Patent Application
20090255246
A hydraulic system may include multiple hydraulic loads, each of which may have different flow and pressure requirements that can vary over time. The hydraulic system may include a pump for supplying a flow of pressurized fluid to the hydraulic loads. The pump may have a variable or fixed displacement configuration. Fixed displacement pumps are generally smaller, lighter, and less expensive than variable displacement pumps. Generally speaking, fixed displacement pumps deliver a definite volume of fluid for each cycle of pump operation. But depending on the configuration of the pump and the precision with which the pump is manufactured, the flow output of the pump may actually decrease as the system pressure level increases due to internal leakage from the outlet side to the inlet side of the pump. The output volume of a fixed displacement pump can be controlled by adjusting the speed of the pump. Closing or otherwise restricting the outlet of a fixed displacement pump will cause a corresponding increase in the system pressure. To avoid over pressurizing the hydraulic system, fixed displacement pumps typically utilize a pressure regulator or an unloading valve to control the pressure level within the system during periods in which the pump output exceeds the flow requirements of the multiple hydraulic loads. The hydraulic system may further include various valves for controlling the distribution of the pressurized fluid to the multiple loads.
Referring now to the discussion that follows and also to the drawings, illustrative approaches to the disclosed systems and methods are shown in detail. Although the drawings represent some possible approaches, the drawings are not necessarily to scale and certain features may be exaggerated, removed, or partially sectioned to better illustrate and explain the present invention. Further, the descriptions set forth herein are not intended to be exhaustive or otherwise limit or restrict the claims to the precise forms and configurations shown in the drawings and disclosed in the following detailed description.
Pump 12 is capable of generating a flow of pressurized fluid that can be used to selectively drive multiple hydraulic loads. For purposes of illustration, hydraulic system 10 is shown to include three separate hydraulic loads, although it shall be appreciated that fewer or more hydraulic loads may also be provided depending on the requirements of the particular application. By way of example, the three hydraulic loads may include a hydraulic cylinder 26, a hydraulic motor 28, and a miscellaneous hydraulic load 30, which may include any of a variety of hydraulically actuated devices. Of course, it shall be appreciated that other types of hydraulic loads may also be used in place of, or in combination with, one or more of the illustrative hydraulic loads 26, 28 and 30, depending on the requirements of the particular application.
Each hydraulic load 26, 28, and 30 may be associated with a separate fluid circuit. A first fluid circuit 32 includes hydraulic cylinder 26; a second fluid circuit 34 includes hydraulic motor 28; and a third fluid circuit 36 includes miscellaneous hydraulic load 30. In the exemplary illustration the three fluid circuits are fluidly connected in parallel to pump discharge passage 22 at fluid junction 38.
Each fluid circuit includes a control valve, illustrated as a digital control valve, for individually controlling the operation of the hydraulic load associated with the respective fluid circuit. The control valve may control a time averaged flow rate passing through each of the respective fluid circuits and the corresponding pressure levels. Each control valve may include an actuator, which when activated opens the respective control valve to allow pressurized fluid to pass through the control valve to the associated hydraulic load. When utilizing a time averaged flow rate approach, the rate at which fluid passes through the control valve is controlled by repetitively cycling the control valve (i.e., opening and closing the valve) using a method commonly known as pulse width modulation (“PWM”). The control valve is either fully open or fully closed at any given time when employing pulse width modulation. The time averaged flow rate through the control valve, and corresponding pressure levels, may be controlled by adjusting the time periods during which the control valve is open and closed, also known as the valve duty cycle. For example, a duty cycle in which the valve is open generally fifty (50) percent of the time will generally produce a time averaged flow rate of approximately fifty (50) percent of the control pump's instantaneous flow output. Inherent fluctuations in the control valve's flow output tend to decrease as the operating frequency of the control valve increases. The inherent fluctuations in the control valve's flow may cause a pressure ripple that may be distributed to the load. The accumulator is generally sized such that the pressure ripples are acceptably small for a given application. Increasing the accumulator size may adversely affect the time required to respond to changes in load pressure. The operating frequency of the duty cycle may be increased, which may reduce the required accumulator size while improving both the response time and the magnitude of the pressure ripple. If the frequency is increased high enough, it may be possible to eliminate the accumulator by taking advantage of the natural compliance of the oil and conveyance to meet the pressure ripple requirement for the load. Valve operating speed limits and increased valve power losses that reduce efficiency may limit the operating frequency of the duty cycle.
Continuing to refer to
The distribution of pressurized fluid to hydraulic cylinder 26 from control valve 40 may be further controlled by a hydraulic cylinder control valve 54, which is fluidly connected to control valve 40 through discharge passage 52. Hydraulic cylinder control valve 54 operates to selectively distribute the pressurized fluid received from control valve 40 between a first chamber 58 and a second chamber 60 of hydraulic cylinder 26. A first supply passage 62 fluidly connects first chamber 58 to hydraulic cylinder control valve 54, and a second supply passage 64 fluidly connects second chamber 60 to hydraulic cylinder control valve 54. A reservoir return passage 66, which is fluidly connected to hydraulic cylinder control valve 54, is provided for returning fluid discharged from hydraulic cylinder 26 to fluid reservoir 18.
