Load-dependent hydraulic fluid flow control system

The present disclosure relates to a load dependent flow control system for directing hydraulic fluid to a hydraulic actuator. The load dependent flow control system includes a closed-center valve device for controlling hydraulic fluid flow to the actuator. The closed-center valve device includes a valve spool and an electro-actuator that adjusts a position of the valve spool to adjust a rate of the hydraulic fluid flow supplied to the hydraulic actuator. A pressure sensor is provided for sensing a pressure of the hydraulic fluid provided to the hydraulic actuator. The system also includes an electronic controller configured to receive an operator flow command from an operator interface. The operator flow command corresponds to a base flow through the closed-center valve device. The electronic controller interfaces with the electro-actuator of the closed-center valve device and with the pressure sensor. At least when the sensed pressure is above a threshold pressure, the electronic controller uses the operator flow command and the sensed pressure to generate a pressure-modified flow command that is sent to the closed-center valve device to control flow through the closed-center valve device. The pressure-modified flow command corresponds to a pressure-modified flow through the closed-center valve device. The pressure-modified flow is less than the base flow through the closed-center valve device.

TECHNICAL FIELD

The present disclosure relates generally to flow control systems for controlling hydraulic fluid flow used for driving one or more hydraulic actuators. More particularly, the present disclosure relates to flow control systems including closed-center valve devices.

BACKGROUND

Flow control systems include valve devices for controlling hydraulic fluid flow within a hydraulic system. A typical valve device has a variable-sized orifice, the orifice area of which can be varied by movement of a valve spool or other structure to vary (e.g., meter) the flow rate of hydraulic fluid provided to and/or from a hydraulic actuator. Valve devices can also be used to reverse the direction of hydraulic fluid flow through an actuator to reverse the direction of movement of the actuator. Example actuators include hydraulic cylinders and hydraulic motors. Common types of valve devices include open-center valve devices and closed-center valve devices.

FIG. 1illustrates an example hydraulic system including a prior art open-center valve device20for controlling the rate of hydraulic fluid flow provided to and from an actuator (e.g., a hydraulic cylinder22) and for proving directional flow control. The hydraulic cylinder22includes a cylinder body24and a piston26that is reciprocated back and forth within the cylinder body24via pressurized hydraulic fluid provided to the cylinder body24by the open-center valve device20. The piston26includes a piston head27and a piston rod28carried with the piston head27. The cylinder body24defines first and second cylinder ports30,32that are respectively in fluid communication with first and second valve ports34,36of the open-center valve device20. The open-center valve device20also includes third and fourth valve ports38,40that are respectively in fluid communication with a hydraulic pump42and a tank44(i.e., a reservoir). The open-center valve device20includes a valve spool45or other type of valve body that reciprocates axially within a valve sleeve47defining the valve ports34,36,38and40. The valve sleeve47can be formed by a valve housing. The valve spool45of the open-center valve device20includes a left section46, a center section48and a right section50each defining different flow paths. By moving the valve spool45axially within the valve sleeve47, the flow paths of the different sections can selectively be placed in fluid communication with the valve ports34,36,38and40. By varying the degree of alignment between the flow paths of the sections46,48and50and the valve ports34,36,38and40, orifice sizes (e.g., the cross-sectional area or areas of an orifice or orifices defined by the valve) of the valve can be varied to meter/vary flow rate through the valve. When valve spool45is positioned such that the flow paths of the left section46of the valve spool45are in fluid communication with the with the valve ports34,36,38and40, the first cylinder port30is placed in fluid communication with the tank44and the second cylinder port32is placed in fluid communication with the high pressure side of the pump42thereby causing the piston26to be driven in a first direction52. When the valve spool45is positioned such that the flow paths of the right section50of the open-center valve device20are in fluid communication with the valve ports34,36,38and40, the second cylinder port32is placed in fluid communication with tank44and the first cylinder port30is placed in fluid communication with the high pressure side of the hydraulic pump42causing the piston26to move in a second direction54relative to the cylinder body24. When the valve spool45is positioned such that the flow paths of the center section48of the open-center valve device20are in fluid communication with the valve ports34,36,38and40(as shown atFIG. 1), the high pressure side of the pump42as well as the first and second cylinder ports30,32are placed in fluid communication with tank44. Open-center valve devices are configured such that the parallel, open-center flow path arrangement provided by the center section48is capable of diverting flow away from the load on the hydraulic cylinder22(e.g., to tank) at higher pressures.

