Abstract:
A method for overpressure control in a hydraulic system having multiple hydraulic pumps, with each hydraulic pump being connected by a respective hydraulic circuit for actuating a single respective hydraulic actuator, includes actuating a first variable displacement hydraulic pump, the first hydraulic pump being fluidly linked by a first hydraulic circuit to a first hydraulic actuator for powering the first hydraulic actuator. Upon detecting a pressure that exceeds a predetermined threshold pressure, the flow rate of the first hydraulic pump is electronically modified to a second flow rate lower than the first flow rate whereby the pressure in the first hydraulic circuit is reduced to a pressure that is below the predetermined threshold pressure.

Description:
TECHNICAL FIELD 
       [0001]    This patent disclosure relates generally to a hydraulic circuit for a double acting piston and cylinder, and, more particularly to arrangements for hydraulic pressure cutoff in a system including a variable flow pump. 
       BACKGROUND 
       [0002]    Unlike a typical hydraulic system having a single pump feeding a plurality of solenoid valves to control an associated plurality of functions, a “meterless” hydraulic control system controls each hydraulic actuator of each function by controlling a flow rate from a dedicated pump associated with that actuator. Thus, while proportional or throttling valves are utilized in prior art metered systems to meter fluid to control movement of each actuator, the flow to each actuator in a meterless system is controlled directly by controlling the associated pump. The dedicated pump or pumps may be of any suitable type including variable displacement or fixed displacement, wherein the flow from the pump to the actuator chambers is varied in order to control the speed and extent of the actuator movement. 
         [0003]    In prior art meterless arrangements, pump controlled circuits known as Displacement Controls (DC) utilize a variable displacement pump with a constant speed driver, while Electro-Hydrostatic Actuators (EHA) utilize a fixed displacement pump with a variable speed driver. In either case, since actuator flow is controlled by the pump, the hydraulic circuit associated with one or more actuators may experience and overpressure condition when the associated actuated element encounters an obstruction. Typical practice is to provide a relief valve through which fluid is vented to relive the excess pressure. In this arrangement, whenever the set release pressure of the valve is reached, the valve opens and the pressure decreases. When the pressure has decreased to below the valve limit, the valve shuts again. 
         [0004]    Although this type of system allows for pressure control, it does so at the expense of fuel efficiency and system. In particular, the release of hydraulic fluid to lower pressure wastes the energy stored in the fluid at that point. 
       SUMMARY 
       [0005]    In one aspect of the disclosure, there is described a method for overpressure control in a hydraulic system having multiple hydraulic pumps. Each hydraulic pump is connected by a respective hydraulic circuit for actuating a single respective hydraulic actuator. The method includes actuating, at a first flow rate, a first variable displacement hydraulic pump of the multiple hydraulic pumps, the first hydraulic pump being fluidly linked by a first hydraulic circuit to a first hydraulic actuator for powering the first hydraulic actuator. After initially detecting a first pressure in the first hydraulic circuit, the first pressure being below a predetermined threshold pressure, the method entails detecting a second pressure in the first hydraulic circuit, the second pressure exceeding the predetermined threshold pressure. In response, the flow rate of the first hydraulic pump is electronically modified to a second flow rate lower than the first flow rate whereby the pressure in the first hydraulic circuit is reduced to a pressure that is below the predetermined threshold pressure. 
         [0006]    In another embodiment, a hydraulic system is described having relief valve-less overpressure control. The hydraulic system includes first and second variable displacement hydraulic pumps, first and second hydraulic actuators, and respective first and second hydraulic circuits connecting the first and second variable displacement hydraulic pumps to the respective first and second hydraulic actuators. A system controller is included and configured to detect that a pressure in one of the first and second hydraulic circuits exceeds a predetermined safe pressure and to destroke the variable displacement hydraulic pump associated with the overpressure hydraulic circuit such that the pressure in the overpressure hydraulic circuit is reduced to less than the predetermined safe pressure. 
