Patent Publication Number: US-2015059860-A1

Title: Method to stabilize pressure in an electro-hydraulically controlled brake charge system

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to a brake charge system. More particularly, the present disclosure relates to a method to stabilize pressure in an electro-hydraulically controlled brake charge system. 
     BACKGROUND 
     Earthmoving and construction work machines often employ hydraulic systems that provide functionality and control to various aspects of the machines. For example, some work machines employ hydraulic brake systems to control driving speeds and fan hydraulic drive systems that control machine cooling. As each system may have separate flow requirements, the hydraulic systems on some work machines are isolated systems, each with a separate fluid pump. Even in a combined system, each hydraulic system may require independent fluid-flow parameters and requirements. To address this, some known systems direct fluid from a common pump to one system or the other system by use of a cut-in/cut-out device. The cut-in/cut-out device may be an electro-hydraulic pump unit with on/off control via set-point. 
     Conventionally, the work machine has a hydraulic system for a brake system. Moreover, the hydraulic system may be adapted to drive other integrated hydraulic systems, such as a cooling system. The hydraulic system may include one or more accumulators, a pump, a charge supply valve, and a fluid reservoir. When a charge cycle is initiated, the fluid is directed from the reservoir to the brake system to charge or fill the one or more accumulators with fluid. When a charge cycle is initiated, the accumulator is said to be in a charging state. Upon completion of the charge cycle, as the accumulator reaches a pre-determined accumulator pressure, the pump having the cut-in/cut-out capability may cut-out or shut-off fluid flow to the brake system. At this point, the brake charge system is said to be in a non-charging state. In addition, the cut-in/cut-out device may direct fluid to the cooling system instead of the brake system. Through normal braking, the fluid in the accumulator may be gradually expended until the accumulator pressure falls below a pre-determined threshold pressure. When this occurs, the cut-in/cut-out device may direct fluid flow to the brake system and the brake charge system will again be in a charging state. When this occurs, the fluid flow to the cooling system is reduced as needed, potentially to zero. Due to the alternate occurrence of cut-in and cut-out of the fluid to the one or more accumulators, the pressure in the brake system fluctuates between a relatively high level and a relatively low level. The brake system may commonly experience pressure spikes and instability. Instability may occur at the end of the charge cycle, which is, at cut-out of fluid flow to the brake system. Instability may be caused by the step change in the fluid flow when the system transitions between the charging state and the non-charging state. Such instabilities may cause component failure due to overpressure and/or pressure acceleration. Also, instabilities may cause audible noise that is irritating to personnel. Further, instabilities in the brake system may be transmitted into other systems that may cause related failures and issues. 
     U.S. Publication No. 2006/181143 discloses an electronically-controlled hydraulic brake system where hydraulic pressure in a wheel cylinder of a brake is controlled by an electric control unit. The brake system is controlled based on an instruction from a brake electronic control unit (ECU) as a hydraulic pressure controller. However, instabilities in the hydraulic brake system may occur and may not be addressed by this reference. 
     The brake charge system disclosed and described herein may overcome one or more of the problems in the existing systems. 
     SUMMARY OF THE INVENTION 
     The present disclosure is related to a method to stabilize pressure in an electro-hydraulically controlled brake charge system. 
     In accordance with the present disclosure, the brake charge system includes a load-sensing pump, an accumulator, a sensor for sensing accumulator pressure, a high pressure cut-off valve, a controller, and a charge supply valve. The method includes pressurization of the accumulator, with the fluid from the load sensing pump. The accumulator pressure, measured by the sensor, is compared with a pre-determined cut-out pressure. When the accumulator pressure, measured by the sensor, exceeds the pre-determined cut-out pressure, a counter-timer is initiated. The counter-timer is continuously compared with a pre-determined time threshold. When the counter-timer becomes equal to the pre-determined time threshold, that is, the pre-determined time threshold has elapsed, a flow of the fluid into the accumulator is minimized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a profile view of a machine (in broken line format) adapted with a brake system (in solid line format), in accordance with the concepts of the present disclosure; 
         FIG. 2  is a schematic and diagrammatic illustration of a hydraulic schematic showing a brake charge system of the machine of  FIG. 1 , in accordance with the concepts of the present disclosure; and 
         FIG. 3  illustrates a flowchart of a disclosed method to stabilize pressure in the electro-hydraulically controlled brake charge system of  FIG. 2 , in accordance with the concepts of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown an exemplary machine  100 . The machine  100  may be a mobile vehicle 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  100  may be a wheel loader (as depicted in  FIG. 1 ), a motor grader, a backhoe, an excavator, a scraper, an off-highway truck, a passenger vehicle, or any other vehicle or machine known in the art. In the illustrated embodiment, the machine  100  includes a frame  102 , an operator station  104 , a power source  106 , a drive system  108 , a lift arm  110 , a lift cylinder (not shown), a tilt cylinder  112 , a work tool  114 , a brake system  116 , and a controller  118 . 
