Abstract:
When a swinging boom driven by a hydraulic cylinder stops, inertia causes continued motion of the boom which increases pressure in a chamber of the hydraulic cylinder. Eventually that pressure reaches a level which causes the boom to reverse direction. Then pressure in an opposite cylinder chamber increases until reaching a level that causes the boom movement to reverse again. This oscillation continues until the motion is dampened by other forces acting on the boom. As a result, an operator has difficulty in properly positioning the boom. To reduce this oscillating effect, a sensor detects when the cylinder chamber pressure increases above a given magnitude and then a determination is made when the rate of change of that pressure is less than a defined threshold. Upon that occurrence, a control value is opened to relieve the pressure in that cylinder chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydraulically powered equipment, such as off-road construction and agricultural vehicles, and more particularly to apparatus for reducing bounce when a hydraulically driven member on the equipment is stopped suddenly. 
     2. Description of the Related Art 
     With reference to FIG. 1, a backhoe  10  is a common type of earth moving equipment that has a bucket  12  attached to the end of an arm  14  which in turn is coupled by a boom  15  to the frame of a tractor  18 . A joint  16  enables the bucket, arm, and boom assembly  17  to pivot left and right with respect to the rear end of the tractor. A hydraulic cylinder  19  is attached on one side of the tractor  18  to the boom  15  and provides the drive force for the pivoting motion. For larger backhoes, a pair of hydraulic cylinders are attached on opposite sides of the tractor  18  to pivot the boom. Hydraulic fluid is supplied to the cylinder  19  through valves that are manipulated by the backhoe operator. This movement of the boom  15  is referred to as “swing” or “slew”. 
     As the boom swings, pressurized fluid is introduced into one chamber of the cylinder  19 , referred to as the “driving chamber”, and fluid is exhausted from the other cylinder chamber, referred to as the “exhausting chamber”. When the operator suddenly stops the boom swing, inertia causes the motion of the backhoe assembly  17  to continue in the direction of the swing. The amount of inertia is a function of the mass of the backhoe assembly  17  and any material carried in the bucket  12 . This continued movement after the control valves have been shut compresses the hydraulic fluid in the previous exhausting chamber of the cylinder  19  and may produce a void, or cavitation, in the previous driving cylinder chamber. Anti cavitation valves typically are provided in the hydraulic system to overcome this latter problem. 
     Eventually the backhoe assembly  17  stops and starts moving in the opposite swing direction due to the relatively high pressure created in the previous exhausting chamber. This subsequent movement produces a reversal of the pressure conditions, wherein the previous driving chamber of the boom swing cylinder  19  becomes pressurized. As a result, the backhoe assembly  17  swing oscillates until inherent dampening provided by other forces eventually brings the assembly to a stop. This phenomenon is known either as “swing bounce” or “swing wag” and increases the time required to properly position the boom  15 , thereby adversely affecting equipment productivity. 
     Various approaches have been utilized to minimize the swing bounce. For example, U.S. Pat. No. 4,757,685 employs a separate relief valve for each hydraulic line connected to the swing cylinder, which valves vent fluid to a tank line when excessive pressure occurs in that cylinder. Additional fluid is supplied from the supply line through makeup valves to minimize voids in the cylinder as the swing stops. 
     U.S. Pat. No. 5,025,626 describes a cushioned swing circuit which also has relief and make-up valves connected to the hydraulic lines for the boom swing cylinder. This circuit also incorporates a cushion valve which in an open position provides a fluid path between the cylinder hydraulic lines. That path includes a flow restriction orifice. The cushion valve is resiliently biased into the shut position by a spring and a mechanism opens the cushion valve for a predetermined time period when the pressure differential between the cylinder chambers exceeds a given threshold. 
     Both of the previous circuits required a number of relatively complex valves. Therefore, it is desirable to provide a more simplified mechanism for reducing swing bounce. 
     SUMMARY OF THE INVENTION 
     A hydraulic system includes a control valve assembly, which selectively couples a pump and a tank to a hydraulic actuator that drives a member on a machine. The system has a device which produces a command designating desired movement of the load. A sensor detects pressure in the hydraulic actuator. 
