Patent Publication Number: US-8523537-B2

Title: Integral plus proportional dual pump switching system

Description:
FIELD OF THE INVENTION 
     This invention generally relates to fluid distribution systems, and, more particularly, to fluid distribution systems capable of operating in a single-pump mode or in a dual-pump mode. 
     BACKGROUND OF THE INVENTION 
     Aircraft turbine engine main fuel pumps are typically high-pressure positive-displacement pumps in which the pump flow rate is proportional to engine speed. At many engine operating conditions the engine flow demand is significantly less than the high amount of flow supplied by the main fuel pump. The excess high-pressure pump flow is typically bypassed back to the low pressure inlet. Raising the pressure of the excess flow and then bypassing it back to low-pressure typically wastes energy. Generally, this wasted energy is converted to heat, which can be potentially useful, results in undesirably high fuel temperatures. 
     One means for reducing this energy loss is to implement a dual-pump system such that the amount of excess flow raised to high pressure is reduced at key thermal conditions. Systems that use two fuel supplies, for example two positive displacement pumps, can minimize the amount of bypass flow at high pressure differentials. This can be done by separating the two supply flows and only bypassing flow from one pump at a high pressure differential (e.g., the second supply pump would be bypassed at a much lower pressure differential). This reduces the wasted energy (i.e., heat) added to the fuel. 
     One problem encountered in implementing fuel distribution systems with dual pump supplies is that when the second pump supply is added (or subtracted) to the first pump supply, the system often generates unacceptable flow disturbances, or transients, resulting from the switch between single-supply and dual-supply operating modes. 
     It would therefore be desirable to have a system and method for dual-supply fuel distribution that reduces the flow disturbances which normally occur during transitions between single-supply and dual-supply operating modes. Embodiments of the invention provide such a system and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, embodiments of the invention provide a dual-pump fluid distribution system that is capable of switching between single-pump mode and dual-pump mode depending on fluid flow demand. In an embodiment, the dual-pump fluid distribution system includes a first pump having an inlet and an outlet, the first pump configured to supply a first flow of fluid, and a second pump having an inlet and an outlet, the second pump configured to supply a second flow of fluid. An embodiment of the fluid distribution system further includes a bypass flow valve having a valve member, a biasing element, and a four-way hydraulic bridge, and the bypass flow valve is configured to initiate the switch between single-pump mode and dual-pump mode based on fluid flow demand. Further, the bypass flow valve is configured such that the position of the bypass flow valve member relative to the four-way hydraulic bridge operates a pump selector valve. In an embodiment, the pump selector valve has a valve member, a biasing element, and a pressure switching port, and the pump selector valve is configured such that the position of the valve member determines whether the second flow of fluid is combined with the first flow of fluid. 
     In another aspect, embodiments of the invention provide a method of supplying fluid using a fluid distribution system capable of alternating between single-pump operation and dual-pump-operation. In an embodiment, the method includes the steps of operating the fluid distribution system in single-pump mode when a flow demand can be satisfied using a first pump, and operating the fluid distribution system in dual-pump mode by adding the flow from a second pump to that of the first pump when the flow demand exceeds the capacity of the first pump to meet the flow demand. In an embodiment, the method further includes alternating between single-pump mode and dual-pump mode by sensing the flow demand based on a pressure at the outlet of the first pump, wherein sensing the flow demand based on a pressure at the outlet of the first pump comprises placing a bypass flow valve between first and second pump outlets and a metering valve. 
     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a schematic diagram of an embodiment of a fluid distribution system, with dual fixed positive-displacement pumps, constructed in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of an embodiment of the fluid distribution system, with dual fixed positive-displacement pumps and variable actuation pressure, constructed in accordance with an embodiment of the present invention; and 
         FIG. 3  is a is a schematic diagram of an embodiment of the fluid distribution system, with a fixed positive-displacement pump and a variable positive-displacement pump, constructed in accordance with an embodiment of the present invention. 
