Patent Publication Number: US-7908852-B2

Title: Control system for recovering swing motor kinetic energy

Description:
TECHNICAL FIELD 
     This patent disclosure relates generally to a hydraulic swing motor control circuit for an excavator or the like and, more particularly, to a hydraulic swing motor control circuit for recovering kinetic energy from the swing motor. 
     BACKGROUND 
     Certain types of machines, such as an excavator, for example, include a swing mechanism which enables an upper structure to be rotated about a base machine on a central pivot by a hydraulic swing motor. The hydraulic swing motor is part of a hydraulic circuit that includes a directional control valve configured to control the swing motor. The large mass and geometry of the upper structure of the machine create high inertial loads when the upper structure is rotated. 
     Many devices have been employed in the hydraulic circuit of such machines to prevent or reduce the inertia-induced hydraulic shock loads on the various parts of the machine and the hydraulic circuit. One such example is disclosed in U.S. Pat. No. 4,586,332, which issued on May 6, 1986, to Lawrence F. Schexnayder. The hydraulic swing motor control circuit described in the &#39;332 patent includes a pair of shunt valves each of which establishes restricted communication between first and second motor conduits leading to the hydraulic swing motor in a particular direction at their normal spring-biased position. This allows limited free swing of the upper structure when the directional control valve is shifted from an operating position to the neutral position. Shifting the directional control valve to an operating position causes an appropriate one of the shunt valves to shift to a blocking position so that no interconnection between the motor conduits exists. The present disclosure is directed to improving machine productivity and fuel efficiency through the swing motor operation. 
     SUMMARY 
     The disclosure describes, in one aspect, a method and a system for controlling a swing motor that recovers kinetic energy generated by the operation of the swing motor, converts the kinetic energy recovered from the swing motor into hydraulic potential energy, and reuses the hydraulic potential energy converted from the kinetic energy recovered from the swing motor for swing motor acceleration. 
     In an aspect of the disclosure, a control circuit includes a pump, a swing motor, first and second motor conduits, and an accumulator system. The swing motor has a first port and a second port. The swing motor moves in a first direction when a flow of hydraulic fluid flows into the swing motor through the first port. The swing motor moves in a second direction when a flow of hydraulic fluid flows into the swing motor through the second port with the second direction being opposite to the first direction. The first motor conduit is connected to the first port of the motor, and the second motor conduit is connected to the second port of the motor. The accumulator system includes a pressure-controlled selection valve and an accumulator. The selection valve is hydraulically connected to the first and second motor conduits and to the accumulator. The selection valve is moveable between a first open position, wherein a flow path between the first port of the swing motor and the accumulator is defined, and a second open position, wherein a flow path between the second port of the swing motor and the accumulator is defined. The selection valve is disposed in the first open position when the pressure in the first motor conduit is greater than the pressure in the second motor conduit and disposed in the second open position when the pressure in the second motor conduit is greater than the pressure in the first motor conduit. 
     In another aspect of the disclosure, a method for controlling a swing motor includes directing a flow of hydraulic fluid through a first motor conduit into a first port of the swing motor and out of a second port of the swing motor into a second motor conduit to move the swing motor in a first direction. The flow of hydraulic fluid through the swing motor into the first port and out the second port can be decelerated. A flow path can be provided from the second port of the swing motor to an accumulator such that at least a portion of the flow of hydraulic fluid exiting the swing motor from the second port is directed into the accumulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view of an excavator. 
         FIG. 2  is a schematic illustration of an embodiment of a hydraulic swing motor control system for recovering kinetic energy therefrom. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to a hydraulic system and method for recovering the kinetic energy generated by the operation of a swing motor, converting the kinetic energy into hydraulic potential energy, and reusing the hydraulic potential energy for swing motor acceleration to improve the machine productivity and fuel efficiency of the overall system. The hydraulic system includes an accumulator for collecting kinetic energy caused by the motion of the swing motor. The accumulator stores exit oil from the swing motor that is pressurized by the inertia torque applied on the moving motor via movement of an upper structure of the machine, such as an excavator. The swing motor deceleration can be dependent upon the accumulator. 