     A method is provided to reduce bounce of the member when it stops. A command is received from the device designating that movement of the member in a given direction is to stop. The signal from the sensor is employed to determine the rate at which the pressure in the hydraulic actuator changes. When the rate of change of the pressure is less than a defined threshold after receiving the command, pressure in the hydraulic actuator is relieved. For example the pressure is relieved by opening a control valve that is connected to the hydraulic actuator. 
     In one application, the present bounce reduction method is used on a machine in which the member is driven by a cylinder that has first and second chambers. It is a well-known practice that this type of installation includes first and second pressure relief valves that are respectively connected to the first and second cylinder chambers. Thus upon receiving the command, pressure in the second chamber is relieved by opening an associated control valve. Then a determination is made whether the first pressure relief valve is open due to excessive pressure in the first chamber. If the first pressure relief valve is found to be open, the bounce reduction method waits for that valve to close, and thereafter opens another control valve that relieves pressure remaining in the first chamber. Otherwise if the first pressure relief valve is found to be closed, the rate of pressure change in the first chamber is determined, and pressure in the first chamber is relieved by opening the other control valve when the rate of pressure change is less than a defined threshold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of a backhoe incorporating the present invention; 
     FIG. 2 is a schematic diagram of a hydraulic circuit for the swing function of the backhoe boom; 
     FIG. 3 is a block diagram of the microcomputer controller in FIG. 2; 
     FIG. 4 is a state diagram depicting operation of a swing bounce reduction routine that is executed by the controller; 
     FIG. 5A graphically depicts pressure changes in a chamber of the hydraulic cylinder that swings the backhoe assembly; and 
     FIG. 5B is a graph of the slope of the changing pressure in FIG.  5 A. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 2, a hydraulic circuit  20  for the backhoe  10  has a pump  2  which forces fluid from a tank  24  into a supply line  26 . A conventional system pressure relief valve  28  opens in the event that the pump pressure exceeds a given safety threshold, thereby relieving that pressurized fluid to the tank  24  via the tank return line  29 . 
     The supply line  26  and tank return line  29  are connected to a plurality of functions on the backhoe tractor  10 . The hydraulic circuit for the boom swing function is shown in detail in FIG. 2. A valve assembly  30  of four solenoid operated, directional control valves  31 - 34  selectively couples the supply line  26  and tank return line  29  to a pair of actuator conduits  35  and  36  which lead to ports of a hydraulic actuator, such as a cylinder  19 , that swings the boom  15 . Specifically, the supply line  26  is connected by the first directional control valve  31  to the first actuator conduit  35  and by the second directional control valve  32  to the second actuator conduit  36 . The tank return line  29  is coupled by the third directional control valve  33  to the first actuator conduit  35  and by the fourth directional control valve  34  to the second actuator conduit  36 . For example, the valve described in U.S. Pat. No. 6,328,275 may be used in valve assembly  30 . However, other types of valves may be utilized to implement the present inventive concept. The four directional control valves  31 - 34  are illustrated in the closed, or shut, position in which the actuator conduits  35  and  36  are disconnected from the pump and tank return lines  26  and  29 . The first and second actuator conduits  35  and  36  also are designated by the letters A and B, respectively and the pressures in the actuator conduits (and the associated cylinder chamber) are designated Pa and Pb. 
     In the exemplary hydraulic circuit  20 , the first actuator conduit  35  is connected to the head chamber  42  of the boom cylinder  19  and the second actuator conduit  36  is connected to the cylinder&#39;s rod chamber  40 . Depending upon which specific ones of the four directional control valves  31 - 34  are activated, hydraulic fluid from the pump  22  is sent to one of the actuator conduits  35  or  36  and the other actuator conduit  36  or  35  is connected to the tank return line  29 . Thus by opening either a combination of the first and fourth directional control valve  31  and  34  or the second and third directional control valves  32  and  33 , the cylinder  19  is driven to extend or retract its piston rod  44  and thus move the backhoe boom  15  right or left. Although the present invention is being described in terms of operating a hydraulic cylinder, it should be understood that the novel concepts can be used with other types of hydraulic actuators, such as a hydraulic motor with a rotating shaft. 