         [0007]    Other features and advantages of the described principles will be apparent from the detailed specification, taken in conjunction with the attached drawing figures, of which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a side elevational view of a machine incorporating aspects of this disclosure; 
           [0009]      FIG. 2  is a schematic view of a hydraulic system according to this disclosure including a hydraulic Circuit, including multiple actuators, pumps and pressure transducers; 
           [0010]      FIG. 3  is a schematic control architecture view of the pump displacement control of  FIG. 2  including data and command signaling; 
           [0011]      FIG. 4  is a simplified plot showing a hydraulic circuit pressure spike and correlated displacement reduction according to the disclosure; and 
           [0012]      FIG. 5  is a flow chart of a process for applying a flow reduction as described herein to alleviate an overpressure condition in a meterless hydraulic circuit such as that shown herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    This disclosure relates to machines  100  that utilize hydraulic actuators (identified generally as  102 ) to control movement of moveable subassemblies of the machine, such as arms, booms, implements, or the like. More specifically, the disclosure relates to such so-called meterless hydraulic systems  104  utilized in machines  100 , such as the excavator  106  illustrated in  FIG. 1 , used to control extension and retraction of such hydraulic actuators  102 . While the arrangement is illustrated in connection with an excavator  106 , the arrangement disclosed herein has universal applicability in various other types of machines  100  as well. The term “machine” may refer to any machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, or any other industry known in the art. For example, the machine may be a wheel loader or a skid steer loader. Moreover, one or more implements may be connected to the machine  100 . Such implements may be utilized for a variety of tasks, including, for example, brushing, compacting, grading, lifting, loading, plowing, ripping, and include, for example, augers, blades, breakers/hammers, brushes, buckets, compactors, cutters, forked lifting devices, grader bits and end bits, grapples, blades, rippers, scarifiers, shears, snow plows, snow wings, and others. 
         [0014]    The excavator  106  of  FIG. 1  includes a cab  108  that is swingably supported on an undercarriage  110  that includes a pair of rotatably mounted tracks  112 . The cab  108  includes an operator station  114  from which the machine  100  may be controlled. The operator station  114  may include, for example, an operator control  115  for controlling the extension and retraction of the hydraulic actuators  102 . The operator control  115  may be of any appropriate design. By way of example only, the operator control  115  may be in the form of joystick, such as illustrated in  FIG. 1 , a dial, a switch, a lever, a combination of the same, or any other arrangement that provides the operator with a mechanism by which to identify the movement commanded. The operator station  114  may further include controls such as a hydraulic lockout switch  113 , or an on/off switch  111 . 
         [0015]    The cab  108  may further include an engine  116 , and at least a portion of the meterless hydraulic system  104 . The engine  116  may be an internal combustion engine or any type power source known to one skilled in the art now or in the future. 
         [0016]    A front linkage  118  includes a boom  120  that is pivotably supported on the cab  108 , a stick  122  pivotably coupled to the boom  120 , and an implement  124  pivotably coupled to the stick  122 . While the implement  124  is illustrated as a bucket  126 , the implement  124  may alternately be, for example, a compactor, a grapple, a multi-processor, thumbs, a rake, a ripper, or shears. 
         [0017]    Movement of the boom  120 , stick  122 , and implement  124  is controlled by a number of actuators  130 ,  132 ,  134 . The boom  120  is pivotably coupled to cab  108  at one end  136 . To control movement of the boom  120  relative to the cab  108 , a pair of actuators  130  are provided on either side of the boom  120 , coupled at one end to the cab  108 , and at the other end to the boom  120 . 
         [0018]    The stick  122  is pivotably coupled to the boom  120  at a pivot connection  138 . Movement of the stick  122  relative to the boom  120  is controlled by the actuator  132  that is coupled at one end to the boom  120 , and at the other end to the stick  122 . The actuator  132  is pivotably coupled to the stick  122  at a pivot connection  140  that is spaced from the pivot connection  138  such that extension and retraction of the actuator  132  pivots the stick  122  about pivot connection  138 . 
         [0019]    The implement  124  is pivotably coupled to the stick  122  at pivot connection  142 . Movement of the implement  124  relative to the stick  122  is controlled by actuator  134 . The actuator  134  is coupled to the stick  122  at one end. The other end of the actuator  134  is coupled to a four-bar linkage arrangement  144  that includes a portion of the stick  122  itself, as well as the implement  124  and a pair of links  146 ,  148 . The actuator  134  is extended in order to move the stick  122  toward the cab (counterclockwise in the illustrated embodiment), and retracted in order to move the implement  124  away from the cab (clockwise in the illustrated embodiment). 