FIG. 2shows a closed-center valve device60incorporated into the hydraulic system ofFIG. 1. The closed-center valve device60includes a valve spool61with a left section62, a center section64and a right section66. The left section62and the right section66control flow to the hydraulic cylinder22in the same way described above with respect to the left section46and the right section50of the open-center valve device20ofFIG. 1. However, the center section64of the closed-center valve device60is different from the center section48of the open-center valve device20. Rather than providing a parallel, open-center flow path like the center section48of the open-center valve device20, the center section64of the closed-center valve device60has a closed (e.g., blocked, terminated, blind, stopped) configuration adapted to block the valve ports34,36,38and40. When the valve spool61is in a position where the center section64is aligned with the valve ports34,36,38and40, the valve ports34,36,38and40are blocked such that the cylinder ports30,32as well as the valve ports34,36are not in fluid communication with either the high pressure side of the pump42or the tank44. Thus, unlike open-center valve devices, closed-center valve devices are not capable of diverting flow to tank in response to higher load pressures.

SUMMARY

Closed-center valve systems are generally more efficient than the open-center valve control systems used in many off-road machines (e.g., excavators, drills). However, in open-center systems, the speed of the load (e.g., the speed of the actuator such as the speed of a driven piston within a cylinder or the speed of a driven motor) is a function of both an operator flow command and the load pressure. This is due to the parallel, open center flow path of the open-center valve structure that is configured to divert flow away from the load at high pressures. This gives the operator visual feedback about the force of the load, since the actuator slows down in a visually perceptible way as the load increases. Aspects of the present disclosure relate to load-dependent flow control systems that provide a load-dependent feel for flow control systems including closed-center valve devices. In certain examples, the load-dependent feel can mimic (e.g., match, imitate) the load-dependent feel provided by flow control systems including open-center valve devices. Thus, aspects of the present disclosure relate to flow control systems having efficiencies of the type associated with closed-center valve systems while also having a load-dependent “feel” of the type typically associated with open-center valve control systems.

In a typical closed-center valve control system (e.g., a load-sense system), an operator flow command which is input by an operator through an operator interface correlates directly to a corresponding flow rate, regardless of the load pressure. Aspects of the present disclosure relate to using a pressure sensor at the actuator to sense load pressure, and to using the sensed load pressure to convert the operator flow command according to some specified function (e.g., a linear function dependent upon sensed load pressure, a curved or exponential function dependent upon sensed load pressure, a function that corresponds to a virtual center orifice function, etc.) to a pressure-modified flow command. The pressure-modified flow command can correspond to a flow rate which is less than the flow rate which would have been established had the operator flow command not been modified. The reduction in flow rate can be directly related to sensed pressure (e.g., higher pressures result in larger reductions in flow rate as compared to lower pressures). In other words, the higher the sensed pressure, the more the operator flow command is reduced. Thus, through the pressure-based command modification, a given operator flow command will result in a lower flow rate at a higher sensed pressure as compared to a lower sensed pressure. In some examples, the pressure-based command modification is only implemented once the sensed pressure reaches or exceeds a threshold pressure. The form of the pressure-dependent flow rate modification function can vary widely, and can be tuned for different original equipment manufacturers (OEMs), operators, soil conditions, etc. This will allow a customized and tunable “feel” for the valve using efficient, closed-center valves. Beyond creating a different “feel”, aspects of the present disclosure can be used in applications such as mining or other applications, where it is desirable to slow down an actuated element when the actuated element encounters harder applications. For example, for mining applications including drilling, it is desirable to reduce the speed of a drill when harder rock is encountered to protect the drill bit or other components of the drill.

Aspects of the present disclosure can relate to a flow control system including an electro-hydraulic flow control valve (e.g., a closed-center valve) and load pressure sensors. An electronic controller can use sensed data from the load pressure sensors to implement a control strategy that mimics a load-dependent feel by reducing the flow demand to the valve based on the magnitude of the load pressure measured at the actuator. In certain examples, this approach can be used on independent metering valves. The approach can be used in flow control systems including load-sense protocol that can be mechanically compensated, electronically compensated, or compensated via a hybrid system that includes a combination of electronics and hydraulics. In certain examples, aspects of the present disclosure relate to a hydraulic control system capable of converting an operator demand from a pure flow command to something closer to a power command.

Aspects of the present disclosure also relate to a hydraulic flow control system having flow-demand modification that can be tunable for different machines, services, operators and/or conditions. For example, the flow-demand modification can be tuned for different operators that might prefer a softer or stiffer feel. The flow-demand modification can also be tuned so that different machine OEMs can use a single valve to provide different, custom feels. In certain examples, flow-demand modification can be adjusted or tuned based on different applications or operating conditions (e.g., soil types).

Aspects of the present disclosure can also be used to limit power demand at individual actuators and across the entire hydraulic system. By limiting the flow demand to a particular service based on pressure, the power to a single service can be capped. By setting power caps for all of the services in the system, the power demand for the entire system can be limited/capped. In one example, the control system operates such that the flow provided to a service will not exceed the maximum power allocated to the service divided by the sensed pressure corresponding to the load at the service. In cases where the pressure is low (e.g., below a pre-set threshold), the flow provided to a service can be set directly by the operator flow command. In cases where the pressure is higher, the flow can be established through a pressure-based command modification protocol that reduces the operator flow command taking into consideration sensed pressure as well as the maximum power allocated to the service. A supervisory controller can communicate with all services and can limit the total power (or torque) of the system. In certain examples, flow to certain valves can be prioritized over other valves.