     A first pressure relief valve  37  is connected to the first actuator conduit  35  to relieve excessive high pressure that may occur in the head chamber  42 . Similarly, a second pressure relief valve  39  is connected to the second actuator conduit  36 . These pressure relief valves  37  and  39  have a conventional design and are set to open at a significantly high pressure threshold. However, if a very heavy load is being carried in the bucket  12  when the boom  15  stops swinging, the pressure in a cylinder chamber due to the inertial load may exceed that threshold causing the associated pressure relief valve to open, as will be described. A pressure relief valve  37  or  39  opens when the pressure Pa or Pb in the respective actuator conduit  35  or  36  exceeds the pressure in the return line  29  plus a relief threshold, determined by force from a valve spring. 
     Pressure sensors are provided throughout the hydraulic circuit  20 . Specifically, a first sensor  46  measures pressure in the supply line  26  and a second sensor  47  is located in the tank return line  29 . Third and fourth pressure sensors  48  and  49  are provided in the first and second actuator conduits  35  and  36 , respectively, and produce electrical signals indicating the pressure within the cylinder chambers  42  and  40  to which those actuator conduits are connected. The electrical signals from the four pressure sensors  46 - 49  are applied to inputs of an electronic controller  50 . The controller  50  also receives input signals from an operator input device, such as a joystick  52 . As will be described, the controller  50  responds to these input signals by producing output signals which activate the solenoids of the four directional control valves  31 - 34  to operate the swing function of the backhoe assembly  17 . 
     Referring to FIG. 3, the controller  50  incorporates a microcomputer  54  which is connected by a set of buses  55  to a memory  56  in which the programs and data for execution by the microcomputer are stored. The set of buses  55  also connect input circuits  57  and output circuits  58  to the microcomputer  54 . Each input circuit  57  for the pressure sensors  46 - 49  includes a first order, low-pass filter which attenuates frequencies above 100 Hz. This filtering removes any noise that might be present on the pressure sensor signals applied to the controller  50 . The output circuits  58  provide signals to devices that indicate the status of the hydraulic system  20  to the backhoe operator. A set of valve drivers  59  controls the application of electricity to the solenoid coils in the four directional control valves  31 - 34 . As will be described, the controller  50  executes software which implements a control algorithm for swinging the backhoe boom  15 . 
     When the backhoe operator activates the joystick  52  to swing the boom  15  to the right or left, the signal generated by the joystick causes the controller  50  to begin executing a boom swing software routine that is stored in the memory  56 . This routine controls selected ones of the four directional control valves  31 - 34  necessary to produce the indicated movement of the boom. On each execution pass through the control software for the backhoe  10 , another routine is executed which detects when the boom swing is stopping and takes action to counter any significant bounce that may occur. 
     With reference to FIG.  2  and the state diagram of FIG. 4, the swing bounce reduction routine  60  commences at State  62  at which the routine remains when the boom is not swinging. In this State  62 , the controller periodically tests to determine whether the boom is moving and if so, in which direction. To do so, the controller  50  examines the velocity command produced from the joystick signal. In the exemplary hydraulic system  20 , a velocity command that is greater than zero indicates that the piston rod  44  is being extended from the cylinder  19 , whereas a negative velocity command indicates that the piston rod is retracting into the cylinder. Assume initially that the velocity command is greater than zero, in which case a transition occurs from the Direction Test State  62  to the Swing Commanded State  64 . 
     The operation of the swing bounce reduction routine  60  remains in this swing commanded State  64  until the operator manipulates the joystick  52  to indicate the boom is either stop or move in the opposite direction. That indication from the operator produces a new velocity command from the joystick which is either zero ora negative value in this situation. That change in the velocity command is detected at State  64  and produces a transition to State  66 . If the velocity command now is zero, the routine for controlling the valve assembly  30  will close all four directional control valves  31 - 34 . 