         [0020]    Movement of the actuator  132  is controlled by the meterless hydraulic system  104 , which is shown in greater detail in  FIG. 2 . While the operation of the hydraulic system  104  is explained below with regard to actuator  132 , this explanation is equally applicable to the other actuators  130 ,  134 , and other actuator operated by a similar meterless hydraulic system  104 . 
         [0021]    The actuator  132  includes a cylinder  162  in which a piston  164  is slidably disposed. A rod  166  is secured to the piston  164 , and extends from the cylinder  162 . In this way, the piston  164  divides the interior of the cylinder  162  into a rod chamber  168  and a cap side chamber  170 . In operation, as the actuator  132  is extended, hydraulic fluid flows from the rod chamber  168  and hydraulic fluid flows into the cap side chamber  170  as the piston  164  and rod  166  slide within the cylinder  162  to telescope the rod  166  outward from the actuator  132 . Conversely, as the actuator  132  is retracted, hydraulic fluid flows into the rod chamber  168  and hydraulic fluid flows out of the cap side chamber  170  as the piston  164  and rod  166  slide within the cylinder  162  to retract the rod  166  into the cylinder  162 . Flow of hydraulic fluid to and from the rod and cap side chambers  168 ,  170  proceeds through a rod side fluid connection  172  and a cap side fluid connection  174 , respectively, that are fluidly coupled to respective ports  176 ,  178  opening in the rod or cap side chambers  168 ,  170  in the cylinder  162 . 
         [0022]    Flow between the rod and cap side chambers  168 ,  170  through the rod side and cap side fluid connections  172 ,  174  is provided by a pump  180  wherein the flow rate from the pump may be varied. In this way, the pump  180  controls the operation of actuator  132 , rather than so-called metering valves. The illustrated pump  180  is a variable displacement pump  180 , which includes a swash plate  181 , the angle of which determines the positive or negative displacement of the pump  180 , and volume of flow from the pump  180 . It will thus be appreciated that the displacement of the pump  180 , and, accordingly, the flow rate is controlled in order to control both the direction and volume of the flow of hydraulic fluid to provide extension and retraction of the actuator  132  as commanded by the operator. While a pump  180  is illustrated, the pump  180  may alternately be a fixed displacement pump wherein the speed may be varied by an associated driving motor. 
         [0023]    The pump  180  may operate as a pump to positively pump fluid from one fluid connection  172 ,  174  to the other  172 ,  174 , or a motor as fluid flows from one fluid connection  172 ,  174  to the other  172 ,  174 . More specifically, as an extension or a retraction of the actuator  132  is commanded against the force of the load  150 , as along the arcs identified as  154  or  158 , respectively, in  FIG. 1 , the pump  180  acts as a pump, pumping hydraulic fluid from one chamber  168 ,  170  to the other  168 ,  170 . Conversely, when an extension or a retraction of the actuator  132  is commanded in the same direction as the force of the load  150 , as in the arcs identified as  156  or  160 , respectively, in  FIG. 1 , the force of the load  150  causes a movement of fluid from one chamber  168 ,  170  to the other  168 ,  170  such that the energy of fluid motion allows the pump  180  to be operated as a motor. 
         [0024]    It will be appreciated by those of skill in the art that the respective volumes of hydraulic fluid flowing into and out of the rod and cap side chambers  168 ,  170  during extension and refraction of the actuator  132  are not equal. This is a result of the difference in surface area of the piston  164  on the rod and cap side chambers  168 ,  170 ; that is, the surface area of the piston  164  where the rod  166  extends from the piston  164  is less than the surface area of the piston  164  facing the cap side chamber  170 . Consequently, during retraction of the actuator  132 , more hydraulic fluid flows from the cap side chamber  170  than can be utilized in the rod chamber  168 . Conversely, during extensions of the actuator  132 , additional hydraulic fluid is required to supplement the hydraulic fluid flowing from the rod chamber  168  in order to fill the cap side chamber  170 . To receive this excess hydraulic fluid and provide this supplemental hydraulic fluid, a charge circuit  182  and make-up hydraulic circuit  184  are provided, as shown in  FIG. 2 . 