Another aspect of the present disclosure relates to a load dependent flow control system for directing hydraulic fluid to a hydraulic actuator. The load dependent flow control system includes a closed-center valve device for controlling hydraulic fluid flow to the actuator. The closed-center valve device includes a valve spool and an electro-actuator that adjusts a position of the valve spool to adjust a rate of the hydraulic fluid flow supplied to the hydraulic actuator. The load dependent flow control system also includes a pressure sensor for sending a pressure of the hydraulic fluid provided to the hydraulic actuator. The load dependent flow control system further includes an electronic controller configured to receive an operator flow command from an operator interface. The electronic controller interfaces with the electro-actuator of the closed-center valve device and with the pressure sensor. At least when the sensed pressure is above a predetermined threshold level, the electronic controller is configured to modify the operator flow command based on sensed pressure to convert the operator flow command into a pressure-based flow command. The pressure-based flow command dictates a position of the valve spool and a corresponding flow rate through the closed-center valve device. The pressure-based flow command is dependent upon and variable with the sensed pressure. In one example, to generate the pressure-based flow command, the operator flow command is modified by reducing the operator flow command in direct dependency with a magnitude of the sensed pressure. When such a flow command modification protocol is in effect, the flow rate through the closed-center valve device for a given operator flow command is indirectly dependent upon the magnitude of the sensed pressure of the actuator load.

A further aspect of the present disclosure relates to a load dependent flow control system for directing hydraulic fluid to a hydraulic actuator. The load dependent flow control system includes a closed-center valve device for controlling hydraulic fluid flow to the actuator. The closed-center valve device includes a valve spool and an electro-actuator that adjusts a position of the valve spool to adjust a rate of the hydraulic fluid flow supplied to the hydraulic actuator. A pressure sensor is provided for sensing a pressure of the hydraulic fluid provided to the hydraulic actuator. The system also includes an electronic controller configured to receive an operator flow command from an operator interface. The operator flow command corresponds to a base flow through the closed-center valve device. The electronic controller interfaces with the electro-actuator of the closed-center valve device and with the pressure sensor. At least when the sensed pressure is above a threshold pressure, the electronic controller uses the operator flow command and the sensed pressure to generate a pressure-modified flow command that is sent to the closed-center valve device to control flow through the closed-center valve device. The pressure-modified flow command corresponds to a pressure-modified flow through the closed-center valve device. The pressure-modified flow is less than the base flow through the closed-center valve device.

A variety of additional aspects will be set forth in the description that follows. The aspects can relate to individual features and to combinations of features. It is to be understood that both the forgoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the broad concepts upon which the examples disclosed herein are based.

DETAILED DESCRIPTION

FIG. 3illustrates a load-dependent flow control system120in accordance with the principles of the present disclosure. The load-dependent flow control system120includes a hydraulic pump122powered by a driver124. The hydraulic pump122has a high pressure side126at which pressurized hydraulic fluid is outputted. The pressurized hydraulic fluid is used to power a plurality of actuators128a,128b. Closed-center valve devices130a,130bare used to control hydraulic fluid flow from the hydraulic pump122to the actuators128a,128b, and to control hydraulic fluid flow from the actuators128a,128bto a tank132(e.g., a reservoir). The load-dependent flow control system120also includes pressure sensors134for sensing (e.g., measuring) load pressures corresponding to the actuators128a,128b. The pressure sensors134interface with an electronic controller136. One or more optional filters138can be used to filter noise from the pressure data sensed by the sensors134. Each of the closed-center valve devices130a,130bincludes two valve spools140and electro-actuators142for moving the valve spools140. The electronic controller136interfaces with the electro-actuators142to control the electro-actuators. By controlling the electro-actuators142, the electronic controller136can control the positions of the valve spools140. The electronic controller136also interfaces with an operator interface144for allowing an operator to generate operator flow commands that are sent to the electronic controller136. Based on the pressure readings provided by the sensors134, the electronic controller136can modify the operator flow commands to convert the operator flow commands into pressure-based flow commands used to control the positions of the valve spools140. The pressure-based flow commands can be dependent upon and variable with the pressures sensed by the pressure sensors134. The sensed pressures are indicative of the loads being handled by the actuators128a,128b.

In certain examples, the hydraulic pump122can include a variable displacement pump. The displacement of the hydraulic pump122can be controlled by the position of a displacement controller such as a swash plate146. The position of the swash plate146can be controlled by a hydraulic actuation arrangement148. The hydraulic actuation arrangement148can be of the type used for load sense control and can include a hydraulic cylinder. The driver124can be coupled to the hydraulic pump122by a mechanical coupling such as a drive shaft150. In certain examples, the driver124can include a power source such as an electric motor, an internal combustion engine (e.g., a diesel or spark ignition engine), a fuel cell or other power source.