     
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, embodiments of the invention are disclosed with respect to their application in a fuel distribution system. However, one having ordinary skill in the art will recognize that embodiments of the invention described herein can be applied to the distribution of a variety of fluids, including but not limited to fuels, where the fluid output supplied by the system is metered. Accordingly, embodiments of the invention include dual-pump systems for the distribution of virtually any fluid that is typically supplied by such a fluid distribution system. 
     In embodiments of the present invention, a fluid distribution system, such as for the distribution of fuel in an aircraft for example, incorporates a dual-pump switching system which allows the discharge flow from the two pumps to be separated when operating in single-pump mode, and then combined when operating in dual-pump mode. Continuing with this example, when the fuel distribution system is operating in single-pump mode, a first pump supplies all of the high-pressure burn flow to the engine combustor. Other required engine flows can be supplied by either the first pump or a second pump depending on how the fuel distribution system is configured. With the system operating in single-pump mode, the discharge pressure of the first pump is typically set by downstream conditions such as fuel nozzle restriction and combustor pressure. 
     Moreover, in an embodiment of the invention, when operating in single-pump mode, the second pump discharge pressure can be controlled independently of the first pump discharge pressure. By minimizing the pressure differential across the first and second pumps when the system is operating in single-pump mode, the system operates efficiently in terms of power consumption, and further adds relatively little thermal energy to the fluid circulating in the system. When the flow demand approaches the capacity of the first pump, the second pump pressure is raised above the first pump pressure and a portion of the second pump flow is supplied to supplement the flow from the first pump. 
       FIG. 1  is a schematic diagram of an embodiment of a fluid distribution system  100  that includes dual fixed positive-displacement pumps, constructed in accordance with an embodiment of the present invention. Fluid distribution system  100  includes a main inlet  102  through which fuel for example, or in an alternate embodiment some other liquid, flows into the fluid distribution system  100 . The main inlet  102  branches off to supply a first pump  104  and a second pump  106 . In the embodiment of  FIG. 1 , both first and second pumps  104 ,  106  are fixed-positive-displacement pumps, though embodiments are contemplated, and will be shown below, in which another type of pump is used. The main inlet  102  is also coupled to a port  108  of a second pump pressurizing valve  110 , which comprises a valve member  112  and a biasing element  114 . The first pump  104  has an inlet  115  and an outlet  116 . The first pump  104  is coupled to a bypass flow valve  118  (also known as an integral plus proportional bypass valve) via flow line  120 . 
     The bypass flow valve  118  includes a bypass flow valve member  122 , a four-way hydraulic bridge  124 , and a biasing element  126 . The four-way hydraulic bridge  124  includes two ports coupled by a flow line  128 , and two remaining ports coupled respectively to two flow lines  130 ,  132 . These flow lines  130 ,  132  couple the two ports of the four-way hydraulic bridge  124  with two ports at opposite ends of a pump selector valve  134 , which comprises a valve member  136 , a biasing element  138 , and a pressure switching port  140 . The four-way hydraulic bridge  124  also includes the bypass flow valve member  122 , which has alternating large-diameter and small-diameter portions. The pressure switching port  140  is coupled to a port of the second pump pressurizing valve  110 . The pump selector valve  134  is coupled to a bypass line  139  configured to provide a path for the discharge flow from the first pump  104  back to the inlet  115  of the first pump  104  when the pump selector valve member  136  is positioned to allow for flow into the bypass line  139 . 
     The second pump  106  includes inlet  141  and outlet  142 , wherein the outlet  142  is coupled to both the second pump pressurizing valve  110  and the pump selector valve  134 . An output line  144 , configured to accept a flow from the output of the second pump  106  via the pump selector valve  134 , is coupled to flow line  120  and thus to the main port  146  of bypass flow valve  118 , wherein the bypass flow valve main port  146  is configured to provide fluid communication between the outlets  116 ,  142  of the first and second pumps  104 ,  106  and a bypass line  148  configured to direct the flow of liquid from first and second pump outlets  116 ,  142  back to the first pump inlet  115 . An actuation supply unit  150  is coupled between the bypass flow valve  118  and a metering valve  152 . The actuation supply unit  150  is configured to supply a flow of pressurized fluid to various devices, such as hydraulic devices, attached to the fluid distribution system  100 . A flow line  154  couples the output of the metering valve  152  to a port  156  at one end of the bypass flow valve  118 . A pressurizing and shutoff valve  158  is also coupled to the output of the metering valve  152 . 
