Patent Publication Number: US-7210291-B2

Title: Priority communication protocol for a distributed electrohydraulic system having multiple controllers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/972,853 filed on Oct. 25, 2004 now abandoned. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electrohydraulic systems for powering components on a vehicle, and more particularly to a distributed hydraulic system having multiple actuators operated by a plurality of electronic controllers that exchange control messages over a communication network on the vehicle. 
     2. Description of the Related Art 
     With reference to  FIG. 1 , a backhoe  10  is a well known type of earth moving vehicle that has a bucket  12  rotatably attached to the end of an arm  14  that in turn is pivotally coupled by a boom  16  to a tractor  18 , thereby forming a boom assembly  15 . A hydraulic boom cylinder  21  raises and lowers the boom  16  with respect to the tractor  18  and a hydraulic arm cylinder  22  pivots the arm  14  about the end of the boom. The bucket  12  is rotated at the remote end of the arm  14  by a hydraulic bucket cylinder  23 . 
     Traditionally, the boom assembly  15  is controlled by valves located within the cab of the tractor  18  and mechanically connected to levers which the operator manipulates to independently move the boom, arm and bucket. A separate valve is provided for each of the cylinders  21 – 23  on the boom assembly  15 . Operating one of the valves controls the flow of pressurized hydraulic fluid from a pump on the tractor to the associated cylinder and controls the return of fluid from that cylinder back to the tank on the tractor. A separate pair of hydraulic conduits runs from each cylinder along the boom assembly to the respective valve in the operator cab. Each of these conduits is subject to fatigue as they flex with motion of the boom assembly. 
     There has been a recent trend away from mechanically operated valves to electrohydraulic valves that are operated by electrical signals. Electrical valve operation enables computerized control of the functions on the machine. In addition, hydraulic control now can be distributed throughout the machine by locating the valves for a given hydraulic function in close proximity to the hydraulic actuator, such as a cylinder or motor for example, being operated by those valves. Such distributed control reduces the amount of plumbing on the machine. A single hydraulic fluid supply conduit and a single fluid return conduit connect all the valve assemblies to the pump and tank on the tractor  18 . 
     The operator in the cab of the tractor  18  with a distributed hydraulic system manipulates joysticks or other input devices to generate electrical control signals for operating the valve assemblies located adjacent each of the boom assembly cylinders  21 ,  22  and  23 . U.S. Pat. No. 6,718,759 describes a velocity based system for controlling a hydraulic system with multiple function in which a velocity command is produced for machine functions in response to the corresponding joystick signals. The velocity command and other signals for a given machine function are transmitted over a shared communication network to a separate function controller which is associated with the valve assembly that controls the hydraulic cylinder for that machine function. Each function controller is located in close proximity to the associated valve assembly. The function controllers also send data and other messages over the communication network to the system controller. 
     A common communication network  56  used in vehicle control systems is the Controller Area Network (CAN) defined by the ISO 11898 standard, promulgated by the International Organization for Standardization in Geneva, Switzerland. In addition to servicing the hydraulic system, the communication network also carries commands and data regarding operation of the engine, transmission and other components on the vehicle. The advantage of using an standardized communication network, as compared to a network that uses a proprietary communication protocol, is that vehicle devices from many manufacturers are able to communicate over that network. However, a drawback of a standardized communication network is that protocol parameters are fixed and can not be varied to meet the requirements of a given device manufacturer. With a distributed hydraulic control system, for example, the data transmission rate can not be changed to enable a greater amount of messages to be communicated between the various controllers in a given time period. Therefore the communication network  56  has a finite bandwidth that limits the number of messages that it is able to carry. As a consequence, if numerous devices are competing for access to the network in order to send a message, a given device may have to wait a relatively long time before sending its message and that message may not arrive at the recipient device in a timely manner. Thus feedback signals and other operations may be delayed which erode the robustness of machine performance. 
     SUMMARY OF THE INVENTION 
     A distributed hydraulic system has a plurality of hydraulic functions at different locations on a vehicle which receive fluid under pressure from a source. Each hydraulic function includes a hydraulic actuator, a valve assembly that controls flow of fluid to the hydraulic actuator, and an electronic function controller which operates the valve assembly. The function controllers send messages over a shared communication network in the vehicle. 