     The valve closure causes pressure within the rod chamber  40 , from which fluid was previously being exhausted, to build up as the rod continues to extend from the cylinder due to the inertia load of the backhoe assembly  17 . In addition, a significant pressure remains momentarily in the head chamber  42 , which aids continued extension of the piston rod  44 . Therefore upon entry into State  66 , the swing bounce reduction routine  60  causes the third directional control valve  33  to open so that the pressure is relieved from the head chamber  42  to the tank return line  29 . This initial pressure relief ensures that the pressure within the head chamber does not contribute to the continued motion of the backhoe assembly  17 . 
     While the swing bounce reduction routine  60  is in State  66 , the controller  50  periodically compares the absolute value of the velocity command to a velocity threshold. When the velocity command exceeds that threshold, the operator is again commanding motion of the backhoe assembly  17  in either direction. In that case, boom swing bounce is not a concern and a transition is made back to the Direction Test State  62  where the direction of the operator commanded boom motion is determined. This transition to State  62  also occurs when the operation remains in State  66  for more than 500 milliseconds. After remaining in State  66  for 180 milliseconds, the controller  50  Begins comparing the pressure level Pb in the rod chamber  40  to a first threshold level (THRESHOLD 1 ) to determine whether the pressure within the previous exhausting cylinder chamber has build up to a significant level indicating that a bounce is likely to occur when the boom motion stops. The 180 millisecond delay prevents a pressure aberrations, which can occur momentarily when a directional control valve closes, from producing a state transition. Therefore, after the 180 milliseconds delay, if the pressure Pb within the rod chamber  40  exceeds the first pressure threshold a transition occurs to State  68 . 
     At State  68  the controller  50  determines when to initiate a pressure relief operation to prevent rebounding of the backhoe assembly  17 . In order to understand how the present swing bounce reduction routine  60  make that determination, reference is made to FIG. 5A which graphically depicts pressure change within the rod chamber  40  following closure of the valves when the piston rod  44  is being extended. Initially that pressure rises until the motion of the boom  15  stops at time T 1 , after which the pressure Pb decreases as the boom moves in the opposite direction. The swing bounce reduction routine  60  makes one of two transitions from State  68  depending on whether the pressure rises to a level that causes the second pressure relief valve  39  to open. That event is indicated by pressure Pb in the second actuator conduit  36  exceeding the valve&#39;s constant relief threshold plus the pressure Pr in the return line  29 , as represented by the input signal from sensor  47 . 
     While the second pressure relief valve  39  remains closed, the swing bounce reduction routine  60  at State  68  uses the rate of change of the pressure Pb to determine when to open the fourth direction control valve  34  to relieve that pressure and prevent rebound of the backhoe assembly  17 . If that control valve is opened too soon, sufficient pressure will not build up in the rod chamber  40  to significantly slow the piston rod  44  and the attached backhoe assembly  17 . In that situation, inertia may cause the boom assembly  17  to continue swinging until striking a stop at one end of the pivot joint  16 . Conversely, if the valve is not opened soon enough, the pressure will not be relieved in time to prevent rebound of the piston and bounce of the backhoe assembly  17 . The rate of change of the pressure Pb in the second actuator conduit  36  is employed as an indicator of when the backhoe assembly  17  has slowed enough that the pressure can be relieved in time to prevent boom bounce. The rate of change corresponds to the slope of the pressure curve in FIG.  5 A and is given mathematically by the derivative of the pressure which is plotted on the graph of FIG.  5 B. 
     Thus, the controller  50  employs the input signal from pressure sensor  49  at State  68  to determine the derivative (dPb/dt) of the pressure Pb in the second actuator conduit  36 . The derivative value is checked to determine whether it is less than a second threshold (THRESHOLD 2 ), indicated by a dotted line, which occurs as the rate of pressure change decreases just prior to the point  67  of maximum pressure. This condition indicates that the hydraulic actuator and the boom assembly attached thereto have slowed a given amount. When this condition exists while the second pressure relief valve  39  is closed (i.e. pressure Pb is less than the relief threshold plus the return line pressure Pr), a transition is made from State  68  to State  70 . 