It is preferred for the load dependent flow control system120to incorporate load-sense control technology. Load-sense control technology relates to an arrangement that ensures the output of the hydraulic pump122has a pressure that exceeds a maximum work pressure in the system120by a predetermined amount (e.g., 10 bars). In essence, in a load sense system, the system is configured such that the pump adjusts flow and pressure to match the load requirements of the system. In the depicted example, the sensed pressures provided by the pressure sensors134are used by the electronic controller136to identify the maximum operating pressure in the overall system120. Based on the maximum operating pressure in the overall system, the electronic controller136controls operation of the hydraulic actuation arrangement148to ensure the output pressure of the hydraulic pump122exceeds the maximum system pressure by the predetermined amount. As indicated above, the hydraulic actuation arrangement148controls the position of the swash plate146and therefore controls the displacement of the hydraulic pump122. In the depicted example, based on the maximum operating pressure sensed by the pressure sensors134, the electronic controller136controls a position of an electronically controlled valve152. The electronically controlled valve152taps into the output of the hydraulic pump122and uses this tapped pressure and flow to control the hydraulic actuation arrangement148. By controlling operation of the electronically controlled valve152, the electronic controller136can control the hydraulic pressure provided to the hydraulic actuation arrangement148and therefore control the position of the swash plate146to ensure the hydraulic pump120outputs sufficient pressure to exceed the maximum operating pressure in the system.

It will be appreciated that the load sense system ofFIG. 3is a hybrid system that uses a combination of electronic components and hydraulic components. The hydraulic actuation arrangement148can include a hydraulic cylinder139that is hydraulically actuated to control a position of the swash plate146. When the closed-center valves are all closed, the pump122is fully de-stroked by the electronic controller136to a stand-by state in which only enough flow to account for system leakage is output by the pump122. The electronic controller136can de-stroke the pump122by opening the valve152causing the hydraulic cylinder139of the actuation arrangement148to be pressurized such that a piston137of the hydraulic cylinder139moves (e.g., extends) against the pressure of a spring135to move the swash plate146to a de-stroked position. When one of the closed-center valve devices is opened, the electronic controller136detects the increase in pressure at the actuator corresponding to the open closed-center valve device and causes the pump122to be fully stroked to a maximum flow output until the flow and pressure output by the pump122matches the load. The electronic controller136can stroke the pump122by closing the valve152. When the valve152is closed, hydraulic fluid in the hydraulic cylinder139drains to tank132through an orifice131thereby reducing the hydraulic pressure in the cylinder139to a level where the piston137and the swash plate146move via the spring force of the spring135to the stroked position. Once the output of the pump matches the load, the pump can be de-stroked (e.g., by metering flow through the valve152) to an operating state where the flow and pressure level match the sensed load. By selectively increasing and decreasing the output of the pump by metering flow through the valve152, a balanced operating state is maintained in which the flow and pressure level output by the pump matches the sensed load. When multiple loads are detected in the system, the pump is set to accommodate the highest load. The system also has a maximum pressure setting. If the output pressure at the pump reaches the maximum pressure setting, the electronic controller fully de-strokes the pump120and the system is maintained at the maximum pressure until the load clears. Once the load clears, the system resumes normal operation.

FIG. 10depicts a pure electronic load sense system where the electronic controller136interfaces electronically with an electronic actuator154that controls position of the swash plate146. The system ofFIG. 10functions in the same manner as the system ofFIG. 3, but does not use hydraulics. The controller136uses the data from the pressure sensors to electronically control the pressure and flow output of the pump. The electronic actuator154can include an actuator such as a solenoid or voice-coil actuator.

FIG. 11Aillustrates a more conventional load-sense system that only involves hydraulics. In this system, a load sense hydraulic circuit155is in fluid communication with the meter-out ports of the closed-center valve devices730a,730b. Through an arrangement of shuttle valves158, the metering port having the highest operating pressure is placed in fluid communication with a hydraulic actuation arrangement157. In one example, shown atFIG. 11B, the hydraulic actuation arrangement157can include a hydraulic cylinder159that controls the position of the pump swash plate. A load sense valve161is in fluid communication with the load sense hydraulic circuit155via a port151. The hydraulic actuation arrangement157also includes a pressure limit valve163. When the closed-center valve devices are closed, pressure from the pump output acts on the load sense valve161and overcomes a spring149(e.g., a 200 pound-per-square inch (psi) spring) of the load sense valve to move the load sense valve161to a position where the hydraulic cylinder159is disconnected from tank and is pressurized by the pump pressure. This causes the pump to be fully de-stroked. For example, the pressure in the hydraulic cylinder159moves the piston of the hydraulic cylinder159against the load of a spring153to move the swash plate to the de-stroked position. When one of the closed-center valve devices is opened, the load sense circuit155is pressurized and acts on the load sense valve161in concert with the spring149to move the valve against the pump pressure to a position where the hydraulic cylinder159is placed in fluid communication with tank. This causes the pressure in the hydraulic cylinder159to drop to a level where the piston of the hydraulic cylinder159is moved by the spring153to a position where the swash plate is in a fully stroked position. In continued operation, the pump pressure and the opposing pressure of the load-sense circuit155continue to act on the load sense valve161such that the valve161meters flow to the hydraulic cylinder159to provide a balanced state in which the output of the pump matches the load. The pressure limit valve163is acted on by the pump output pressure. When the pump pressure reaches a pressure limit, the pump output pressure overcomes a spring147(e.g., a 3000 psi spring) of the pressure limit valve163to place the hydraulic cylinder159in fluid communication with pump pressure causing the pump to be fully de-stroked until the pump pressure reduces.