         [0025]    The charge circuit  182  includes at least one hydraulic fluid source, two of which are provided in the illustrated embodiment. The illustrated charge circuit  182  includes an accumulator  186  that may be utilized to provide a source of pressurized hydraulic fluid or that may be charged with excess hydraulic fluid through a charge conduit  188 . The illustrated charge circuit  182  additionally includes a tank  190  from which hydraulic fluid may be provided by a second pump  192  through the charge conduit  188 . Excess hydraulic fluid, either from the second pump  192  or operation of the actuator  132  may be returned to either the accumulator  186 , or to the tank  190  by way of a charge pilot valve  198  disposed in a charge pilot conduit  200 , which is fluidly connected to return conduit  201 . The charge pilot valve  198  is operated as a result of fluid pressure in the conduit  200  along the inlet side of the charge pilot valve  198 , although an alternate method of operation may be provided. In this embodiment, the pump  180  and the second pump  192  are both operated by a prime mover  194 , such as the engine  116 , through a gearbox  196 . In an alternate embodiment, one or both of the pumps  180 ,  192  may connected directly to the engine  116  or prime mover  194  shaft with no speed ratio change. The pump  180  and/or the second pump  192  may alternately be operated by a battery or other power storage arrangement. It will further be appreciated that the second pump  192  may be selectively operated, or continuously operated, as in the illustrated embodiment, depending upon the arrangement provided. 
         [0026]    The make-up hydraulic circuit  184  includes a make-up conduit  202  that is fluidly coupled to the charge conduit  188 , a make-up valve  204 , a rod side make-up conduit  206  and a cap side make-up conduit  208 , which are fluidly coupled to the rod side fluid connection  172  and the cap side fluid connection  174 , respectively. The make-up valve has three positions. The first, central default position  210  prevents flow to or from each of conduits  202 ,  206 ,  208 . Alternatively, the central default position may be constructed such that conduit  208  is connected to conduit  202  by an orifice (not shown), and conduit  206  is connected to conduit  202  by an orifice (not shown); this connection using orifices may be desirable if the pump  180  does not return to a perfect zero displacement when commanded to neutral. 
         [0027]    For the purposes of this disclosure, however, any reference to the central default position  210  being considered a no-flow position is intended to include both illustrated design wherein no connections is made, and a situation wherein orifices are disposed between the conduits  208 ,  206  and the conduit  202  to severely limit any flow therethrough. The second position  212  fluidly couples the make-up conduit  202  and the rod side make-up conduit  206  to allow flow therethrough, and prevent flow to or from the cap side make-up conduit  208 . The third position  214  fluidly couples the make-up conduit  202  and the cap side make-up conduit  208  to allow flow therethrough, and prevent flow to or from the rod side make-up conduit  206 . 
         [0028]    In order to operate the make-up valve  204 , pilot connections  216 ,  218  are provided from the rod and cap side make-up conduits  206 ,  208 , respectively. Thus, the make-up valve  204  is operative as a result of a minimum pressure differential between the pilot connections  216 ,  218 . While very little flow occurs through the pilot connections  216 ,  218 , it will be appreciated that the pressure from the rod side fluid connection  172  is applied to the pilot connection  216  by way of the rod side make-up conduit  206 . Similarly, the pressure from the cap side fluid connection  174  is applied to the pilot connection  218  by way of the cap side make-up conduit  208 . 
         [0029]    When the pressure on the cap side pilot connection  218  is sufficiently greater than the pressure on the rod side pilot connection  216 , the make-up valve  204  will move to its second position  212 . Conversely, when the pressure on the rod side pilot connections  216  is sufficiently greater than the pressure on the cap side pilot connection  218 , the make-up valve  204  will move to its third position  214 . 
         [0030]    It will be noted that the make-up circuit  184  may include additional valving arrangements. By way of example, the make-up circuit  184  may include check valves  220 ,  222  that are operative at set pressure differentials between the make-up conduit  202  and the rod side and cap side fluid connections  172 ,  174 , respectively. It will be appreciated that the check valves  220 ,  222  will unseat to permit flow if the pressure within the make-up conduit  202  is sufficiently greater than the pressures in rod side and cap side fluid connections  172 ,  174 , respectively. The check valves  220 ,  222  may include any device for limiting flow in a piping system to a single direction known by one skilled in the art now and in the future. 