     In operation, fuel, or in an alternate embodiment some other liquid, flows into the main inlet  102  of fluid distribution system  100  and to the inlets  115 ,  141  of the first and second pumps  104 ,  106 . The bypass flow valve  118  is configured to sense the pressure differential across the metering valve  152  and to regulate that pressure differential by controlling the amount of total pump (i.e., first and second pump) bypass flow. In at least one embodiment, a fuel valve, for example an electrohydraulic servo valve  160 (EHSV) has two inputs  162 : one coupled to the main inlet  102  and one coupled to the output flow of the first pump  104 , or to the output flow of the first and second pumps  104 ,  106  when their flows are combined. The EHSV  160  has two outputs  164  corresponding to the two inputs  162 . The EHSV outputs  164  are coupled to ports at opposite ends of the metering valve  152 . Flows from the EHSV outputs  164  enter the corresponding ports on the metering valve  152  and, depending on the pressure differential in the flow from the EHSV outputs  164 , may cause a metering valve member  153  to move toward the port having the lower pressure. As can be seen from  FIG. 1 , when pressure differential becomes large, the metering valve member  153  is moved in the upward direction (pictorially) reducing the flow through the pressurizing and shutoff valve  158  to the engine (not shown). This increases the pressure on bypass flow valve member  122  at the bypass flow valve main port  146 , moving the bypass flow member  122  downward (pictorially) such that the flow through the bypass flow valve main port  146  and through the bypass flow line  148  increases. This increased bypass flow reduces the pressure at the outlet  116 , thus reducing the pressure differential seen by the metering valve  152 . 
     The bypass flow valve  118  senses the differential pressure across the metering valve  152  and regulates that pressure differential by controlling the amount of total pump bypass flow. The bypass flow valve main port  146  normally maintains a minimal amount of pump bypass flow. The bypass flow into flow line  131  and into flow line  128  is available for quick response in advance of the slower high gain integral system. The integrating portion of the bypass flow valve  118  consists of a four-way hydraulic bridge  124  to regulate the pressures in flow line  130  and flow line  132  based on the position of the bypass flow valve member  122 . 
     When the fluid distribution system  100  is in equilibrium (i.e., the discharge pressures of first and second pumps  104 ,  106  are approximately equal), the bypass flow valve member  122  is in a “null position” as shown in  FIG. 1 . The four-way hydraulic bridge  124  is located such that its null position corresponds to a set amount of proportional port area. As the bypass flow valve member  122  moves from the null position, flow line  130  and flow line  132  pressures change to position (integrate) the pump selector valve  134 . Depending on the position of the pump selector valve  134 , flow is either added from the second pump  106  to supplement the first pump  104 , or no flow is added from second pump  106  and an additional bypass port is opened on the pump selector valve  134  to provide a second path for first pump  104  bypass flow. 
     Referring to  FIG. 1 , an excess of pump metered flow causes an increase in pressure from the first pump  104  relative to that of the second pump  106 , which causes the bypass flow valve main port  146  area to increase and moves the bypass flow valve member  122  away from its null position in the downward direction (pictorially). The movement of the valve member  122  leads to an increase in flow line  130  pressure and a decrease in flow line  132  pressure and results in an upward movement of the pump selector valve member  136 . Depending on the pump selector valve member  136  position, this either increases the amount of flow from the first pump  104  bypassed through the pump selector valve  134 , or decreases the amount of flow from the second pump  106  added to supplement flow from the first pump  104 . This results in lower metered flow, which returns the bypass flow valve member  122  to its null position. 