     The frame  102  may include any structural member or assembly of members that support movement of the machine  100 . The frame  102  may support the operator station  104 , which may contain controls necessary to operate the machine  100 . Controls that may operate the machine  100  may include input devices (not shown) for propelling the machine  100  and other machine components. The input devices (not shown) may be adapted to receive input from the operator indicative of a desired work tool  114  or machine movement. The input devices (not shown) may include a steering wheel, single or multi-axis joysticks, switches, knobs, or other known devices that are located proximal to an operator seat. The input devices (not shown) may be configured to generate control signals indicative of a desired position, force, velocity, and/or acceleration of the lift cylinder (not shown) and the tilt cylinder  112 . The lift cylinder (not shown) and the tilt cylinder  112  may be operably coupled to the lift arm  110 . The lift cylinder (not shown) and the tilt cylinder  112  are connected to the frame  102  at one end and coupled to the work tool  114  at as a second end. Expansion of the lift cylinder (not shown) may result in elevation of the lift arm  110 . Retraction of the lift cylinder (not shown) results in lowering of the lift arm  110 . 
     The frame  102  may support the power source  106 . The power source  106  may be an engine, such as a diesel engine, a gasoline engine, a gaseous fuel-powered engine, a natural gas engine, or other engine known to one skilled in the art. It is contemplated that the power source  106  may alternatively embody a non-combustion source of power, such as a fuel cell, a power storage device, or another suitable source of power. The power source  106  may produce a mechanical or electrical power output that may be converted to hydraulic power. The power source  106  may power the drive system  108  that may include a pair of front wheels  120  and a pair of rear wheels  122 , adapted to support the machine  100 . The front wheels  120  and the rear wheels  122  may be adapted to steer and maneuver the machine  100  and to propel the machine  100  in forward and reverse directions. 
     The machine  100  further includes the brake system  116 , operatively connected to the controller  118 . The brake system  116  may be adapted to decelerate the movement of the machine  100 , when the machine  100  is in motion. The controller  118  may be operatively connected to the power source  106 , the brake system  116 , and the operator station  104 . The controller  118  may be adapted to receive signals from the input devices (not shown) associated with the operator station  104 . The controller  118  may monitor and provide appropriate output signals to various systems to control the movement of the machine  100  and the work tool  114 , or to perform various other functions and tasks. 
     The brake system  116  may be associated with the front wheels  120  and the rear wheels  122 . The brake system  116  may further be operable with other input devices, such as a brake pedal  124  within the operator station  104 . The brake system  116  may be hydraulically driven. The brake system  116  may include front brakes  126  and rear brakes  128 . The front brakes  126  and rear brakes  128  may, respectively, be operatively associated with the front wheels  120  and rear wheels  122 . The front brakes  126  and rear brakes  128  may selectively decelerate movement of the machine  100 . In an embodiment, each of the front brakes  126  and the rear brakes  128  may include a hydraulic pressure-actuated wheel brake, such as a disk brake or a drum brake. The front brakes  126  and the rear brakes  128  are disposed intermediate to the front wheels  120 , the rear wheels  122 , and a final drive assembly (not shown) of the machine  100 . When actuated, pressurized fluid within the front brakes  126  and the rear brakes  128  may increase the rolling friction of the machine  100 , which retards the movement of the machine  100 . The front brakes  126  and the rear brakes  128  may be operated in a known manner, such as by the brake pedal  124  disposed within the operator station  104  of the machine  100 . The brake pedal  124  may be associated with the front brakes  126  and the rear brakes  128 , for manual control of the front brakes  126  and the rear brakes  128 . As an operator depresses the brake pedal  124  along a braking range, pressurized fluid may be directed to the front brakes  126  and the rear brakes  128 . The degree of the brake pedal  124  depression proportionally controls the pressure of the fluid that is supplied to the front brakes  126  and the rear brakes  128 . 