     The supply of pressurized oil in the accumulator can be reused to accelerate the swing motor by supplying pressurized oil to the selected motor port. The accumulator can be connected to the swing motor in parallel with the hydraulic pump that operates the swing motor for turbo-charging the swing motor. A pressure-controlled selector valve can be included to ensure that the accumulator is connected to the appropriate side of the swing motor. 
       FIG. 1  schematically illustrates a machine  4 , such as a hydraulic excavator. The machine  4  includes an upper structure  6  that is rotatable relative to a base machine  8  about a central axis (not shown). The upper structure  6  rotates under the control of a swing motor  11 . In the illustrated embodiment, the upper structure  6  includes a boom  9  extending therefrom that supports a work tool  13 , in this case a bucket, as will be understood by those skilled in the art. 
       FIG. 2  illustrates a hydraulic circuit  10  adapted to control the hydraulic swing motor  11  adapted to drivingly rotate the upper structure  6  of the machine  4 . The hydraulic circuit  10  can include a pump  14  connected to a tank  16 , a control valve  17  connected to the pump  14  via a pump conduit  18 , first and second motor conduits  19 ,  21  connecting the control valve  17  to opposite sides of the hydraulic swing motor  11 , and an accumulator system  23 . The accumulator system  23  is connected to the hydraulic swing motor  11  via first and second selector conduits  25 ,  26  which in turn are connected to the first and second motor conduits  19 ,  21 , respectively. An operator input mechanism  28 , or swing lever, can be provided to allow a user to operate the swing motor  11 . Specifically, the operator input mechanism  28  is connected to a controller  30  adapted to receive input command signals from the operator mechanism  28 . The controller  30  operates in a logical fashion to provide output control signals for adjusting the fluid applied to the swing motor  11 . 
     In an embodiment, the swing motor  11  includes a first port  40  and a second port  42 . The swing motor  11  can move in a first direction when a flow of hydraulic fluid flows into the swing motor  11  through the first port  40 . The swing motor  11  can move in a second direction when a flow of hydraulic fluid flows into the swing motor  11  through the second port  42 . The second direction is in opposing relationship to the first direction in an embodiment. In a further embodiment, the swing motor  11  can move the upper structure  6  in a clockwise direction (when viewed from above) when the swing motor  11  is operated in the first direction and a counterclockwise direction (when viewed from above) when the swing motor  11  is operated in the second direction. 
     The pump  14  can be any suitable pump and is shown as a variable displacement pump. The pump  14  can be adapted to selectively supply a flow of pressurized hydraulic fluid to the swing motor  11  through one of the first and second motor conduits  19 ,  21  via the control valve  17 . The pump conduit  18  can have a one-way check valve  45  disposed therein to define a one-way flow path from the pump  14  to the control valve  17 . 
     The control valve  17  can be hydraulically connected to the pump  14  and to the first and second motor conduits  19 ,  21 . The control valve can be movable between a first open position, wherein a flow path between the pump  14  and the first port  40  of the swing motor  11  is defined, a second open position, wherein a flow path between the pump  14  and the second port  42  of the swing motor  11  is defined, and a closed position, wherein the pump  14  and the swing motor  11  are hydraulically blocked from each other. 
     The control valve  17  can be an independent metering valve (IMV) system that includes four independently-operated valves that can be considered to act as a flow divider  48  and a pair of throttle-check valves  50 ,  51 . The flow divider  48  can have an inlet  54  hydraulically connected to the pump  14  via the pump conduit  18 , a first outlet  55  hydraulically connected to the swing motor  11  via the first motor conduit  19 , and a second outlet  56  hydraulically connected to the swing motor  11  via the second motor conduit  21 . The flow divider of the control valve  17  can include first and second variable restrictors  58 ,  59 . The first variable restrictor  58  can be disposed between the inlet  54  of the control valve  17  and the first outlet  55  thereof. The second variable restrictor  59  of the flow divider can be disposed between the inlet  54  of the control valve and the second outlet  56  thereof. The first variable restrictor  58  of the flow divider can define a variable pump to motor one-way flow path for the first port  40  of the swing motor  11 . The second variable restrictor  59  of the flow divider can define a variable pump to motor cylinder one-way flow path for the second port  42  of the swing motor  11 . 