A digital valve controlled using pulse width modulation generally does not produce a continuous flow output, but rather produces a cyclic output in which a volume of fluid is discharged from the valve followed by a period in which no fluid discharge is produced. To help compensate for the cyclic output of the control valve and deliver a more uniform flow of pressurized fluid to the hydraulic load, an accumulator 68 may be provided. Accumulator 68 stores pressurized fluid discharged from control valve 40 during the discharge stage of the valve duty cycle. The stored pressurized fluid can be released during the period in which control valve 40 is closed to compensate for the cyclic discharge of control valve 40 and deliver a more constant flow of pressurized fluid to hydraulic load 26.
Accumulator 68 may have any of a variety of configurations. For example, one version of accumulator 68 may include a fluid reservoir 69 for receiving and storing pressurized fluid. Reservoir 69 can be fluidly connected to discharge passage 52 at a fluid junction 71 through a supply/discharge passage 73. Accumulator 68 may include a moveable diaphragm 75. The location of diaphragm 75 within accumulator 68 can be adjusted to selectively vary the volume of reservoir 69. A biasing mechanism 79 urges diaphragm 75 in a direction that tends to minimize the volume of reservoir 69 (i.e., away from biasing mechanism 79). Biasing mechanism 79 exerts a biasing force that opposes the pressure force exerted by the pressurized fluid present within reservoir 69. If the two opposing forces are unbalanced, diaphragm 75 will be displaced to either increase or decrease the volume of reservoir 69, thereby restoring balance between the two opposing forces. For example, when control valve 40 is opened the pressure level at fluid junction 71 will tend to increase. Generally speaking, the pressure level within reservoir 69 corresponds to the pressure at fluid junction 71. If the pressure force within reservoir 69 exceeds the opposing force generated by biasing mechanism 79, diaphragm 75 will be displaced toward biasing mechanism 79, thereby increasing the volume of the reservoir and the amount of fluid that can be stored in reservoir 69. As reservoir 69 continues to fill with fluid, the opposing force generated by biasing mechanism 79 will also increase to the point at which the biasing force and the opposing pressure force exerted from within reservoir 69 are substantially equal. The volumetric capacity of reservoir 69 will remain substantially constant when the two opposing forces are at equilibrium. On the other hand, closing control valve 40 will generally cause the pressure level at fluid junction 71 to drop below the pressure level within reservoir 69. This coupled with the fact that the pressure forces across diaphragm 75 are now unbalanced will cause fluid stored in reservoir 69 to be discharged through supply/discharge passage 73 to discharge passage 52 and delivered to hydraulic load 26.
Hydraulic system 10 may also include a second control valve 70 for controlling the distribution of pressurized fluid from pump 12 to second fluid circuit 34, and in particular, to hydraulic motor 28. Control valve 70 may also be a high frequency digital valve that can be operated in the manner described previously using pulse width modulation. Although illustrated schematically in
Fluidly connected to discharge port 78 of control valve 70 is a hydraulic motor supply passage 80 in fluid communication with hydraulic motor 28. In turn hydraulic fluid may be discharged from hydraulic motor 28 through a discharge passage 82 fluidly connected to reservoir return passage 66 at fluid junction 83. A second accumulator 84 may be provided within supply passage 80 to store pressurized fluid in much the same manner as previously described with respect to accumulator 68. Accumulator 84 can be fluidly connected to hydraulic motor supply passage 80 at a fluid junction 85 through a supply/discharge passage 87. Pressurized fluid discharged from control valve 70 can be used to charge accumulator 84 during the discharge stage of control valve 70. The stored pressurized fluid can be released during the period in which control valve 70 is closed to help minimize fluctuations in the flow of pressurized fluid being delivered to hydraulic load 28.
Hydraulic system 10 may also include a third control valve 86 for controlling the distribution of pressurized fluid from pump 12 to third fluid circuit 36. Similar to control valves 40 and 70, control valve 86 may also be a high frequency digital valve that can be operated in the manner described previously using pulse width modulation. Although illustrated schematically in
A hydraulic load supply passage 94 fluidly connects discharge port 96 of control valve 86 to hydraulic load 30. Pressurized hydraulic fluid may be discharged from hydraulic load 30 through a discharge passage 98 fluidly connected to reservoir return passage 66 at fluid junction 103. An accumulator 95 may be provided to store pressurized fluid in much the same manner as previously described with respect to accumulator 68. Accumulator 95 may be fluidly connected to hydraulic load supply passage 94 at a fluid junction 97 through a supply/discharge passage 99. Pressurized fluid discharged from control valve 86 may be used to charge accumulator 95 during the discharge stage of control valve 86. The stored pressurized fluid may be released when control valve 86 is closed to help offset fluctuations in the flow of pressurized fluid to hydraulic load 30.