         [0031]    Turning now to  FIG. 3 , this figure is a schematic view of the control architecture  400  of the pump displacement control of  FIG. 2  including data and command signaling. In particular, the illustrated control architecture  400  includes a human machine interface (HMI)  401  which allows the machine to receive operator commands and translate them into a machine operable form such as a digital or analog command or signal. Examples of the HMI  401  include the related structures of  FIG. 1 , namely operator control  115  for controlling the extension and retraction of the hydraulic actuators  102 , which control may be in the form of a joystick, a dial, a switch, a lever, a combination of the same, or any other arrangement by which the operator may command a movement, as well as a hydraulic lockout switch  113 , on/off switch  111 , etc. 
         [0032]    In addition to the HMI  401 , the architecture  400  includes a controller  403  for receiving an interface command  402  from the HMI  401 . The controller  403  may comprise one or more processors, e.g., microprocessors, for generating and transmitting control signals  404 ,  405  based on received data and commands. The controller  403  may operate specifically by the computerized execution of computer-readable instructions stored on a nontransitory computer-readable medium such as a RAM, ROM, PROM, EPROM, optical disk, flash drive, thumb drive, etc. 
         [0033]    The controller  403  is operable to receive commands and data from the HMI  401  and to receive pressure data from another source, to be discussed, and control a pump flow on that basis. In particular, the commands  404 ,  405  output from the controller  403  are provided to a first hydraulic pump  406  and to a second hydraulic pump  407  respectively. Each of the first hydraulic pump  406  and the second hydraulic pump  407  is configured to provide pressurized fluid at a commanded rate. The first hydraulic pump  406  is fluidly linked via hydraulic circuit  410  to supply pressurized fluid to a first hydraulic actuator  408 , while the second hydraulic pump  407  is fluidly linked via hydraulic circuit  411  to supply pressurized fluid to a second hydraulic actuator  409 . As discussed above, the hydraulic actuators  408 ,  409  may be situated to power various machine functions depending upon the type of machine being operated. 
         [0034]    Depending upon the ease with which each actuator moves, i.e., in an encumbered on unencumbered manner, the pressure within each hydraulic circuit  410 ,  411  will vary over time. While some pressure variation is thus to be expected, an excessive rise in pressure, e.g., due to striking an obstacle with the associated operated implement or function, may severely damage the hydraulic actuator, the associated hydraulic circuit, and/or the associated hydraulic pump. While it is known to use simple pressure relief valves to buffer such pressure spikes, this technique, while simple, has certain drawbacks. For example, the release of pressurized fluid through a relief valve has the affect of dumping energy out of the system and thus lowering fuel efficiency. 
         [0035]    Thus, the disclosed principles allow a meterless hydraulic supply system that operates in the absence of a pressure relief valve. In an embodiment, this is accomplished by reducing the pressure in the affected hydraulic circuit by lossless means. In particular, each hydraulic circuit  410 ,  411  embodies a dedicated pressure sensor  412 ,  413 , which may be a pressure transducer of other mechanism, for sensing a pressure and outputting a signal repeatably related to the sensed pressure. 
         [0036]    Each pressure sensor  412 ,  413  senses a pressure in the associated hydraulic circuit  410 ,  411 , and provides a respective pressure signal  414 ,  415  to the controller  403 , from which the controller  403  is able to identify the existence and extent of any over-pressure condition in the associated circuit  410 ,  411 . Thus, for example, the signal from each pressure sensor  412 ,  413  may be an analog or digital representation of the hydraulic pressure in the associated hydraulic circuit  410 ,  411 . 
         [0037]    As will be discussed in greater detail hereinafter, the controller  403  responds to the received pressure signals  414 ,  415  by modifying one or both of the pressure commands  404 ,  405  under certain circumstances to eliminate a circuit overpressure condition. In particular, the quantitative behavior of the system during a pressure spike will be discussed with reference to  FIG. 4 , and then the operations of the controller  403  to alleviate pressure spikes will be discussed with reference to  FIG. 5 . 
         [0038]    Thus, turning now to  FIG. 4 , this figure illustrates a set of simplified plots showing a hydraulic circuit pressure spike and correlated displacement reduction according to an embodiment of the disclosure. The bottom curve  450  plots hydraulic pressure in one hydraulic circuit of interest as a function of elapsed time. This plot  450  represents the pressure signal received from an appropriate pressure sensor associated with the hydraulic circuit. 