     Each throttle-check valve  50 ,  51  can include a variable restrictor  62 ,  63  and a one-way check valve  64 ,  65 . The first and second throttle-check valves  50 ,  51  are hydraulically connected to the tank  16 . The first throttle check valve  50  and second throttle check valve  51  are connected in parallel to a tank conduit  68 , which, in turn, is connected to the tank  16 . A one-way check valve  69  can be disposed in the tank conduit  68  to help establish back pressure in the tank conduit  68 . 
     The first throttle-check valve  50  can be hydraulically connected to the first motor conduit  19 . The third variable restrictor  62  can be hydraulically connected to the first motor conduit  19  and to the tank  16  via the tank conduit  68 . The one-way check valve  64  can be connected in parallel relationship with the third variable restrictor  62 . The check valve  64  can be connected to the first motor conduit  19  and the tank  16  via the tank conduit  68  to define a one-way fluid flow path from the tank  16  through the check valve  64  to the swing motor  11  via the first motor conduit  19 . 
     The second throttle-check valve  51  can be hydraulically connected to the second motor conduit  21 . The fourth variable restrictor  63  can be hydraulically connected to the second motor conduit  21  and to the tank  16  via the tank conduit  68 . The one-way check valve  65  can be connected in parallel relationship with the fourth variable restrictor  63 . The check valve  65  can be connected to the second motor conduit  21  and the tank  16  via the tank conduit  68  to define a one-way fluid flow path from the tank  16  through the check valve  65  to the swing motor  11  via the second motor conduit  21 . 
     The first throttle-check valve  50  can define a variable motor cylinder-to-tank one-way flow path for the first port  40  of the swing motor  11  with the check valve  64  providing an anti-cavitation feature for the swing motor  11 . The second throttle-check valve  51  can define a variable motor cylinder-to-tank one-way flow path for the second port  42  of the swing motor  11  with the associated check valve  65  providing an anti-cavitation feature for the swing motor  11 . 
     The control valve  17  can be electrically connected to the controller  30 . The motor speed can be controlled using the control valve  17  to control the flow of hydraulic oil into the swing motor  11  from the pump  14 . Each of the variable restrictors  58 ,  59 ,  62 ,  63  of the control valve  17  can be independently operated via the controller  30 . In other embodiments, a solenoid-operated directional control valve as is known in the art can be used to control the flow of hydraulic oil from the pump  14  to the swing motor  11 . 
     The first motor conduit  19  is hydraulically connected to the control valve  17  and to the first port  40  of the swing motor  11 . The second motor conduit  21  is hydraulically connected to the control valve  17  and to the second port  42  of the swing motor  11 . A pair of cross-line pressure relief valves  72 ,  73  can be provided to interconnect the motor conduits  19 ,  21  in the usual manner so that excessive pressure above a predetermined value in one of the first and second motor conduits  19 ,  21  is relieved to the other of the first and second motor conduits  19 ,  21 . 
     The accumulator system  23  can included a selection valve  80  connected to the first and second motor conduits  19 ,  21 , a modulation valve  82  connected in series to the selection valve  80  via a first accumulator conduit  83 , an accumulator charge valve  85  connected in series to the modulation valve  82  via a second accumulator conduit  86 , and a hydraulic accumulator  88  connected in series to the accumulator charge valve  85  via a third accumulator conduit  89 . A pressure sensor  91  can be disposed between the accumulator charge valve  85  and the accumulator  88 . 