     A method for controlling the distributed hydraulic system comprises determining a given electronic function controller on the vehicle as generating messages which have a higher priority than messages from other electronic function controllers. The given function controller is enabled to send messages over the communication network more frequently than the other function controllers. Specifically the given function controller is able to send messages as often as periodically at a first time interval. The other electronic function controllers on the vehicle are limited to sending messages over the communication network no more often than once every second time interval, that is longer than the first time interval. 
     In one embodiment of this control method, a hydraulic fluid pressure level required by each hydraulic function is determined, thereby forming a plurality of hydraulic fluid pressure levels. Then a given hydraulic function that requires the greatest one of the plurality of hydraulic fluid pressure levels is identified. The function controller associated with that given hydraulic function is selected as the given electronic function controller that may send messages more frequently over the communication network. 
     In another aspect of the present control method, the greatest one of the plurality of hydraulic fluid pressure levels is employed to control a pressure level produced by the source. In particular, that greatest one of the plurality of hydraulic fluid pressure levels is communicated to a controller that operates an unloader valve in the source to selectively connect an outlet of a pump to a tank of the distributed hydraulic system on the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a backhoe incorporating the present invention; and 
         FIG. 2  is a schematic diagram of a hydraulic system for moving a boom, an arm and a bucket on the backhoe. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIG. 2 , a hydraulic system  30  for controlling operation of the backhoe boom assembly  15  includes a fluid source  31  that has a fixed displacement pump  32  which draws fluid from a tank  33  and forces that fluid under pressure into a supply conduit  34 . The supply conduit  34  furnishes pressurized fluid to a boom function  41 , an arm function  42 , and a bucket function  43 , which respectively operate the boom cylinder  21 , the arm cylinder  22  and the bucket cylinder  23 . Fluid returns from these three functions  41 – 43  to the tank  33  via a return conduit  40 . The supply conduit  34  and the return conduit  40  extend from the pump and tank  32  and  33  located in the tractor  18  of the backhoe  10  along both the boom  16  and the arm  14 . Other functions, such as for swinging the boom assembly  15  or operating stabilizers, also can be connected to the supply and return conduits  34  and  40 . Although the present method is being described in the context of a machine that employs hydraulic cylinders, it should be understood that the inventive concepts can be used with other types of hydraulic actuators, such as a motor that produces rotational motion, for example. 
     The outlet pressure Ps from the pump  32  is measured by a first sensor  35 , which provides a signal indicating that pressure to a system controller  50 . An unloader valve  36  is operated by the system controller  50  to regulate pressure in the supply conduit  34  by relieving some of the fluid to the tank  33 . Other hydraulic systems utilize a variable displacement pump, which would be operated by the system controller  50 . The system controller  50  also receives a signal from a second pressure sensor  38  that measures the pressure Pr in the tank return conduit  40 . 
     Each hydraulic function  41 – 43  respectively includes one of the hydraulic cylinders, a valve assembly, and an electronic function controller. Specifically, the boom function  41  has a first valve assembly  44  that selectively applies the pressurized fluid from the supply conduit  34  to one of the chambers of the boom cylinder  21  and drains fluid from the other cylinder chamber to the return conduit  40 . A second valve assembly  45  in the arm function  42  controls the flow of hydraulic fluid to and from the arm cylinder  22  and the supply and return conduits  34  and  40 . The bucket function  43  has a third valve assembly  46  that couples the chambers of the bucket cylinder  23  to the supply and tank conduits  34  and  40 . Each of the valve assemblies,  44 – 46  is located adjacent the respective hydraulic cylinder  21 ,  22  and  23  to form a distributed control system. Any of a number of conventional configurations of electrical operated valve elements can be employed in each valve assembly  44 – 46 , such as described in U.S. Pat. No. 6,328,275. 