The operator interface144is configured for allowing an operator to input an operator flow command to the electronic controller136. In certain examples, the operator interface can include one or more input structures such as joysticks, toggles, dials, levers, touch screens, buttons, switches, rockers, slide bars or other control elements that can be manipulated by the operator for allowing the operator to control movement of the actuators128a,128b. Separate input structures can be provided at the operator interface144for each of the actuators128a,128b(e.g., separate input structures can be provided for controlling each of the closed-center valve devices130a,130b). It will be appreciated that the position of the manipulated control element can correspond to the magnitude of the operator flow command generated by the operator interface. For example, in the case of a joystick300(seeFIG. 4), if the operator wants the actuator to stop, the joystick may be positioned at a neutral, central position302. If the operator wants the actuator to extend at full speed, the joystick300may be moved to a full right position304. If the operator wants the actuator to retract at full speed, the joystick300may be moved to a full left position306. Between the center position and the full left position or the full right position are intermediate positions (e.g., see example intermediate positions308,310,312,314). The magnitude of the operator flow command signal may vary proportionately with the position of the joystick. Thus, in certain examples, the magnitude of the operator flow command will vary proportionately with a position of a component of the operator interface.

In certain examples, the filter138can be used to filter noise from the pressure data generated by the pressure sensors134. In this way, relatively small variations in the sensed pressure can be filtered out to provide for more smooth control of the hydraulic actuators128a,128b. Filters can thus be used to shape the dynamics of flow rate modification.

The hydraulic actuators128a,128bare depicted as hydraulic cylinders. In other examples, the hydraulic actuators can include hydraulic motors or other types of actuators. Each of the hydraulic actuators128a,128bincludes a cylinder body160defining first and second cylinder ports162,164. Each of the actuators128a,128balso includes a piston arrangement including a piston head166and a piston rod168. It will be appreciated that the cylinder body160and/or the piston rod168is adapted for connection to a load. The actuators can provide various functions such as boom swinging, boom lifting, bucket or blade manipulation, vehicle propulsion, boom pivoting, vehicle lifting, vehicle tilting, drill propulsion, drill rotation or other functions.

Each of the closed-center valve devices130a,130bincludes two of the valve spools140. Each of the valve spools140corresponds to one of the cylinder ports162,164of the corresponding actuator128a,128b. Thus, the valve spools140each independently control flow to each of the cylinder ports162,164, since separate valve spools140are provided for each of the ports162,164.

With respect to each of the valve spools140, the closed-center valve devices130a,130binclude a first valve port170corresponding to one of the cylinder ports162,164, a second valve port172hydraulically connected to the high pressure side of the hydraulic pump122and a third valve port174coupled in fluid communication with tank132. It will be appreciated that the valve ports170,172,174can be defined within valve housings defining valve sleeves175of the closed-center valve devices130a,130b. The valve spools140are axially moveable within the valve sleeves175to change the positions of the valve spools140relative to the ports170,172,174. Movement of the valve spools140can be implemented through operation of the electro-actuators142. In certain examples, the electro-actuators142can include actuators such as solenoid actuators, voice coil actuators, combined hydraulic and electronic actuators or other type of actuators.

Each of the valve spools140includes a left section176, a center section178, and a right section180. The center section178has a closed-center arrangement adapted to block fluid communication between the first valve port170and the second and third valve ports172,174when the valve spool140is in a central position. With the valve spool140in the central position, the second and third valve ports172,174are isolated from one another. The left and right sections176,180have flow paths for controlling directional flow to the actuators. The valve spools140slide within the sleeves175and can function as metering valves for controlling fluid flow rates based on the positions of the spools140within the sleeve175. By controlling the degree of alignment between the flow paths of the valve sections176,180and the valve ports170,172,174, the orifice size through the valve can be controlled to control flow rates through the flow paths.