     The selection valve  80  can be hydraulically connected to the first and second motor conduits  19 ,  21  and to the accumulator  88  (through the modulation valve  82  and the accumulator charge valve  85  as illustrated). The selection valve  80  can be a pressure-operated, directional control 2/2-way valve. The selection valve  80  can respond to the differential pressure between the first and second motor conduits  19 ,  21  such that the selection valve  80  opens a flow path between the first accumulator conduit  83  and the motor conduit having the greater relative pressure via the associated selector conduit. 
     The selection valve  80  can be movable between a first open position, wherein a flow path between the first port  40  of the swing motor  11  and the accumulator  88  is defined, and a second open position, wherein a flow path between the second port  42  of the swing motor  11  and the accumulator  88  is defined. The selection valve  80  can be disposed in the first open position when the pressure in the first motor conduit  19  is greater than the pressure in the second motor conduit  21 . The selection valve  80  can be disposed in the second open position when the pressure in the second motor conduit  21  is greater than the pressure in the first motor conduit  19 . 
     The modulation valve  82  can be a normally-closed proportional flow control valve. The modulation valve  82  can be hydraulically connected to the selection valve  80  and the accumulator  88  (through the accumulator charge valve  85  as illustrated). The modulation valve  82  can be disposed in series between the selection valve  80  and the accumulator  88 . The modulation valve  82  can be disposed in series between the selection valve  80  and the accumulator charge valve  85 . The modulation valve  82  can be variably movable over a range of travel between a fully open position, wherein a flow path between the first accumulator conduit  83  and the second accumulator conduit  86  is defined, and a fully closed position, wherein the first accumulator conduit  83  and the second accumulator conduit  86  are hydraulically blocked from each other. 
     Intermediate positions between the fully open position and the fully closed position can define a restricted flow path relative to the fully open position according to a relationship between the relative position of the modulation valve  82  with respect to the fully open position. The modulation valve  82  can be variably movable over a range of travel between a fully open position, wherein a flow path between the selection valve  80  and the accumulator  88  (through the accumulator charge valve  85  as illustrated) is defined, and a fully closed position, wherein the selection valve  80  and the accumulator  88  are hydraulically blocked from each other. 
     The modulation valve  82  can include a solenoid  94  and a spring  95 . The solenoid  94  and the spring  95  can be adapted to move the modulation valve  82  over the range of travel between the fully open position and the fully closed position. In the illustrated embodiment, the spring  95  positions the modulation valve  82  in the fully closed position when the solenoid  94  is de-energized. The solenoid  94  of the modulation valve  82  can be electrically connected to the controller  30 . The controller  30  can adjust the position of the modulation valve  82  based upon the pressure detected by the pressure sensor  91  associated with the accumulator  88 , the pressure sensor  91  also being electrically connected to the controller  30 . The pressure sensor  91  can be operably arranged with the accumulator  88  to sense the pressure within the accumulator  88 . 
     The controller  30  can be adapted to receive a variable signal from the pressure sensor  91  with the signal being variable to indicate the pressure in the accumulator  88  sensed by the pressure sensor  91 . The controller  30  can operate the solenoid of the modulation valve to position the modulation valve  82  based on the pressure sensed by the pressure transducer  91 . 
     In certain embodiments, when the accumulator is undergoing a charging operation, the controller  30  can be adapted to maintain the modulation valve  82  in the fully open position while the pressure in the accumulator  88  is at or below a predetermined level. Once the pressure transducer  91  indicates that the pressure in the accumulator  88  exceeds the predetermined level, the controller  30  can position the modulation valve  82  in an intermediate position between the fully open position and the fully closed position based on the pressure sensed by the pressure transducer  91 . Once the pressure transducer  91  senses that the pressure in the accumulator  88  is at a second predetermined level, which is higher than the first predetermined level, the controller  30  can position the modulation valve  82  in the fully closed position. 