     Operation of the valve assemblies  44 ,  45 , and  46  is controlled by a separate function controller  51 ,  52  and  53 , respectively. Each function controller is co-located along the boom assembly  15  with the associated valve assembly. The function controllers  51 – 53  receive operational commands from the system controller  50  and the joysticks  25  and send data to the system controller. Those commands and data are exchanged via a communication network  56 , such as the Controller Area Network (CAN) serial bus that uses the communication protocol defined by ISO 11898 promulgated by the International Organization for Standardization in Geneva, Switzerland, for example. Communication network  56  also carries other messages between the engine, transmission, other components, and computers on the vehicle. 
     The system controller  50  and the function controllers  51 – 53  incorporate microcomputers that execute software programs which perform specific tasks assigned to the respective controller. The system controller  50  supervises the overall operation of the hydraulic system  30 . To produce movement of a given hydraulic cylinder  21 – 23  on the boom assembly  15 , the backhoe operator manipulates the corresponding joystick  58  to produce a command that indicates the movement desired. Each joystick  58  has circuitry that transmits its command via the communication network  56  to the function controller  51 ,  52  or  53  that operates the respective hydraulic cylinder  21 ,  22  or  23 . The joystick commands also are received by the system controller  50 . 
     Each function controller  51 ,  52  and  53  has a look-up table that converts the joystick command into a velocity command specifying the desired direction and speed that the associated hydraulic cylinder is to move. A given function controller responds to that velocity command and to pressures sensed at the ports of the associated valve assembly  44 ,  45  or  46 , respectively, by determining how to operate that valve assembly in order to achieve the commanded velocity of the designated cylinder. A given machine function  41 – 43  may operate in different metering modes depending upon the external force acting on the cylinder and the desired direction of motion. In powered extension and retraction metering modes, fluid from the supply conduit  34  is applied to one chamber of the function&#39;s cylinder  21 – 23  and all the fluid exhausting from the other cylinder chamber flows into the return conduit  40 . In the high side regeneration mode, fluid exiting one cylinder chamber is provided to the other cylinder chamber through a valve assembly node that is connected to the supply conduit  34 . In the low side regeneration mode, fluid exiting one cylinder chamber is supplied to the other cylinder chamber through a valve assembly node that is connected to the return conduit  40 . Additional fluid required by the cylinder in a regeneration mode is obtained from either the supply or return conduit  34  and  40 , and excess exhausted fluid is fed into the other conduit. 
     Once the metering mode to use has been determined, the function controller  51 ,  52  or  53  employs the commanded velocity and pressure input signals to derive an equivalent flow coefficient which characterizes either fluid flow resistance or the conductance of the conduits, valves, cylinder and other hydraulic components in the associated function. From that equivalent flow coefficient, a separate valve flow coefficient is derived for each valve element in the corresponding valve assembly  44 – 46 . The valve flow coefficients define the degree to which the respective valve element must open to provide the requisite amount of fluid flow to the hydraulic cylinder  21 – 23  being operated. Based on each valve flow coefficient, an electrical current is produced and applied to the electrical actuator of the corresponding valve element. The operation of the system controller  50  and the function controllers  48 – 52  is described in U.S. Pat. No. 6,718,759, which description is incorporated by reference herein. 
     In order that each machine function  41 – 43  achieves the commanded velocity, the system controller  50  must operate the unloader valve  36  to produce a pressure level in the supply conduit  34  which meets the requirements of all the functions. This is accomplished by each function controller  51 – 53  deriving a function supply pressure setpoint, which designates the pressure level required from the supply conduit  34  by the respective function. Derivation of the function supply pressure setpoint is based on the respective function&#39;s commanded velocity, selected metering mode, equivalent flow coefficient, cylinder characteristics, and pressures at ports of the associated valve assembly using a process described in the aforementioned patent. Each function controller  51 – 53  sends its function supply pressure setpoint to the system controller  50  over the communication network  56 . The greatest of these function supply pressure setpoints is selected by the system controller as the source supply pressure setpoint, that is used in operating the unloader valve  36  to produce the specified pressure level in the supply conduit  34 . Regulating the supply conduit pressure at the greatest function supply pressure setpoint ensures that all the functions receive fluid at a pressure that is sufficient to meet their needs. The function controllers  51 – 53  are informed via the communication network  56  of each change of the source supply pressure setpoint. 