When one of the valve spools140is positioned such that flow path of the left section176of the valve spools140is in fluid communication with the valve ports170and172, the valve port170is placed in fluid communication with the high pressure side of the hydraulic pump122and the port174is blocked. When one of the valve spools140is positioned such that flow path of the right section180of the valve spools140is in fluid communication with the valve ports170and174, the valve port170is placed in fluid communication with tank and the port172is blocked.

The electro-actuators142control the positions of the valve spools140. It will be appreciated that the electro-actuators142can move the valve spools140to change the direction of movement of the pistons (i.e., the valves can be directional valves). For example, as shown atFIG. 3, the valve spools140of the closed-center valve device130aare in a position where the piston head166of the actuator128ais driven in an upward (or leftward) direction. In this configuration, the upper spool140of the device130ais positioned with the right section180at the valve ports170,172,174and the lower spool140of the device130ais positioned with the left section176at the valve ports170,172, and174. By moving the valve spools140with the electro-actuators142, the direction of flow through the actuator128can be reversed to reverse the direction of movement of the piston head166. The closed-center valve device130bis shown with the valve spools reversed to cause the piston head166of the actuator128bto be driven in a downward (or rightward) direction. In this configuration, the upper spool140of the device130bis positioned with the left section176at the valve ports170,172,174and the lower spool140of the device130bis positioned with the right section180at the valve ports170,172, and174. In addition to moving the valve spools140to alter the direction of flow through the actuators128a,128b, the electro-actuators142can also move the valve spools140to meter flow through the first valve ports170to control the flow rate provided to the actuators128a,128band to thus control the speed of the actuators128a,128b. In other words, the electro-actuators142can be used to control the orifice size provided at the first valve ports170to control the flow rates provided to and from the actuators128a,128b. By enlarging the orifice size, the flow rate is increased. By reducing the orifice size, the flow rate is decreased. Thus, the closed-center valve devices preferably function as directional valves and metering valves.

It will be appreciated that the flow rates through the closed-center valve devices are dependent upon the spool positions and the orifice sizes corresponding to the spool positions. In certain examples, the system can be configured such that the closed-center valve devices are pressure compensated so that the pressure drops across the valve devices remain constant regardless of changes in the load pressure. With pressure compensated valves of this type, a given orifice size will always provide a given flow since the pressure drop across the orifice is constant regardless of load pressure. In other examples, the system can sense the pressure drop across a given closed-center valve device and can adjust the orifice size based on pressure drop to achieve a controller commanded flow rate established by the electronic controller136. It will be appreciated that the controller commanded flow rate established by the electronic controller136can be dependent upon a magnitude of an operator flow command from the operator interface144. In certain examples, the electronic controller136will be capable of commanding different flow rates for a given operator flow command dependent on a measured pressure at the actuator controlled by the closed-center valve device at issue. In cases where actuator pressure is taken into account for determining the controller commanded flow rate through the valve, the electronic controller136can modify the operator flow command based on sensed pressure at the actuator to generate the controller commanded flow rate (e.g., the controller commanded flow rate is dependent on 2 variables, namely, the sensed load pressure and the magnitude of the operator flow command). In cases where actuator pressure is not taken into account for determining the controller commanded flow rate through the valve, the controller commanded flow rate is only based on the operator flow command (e.g., the operator flow command is the only variable upon which the controller commanded flow rate depends).

It will be appreciated that the electronic controller136can include software, firmware and/or hardware. Additionally, the electronic controller136can include memory. In certain examples, the electronic controller can interface with memory (e.g., random access memory, read-only memory, or other data storage means) that stores algorithms, look-up tables, look-up graphs, look-up charts, control models, empirical data, control maps or other information that can be accessed for use in controlling operation of the flow control system. The electronic controller can include one or more microprocessors or other data processing devices. A Controller Area Network (CAN bus) can be used to provide an architecture that allows the processors (e.g., micro-processors), sensors, actuation devices, and other devices to communicate with one another.

Referring toFIG. 5, the electronic controller136includes digital or analog processing capability for providing pressure monitoring functionality181, valve control183and pump control185. Suitable electronic processing capability and data storage capability (e.g., memory) can be used or dedicated for each function. A combined electronic processing unit can be used to implement the various functions, or multiple separate processing units/processors can work together and can be used or dedicated for the different functions. The electronic controller136interfaces with the pressure sensors134to provide the pressure monitoring functionality181. For example, the electronic controller136receives sensed pressure data from the pressure sensors134. The sensed pressure data corresponds to the sensed pressures at the ports162,164of the actuators128a,128b. The sensed pressures depend upon and are indicative of load on the actuators128a,128b. The electronic controller136uses the sensed pressure data generated by the pressure sensors134for both pump control185and valve control183.