     The preferred embodiment of the swing bounce reduction routine  60  employs the rate of pressure change to determine when the hydraulic actuator and the boom assembly have slowed to a point at which action to reduce bounce can be taken. However, other methods for making that determination can be used instead, For example, a sensor can provide a signal indicating the swing position of the boom and the rate of position change used to determine when to implement bounce reduction. A velocity sensor or an accelerometer alternatively could be employed to detect when motion of the hydraulic actuator or the boom assembly has slowed to the point at which bounce reduction can be implemented. 
     At State  70 , the controller  50  opens the fourth directional control valve  34  to relieve the pressure in the rod chamber  40  of cylinder  19  to the tank  24  via the return line  29 . This prevents the pressure which has previously built up by the continued extension of the piston rod  44  from causing the piston rod to bounce back in the opposite direction. The fourth directional control valve  34  remains open for a fixed period of time (e.g. 40 milliseconds) after which the control valve is closed and a transition returns the swing bounce reduction routine to the Direction Test State  62 . 
     However, if a determination is made at State  68  that the second pressure relief valve  39  has opened, i.e. pressure Pb exceeds that valve&#39;s relief threshold plus the pressure Pr within the tank return line  29 , a transition occurs to State  72 . Because opening of the second pressure relief valve  39  provides a path which relieves pressure from the rod chamber  40 , the swing bounce reduction routine  60  remains in State  72  until a closure of the second pressure relief valve  39  is detected. That closure is indicated by a the pressure Pb within the second actuator conduit  36  decreasing below the relief threshold plus the pressure in the tank return line  29 , or by a pressure drop in the second actuator conduit  36  accompanied by a pressure increase in the first actuator conduit  35  as transpires when the piston rod  44  rebounds and moves in the opposite direction. When either of these conditions occurs, the swing bounce reduction routine  60  makes a transition from State  72  to State  74 . 
     The controller  50  in State  74  opens the fourth directional control valve  34  to relieve any residual pressure within the rod chamber  40  for a predefined period (e.g. 30 milliseconds) after which the fourth directional control valve is closed. This action relieves the pressure within the cylinder  19  due to the inertial motion of the backhoe assembly  17  thereby preventing rebound of the piston and bounce of the backhoe boom  15 . The swing bounce reduction routine  60  remains in State  74  for a total of 500 milliseconds after which a transition occurs back to the Direction Test State  62 . 
     While in State  62 , when the operator desires that the boom  15  swing in the opposite direction, as indicated by the joystick  52  producing a negative velocity command, a transition is made to State  76 . State  76  is the reciprocal of State  74  and operation of the anti-bounce routine is similar thereto with the understanding that the boom  15  is moving in the opposite direction. Therefore, when the velocity command is zero or greater, as occurs when the operator intends to stop the boom or reverse its direction, another transition occurs to State  74 . Because in this mode of operation the piston rod  44  is retracting into the cylinder  19 , pressurized fluid from the pump  22  was previously applied to the rod chamber  40 . Therefore at State  74 , the fourth direction control valve is opened by the controller  50  to relieve that pressure Pb so that it does not contribute to the continued motion of the boom  15 . Operation at this time is similar to that which occurred at State  66  when motion in the opposite direction was stopping. Therefore, under similar transition conditions, if the operator&#39;s movement of the joystick produces a new velocity command or 500 milliseconds have elapsed, a transition occurs back to the Direction Test State  62 . Otherwise, the swing bounce reduction routine  60  eventually makes a transition to State  78 . 
     In State  78 , if the first pressure relief valve  37  is not detected as opened, the anti-bounce routine enters State  80  where the pressure in the head chamber is relieved by opening the third directional control valve  33 . Thereafter, the operation returns to the Direction Test State  62 . Otherwise, when the pressure Pa in the head chamber  42  is great enough to open the first pressure relief valve  37 , a transition occurs to State  82  where the operation remains until the relief valve closure is detected. At that time, operation moves into State  66  where residual pressure within the head chamber  42  is relieved by opening the third direction control valve  33  for a predefined period before transitioning back to the Direction Test State  62 . 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of that embodiment. For example, although the invention has been described in the context of reducing swing bounce of a backhoe assembly, the novel technique can be applied to other types of motion by a variety of machine members. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above description.