     When the pressure in the accumulator  88  is between the first predetermined level and the second predetermined level, the controller  30  can position the modulation valve  82  in an intermediate position between the fully open and the fully closed position that corresponds to the pressure level in the accumulator  88  relative to the first and second predetermined levels. For example, if the pressure in the accumulator  88  is halfway between the first and second predetermined levels, the modulation valve  82  can be placed in an intermediate position that restricts the flow through the modulation valve  82  by a predetermined ratio when the modulation valve  82  is in the fully open position. 
     The accumulator charge valve  85  can be hydraulically connected to the selection valve  80  (through the modulation valve  82  as illustrated) and to the accumulator  88 . The accumulator charge valve  85  can be disposed in series between the selection valve  80  and the accumulator  88 . The accumulator charge valve  85  can be disposed in series between the modulation valve  82  and the accumulator  88 . 
     The accumulator charge valve  85  can be movable between a first open position, or a charge position, wherein a one-way flow path into the accumulator  88  is defined, and a second open position, or a discharge position, wherein a one-way flow path out of the accumulator  88  is defined. When the accumulator charge valve  85  is in the charge position, a one-way flow path from the selection valve  80  through the modulation valve  82  to the accumulator  80  can be defined. When the accumulator charge valve  85  is in the discharge position, a one-way flow path from the accumulator  88  through the modulation valve  85  to the selection valve  80  can be defined. 
     The accumulator charge valve  85  can include a solenoid  97  and a spring  98 . The solenoid  97  and the spring  98  of the accumulator charge valve  85  can be adapted to move the accumulator charge valve  85  between the first open position and the second open position. In the illustrated embodiment, the spring  98  positions the accumulator charge valve  85  in the charge position when the solenoid  97  is de-energized. The solenoid  97  of the accumulator charge valve  85  can be electrically connected to the controller  30 . The position of the accumulator charge valve  85  can be a function of the operator swing motor lever  28 , which is also electrically connected to the controller  30 . 
     The accumulator charge valve  85  can be normally in the charge position as shown in  FIG. 2  for swing motor deceleration. In some embodiments, the controller  30  can operate the solenoid  97  of the accumulator charge valve  85  to move the accumulator charge valve  85  to the discharge position when the user positions the operator input mechanism  28  in a position at or above a predetermined threshold that calls for the swing motor  11  to accelerate. 
     The operator input mechanism  28  can be located within the upper structure  6  of the machine  4 , for example. The operator input mechanism  28  can be adapted to selectively indicate the direction and degree of swing motor operation. The direction can include the first and second directions of the swing motor  11 , and the degree can include a range between a lower limit and an upper limit of swing motor operation. In one embodiment, the operator input mechanism  28  can be moved from a neutral position (as shown in  FIG. 2 ) in a left direction  99  to indicate the first direction and from the neutral position in a right direction  100  to indicate the second direction. In one embodiment, the operator input mechanism  28  can be moved a predetermined amount from the neutral position to the left and to the right to a full left position and a full right position, respectively. Also, the rate of movement of the operator input mechanism  28 , together with its direction, can be used to indicate the motor acceleration or deceleration. 
     The degree, or percentage, the operator input mechanism  28  is moved from the neutral position, either to the left or the right, can be used to indicate the degree of operation of the swing motor  11  (which can be expressed as a percentage of maximum allowed swing motor operation). In some embodiments, the operator can signal the swing motor  11  to operate at 100% allowed capacity in the first direction by moving the operator input mechanism  28  to the full left position. Similarly, the operator can signal the swing motor  11  to operate at 100% allowed capacity in the second direction by moving the operator input mechanism to the full right position. Intermediate positions between the full left position and the neutral position can indicate a correlating percentage of operation in the first direction. Intermediate positions between the full right position and the neutral position can indicate a correlating percentage of operation in the second direction. 