     In the case of too little flow from the first pump  104  to meet engine flow demand, the drop in pressure causes the bypass flow valve main port  146  area to decrease and moves the bypass flow valve member  122  away from its null position in the upward direction (pictorially). The movement of the valve member  122  leads to a decrease in flow line  130  pressure and an increase in flow line  132  pressure and results in a downward movement of the pump selector valve member  136 . Depending on the pump selector valve member  136  position, this either decreases the amount of flow from the first pump  104  bypassed through the pump selector valve  134 , or increases the amount of flow from the second pump  106  added to supplement flow from the first pump  104 . This results in greater metered flow and returns the bypass flow valve member  122  to its null position. 
     Whether the engine flow demand is greater or lesser than that provided by the fluid distribution  100  at a particular time, the bypass flow valve  118  proportional ports coupled to flow line  128  provide a rapid response to change in metering valve  152  differential pressure. The integrating section, which include those ports coupled to flow lines  130 ,  132 , then responds to bring the bypass flow valve member  122  back to its null position. Since the bypass flow valve member  122  returns to its null position, the steady state bypass port area of the bypass flow valve main port  146  remains nearly constant. 
     Another feature of the fluid distribution system  100  is the pressure switching port  140  on the pump selector valve  134 . The pressure switching port  140  controls the second pump pressurizing valve  110  reference pressure, and therefore second pump  106  discharge pressure as a function of pump selector valve  134  position. The pressure switching port  140  is timed such that the second pump  106  discharge pressure is increased to be at least equal to the first pump  104  discharge pressure prior to opening the flow path from the second pump  106  to the first pump  104 . This feature eliminates backflow from first pump  104  to second pump  106  when switching from single-pump operation to dual-pump operation, which is a key source of flow disturbances during switching. Furthermore, when operating in single-pump mode, the pump selector valve  134  operates the pressure switching port  140  to lower the second pump  106  discharge pressure to the minimum required value, thus reducing the amount of work done by the second pump  106 . 
     Additionally, it is a feature of the fluid distribution system  100 , and of those fluid distribution systems described below, that an abrupt increase or decrease in the flow demand can be accommodated without the flow disturbance, and the resulting metering problems, that might occur in conventional dual-pump fuel distribution systems due to the operation of the bypass flow valve  118  with its four-way hydraulic bridge  124 . The configuration of the bypass flow valve  118  allows for the rapid increase or decrease fluid flow in response to flow demand via control of the pump selector valve  134  and second pump pressurizing valve  110 . This type of control typically results in less wasted energy and less heat added to the fluid in the system than in conventional fluid distribution systems. 
       FIG. 2  is a schematic diagram illustrating an alternate embodiment of a fluid distribution system  200  with variable actuation pressure, constructed in accordance with an embodiment of the invention. Fluid distribution system  200  includes a main inlet  202  through which fuel, or in an alternate embodiment some other liquid, flows into the fluid distribution system  200 . The main inlet  202  branches off to supply a first pump  204  and a second pump  206 . In the embodiment of  FIG. 2 , both first and second pumps  204 ,  206  are fixed-positive-displacement pumps, though embodiments are contemplated in which other types of pumps are used. The main inlet  202  is also coupled to a variable pressure regulator  208 , which, in turn, is coupled to an outlet  222  of the second pump  206 . The variable pressure regulator  208  includes a port  210  coupled to a pressure switching port  212  of a pump selector valve  214 , which comprises a valve member  216  and biasing element  218 . The pump selector valve  214  is coupled to a bypass line  220  configured to provide a path for the discharge flow from the first pump  204  back to an inlet  221  of the second pump  206  when the pump selector valve member  216  is positioned to allow for flow into the bypass line  220 . 
     The second pump  206  includes inlet  221  and outlet  222 , wherein the outlet  222  discharges into flow line  223 , which is coupled to both the variable pressure regulator  208  and an actuation supply unit  224 . Flow line  223  is also coupled to pump selector valve  214  such that, depending on the position of pump selector valve member  216 , flow output from the second pump  206  can flow through the pump selector valve  214  to flow line  226  to combine with flow from the first pump  204 . 