Closing or otherwise restricting the outlet of fixed displacement pump 12 can cause the pressure within hydraulic system 10 to reach undesirable levels. To avoid over pressurizing the hydraulic system during periods in which the pump output exceeds the flow requirements of the hydraulic loads, a bypass control valve 100 associated with a bypass fluid circuit 101 may be provided. An inlet port 102 of bypass control valve 100 may be fluidly connected to pump discharge passage 22 at a fluid junction 104 through an inlet passage 106. Bypass control valve 100 is operable to selectively allow excess flow generated by pump 12 to be dumped to fluid reservoir 18. A bypass discharge passage 108 is fluidly connected to a discharge port 110 of bypass control valve 100 and reservoir return passage 66 at fluid junction 111. Bypass control valve 100 also includes an actuator 112 operable for selectively opening and closing a fluid path between inlet port 102 and discharge port 110 of bypass valve 100 in response to a control signal. Actuator 112 may be configured to open bypass control valve 100, but not close it, in which case a second actuator 113 may be employed to selectively close the valve. Actuators 112 and 113 may have any of a variety of configurations, including but not limited to, a pilot valve, a solenoid, and a biasing member, such as a spring.
A controller 114 may be provided for controlling the operation of control valves 40, 70, 86 and 100. More generally, controller 114 may form a portion of a more general system based Electronic Control Unit (ECU) or may be in operational communication with such an ECU. Controller 114 may include, for example, a microprocessor, a central processing unit (CPU), and a digital controller, among others.
More specifically controller 114 and any associated ECU is an example of a device generally capable of executing instructions stored on a computer-readable medium, such as instructions for performing one or more of the processes discussed herein. Computer-executable instructions may be compiled or interpreted from computer programs created using a variety of known programming languages and/or technologies, including, without limitation, and either alone or in combination, Java, C, C++, Visual Basic, Java Script, Perl, etc. In general, a processor (e.g., a microprocessor) receives instructions, e.g., from a memory, a computer-readable medium, etc., and executes these instructions, thereby performing one or more processes, including one or more of the processes described herein. Such instructions and other data may be stored and transmitted using a variety of known computer-readable media.
A computer-readable medium (also referred to as a processor-readable medium) includes any tangible medium that participates in providing data (e.g., instructions) that may be read by a computer (e.g., by a processor of a computer, a microcontroller, etc.). Such a medium may take many forms, including, but not limited to, non-volatile media and volatile medial. Non-volatile media may include, for example, optical or magnetic disks, read-only memory (ROM), and other persistent memory. Volatile media may include, for example, dynamic random access memory (DRAM), which typically constitutes a main memory. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other tangible medium with patterns of holes, a RAM, a PROM, an EPROM, a FLASH-EEPROM, any other memory chip or cartridge, or any other medium from which a computer can read.
A transmission media may facilitate the processing of instructions by carrying instructions from one component or device to another. For example, a transmission media may facilitate electronic communication between mobile device 110 and telecommunications server 126. Transmission media may include, for example, coaxial cables, copper wire and fiber optics, including the wires that comprise a system bus coupled to a processor of a computer. Transmission media may include or convey acoustic waves, light waves, and electromagnetic emissions, such as those generated during radio frequency (RF) and infrared (IR) data communications.
A digital controller 14 is illustrated. A first control link 116 operably connects controller 114 to actuator 42 of control valve 40. A second control link 117 operably connects controller 114 to actuator 43 of control valve 40. A third control link 118 operably connects controller 114 to actuator 77 of control valve 70. A fourth control link 119 operably connects controller 114 to actuator 81 of control valve 70. A fifth control link 120 operably connects controller 114 to actuator 93 of control valve 86. A sixth control link 121 operably connects controller 114 to actuator 91 of control valve 86. A first bypass control link 122 operably connects controller 114 to actuator 112 of bypass control valve 100. A second bypass control link 123 operably connects controller 114 to actuator 113 of bypass control valve 100. Controller 114 may be configured to control operation of the control valves in response to various system inputs, such as the pressure and flow requirements of the hydraulic loads, pump speed, pump exit pressure, and the discharge fluid flow rate from pump 12, among others. Depending on the requirements of the particular application, hydraulic system 10 may include various sensors for monitoring various operating characteristics of the system, and may include a speed sensor 124, a pressure sensor 126, and a flow sensor 128, as well as others.