         [0039]    The plot illustrates three regions, namely an initial normal region  451 , a high-pressure spike region  452 , and a subsequent normal pressure region  453 . The top plot  460  illustrates the progression of circuit flow rate, i.e., pump flow rate, during the same periods. As can be seen from the plots  450 ,  460 , the initial system pressure during the initial period  451  is P i , with an associated hydraulic flow of F. As time progresses, an obstacle or other hindrance slows the actuator, increasing hydraulic pressure, without changing the hydraulic flow. During this period, the hydraulic pressure increase, but is beneath an overpressure threshold P t . However, in time, as the hydraulic pressure continues to increase, it passes the overpressure threshold P t  at the start of high-pressure spike region  452 . 
         [0040]    Once the hydraulic pressure has passed the overpressure threshold P t , the controller  403  reacts by decreasing the hydraulic flow, as can be seen in plot  451  during high-pressure spike region  452 . Initially, the decrease in hydraulic flow does not reduce the hydraulic pressure to below the overpressure threshold P t , and indeed the hydraulic pressure reaches its peak P p  during this period. However, eventually, the decrease in hydraulic flow reverses the pressure spike, and the hydraulic pressure falls to or below the overpressure threshold P t  at the start of subsequent normal pressure region  453 . Throughout this region  453 , the hydraulic pressure remains stable at P t  and the hydraulic flow remains stable at F s . 
         [0041]    The controller function that provides this pressure-ameliorating behavior will be discussed in greater detail with respect to the flow chart of  FIG. 5 . In particular,  FIG. 5  is a flow chart of a process  500  for applying a flow reduction as described herein to alleviate an overpressure condition in a meterless hydraulic circuit such as that shown above. At stage  501  of process  500 , the controller  403  establishes an initial flow rate based on a user command and/or automated response. In the case wherein the hydraulic pump is a variable displacement hydraulic pump, the controller sets the flow of the variable displacement hydraulic pump by setting the angle of a swash plate associated with the variable displacement hydraulic pump. In an alternative embodiment wherein the hydraulic pump is a fixed displacement electrically-driven hydraulic pump, the controller sets the flow of the fixed displacement electrically-driven hydraulic pump by setting a speed of the associated electric drive mechanism (not shown) such as an electric motor. 
         [0042]    As the process  500  continues, the controller  403  monitors the pressure signal received from the pressure sensor associated with the hydraulic circuit being measured at stage  502 . It will be appreciated that the illustrated process  500  is executed in parallel for each monitored circuit. If the monitored pressure has not exceeded a predetermined limit, e.g., the overpressure threshold P t , then the process  500  continues from stage  502  back to stage  501  to execute any changes in commanded flow. 
         [0043]    If, however, it is determined at stage  502  that the monitored pressure has exceeded the predetermined limit, the process  500  branches to stage  503 , wherein the controller  403  calculates a reduction factor for the hydraulic flow. In an embodiment, in order to provide a smooth but sufficiently rapid reduction in pressure, the reduction factor is related to extent to which the hydraulic pressure has exceeded the predetermined limit, and in a further embodiment is proportional to the extent to which the hydraulic pressure has exceeded the predetermined limit. Thus, for example, if the circuit pressure has gone from below the predetermined limit to 50% beyond the limit in one checking interval, the reduction factor would be much greater than if during the same interval the pressure had risen to only 20% beyond the limit. 
         [0044]    Having calculated the reduction factor, the controller applies the reduction factor in stage  504  to reduce the circuit pressure. In the case of a variable displacement hydraulic pump, the pump swash plate may be destroked by an amount set by the reduction factor. In an alternative embodiment, if a fixed displacement electrically-driven hydraulic pump is used, the pump speed may be decreased by the reduction factor. The reduction factor may be in any suitable form, i.e., multiplicative, subtractive, etc. After the reduction factor is applied and the flow reduced, the process returns to stage  501  to apply any updated control commands. 
       INDUSTRIAL APPLICABILITY 
       [0045]    The described system and method may be applicable to any meterless hydraulically actuated machine having one or more variable flow pumps, e.g., excavators, motorgraders, dozers, etc. The described system and method may avoid the use of pressure relief valves, which tend to waste energy when triggered. The described system may also allow a temporary increase in pressure where such may be beneficial without being damaging, whereas relief valve systems open as soon as the limit pressure is reached. 
         [0046]    It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
         [0047]    Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
         [0048]    Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.