     First pump  204  has an inlet  229  and an outlet  230 , which discharges into flow line  232 . Flow line  232  is coupled to flow line  226 , to metering valve  233 , and to a main port  234  of a bypass flow valve  236  (also known as an integral plus proportional bypass valve), which comprises a valve member  238  and a biasing element  240 . The bypass flow valve  236  also includes a four-way hydraulic bridge  242 . The four-way hydraulic bridge  242  includes two ports coupled by a flow line  244 , and two additional ports coupled, respectively, to flow lines  246 ,  248 . The flow lines  246 ,  248  couple the two additional ports of four-way hydraulic bridge  242  with two ports at the opposite ends of a pump selector valve  214 . The four-way hydraulic bridge  242  also includes the bypass flow valve member  238 , which has alternating large-diameter and small-diameter portions. The main bypass flow valve port  234  is configured to provide fluid communication between the outlets  222 ,  230  of the first and second pumps  204 ,  206  and a bypass line  250  configured to direct the flow of liquid from first and second pump outlets  222 ,  230  back to the first pump inlet  221 . 
     Liquid flows into the metering valve  233  from flow line  232  and flows out of the metering valve  233  into flow line  252 , which is coupled to a pressurizing and shutoff valve  254 , and to a port  256  at one end of the bypass flow valve  236 . In an embodiment of the invention in which the fluid distribution system  200  operates as a fuel distribution system aboard an aircraft, for example, the output of the pressurizing and shutoff valve  254  flows to the engine (not shown). 
     In this fluid distribution system  200 , servo and actuation flow for all conditions is supplied to the actuation supply unit  224  by the second pump  206 . The actuation supply unit  224  is configured to provide a flow of pressurized fluid to various devices, such as hydraulic devices, coupled to the fluid distribution system  200 . The variable pressure regulator  208  is configured to actively control the discharge pressure of the second pump  206  to the minimum pressure required to supply the actuation supply unit  224  demands. Operation of the switching system (i.e., alternating between single-pump mode and dual-pump mode) is very similar to the operation described for the fluid distribution system  100  of  FIG. 1 . One of the differences in the implementation shown in  FIG. 2  is that the pressure switching port  212  on the pump selector valve  214  is configured to provide an override signal to the variable pressure regulator to insure that the second pump  206  discharge pressure is maintained above the first pump  204  discharge pressure when operating in dual-pump mode. 
       FIG. 3  is a schematic diagram illustrating yet another embodiment of a fluid distribution system  300 , constructed in accordance with an embodiment of the invention. In this embodiment, fluid distribution system  300  has both a fixed-positive-displacement pump and a variable-positive-displacement pump.  FIG. 3  shows a first pump  304  having fixed positive displacement, and a second pump  306  having variable positive displacement. In at least one embodiment, fuel, or in an alternate embodiment, some other liquid flows into fluid distribution system  300  at a main inlet  302 , which supplies the first and second pumps  304 ,  306 . The main inlet  302  is also coupled to multiple ports on a second pump pressurizing valve  308 , which comprises a valve member  310 , a biasing element  312 , a main port  314 , and a four-way hydraulic bridge  316 . 
     The four-way hydraulic bridge  316  includes two ports on the second pump pressurizing valve  308 , the two ports coupled by a flow line  318 . The flow line  318  is, in turn, coupled to a flow line  320  and configured to accept a bypass flow from the outlet  322  of the second pump  306 . Flow line  320  is configured to direct the bypass flow from the outlet  322  of the second pump  306  back to an inlet  321  of the second pump  306 . The four-way hydraulic bridge  316  further includes two ports coupled via respective flow lines  323 ,  325  to ports at opposite ends of a displacement-control valve  324  coupled to the second pump  306 . The displacement control valve  324  also includes a piston  328 , and a biasing element  330 . Further, the four-way hydraulic bridge  316  includes the bypass flow valve member  310 , which has alternating large-diameter and small-diameter portions. 
