Abstract:
A fluid control system may comprise a pump, a tank, and an actuator including a working chamber. The system may be operative to control rotational movement of a swing structure and movement of an least one implement. A valve assembly may be configured to control fluid communication between the working chamber and the tank and to control fluid communication between the working chamber and the pump. An input device may be operative to selectively control movement of the swing structure. The system may include a controller in communication with the valve assembly and the input device. The controller may be configured to control a flow condition of the working chamber through a sensed pressure condition of the working chamber and a command from the input device.

Description:
TECHNICAL FIELD 
     The invention relates generally to a fluid control system and, more particularly, to a swing control algorithm for a hydraulic circuit. 
     BACKGROUND 
     Conventional hydraulic systems, for example, those implemented in large excavators, typically include an open center system to control swinging movement of an arm attached to a cab, for example. Such a system is commonly referred to as a swing circuit. In contrast, a closed center system is typically used to control implements. In such hydraulic systems, the open center system and the closed center system each include a dedicated pump; a fixed displacement pump for the open center system and a variable displacement pump for the closed center system. The open center system provides the operator with a feel for how much of a load is on the swing circuit motor, whereas the closed center system does not. However, the open center system is generally less efficient than a closed center system because some fluid flow in the open center system usually gets to tank without performing any work. 
     One typical hydraulic swing circuit, as shown in U.S. Pat. No. 5,575,149, includes an open center system with a fixed displacement pump. This swing circuit employs a control valve, a pair of pilot operated, dual level pressure relief valves, and a pair of pilot operated counter balance valves. The circuit does not provide a mechanism for assisting with determination of when the arm controlled by the swing circuit runs up against a wall. In addition, such a complex system that lacks the efficiency of a closed center system may not be desirable. 
     A fluid control system and swing control algorithm for effectively and efficiently providing an open center feel to a closed center hydraulic system is desired. The present invention is directed to solving one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, a fluid control system operative to control rotational movement of a swing structure and movement of at least one implement may comprise a pump, a tank, and an actuator including a working chamber. A valve assembly may be configured to control fluid communication between the working chamber and the tank and to control fluid communication between the working chamber and the pump. An input device may be operative to selectively control movement of the swing structure. The system may include a controller in communication with the valve assembly and the input device. The controller may be configured to control a flow condition of the working chamber through a sensed pressure condition of the working chamber and a command from the input device. 
     According to another aspect of the invention, a method is provided for controlling a hydraulic system. The method may include receiving an input command from an input device, generating a desired pressure value based on the input command, generating a flow limit based on the input command, and causing incremental movement of an actuator. A magnitude of the movement over a predetermined time interval may be based on the desired pressure value and the flow limit. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a schematic illustration of a hydraulic circuit in accordance with one embodiment of the present invention. 
     FIG. 2 is a block diagram in accordance with one embodiment of the present invention. 
     FIG. 3 is a graph of desired pressure versus lever position in accordance with one embodiment of the present invention. 
     FIG. 4 is a graph of flow limit versus lever position in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Referring to FIG. 1, a fluid control system, for example, hydraulic circuit  100 , includes a valve assembly, for example, an independent metering valve arrangement  110 , a pump  112 , a tank  114 , and an actuator, for example, a hydraulic motor  116 . In this exemplary embodiment, the hydraulic motor may be a reversible, equal-displacement motor, having a first end chamber  118  and a second end chamber  120 . The pump  112  may comprise, for example, a variable-displacement, high pressure pump. 
     The independent metering valve arrangement  110  includes a plurality of independently-operated, electronically-controlled metering valves  122 ,  124 ,  126 ,  128 . The metering valves  122 ,  124 ,  126 ,  128  control fluid flow between the pump  112 , the tank  114 , and the hydraulic motor  116 . The metering valves may be spool valves, poppet valves, or any other conventional type of metering valve that would be appropriate. The metering valves are referred to individually as a chamber-to-tank first end (CTFE) metering valve  122 , a pump-to-chamber first end (PCFE) metering valve  124 , a pump-to-chamber second end (PCSE) metering valve  126 , and a chamber-to-tank second end (CTSE) metering valve  128 . The independent metering valve arrangement  110  also includes an input port  130 , an output port  132 , a first end control port  134 , and a second end control port  136 . 