Control valves 40, 70, 86, and 100 may be digitally controlled using pulse width modulation. Generally, the control valves are either fully open or fully closed when employing pulse with modulation. Also, typically only one control valve is fully open at any given instance, although a portion of the opening and closing sequences of consecutive valves may occur simultaneously, which is discussed in more detail subsequently. Substantially the entire quantity of fluid discharged from pump 12 passes through the control valve when the valve is open. Operating the control valve in this manner results in a generally cyclic fluid output, in which either the entire fluid output of pump 12 is discharged from the control valve or none at all. Control valves 40, 70, 86, and 100 are typically operated at a relatively high operating frequency. The operating frequency is defined as the number of duty cycles completed per unit of time, typically expressed as cycles/sec or Hertz.
The effective flow rate of fluid passing through control valves 40, 70, 86 and 100 can be controlled by adjusting the respective valve duty cycle. A complete duty cycle includes one opening and one closing of the control valve. The duty cycle can be expressed as the ratio of the time period that the control valve is open and the duty cycle operating period. The duty cycle operating period may be defined as the time required to complete one duty cycle. The duty cycle is typically expressed as a percentage of the operating period. For example, a seventy-five percent (75%) duty cycle results in the control valve being open approximately seventy-five percent (75%) of the time and closed twenty-five percent (25%) of the time. The term “effective flow rate” refers to the time averaged flow rate of fluid discharged from the control valve over one complete duty cycle expressed as a percentage of the flow output of pump 12. The effective flow rate is determined by dividing the total quantity of fluid discharged from the control valve over one complete duty cycle by the duty cycle operating period. For example, operating the control valve at a seventy-five percent (75%) duty cycle will produce an effective discharge flow rate of seventy-five percent (75%) of the flow output of pump 12.
Exemplary duty cycles for control valves 40, 70, 86 and 100 are shown in
The duty cycles employed with each of the control valves 40, 70, 86, and 100, may be reevaluated for each operating cycle and adjusted as necessary to accommodated changing load conditions. Factors that may be considered in determining the appropriate duty cycles for control valves 40, 70, 86 and 100 may include the flow and pressure requirements of hydraulic loads 26, 28 and 30, the flow output of pump 12, the discharge pressure of pump 12, and the operating speed of pump 12, as well as others.
The duty cycle tracks a generally square waveform represented by a solid line in
The effective flow rate of control valves 40, 70, 86 and 100 may be controlled by varying their respective duty cycles. The duty cycle for each of the control valves 40, 70, 86 and 100 may be continuously varied to accommodate changing load conditions. Controller 114 may be configured to determine the duty cycle for each of the control valves. Controller 114 may also be configured to transmit a control signal corresponding to the desired duty cycle that may be used to control operation of the respective control valve. Controller 114 may include logic for determining an appropriate duty cycle based on a variety of inputs.
The control strategy employed by controller 114 may be based on an open-loop or closed-loop control scheme. In a closed-loop system, controller 114 may receive feedback information from a variety of sensors used to monitor various operating parameters, such as pressure, temperature, and speed, to name a few. Controller 114 may use the information received from the sensors to adjust, if necessary, the duty cycle of the respective control valve to achieve a desired load performance. A closed-loop system may allow various operating parameters, such as pressure, speed, and flow, to be controlled more precisely. A closed loop system may be used, for example, to control the pressure applied to hydraulic load 30. Controller 114 may receive feedback information from a pressure sensor 138 regarding the actual pressure applied to hydraulic load 30. A communication link 139 operably connects pressure sensor 138 to controller 114. Controller 114 may use the pressure data to compute a pressure error corresponding to the difference between the pressure commanded by controller 114 and the pressure applied to hydraulic load 30, as detected by pressure sensor 138. If the pressure error falls outside a selected error range controller 114 can modify the duty cycle of control valve 86 to achieve the desired pressure at hydraulic load 30.
A closed loop system may also be used to implement a load sensing control scheme. A hydraulic system employing load sensing has the ability to monitor the system pressures and to make appropriate adjustments as necessary to provide a desired flow rate at a pressure required to operate the hydraulic load. Load sensing may be implemented by monitoring a pressure drop across an orifice positioned within a passage supplying pressurized fluid to the hydraulic load. The pressure drop across the orifice is generally set at a predetermined fixed value. With the pressure drop across the orifice fixed, the flow rate through the orifice is only dependent on the flow area of the orifice. This enables the rate at which fluid is delivered to the hydraulic load to be controlled by adjusting the cross-sectional flow area of the orifice while maintaining the desired constant pressure drop. Increasing the orifice cross-sectional flow area increases the flow rate, whereas decreasing the orifice cross-sectional flow area decreases the flow rate. A change in the pressure drop across the orifice, which may be due for example, to an increase in the working load being moved by the hydraulic load, will cause a corresponding change in the flow rate of fluid delivered to the hydraulic load. The change in pressure drop across the orifice may be detected and compensated for by adjusting the upstream orifice pressure to achieve the desired pressure drop.