The valve control183of the electronic controller136is adapted to receive operator flow commands from an input structure of the operator interface144and to process the operator flow commands according to flow command logic182(seeFIG. 6). As shown atFIGS. 5-6, the electronic controller136initially receives an operator flow command from the operator interface144(see box184). Next, at box186, the electronic controller136compares the sensed load pressure Psfor the actuator128a,128bto which the operator flow command corresponds with a threshold pressure PT. In one non-limiting example, the threshold pressure PTis at least 20 Bars, or at least 30 Bars. If the sensed pressure Psis less than the threshold pressure PT, then the flow command logic dictates that the controller flow command generated and output by the electronic controller136is based only on the magnitude/value of the operator flow command (see box800). Hence, the flow commanded by the controller136at the valve of the actuator is not pressure dependent, but instead is only dependent on a single variable, namely, the value of the operator flow command. The controller flow command, based only on the value of the operator flow command, is sent to the electro-actuators142of the closed-center valve device130aor130bbeing controlled by given input structure of the operator interface144to control the flow to the corresponding actuator128aor128b. If the sensed pressure Psis greater than the threshold pressure PT, then the flow command logic dictates that the controller generated flow command is dependent upon two separate variables which include: sensed pressure Psand the value of the operator flow command (see box802). For example, the flow that would have been commanded based on the value of the operator flow command if the sensed pressure Pswas less than the threshold pressure PT(i.e., a base flow) is reduced a particular amount based on the sensed pressure Ps. The amount the base flow is reduced can be dependent upon the sensed pressure Psand can be derived/calculated by a function that includes the sensed pressure Psas a variable. The pressure-based controller flow command is sent to the electro-actuators142of the closed-center valve device130aor130bbeing controlled by given input structure of the operator interface144to control the flow to the corresponding actuator128aor128b. By using the sensed pressure Psas a factor in determining the commanded flow rate through the closed-center valve being controlled, the system can provide a load dependent feel to the operator at load pressures above the threshold pressure PT.

In other examples, the system may be designed so that the controller flow command always takes into consideration both the operator flow command and the sensed load pressure of the actuator being controlled. In this situation, the threshold pressure PTis essentially set to zero.

It will be appreciated that a function (e.g., formula, equation, relationship, etc.) can be used to generate pressure-based flow control command based on the value of the operator flow command and the sensed pressure Ps. The controller can apply the function directly to determine the controller flow commands, or can use data maps or like tools based on the function to determine the controller flow commands. In one example, the function can include a linear function that includes pressure as a variable and that reduces the flow established only by the operator flow command by an amount dependent on sensed pressure Ps. In other examples, the functions can include curved functions (e.g., exponential functions) based on pressure, more complex polynomial functions (e.g., quadratic functions), and/or specialized functions (e.g., a function defining a virtual center orifice).

The following formula (1) is an example linear pressure-based flow modification function:
Q2=Q1−f(Ps), wheref(Ps)=aPs(1)

In formula (1), Q2is the flow dictated by the electronic controller flow command, Q1is the flow that would have been dictated by the controller based only on the value of the operator flow command (e.g., a base flow), a is a constant, and Psis the sensed load pressure.

The following formula (2) is an example exponential pressure-based flow modification function:
Q2=Q1−f(Ps), wheref(Ps)=aPsn(2)

In formula (2), Q2is the flow dictated by the electronic controller flow command, Q1is the flow that would have been dictated by the controller based only on the value of the operator flow command (e.g., a base flow), a is a constant, Psis the sensed load pressure, and n is a whole number greater than 1.

The following formula (3) is an example of a more complicated polynomial pressure-based flow modification function such as a quadratic function:
Q2=Q1−f(Ps), wheref(Ps)=a1Ps1+ . . . +anPn(3)

In formula (3), Q2is the flow dictated by the electronic controller flow command, Q1is the flow that would have been dictated by the controller based only on the value of the operator flow command (e.g., a base flow), the a1. . . anvalues are different constants, Psis the sensed load pressure, and n is a whole number greater than 1.

The following formula (4) is an example of a modification function that defines a virtual center orifice:

In formula (4), Q2is the flow dictated by the electronic controller flow command, Q1is the flow that would have been dictated by the controller based only on the value of the operator flow command (e.g., a base flow), ρ is a constant determined by the density of the hydraulic fluid of the system, Psis the sensed load pressure, and A(Q1) is a virtual center orifice area profile for the valve.