     The hydraulic control system  100  also includes a first end pressure sensor  150 , a second end pressure sensor  152 , a controller  160 , and an operator input device  170 . The first and second end pressure sensors  150 , 152  are configured to communicate with the controller  160 . The input device  170  also communicates with the controller and allows an operator to control the hydraulic circuit  100 . For example, the input device  170  allows the operator to swing a load, for example, a pivotal operator cab with a work arm and/or a work implement  190 . Alternatively, the input device  170  may represent a source of input commands from, for example, a computer used to automatically control the hydraulic motor  116  without an operator. 
     As shown in FIG. 1, the controller  160  communicates electronically with the input device  170 , the metering valves  122 ,  124 ,  126 ,  128 , and the pressure sensors  150 ,  152 . The controller  160  may receive information from the input device  170 , for example, a swing direction command, as well as from the pressure sensors  150 ,  152 . Based on the commands from the input device  170  and the pressure sensors  150 ,  152 , the controller may determine a desired operation for the hydraulic circuit  100  and an appropriate set of outputs  165  to the metering valves  122 ,  124 ,  126 ,  128 . In one embodiment, the outputs  165  may represent currents to each of the metering valves  122 ,  124 ,  126 ,  128 . 
     FIG. 2 is an exemplary operation  200  of the controller  160  in accordance with a first exemplary embodiment of the hydraulic circuit  100 . It should be appreciated that the numerical pressure error limits and ranges used throughout this exemplary operation  200  may differ depending on the machinery employing an algorithm according to the invention. 
     Referring to FIG. 2, control commences with step  202  when the controller  160  receives a command from the input device  170 . In step  204 , the controller  160  determines a desired pressure to be applied to a working chamber and a flow limit of fluid to the working chamber, for example, by extrapolation from the graphs shown in FIGS. 3 and 4. For example, a forward push on the input device  170 , for example, an operating lever, may be associated with the positive lever positions of FIGS. 3 and 4 and may cause the application of pressurized fluid to the first end chamber  118  of the hydraulic motor  116  to rotate the motor  116  in a clockwise direction. In this situation, the first end chamber  118  would be the working chamber. It should be appreciated that the opposite may be true for a rearward pull on the input device  170 . Further, it should be appreciated that the effect of the directional movement of the input device  170  may be reversed as may be the association of the lever positions of FIGS. 3 and 4. 
     Control then continues to step  206  where the controller  160  determines the pressure error at the working chamber, i.e., first end chamber  118  or second end chamber  120 . The pressure error may be determined by subtracting the pressure sensed by the corresponding pressure sensor  150 ,  152  at the working chamber from the desired pressure determined by the position of the input device  170 . 
     Then, in step  208 , the controller  160  determines whether the pressure error is greater than a first predetermined positive pressure error limit of, for example, 50 KPa (7.252 psi). If the pressure error is greater than 50 KPa, control proceeds to step  210 . Otherwise, control skips to step  230 . 
     In step  210 , the controller  160  increases the fluid flow to the working chamber. The fluid flow to the working chamber may be controlled by operating the pump-to-chamber metering valve  124 ,  126  associated with the working chamber. For example, if the first end chamber  118  is the working chamber, the PCFE metering valve  124  is controllably opened to increase the fluid flow to the first end chamber  118 . The amount that the pump-to-chamber metering valve  124 ,  126  associated with the working chamber is opened may be determined by a predetermined algorithm or look-up table. Gradual ramping of the fluid flow to the working chamber may provide a more controlled and/or smoother movement of the load. The ramping may be a linear or non-linear function. 
     In addition, the chamber-to-tank metering valve associated with the non-working chamber may meter flow out of the non-working chamber as the pump-to-chamber metering valve associated with the working chamber controls fluid flow to the working chamber. Metering of the chamber-to-tank valve associated with the non-working chamber may provide extra resistance to the working chamber and facilitate a quicker pressure buildup of fluid pressure at the working chamber. 
     Then, in step  212 , the controller  160  determines whether the increased fluid flow to the working chamber is greater than the flow limit determined by the position of the input device  170 . If the increased fluid flow is greater than the flow limit, control continues to step  214 . Otherwise, control skips to step  216 . 
     In step  214 , the controller  160  sets the increased fluid flow equal to the flow limit determined by the position of the input device  170 . Control continues to step  216 . 