Load sensing capabilities may be advantageous when trying to control a hydraulic device requiring a particular flow while maintaining a particular pressure drop across a metering orifice. Hydraulic cylinder 26 is an example of such a device. Hydraulic cylinder 26 may be used in a variety of applications. By way of example and for purposes of discussion, hydraulic cylinder 26 will be described in the context of a power steering system, although it shall be appreciated that other applications of hydraulic cylinder 26 may also be possible. Hydraulic cylinder 26 may include a piston 140 slidably disposed in a cylinder housing 141. An end 142 of piston 140 is connected through a series of links to a wheel of the vehicle. Piston 140 may be slid longitudinally within cylinder housing 141 by selectively delivering pressurized fluid to first and second chambers 58 and 60. The rate at which the fluid is delivered to the respective chambers determines the speed at which piston 140 moves. Hydraulic cylinder control valve 54 operates to distribute the pressurized fluid between fluid chambers 58 and 60 of hydraulic cylinder 26. Hydraulic cylinder control valve 54 includes a variable orifice that controls the rate at which fluid is delivered to hydraulic cylinder 26. Hydraulic cylinder control valve 54 is responsive to a user input that causes the valve to adjust the orifice size to achieve a desired flow rate and to direct the flow to the appropriate chamber in hydraulic cylinder 26.
A load sensing control scheme may be implemented by arranging a pair of pressure sensors 144 and 146 upstream and downstream, respectively, of hydraulic cylinder control valve 54. A first communication link 145 and a second communication link 147 may operably connect pressure sensors 144 and 146, respectively, to controller 114. The pressure sensors may be configured to send a pressure signal to controller 114 indicative of the pressure at the respective sensor locations. Controller 114 uses the pressure data to formulate an appropriate control signal, using logic included in controller 114, for controlling the operation of control valve 40. The control signal includes a pulse width modulated signal that can be sent to actuator 42 across control link 116. Actuator 42 opens and closes control valve 40 in response to the received signal. Controller 114 determines an appropriate pulse width for the control signal that is calculated to deliver a desired flow at a desired pressure margin to hydraulic cylinder control valve 54. Controller 114 monitors the pressure drop across the orifice in hydraulic cylinder control valve 54 and may adjust the control signal as necessary to maintain the desired pressure drop across the orifice. For example, increasing the opposing force applied to end 142 of piston 140 may cause a corresponding increase in the downstream pressure monitored by pressure sensor 146 and a corresponding decrease in the pressure drop across the orifice in hydraulic cylinder control valve 54. The decreased pressure drop may also result in a corresponding decrease in the flow rate of fluid to hydraulic cylinder 26. To compensate for the decrease in flow, controller 114 may increase the pressure at the inlet to hydraulic cylinder control valve 54, which is monitored using pressure sensor 144, by adjusting the duty cycle of the control signal that controls the operation of control valve 40. The pressure to the inlet may be increased an amount sufficient to achieve the same pressure drop across the orifice that was present before the opposing force applied to end 142 of piston 140 was increased. In this way, the desired flow rate delivered to hydraulic cylinder 26, and thus the actuating speed of the piston, can be maintained at the desired level notwithstanding the fact the forces acting against the piston are continuously fluctuating.
A closed loop system may also be used to control the speed of a hydraulic device, such as hydraulic motor 28. Controller 114 may receive feedback information from a speed sensor 148 indicating the rotational speed of hydraulic motor 28. A communication link 149 operably connects speed sensor 148 to controller 114. Controller 114 may use the speed data to compute a speed error corresponding to the difference between a speed commanded by controller 114 and the actual rotational speed of hydraulic motor 28, as detected by speed sensor 148. If the speed error falls outside a selected error range, controller 114 may modify the duty cycle of control valve 70 in order to operate hydraulic motor 28 at the desired speed.
A closed loop system may also be used to control the flow rate of hydraulic fluid delivered to a hydraulic device, such as hydraulic device 30. Controller 114 may receive feedback information from a flow sensor 150 indicating the flow rate of fluid delivered to hydraulic device 30. A communication link 151 operably connects flow sensor 150 to controller 114. Controller 114 may use the flow data to compute a flow error corresponding to the difference between a flow rate commanded by controller 114 and an actual flow rate as detected by flow sensor 150. If the flow error falls outside a selected error range, controller 114 may modify the duty cycle of control valve 86 to achieve the desired flow rate.
Controller 114 may also include logic for controlling a maximum standby pressure. The maximum standby pressure represents the maximum pressure that can be applied to a hydraulic load. Digital high pressure standby control generally serves the same purpose as a high standby relief valve employed in an analog hydraulic system. A pressure relief valve may, however, be used in conjunction with a digital high pressure standby control as a backup measure. The maximum standby pressure setting is typically set lower than the pressure setting of a pressure relief valve, if one is used. This prevents the pressure relief valve from opening under normal operating conditions, which may result in an undesirable loss of energy. Once the pressure reaches the maximum allowable level, controller 114 may adjust the pulse width of the control signal used to control operation of the control valve associated with the hydraulic load to zero. Doing so closes the control valve to prevent any further increase in pressure.