     Key to the performance of the hydraulic system  30  is the efficient and timely exchange of the command and data messages over the communication network  56 . For example, if a function controller  51 – 53  does not promptly receive the velocity command from the associated joystick  58 , the respective hydraulic cylinder  21 – 23  will not operate in a timely manner. Similarly if the system controller  50  does not promptly receive the function supply pressure setpoints from the function controllers  50 , the supply conduit  34  will not convey fluid at the proper pressure required to operate the hydraulic cylinders at the commanded velocities. 
     On previous machines, every controller sent its data to the other devices periodically at constant intervals. However, the typical vehicle communication network  56  used in vehicles, such as the ISO 11898 Controller Area Network, requires that the messages conform to a well defined protocol, that among other defined parameters limits the rate at which message bits can be transmitted. That transmission rate thereby limits how often a given device on the network is able to transmit its data and commands without interfering with the ability of the other devices to transmit their messages. This limitation can prevent the function controllers on a machine from being able to send their data frequently enough so that the source  31  can be operated by the system controller to provide the requisite supply conduit pressure. 
     The possibility of that operational deficiency is minimized in the present hydraulic system  30  by designating certain controllers with high priority control requirements as being able to communicate over the network  56  more frequently than other controllers. Whether a given function controller  51 – 53  qualifies as having a high priority control requirement changes from time to time and thus its communication incidence also changes. 
     As described previously, the system controller  50  selects the greatest supply pressure setpoint required by the functions  41 – 43  as the source supply pressure setpoint for use in controlling the unloader valve  36 . This necessitates that the system controller monitors that greatest pressure level more often than pressure levels demanded by the other functions. As a result, the function controller  51 ,  52  or  53  associated with the function demanding the greatest supply pressure be allowed to communicate with the system controller  50  relatively often, i.e. more often that other function controllers. 
     Every function controller  51 – 53  has an network access rate variable stored in its memory that specifies the incidence at which that controller may send a message to over the communication network  56 . When a given function controller is informed that it has the greatest supply pressure setpoint, that function controller sets its network access rate variable to a predefined value which corresponds to a relatively short message transmission interval (e.g. 10 milliseconds), referred to herein as a first interval. The other function controllers, including the one that previously sent data at the first interval, upon determining that they are not demanding the greatest supply pressure, set their network access rate variables to a longer transmission interval (e.g. 100 milliseconds), referred to herein as a second interval. The given function controller thus is able to send messages as often as once every first interval, while the other function controllers may send messages only as often as once every second interval. Therefore, the given function controller, whose pressure requirements are being used to control the supply conduit pressure PS at the source  31 , sends its pressure requirement over the communication network  56  more frequently than the other function controllers. The pressure requirements of those other function controllers are less important for system control purposes as long as one of them is not demanding the greatest supply conduit pressure, in which case that one other function controller will be designated for more frequent message transmission. 
     In some hydraulic systems, pressure in the return conduit  40  also is regulated by operating an additional proportional control valve that couples the return conduit to the tank  33 . In this instance, the return conduit pressure required by each function also is determined and the greatest of those pressure requirements is used to operate the tank control valve. Here another function controller, for the function demanding the greatest return conduit pressure, also may be designated to send messages more frequently over the communication network  56 . 
     In other situations, a particular controller  50 – 54  may always have relatively high priority messages to send via the communication network  56  because of the function being controlled or operations being performed. In which case, that particular controller always will be enabled to transmit messages more often at the first interval. There may be more that two different communication intervals to which a particular controller can set its incidence of communication depending upon the relative priority of that controller&#39;s operations. 
     Therefore, the present process dynamically designates which controller or controllers  50 – 54  need high priority access to the communication network  56  and enable those controllers to send messages more frequently that other network devices. 
     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 embodiments of the invention. For example, the present hydraulic system control method can be utilized to control other functions than those associated with a boom assembly, and on other types of machines, than just backhoes. In addition, a greater or lesser number of functions than that provided in the exemplary hydraulic system  30  can be controlled. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.