FIG. 7is a graph showing data corresponding to a linear function used by the electronic controller to generate controller flow commands. The graph includes three plots500,502,504showing flow rates commanded by the electronic controller136verses sensed load pressure. The plot500shows controller commanded flow verses sensed pressure for an operator flow command having a first value. In one example, the operator flow command having the first value can be generated when an operator control such as the joystick300is in the maximum position304(seeFIG. 4). The plot502shows controller commanded flow verses sensed pressure for an operator flow command having a second value less than the first value. In one example, the operator flow command having the second value can be generated when an operator control such as the joystick300is in the intermediate position310(seeFIG. 4). The plot504shows controller commanded flow verses sensed pressure for an operator flow command having a third value less than the second value. In one example, the operator flow command having the third value can be generated when and operator control such as the joystick300is in the intermediate position308(seeFIG. 4). As shown byFIG. 7, when the sensed pressure is less than the threshold pressure, the flows commanded by the controller136are not pressure dependent. For sensed pressures less than the threshold pressure, the plots500,502,504are horizontal indicating that the flows commanded by the electronic controller are constant for each of the first, second and third operator flow command values across the range of pressures less than the threshold pressure. For sensed pressures greater than the threshold pressure, the plots500,502,504angle linearly downwardly as the sensed pressure increases indicating that the flows commanded by the electronic controller are progressively reduced for each of the first, second and third operator flow command values across the range of pressures greater than the threshold pressure as the sensed pressures increase.

FIG. 8is another graph showing data corresponding to a linear function used by the electronic controller to generate controller flow commands. The graph includes three plots P1, P2and P3showing flow rates commanded by the electronic controller136verses the position of the operator control that generates operator flow control commands. Plot P1is for a sensed pressure less than the threshold pressure and represents base line600for flow data. When the sensed pressure is less than the threshold pressure, the base line600establishes the flow commanded by the electronic controller for a given position of the operator control. Plot P2is for a sensed pressure greater than the threshold pressure and represents a controller flow command line602for the pressure P2. When the sensed pressure is at P2, the controller flow command line602establishes the flow commanded by the electronic controller for a given position of the operator control. It is noted that the flow commanded by the controller136at the pressure P2for a given operator flow command is less than the flow commanded by the controller136at the pressure P1for the same operator flow command. Plot P3is for a sensed pressure greater than the pressure P2and represents a controller flow command line604for the pressure P3. When the sensed pressure is at P3, the controller flow command line604establishes the flow commanded by the electronic controller for a given position of the operator control. It is noted that the flow commanded by the controller136at the pressure P3for a given operator flow command is less than the flow commanded by the controller136at the pressure P2for the same operator flow command.

FIGS. 9A-9Dare graphs which plot various operating characteristics of an actuator controlled by a control system having flow control logic of the type disclosed herein. InFIGS. 9A-9D, the value of the operator flow command remains constant over the time period involved (e.g., the operator maintains the controller of the operator interface in the same position over the time period). In one example, the actuator can be coupled to an excavator arm.FIG. 9Ais a plot showing sensed load pressure versus time. Initially, from zero to about two seconds, the arm is lowered toward the ground. During this time period, the sensed pressure is less than the threshold pressure. Just after two seconds, the arm contacts the ground thereby causing the sensed load pressure to increase to a value over the threshold pressure. At just before five seconds, the excavator arm encounters harder soil and the sensed load pressure again increases.

FIG. 9Bshows the flow rate provided to the actuator over the same time period ofFIG. 9A. As shown atFIG. 9B, when the load pressure increases above the threshold pressure just after the two second mark, the flow rate is reduced to reduce the speed of the actuator. Similarly, when the pressure increases just before the five second mark, the flow rate is again reduced in a manner proportional to the increase in the load pressure.

FIG. 9Cshows the position of the excavation arm with respect to ground level over the same time period as the graphs ofFIGS. 9A and 9B. Based on the slopes of the lines ofFIG. 9C, the downward speed of the excavation arm is reduced slightly after the two second mark when the load pressure increases above the threshold pressure, and is further reduced just before the five second mark.

FIG. 9Dillustrates the velocity of the cylinder over the same time period asFIGS. 9A-9D. Similar toFIG. 9C,FIG. 9Dshows the velocity of the cylinder reducing slightly after the two second mark and then again reducing slightly before the five second mark in reaction to the change in cylinder pressure. It will be appreciated that the change in speed is a result of applying a linear function dependent upon pressure to the base line flow demand input by the operator from the operation interface.

The pump control185of the electronic controller136controls operation of the variable displacement pump122. The pump control185can include load sense control logic187that uses pressure information from the pressure sensors to control the pump12such that the pump122adjusts flow and pressure to match the load requirements of the system. In certain examples, the pump control185can also include supervisory control logic189that can use the pressures sensed at the actuators to selectively limit the flow provided to one or more of the actuators. In certain examples, certain actuators can be prioritized over other actuators. By limiting the flow demand based on pressure, the power to a single service can be capped. A supervisory controller can communicate with all services and can limit the total power (or torque) of the system. By measuring the maximum pressure of the actuators in the system, the supervisory controller can limit the sum of the flow demands to all the valves.

The various examples described above are provided by way of illustration only and should not be construed to limit the scope of the present disclosure. Those skilled in the art will readily recognize various modifications and changes that may be made without following the example examples and applications illustrated and described herein, and without departing from the true spirit and scope of the present disclosure.