     In step  216 , the controller  160  determines whether the pressure error is greater than a second predetermined positive pressure error limit of, for example, 5000 KPa (725.189 psi). If the pressure error is greater than 5000 KPa, control continues to step  218 . Otherwise, control goes to step  220 . 
     In step  218 , the controller  160  sets the bleed flow associated with the working chamber equal to zero. The bleed flow may be controlled by the chamber-to-tank metering valve  122 ,  128  associated with the working chamber. For example, if the first end chamber  118  is the working chamber, the bleed flow may be controlled by the CTFE metering valve  122 . In step  218 , the controller  160  may close the appropriate chamber-to-tank metering valve to achieve zero bleed flow. Control then continues to step  240 , where control is returned to step  202 . 
     In step  220 , when the controller  160  determines that the pressure error is not greater than 5000 KPa in step  216 , the controller  160  determines whether the bleed flow is equal to zero. If the bleed flow equals zero, control continues to step  222 . Otherwise, control goes to step  224 . 
     In step  222 , the controller  160  sets the bleed flow equal to a minimum bleed flow. The minimum bleed flow may be any predetermined amount, for example, 3 liters/min (lpm), 5 lpm, 10 lpm, etc. The minimum bleed flow may differ depending on the machinery employing an algorithm according to the invention. Control then continues to step  240 , where control is returned to step  202 . 
     In step  224 , when the controller  160  determines that the bleed flow is not equal to zero in step  220 , the controller reduces the amount of bleed flow. Then, in step  226 , the controller  160  determines whether the reduced bleed flow is less than the minimum bleed flow. If the reduced bleed flow is less than the minimum bleed flow, control continues to step  228 . Otherwise, control skips to step  240 , where control is returned to step  202 . 
     In step  228 , when the controller  160  determines that the reduced bleed flow is less than the minimum bleed flow, the controller  160  sets the reduced bleed flow equal to the minimum bleed flow. Control then continues to step  240 , where control is returned to step  202 . 
     In step  230 , after determining that the pressure error is not greater than 50 KPa in step  208 , the controller  160  determines whether the pressure error is less than a first predetermined negative pressure error limit of, for example, −50 KPa (−7.252 psi). If the pressure error is less than −50 KPa, control continues to step  232 . Otherwise, control goes to step  234 . 
     In step  232 , the controller  160  decreases the fluid flow to the working chamber and increases the bleed flow. Control then continues to step  240 , where control is returned to step  202 . 
     In step  234 , after determining that the pressure error is not less than −50 KPa, the controller maintains the present fluid flow to the working chamber and the present bleed flow. Control then continues to step  240 , where control is returned to step  202 . 
     Referring to FIGS. 3 and 4, the maximum and minimum desired pressures and the maximum flow limit may differ depending on the machinery employing an algorithm according to the invention. In addition, the relationship between desired pressure and lever position may be non-linear. Furthermore, the relationship between flow limit and lever position may be linear or another nonlinear configuration. 
     INDUSTRIAL APPLICABILITY 
     In use, the metering valves  122 ,  128  control chamber-to-tank fluid flow while the metering valves  124 ,  126  control pump-to-chamber fluid flow. Conventional rotation of the motor  116  in one direction may be achieved, for example, by selective, operator-controlled actuation of the metering valves  124 ,  128  and rotation in a second, opposite direction may be achieved, for example, by simultaneous operator controlled actuation of the metering valves  122 ,  126 . 
     Referring to FIG. 1, the input device  170  may be positioned to provide an input to initiate the exemplary control operation shown in FIG.  2 . The input may include a desired pressure at the working chamber and a flow limit to the working chamber based on the lever position in accordance with the exemplary graphs shown in FIGS. 3 and 4. 
     For example, an operator may initially move the input device  170  to a 100% position corresponding to clockwise rotation of the motor  116  to position a cab with an attached swing arm. Accordingly, the desired working pressure to be applied to a working chamber may be equal to a maximum desired pressure and the flow limit may be equal to a maximum flow limit. 
     As the exemplary operation  200  proceeds, the maximum desired pressure is compared to the sensed pressure at the working chamber, for example, first end chamber  118 , associated with clockwise rotation. Since the pressure error will likely be greater than, for example, 5000 KPa, or some other second predetermined positive pressure limit, the controller  160  may operate the PCFE metering valve  124  to increase the fluid flow to the first end chamber  118  up to the maximum flow limit associated with the position of the input device  170 . The controller  160  may also operate the CTFE metering valve  122  to provide zero bleed flow. This operation may continue until the position of the input device  170  is changed or until the pressure error is less than 5000 KPa. 