     The first pump  304  has an inlet  333  and an outlet  334  which discharges into flow line  336  which is coupled to an actuation supply unit  338  and to a main port  340  of a bypass flow valve  342  (also known as an integral plus proportional bypass valve). The actuation supply unit  338  is configured to supply a pressurized fluid flow to various devices, such as hydraulic devices, coupled to the fluid distribution system  300 . The bypass flow valve  342  comprises a valve member  344 , a biasing element  345 , and a four-way hydraulic bridge  348 . The bypass flow valve main port  340  provides fluid communication between the outlet  334  of the first pump  304 , and a bypass line  346  configured to direct the bypass flow from the outlet  334  of the first pump  304  back to the inlet  333  of the first pump  304 . Bypass flow line  346  is coupled to two ports of the four-way hydraulic bridge  348  via flow line  350 . The other two ports of the four-way hydraulic bridge  348  are coupled, via flow lines  352 ,  354  to ports at opposite ends of a pump selector valve  358 , which comprises a valve member  360 , a biasing element  362 , and a pressure switching port  364  coupled to a port  366  at one end of the second pump pressurizing valve  308 . The four-way hydraulic bridge  348  also includes the bypass flow valve member  344 , which has alternating large-diameter and small-diameter portions. The pump selector valve  358  is coupled to a bypass line  368  configured provide a path for the discharge flow from the first pump  304  back to an inlet  321  of the second pump  306  when the pump selector valve member  360  is positioned to allow for flow into the bypass line  368 . 
     The second pump outlet  322  discharges into flow line  370  which directs the flow from the second pump  306  through the pump selector valve  358  (depending on the position of valve member  360 ) to flow line  372  which is coupled to flow line  336  allowing for the combination of output flows from the first and second pumps  304 ,  306 . Actuation supply unit  338  is disposed between flow lines  336 ,  372  and a metering valve  374 . Liquid flows into the metering valve  374  from flow lines  336 ,  372  and flows out of the metering valve  374  into flow line  376 , which is coupled to a pressurizing and shutoff valve  378 , and to a port  380  at one end of the bypass flow valve  342 . In an embodiment of the invention in which the fluid distribution system  300  operates as a fuel distribution system aboard an aircraft, for example, the output of the pressurizing and shutoff valve  378  flows to the engine (not shown). 
     Operation of the fluid distribution system  300  is very similar to the operation of fluid distribution system  100 , described for  FIG. 1 . One of the differences is that, along with the second pump  306  discharge pressure, the displacement of the second pump  306  can be varied as well. In single-pump mode, first pump  304  supplies all engine flow demand. The pressure switching port  364  on the pump selector valve  358  is configured to minimize the discharge pressure at the outlet  322  of the second pump  306 . In addition, the second pump pressurizing valve  308  is configured to regulate the displacement of the second pump  306  such that minimal second pump  306  flow is generated. 
     When the engine flow demand approaches the capacity of first pump  304 , the bypass flow valve  342  operates to raise the second pump  306  pressure above the first pump  304  pressure, such that a portion of the second pump  306  flow is supplied to supplement the first pump  304  flow. The four-way hydraulic bridge  316  on the second pump pressurizing valve  308  controls the displacement of second pump  306  to supplement the flow from the first pump  304  when necessary, and to maintain a minimal amount of bypass flow through the second pump pressurizing valve  308 . 
     As stated above, embodiments of the fuel distribution system described herein may be used in the distribution of fluids other than those used as fuel. One of ordinary skill in the art will recognize that embodiments of the invention may encompass uses in a variety of fluid distribution systems. However, that said, one of ordinary skill in the art will also recognize that embodiments of the invention are well-suited to aircraft fuel distribution systems where the efficiencies provided by the aforementioned embodiments may result in systems that are lighter and less costly than conventional aircraft fuel distribution systems. Further, aircraft fuel distribution systems incorporating an embodiment of the invention may be more thermally efficient than conventional fuel distribution systems, in which case, the need for cooling systems is greatly reduced, resulting in additional weight and cost savings. 
     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. 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. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention 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 invention unless otherwise indicated herein or otherwise clearly contradicted by context.