Controller 114 may also include logic for controlling a low standby pressure. Low standby pressure control operates to help insure that a predetermined minimum pressure is always delivered to a hydraulic load when the load does not require any flow. Maintaining a minimum standby pressure may enable the hydraulic load to react in a predictable and reasonably responsive manner. The low standby pressure can be maintained by controller 114 generating a pulse width modulated control signal having narrow pulse width for controlling the control valve associated with the hydraulic load. The narrow pulse width control signal causes the valve to have an effective opening that is large enough to allow sufficient flow to pass through the control valve to compensate for system leakage while maintaining pressure at the minimum standby pressure level.
Low pressure standby control may be used, for example, in conjunction with a power steering system employing hydraulic cylinder 26. The low standby pressure typically occurs when the power steering system is positioned in the neutral position. With the power steering system in the neutral position, controller 114 may issue a low standby pressure command signal for instructing hydraulic cylinder control valve 54 to deliver the requested pressure to hydraulic cylinder 26. The low standby pressure is sufficient to allow the hydraulic cylinder 26 to firmly maintain the desired steering geometry of the vehicle and to enable quick actuation of the steering mechanism. In practice, controller 114 may formulate the pulse width modulated control signal for operating the control valve based on a maximum of the requested pressure level and the low standby pressure level, whichever is higher.
With continued reference to
With continued reference to
When operating hydraulic system 10 there may be instances in which the flow requirements of the hydraulic loads exceeds the flow output of pump 12. When that occurs a determination may be made as to what proportions the available flow will be distributed between the hydraulic loads. This may be accomplished by assigning each hydraulic load a priority level. For example, a priority level one (1) may be considered the highest priority, a priority level two (2) the second highest priority, and so forth. Each hydraulic load may be assigned a priority level. The bypass circuit is typically assigned the lowest priority level.
Various criteria may be used to determine the priority assignments, including but not limited to safety concerns, efficiency considerations, operator convenience, among others. Each hydraulic load may be assigned a separate priority level or multiple hydraulic loads may be assigned the same priority level depending on the requirements of the particular application. The priority level assignment for each load may be saved in controller 114 such as by way of memory 153, or in the memory or other tangible storage mechanism of a system level electronic control unit (ECU) in operational communication with controller 114.
The available flow may be distributed to the hydraulic loads based on their priority level ranking, with the hydraulic loads assigned the highest priority level (i.e., priority level 1) receiving all of the flow they require, and the remaining hydraulic loads receiving either a reduced flow or no flow at all. An example of possible priority level assignments for fluid circuits 32, 34, 36 and 101, and a resulting flow distribution based on the priority level assignments is shown in Table 1 below. For purposes of this example, it is assumed that hydraulic pump 12 has a maximum output of one-hundred fifty (150) liters/min. For illustrative purposes, first fluid circuit 32, which includes hydraulic cylinder 26, is assigned a priority level one. Second and third fluid circuits 34 and 36 are assigned a priority level two. Bypass fluid circuit 101, which is typically assigned the lowest priority level, is assigned priority level three. In this example, the first fluid circuit requires two-thirds (66.7 percent) of the total available flow, or 100 liters/min. The second and third fluid circuits both require one-third (33.3 percent) of the available flow, or 50 liters/min. Since the total flow requirement of all three fluid circuits exceed the available flow from pump 12, the second and third fluid circuits, which are assigned a lower priority than the first fluid circuit, will receive only a portion of their required flow. The first fluid circuit will receive its total flow requirement of 100 liters/min. This leaves 50 liters/min. to be distributed between the second and third fluid circuits. Since the second and third fluid circuits have the same priority level, the remaining 50 liters/min. is divided evenly between the two fluid circuits, with each circuit receiving 25 liters/min. The bypass fluid circuit receives no fluid in this example since all of the available fluid is distributed between the other three fluid circuits.
The order in which the control valves are actuated may have an effect on the efficiency of the hydraulic system. The valves may be actuated in sequential order based on various selected criteria, for example, in order of decreasing or ascending pressure. The order in which the control valves are actuated may be determined based on the pressure requirements of the hydraulic loads, for example, hydraulic loads 26, 28, and 30. Typically, the control valve supplying the hydraulic load with the highest pressure requirement is actuated first, followed by the control valve supplying the hydraulic load with the next highest pressure requirement and so forth down the line until all of the control valves have been actuated. If a particular hydraulic load does not require pressure, the control valve associated with the non-operational hydraulic load will not be opened during that particular operating cycle. Bypass control valve 100 is typically actuated last, if at all, after all of the remaining control valves (i.e., control valves 40, 70, and 86) have been actuated. Once all the control valves have been actuated the present operating cycle is completed and the next operating cycle may be commenced.