     If the load is swinging freely, i.e., without resistance of a wall, barrier, or the like, the pressure error may eventually be less than 5000 KPa. However, when swinging freely, it is unlikely that the pressure error will be less than the first predetermined positive pressure limit, for example, 50 KPa, or some other first predetermined positive pressure limit, even though the operator maintains the input device  170  in the 100% clockwise position. When the pressure error is between the first and second predetermined positive pressure limits, the controller  160  may continue to increase the fluid flow to the first end chamber  118  up to the maximum flow limit associated with the position of the input device  170 . The controller  160  may operate the CTFE metering valve  122  to decrease the bleed flow to or maintain a bleed flow at a minimum bleed flow equal to a predetermined value. This operation may continue until the position of the input device  170  is changed or until the pressure error is less than 50 KPa or greater than 5000 KPa. Again, the specific pressures identified are exemplary only. The present invention is not limited to operation in accordance with specific pressures. 
     If the load continues to swing freely, it is likely that the pressure error will drop below the second predetermined positive pressure error limit, for example, 5000 KPa, while overcoming friction forces encountered to get the load swinging. After overcoming the friction forces, the pressure error will likely increase and become greater than the second positive limit, for example, 5000 KPa. After exceeding the second positive limit, the controller  160  may operate the PCFE metering valve  124  to increase the fluid flow to the first end chamber  118  once again, up to the maximum flow limit associated with the position of the input device  170 . The controller  160  may also operate the CTFE metering valve  122  to provide zero bleed flow. This situation may continue until the position of the input device  170  is changed or until the pressure error is less than 5000 KPa. 
     If, after initiating movement, the load encounters an obstruction, for example, a wall or any other barrier, the pressure error may decrease below the first predetermined positive pressure error limit, for example, 50 KPa. Until that time, the system  100  may operate as described above for the situation where the pressure error is greater than the second positive limit and the situation where the pressure error is between the first and second positive limits. 
     When the pressure error becomes less than the first predetermined positive pressure error limit, for example, 50 KPa, and greater than a first predetermined negative pressure error limit, for example, −50 KPa, the controller  160  may operate the PCFE metering valve  124  to maintain the present fluid flow to the first end chamber  118  up to the maximum flow limit associated with the position of the input device  170 . The controller  160  may also operate the CTFE metering valve  122  to maintain the previous bleed flow. These controller operations may keep the pressure error near zero. This situation may continue until the position of the input device  170  is changed or until the pressure error is greater than 50 KPa or less than −50 KPa. 
     If the pressure error becomes less than the first predetermined negative pressure error limit, for example, −50 KPa, the controller  160  may operate the PCFE metering valve  124  to decrease the present fluid flow to the first end chamber  118 . The controller  160  may also operate the CTFE metering valve  122  to increase the bleed flow. These controller operations may force the pressure error back toward zero. This situation may continue until the position of the input device  170  is changed or until the pressure error is greater than −50 KPa. 
     It should be appreciated that the bleed flow may provide damping and stability to the system. However, upon initiation by the operator, the chamber-to-tank metering valve  122 ,  128  associated with the working chamber  118 ,  120 , which controls the bleed flow, may remain closed to eliminate the possibility of the load swinging in the opposite direction, also referred to as backdriving. It should also be appreciated that, at a full lever command, the pressure error may be large and the bleed flow may be reduced to zero, thereby improving fuel efficiency. 
     Thus, the present invention provides a swing control algorithm for a hydraulic circuit, which may provide both flow and pressure control on a closed center system without the use of a dedicated swing pump. The swing control algorithm may simplify the hydraulic control system, offer a cost savings, and/or provide an open center feel to a closed center hydraulic system. 
     As shown in FIG. 1, the operation of an exemplary embodiment of this invention may be implemented on a controller  160 . The controller  160  may include a general purpose or special purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit elements, an ASIC or other integrated circuit, a hardware electronic or logic circuit such as a discrete element circuit, a programmable logic device such as a PLD, PLA, FPGA or PAL, or the like. In general, any device on which a finite state machine capable of implementing the flowchart shown in FIG. 2 can be used to implement the controller functions of this invention. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the hydraulic control system and/or the swing control algorithm without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.