     The controller  30  can be electrically connected to the operator input mechanism  28  and the solenoid  97  of the accumulator charge valve  85 . The controller  30  can be adapted to receive a variable signal from the operator input mechanism  28  with the signal variable to indicate the direction and degree of swing motor operation selected by the operator. The controller  30  can operate the solenoid  97  of the accumulator charge valve to place the accumulator charge valve  85  in one of the charge position and the discharge position based on the signal from the operator input mechanism  28  and/or another signal, such as motor pressure, for example. The controller  30  can be adapted to operate the IMV  17  (or in other embodiments, the directional control valve, for example) based on the input received from the operator input mechanism  28 . 
     The controller  30  can place the accumulator charge valve in the discharge position once the operator calls for operation of the swing motor  11  within a predetermined amount of the full left position or the full right position. For example, in one embodiment, the controller  30  can place the accumulator charge valve  85  in the discharge position when the operator input mechanism  28  indicates a clockwise direction with a predetermined percentage, such as ninety percent, or more of the maximum allowed operation of the swing motor  11 . Similarly, the controller  30  can place the accumulator charge valve  85  in the discharge position when the operator input mechanism  28  indicates a counterclockwise direction with a predetermined percentage, such as ninety percent, or more of the maximum allowed operation of the swing motor  11 . Once the accumulator charge valve  85  is placed in the discharge position, the controller  30  can maintain it in the discharge position until the operator input mechanism  28  is placed at or below a predetermined range encompassing the neutral position. For example, the controller  30  can be adapted to maintain the accumulator charge valve  85  in the discharge position until the operator input mechanism  28  is in a position within twenty percent of the neutral position either from the left or from the right directions  99 ,  100 . 
     In some embodiments, when the accumulator is undergoing a discharge operation, the controller  30  can be adapted to disable the accumulator discharge function when the pressure in the accumulator  88  is below a predetermined level, such as below a pressure level where the pressurized fluid in the accumulator would be close to empty. In such instances, the controller  30  can maintain the accumulator charge valve  85  in the charge position even though the operator input mechanism  28  is calling for the swing motor  11  to operate above the predetermined threshold. 
     In another aspect of the disclosure, a method for controlling a swing motor  11  can include a charging operation to convert the kinetic energy generated by the swing motor  11  into pressurized hydraulic fluid stored in the accumulator  88 . In one embodiment, a flow of hydraulic fluid can be directed through the first motor conduit  19  into the first port  40  of the swing motor  11  and out of the second port  42  of the swing motor  11  into the second motor conduit  21  to move the swing motor  11  in the first direction. The flow of hydraulic fluid through the swing motor  11  into the first port  40  and out the second port  42  can be decelerated. A flow path can be provided from the second port  42  of the swing motor  11  to the accumulator  88  such that at least a portion of the flow of hydraulic fluid exiting the swing motor  11  from the second port  42  is directed into the accumulator  88 . 
     The method for controlling a swing motor can include an accelerating operation, or a discharging operation, to use the pressurized hydraulic fluid stored in the accumulator  88  to accelerate the swing motor  11 . In one embodiment, the flow of hydraulic fluid through the swing motor  11  into the first port  40  and out the second port  42  can be accelerated as needed. The flow path from the second port  42  of the swing motor  11  to the accumulator  88  can be blocked. A flow path can be provided from the accumulator  88  to the first port  40  of the swing motor  11  such that at least a portion of the flow of hydraulic fluid stored in the accumulator  88  flows through the swing motor  11  into the first port  40  and out the second port  42 . 
     The accelerating operation can be used when the swing motor  11  is operated in the second direction, as well. In one embodiment, the flow of hydraulic fluid into the first port  40  of the swing motor  11  and out the second port  42  thereof can be blocked. A flow of hydraulic fluid can be directed through the second motor conduit  21  into the second port  42  of the swing motor  11  and out of the first port  40  of the swing motor  11  through the first motor conduit  19  to move the swing motor  11  in the second direction. The flow of hydraulic fluid into the second port  42  of the swing motor  11  and out the first port  40  can be accelerated as needed. A flow path from the accumulator  88  to the second port  42  of the swing motor  11  can be provided such that at least a portion of the flow of hydraulic fluid stored in the accumulator  88  flows through the swing motor  11  into the second port  42  and out the first port  40 . 