An example of a possible sequencing order for control valves 40, 70, 86, and 100 is illustrated graphically in
The order in which the control valves are sequenced may not always be consistent. The sequencing order may be varied between operating cycles, and in some cases midway through an operating cycle, to accommodate changes in operating conditions, such as load pressure requirements. If the pressure requirement of a hydraulic load becomes higher than the pressure requirement of one or more of the remaining hydraulic loads, the sequencing order may be reordered so that the control valves continue to be sequenced from the highest pressure requirement to the lowest pressure requirement. For example, in
Improvements in overall system performance may be realized by adjusting the pulse width of a control valve midway through an operating cycle to accommodate changes in the flow requirements of the hydraulic load. This is in contrast to determining the pulse width for each hydraulic load at the start of an operating cycle and maintaining the same pulse width for the duration of the operating cycle. Progressive pulse width control, in which the pulse width is adjusted midway through the operating cycle, may improve system bandwidth, which is directly influenced by the system's operating cycle frequency. An exemplary implementation of progressive pulse width control is illustrated graphically in
Referring again to
Factors that may be considered in determining an appropriate time delay may include the volume and the compliance of the fluid supply circuit between pump 12 and control valves 40, 70, 86 and 100. The time delay is also a function of the pressure difference between fluid circuits.
If the time delay between commencing closing one control valve and commencing opening the next successive control valve is too long, energy may be wasted as the fluid present in the supply circuit leading to the control valve is compressed, thereby causing a spike in system pressure. This phenomenon is depicted graphically in
If the delay between commencing closing one valve and commencing opening the next successive valve is too short, fluid may flow backward from the previous hydraulic load (valve 1) to the next hydraulic load (valve 2). This phenomenon is depicted graphically in
The time delay (Δt) may be determined using the following equation:
Δt=α*ΔP+TimeDelayAdder
By way of example, in instances where a is dependent on manifold volume, pump flow rate, and effective bulk modulus of the hydraulic fluid, the time delay (Δt) may be determined using the following equation:
The bulk modulus may be determined using the following equation:
The bulk modulus varies non-linearly with pressure. The bulk modulus of the hydraulic fluid is a function of temperature, entrained air, fluid composition and other physical parameters. The bulk modulus of the hydraulic system is representative of the volume and rigidity of the hydraulic system hardware and is a factor in determining an appropriate time delay. The effective bulk modulus of a hydraulic system is a compilation of the bulk modulus of the fluid and the bulk modulus of the system hardware. In practice, the bulk modulus may vary significantly, and if possible, may be measured to obtain an accurate bulk modulus for use in computing the time delay. Measurement of the effective bulk modulus may be accomplished, for example, by monitoring a pressure rise in hydraulic system 10 as a function of fluid flow from pump 12 with all the control valves 40, 70, 86 and 100, closed. The pump flow may be approximated using the following equation:
Pump Flow=(Pump Revolutions Per Minute (RPM))×(Pump Displacement per Pump Revolution)×(Approximate Volumetric Efficiency)
Pressure rise may be monitored using a pressure sensor (i.e., pressure sensor 126 in
The bulk modulus can be mapped during an initial start-up of the hydraulic system to provide an initial operating map. The bulk modulus can be measured periodically as the hydraulic fluid heats up until a steady state condition is reached. Bulk modulus maps for similar system conditions obtained during previous operating cycles may be compared and used to evaluate the status of the hydraulic system. For example, a substantial decrease in bulk modulus may indicate a significant increase in entrained air in the hydraulic fluid, or an impending failure in a hydraulic system hose or pipe.
The TimeDelayAdder parameter included in the equation for computing the time delay (Δt) is a correction factor for optimizing the time delay (Δt). The a parameter and the TimeDelayAdder parameter may be determined empirically. The a term of the time delay equation, which may correspond, for example, to the equation (ΔPV/βQ), or another functional relationship, provides an estimate of the amount of delay between commencing to close one control valve and commencing to open the next successive valve. Since it is only an estimate, however, the computed time delay (Δt) may not produce an optimum balance between minimizing system pressure spikes and backflow occurring between successively actuated control valves.
The effectiveness of the time delay (Δt) estimate may be assessed by computing a corresponding Time Delay Pressure Error that at least partially accounts for the losses associated with both spikes in system pressure and backflow from one control valve to the next. The Time Delay Pressure Error may be computed using the following equation:
Time Delay Pressure Error=MAX[(Ppump−(Pload−ΔPvalve),0]+ABS(MIN[Ppump−Pload,0])
The steady state pressure drop across the control valve (ΔPvalve) may be obtained from a look-up table stored in memory 153 of controller 114, wherein the steady state pressure drop is correlated to the flow rate of pump 12. The flow rate of pump 12 may be computed using a measured pump RPM, which may be detected, for example, using speed sensor 124, and the previously described equation for determining Pump Flow.
