from swing motor  49  during the swing-to-dig deceleration segment, for a total of three charging segments and one discharging segment. During mode 6, controller  100  may cause first accumulator  108  to receive fluid from pump  58  during the swing-to-dump acceleration segment, receive fluid from swing motor  49  during the swing-to-dump deceleration segment, discharge fluid to swing motor  49  during the swing-to-dig acceleration segment, and receive fluid from swing motor  49  during the swing-to-dig deceleration segment. 
     It should be noted that controller  100  may be limited during the charging and discharging of first accumulator  108  by fluid pressures within first chamber conduit  84 , second chamber conduit  86 , and first accumulator  108 . That is, even though a particular segment in the work cycle of machine  10  during a particular mode of operation may call for charging or discharging of first accumulator  108 , controller  100  may only be allowed to implement the action when the related pressures have corresponding values. For example, if sensors  102  indicate that a pressure of fluid within first accumulator  108  is below a pressure of fluid within first chamber conduit  84 , controller  100  may not be allowed to initiate discharge of first accumulator  108  into first chamber conduit  84 . Similarly, if sensors  102  indicate that a pressure of fluid within second chamber conduit  86  is less than a pressure of fluid within first accumulator  108 , controller  100  may not be allowed to initiate charging of first accumulator  108  with fluid from second chamber conduit  86 . Not only could the exemplary processes be impossible to implement at particular times when the related pressures are inappropriate, but an attempt to implement the processes could result in undesired machine performance. 
     During the discharging of pressurized fluid from first accumulator  108  to swing motor  49 , the fluid exiting swing motor  49  may still have an elevated pressure that, if allowed to drain into tank  60 , may be wasted. At this time, second accumulator  110  may be configured to charge with fluid exiting swing motor  49  any time that first accumulator  108  is discharging fluid to swing motor  49 . In addition, during the charging of first accumulator  108 , it may be possible for swing motor  49  to receive too little fluid from pump  58  and, unless otherwise accounted for, the insufficient supply of fluid from pump  58  to swing motor  49  under these conditions could cause swing motor  49  to cavitate. Accordingly, second accumulator  110  may be configured to discharge to swing motor  49  any time that first accumulator  108  is charging with fluid from swing motor  49 . 
     As described above, second accumulator  110  may discharge fluid any time a pressure within drain passage  78  falls below the pressure of fluid within second accumulator  110 . Accordingly, the discharge of fluid from second accumulator  110  into first circuit  52  may not be directly regulated via controller  100 . However, because second accumulator  110  may charge with fluid from first circuit  52  whenever the pressure within drain passage  88  exceeds the pressure of fluid within second accumulator  110 , and because control valve  56  may affect the pressure within drain passage  88 , controller  100  may have some control over the charging of second accumulator  110  with fluid from first circuit  52  via control valve  56 . 
     In some situations, it may be possible for both first and second accumulators  108 ,  110  to simultaneously charge with pressurized fluid. These situations may correspond, for example, with operation in the peak-shaving modes (i.e., in modes 5 and 6.). In particular, it may be possible for second accumulator  110  to simultaneously charge with pressurized fluid when pump  58  is providing pressurized fluid to both swing motor  49  and to first accumulator  108  (e.g., during the swing-to-dig acceleration segment of mode 5 and/or during the swing-to-dump acceleration segment of mode 6). At these times, the fluid exiting pump  58  may be directed into first accumulator  108 , while the fluid exiting swing motor  49  may be directed into second accumulator  110 . 
     Second accumulator  110  may also be charged via second circuit  54 , if desired. In particular, any time waste fluid from second circuit  54  (i.e., fluid draining from second circuit  54  to tank  60 ) has a pressure greater than the threshold pressure of second accumulator  110 , the waste fluid may be collected within second accumulator  110 . In a similar manner, pressurized fluid within second accumulator  110  may be selectively discharged into second circuit  54  when the pressure within second circuit  54  falls below the pressure of fluid collected within second accumulator  110 . 
     INDUSTRIAL APPLICABILITY 
     The disclosed hydraulic control system may be applicable to any excavation machine that performs a substantially repetitive work cycle, which involves swinging movements of a work tool. The disclosed hydraulic control system may help to improve machine performance and efficiency by assisting swinging acceleration and deceleration of the work tool during different segments of the work cycle based on a current mode of operation. Specifically, the disclosed hydraulic control system may partition the work cycle into segments and, based on the current mode of operation, selectively store pressurized waste fluid or release the stored fluid to assist movement of a swing motor during the partitioned segments. 
     Several benefits may be associated with the disclosed hydraulic control system. First, because hydraulic control system  50  may utilize a high-pressure accumulator and a low-pressure accumulator (i.e., first and second accumulators  108 ,  110 ), fluid discharged from swing motor  49  during acceleration segments of the excavation work cycle may be recovered within second accumulator  110 . This double recovery of energy may help to increase the efficiency of machine  10 . Second, the use of second accumulator  110  may help to reduce the likelihood of voiding at swing motor  49 . Third, the ability to adjust accumulator charging and discharging based on a current segment of the excavation work cycle and/or based on a current mode of operation, may allow hydraulic control system  50  to tailor swing performance of machine  10  for particular applications, thereby enhancing machine performance and/or further improving machine efficiency. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed hydraulic control system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed hydraulic control system. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.