     Similarly, the charging operation to convert the kinetic energy generated by the swing motor  11  into pressurized hydraulic fluid stored in the accumulator  88  can be used when the swing motor  11  is operated in the second direction, as well. In one embodiment, the flow of hydraulic fluid into the second port  42  of the swing motor  11  can be decelerated. The flow path from the accumulator  88  to the second port  42  of the swing motor  11  can be blocked. A flow path from the first port  40  of the swing motor  11  to the accumulator  88  can be provided such that at least a portion of the flow of hydraulic fluid exiting the swing motor  11  from the first port  40  is directed into the accumulator  88 . 
     The charging operation and the discharging operations can be performed in repeated fashion alternately to fill the accumulator  88  with more pressurized fluid and increase the pressure in the accumulator  88  and to accelerate the swing motor  11  by discharging the pressurized fluid in the accumulator  88  through the swing motor  11  in the desired direction. 
     The method for controlling a swing motor can include an accumulator discharge blocking operation which can disable the discharging of the pressurized fluid in the accumulator  88  when the pressure in the accumulator  88  is below a predetermined level. In one embodiment, the flow of hydraulic fluid through the swing motor  11  into the first port  40  and out the second port  42  can be accelerated. The pressure of the hydraulic fluid stored in the accumulator  88  can be sensed. The flow path from the second port  42  of the swing motor  11  to the accumulator  88  can be blocked. A flow path from the accumulator  88  to the first port  40  of the swing motor  11  can be provided such that at least a portion of the flow of hydraulic fluid stored in the accumulator  88  flows through the swing motor  11  into the first port  40  and out the second port  42  when the pressure in the accumulator  88  exceeds a first predetermined pressure. The flow path from the accumulator  88  to the first port  40  of the swing motor  11  can be blocked when the pressure in the accumulator  88  is less than a second predetermined pressure, the second predetermined pressure being less than the first predetermined pressure. 
     The method for controlling a swing motor can include an accumulator charge blocking operation which can restrict and the charging of the pressurized fluid into the accumulator when the pressure in the accumulator is above a predetermined level and which can disable the charging of the accumulator when the pressure in the accumulator is above a second predetermined level, which is higher than the first predetermined level. In one embodiment, the pressure of the hydraulic fluid stored in the accumulator  88  can be sensed. The flow path from the swing motor  11  to the accumulator  88  can be restricted when the pressure in the accumulator  88  exceeds a first predetermined pressure. The flow path from the swing motor  11  to the accumulator  88  can be blocked when the pressure in the accumulator  88  exceeds a second predetermined pressure, the second predetermined pressure being higher than the first predetermined pressure. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to control a swing motor  11  of a machine  4 , such as an excavator, for example. The swing motor  11  can be adapted to drivingly rotate the upper structure  6  of the machine  4  in either a clockwise direction or a counterclockwise direction. The accumulator  88  stores exit oil from the swing motor  11  that is pressurized by the inertia torque applied on the moving motor  11  via movement of the upper structure  6  of the excavator  13 . The swing motor deceleration can be controlled via the accumulator  88 . The supply of pressurized oil in the accumulator  88  can be reused to accelerate the swing motor  11  by supplying pressurized oil to the selected motor port  40 ,  42 . The pressure-controlled selector valve  80  can be included to ensure that the accumulator  88  is connected to the appropriate side of the swing motor  11 . 
     The advantages provided by the disclosed swing motor arrangement and method of operation will be appreciated upon consideration of the teachings herein. For example, the system and method enables recovery of kinetic energy generated by the operation of the swing motor through conversion thereof into hydraulic potential energy. The converted hydraulic energy may thereafter be reused for providing swing motor acceleration. It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.