The substance of the Time Delay Pressure Error may be better understood with reference to
As can be observed from
Continuing to refer to
With continued reference to
The time delay (Δt) may be optimized by minimizing the Time Delay Pressure Error. This may be accomplished by incrementally varying the TimeDelayAdder parameter in the time delay (Δt) equation until a minimum Time Delay Pressure Error is achieved. A new time delay (Δt) is computed for each TimeDelayAdder value. The corresponding control valve is then operated using the modified time delay (Δt) and the resulting pressure drop across the control valve is tracked. A new Time Delay Pressure Error is computed based on the latest pressure drop data and compared with the previously computed Time Delay Pressure Error. This process continues until a minimum Time Delay Pressure Error is determined. An optimum TimeDelayAdder corresponding to the minimum Time Delay Pressure Error, along with the corresponding pressure and flow rate, may be stored in memory 153 of controller 114 in the form of a lookup table for future reference.
With reference to
Continuing to refer to
Upon closing control valve 40, controller 114 may send a control signal to actuator 77 instructing the actuator to open control valve 70 and establish a fluid connection between inlet port 72 and discharge port 78. Based on a thirty percent (30%) duty cycle, control valve 70 will remain open for a period of approximately six (6) milliseconds, starting at approximately eight (8) milliseconds and ending at approximately fourteen (14) milliseconds. With control valve 70 in the open position, the entire flow of fluid discharged from pump 12 will pass through control valve 70 (see
As shown in
Upon closing control valve 70, controller 114 may send a control signal to actuator 93 instructing the actuator to open control valve 86 to establish a fluid connection between inlet port 88 and discharge port 96. Based on a twenty percent (20%) duty cycle, control valve 86 will remain open for a period of approximately four (4) milliseconds, starting at approximately fourteen (14) milliseconds and ending at approximately eighteen (18) milliseconds. With control valve 86 in the open position, the entire flow of fluid discharged from pump 12 will pass through control valve 86 (see
Upon closing control valve 86, control valve 100 may be selectively opened to dump any excess pressure present within pump discharge passage 22 to fluid reservoir 18. Controller 114 may send a control signal to actuator 112 instructing the actuator to open bypass control valve 100 to establish a fluid connection between inlet port 102 and discharge port 110. Based on a ten percent (10%) duty cycle, control valve 86 will remain open for a period of two (2) milliseconds, starting at eighteen (18) milliseconds and ending at twenty (20) milliseconds. The closing of control valve 86 at approximately twenty (20) milliseconds corresponds to the end of the current operating cycle and the beginning of the subsequent operating cycle. With control valve 100 in the open position, the entire flow of fluid discharged from pump 12 will pass through control valve 100 (see
The current exemplary operating sequence is completed when bypass control valve 100 is closed. A subsequent operating sequence may be commenced by actuating control valve 40 and repeating the previously described operating sequence. If there a change in operating conditions, for example, wherein a pressure requirement of a hydraulic load has increased or decreased, the affected control valve duty cycle may be reevaluated and adjusted as necessary to accommodate the changed operating condition.
With regard to the processes, systems, methods, etc. described herein, it should be understood that, although the steps of such processes, etc. have been described as occurring according to a certain ordered sequence, such processes could be practiced with the described steps performed in an order other than the order described herein. It further should be understood that certain steps could be performed simultaneously, that other steps could be added, or that certain steps described herein could be omitted. In other words, the descriptions of processes herein are provided for the purpose of illustrating certain embodiments, and should in no way be construed so as to limit the claimed invention.
It is to be understood that the above description is intended to be illustrative and not restrictive. Many embodiments and applications other than the examples provided would be apparent to those of skill in the art upon reading the above description. The scope of the invention should be determined, not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. It is anticipated and intended that future developments will occur in the arts discussed herein, and that the disclosed systems and methods will be incorporated into such future embodiments. In sum, it should be understood that the invention is capable of modification and variation and is limited only by the following claims. All terms used in the claims are intended to be given their broadest reasonable constructions and their ordinary meanings as understood by those skilled in the art unless an explicit indication to the contrary in made herein. In particular, use of the singular articles such as “a,” “the,” “said,” etc. should be read to recite one or more of the indicated elements unless a claim recites an explicit limitation to the contrary.
| Number | Date | Country | Kind |
|---|---|---|---|
| PCT/US09/40219 | Apr 2009 | US | national |
This application claims the benefit of U.S. Provisional Application 61/044,337 filed on Apr. 11, 2008 and PCT application PCT/US09/40219 filed on Apr. 10, 2009.
| Number | Date | Country | |
|---|---|---|---|
| 61044337 | Apr 2008 | US |