     The brake system  116  may further include a brake charge system  130  associated with at least one of the front brakes  126  or the rear brakes  128 . The brake charge system  130  may include a plurality of fluid components and electrical components. The brake charge system  130  may be operatively connected to control the braking capacity of the brake system  116 . Hence, the brake charge system  130  controls the braking capacity of the machine  100 . In the illustrated embodiment, the brake charge system  130  is operatively connected to the controller  118  to regulate the flow of pressurized fluid directed to the front brakes  126  and rear brakes  128 . In an embodiment, the brake charge system  130  may be adapted to drive other integrated hydraulic systems, such as a cooling system, which may operate from a common fluid source. The fluid components and electrical components may cooperate to control the braking and other capacities of the machine  100 . 
     Referring to  FIG. 2 , there is shown a schematic of the brake charge system  130 . The brake charge system  130  may include a tank  200 , a load-sensing pump  202 , a margin valve  204 , a high pressure cut-off valve  206 , a cylinder  208 , a charge supply valve  210 , the controller  118 , a first accumulator  212 , a second accumulator  214 , a sensor  216 , a first brake control valve  218 , and a second brake control valve  220 . 
     The brake charge system  130  is adapted to draw fluid from and return fluid to the tank  200 . The tank  200  may be adapted to hold a supply of fluid. For example, the tank  200  may constitute a low-pressure reservoir adapted to hold the supply of the fluid. The fluid may include dedicated hydraulic oil, engine lubrication oil, transmission lubrication oil, or any other fluid known in the art. One or more hydraulic systems within the machine  100  may draw fluid from and return fluid to the tank  200 . The tank  200  is in fluid communication with the load-sensing pump  202 . The load-sensing pump  202  is adapted to provide pressurized fluid to the brake charge system  130 . The load-sensing pump  202  may be adapted to pressurize fluid drawn from the tank  200  and direct the pressurized fluid to the first accumulator  212  and the second accumulator  214 . In the illustrated embodiment, the load-sensing pump  202  embodies a variable displacement piston pump with load-sensing capabilities, which may permit the load-sensing pump  202  to operate or provide fluid flow only when necessary, thus improving the efficiency of the machine  100 . In an embodiment, the load-sensing pump  202  may embody a fixed displacement pump adapted to produce a flow of pressurized fluid proportional to a rotational input speed. The load-sensing pump  202  may be directly driven by an electric motor (not shown). The load-sensing pump  202  may or may not be a fixed delivery pump, that is, a pump that delivers a constant flow rate of pressurized fluid per input revolution. 
     The load-sensing pump  202  is operated by at least one of the margin valve  204  and the high pressure cut-off valve  206 . The margin valve  204  is adapted to take an accumulator pressure as reference signal, or load-sense signal, via a load-sense line  222 . The margin valve  204  is adapted to add a margin pressure to the accumulator pressure and generate a margin valve output signal for the cylinder  208 . Based on the margin valve output signal, which corresponds to the margin valve output pressure, the cylinder  208  actuates a swash plate of the load-sensing pump  202 . Accordingly, the load-sensing pump  202  delivers the fluid to the charge supply valve  210  at an output pressure that corresponds to the margin valve output pressure. 
     The high pressure cut-off valve  206  has a pre-determined cut-off pressure at which the high pressure cut-off valve  206  initiates operation. The pre-determined cut-off pressure of the high pressure cut-off valve  206  is set as a sum of spring biasing pressure and tank pressure. The high pressure cut-off valve  206  is adapted to operate the load-sensing pump  202  when the output pressure corresponds to the margin valve output pressure and exceeds the pre-determined cut-off pressure. The high pressure cut-off valve  206  actuates the cylinder  208  to displace the load-sensing pump  202  to deliver the fluid to the charge supply valve  210  at the output pressure equal to the pre-determined cut-off pressure. During charging one of the first accumulator  212  and the second accumulator  214 , via the charge supply valve  210 , when the accumulator pressure becomes equal to the pre-determined cut-out pressure, the controller  118  initiates a counter-timer. The counter-timer is initiated for a delay to be implemented (in unit of time) for a cut-out event. When charging of one of the first accumulator  212  and the second accumulator  214 , via the charge supply valve  210  is stopped, the condition is referred to as the cut-out event. 
     The charge supply valve  210  is disposed between the load-sensing pump  202  and each of the first accumulator  212  and the second accumulator  214 . The charge supply valve  210  may be adapted to deliver the fluid from the load-sensing pump  202  to the brake charge system  130 , such that the fluid pressure of the first accumulator  212  and the second accumulator  214  is maintained between cut-in and cut-out during normal operating conditions. The first accumulator  212  and the second accumulator  214 , respectively, are in fluid communication with the first brake control valve  218  and the second brake control valve  220 , to control braking during machine operation. The charge supply valve  210  may include a priority valve  224 , a relief valve  226 , a screen  228 , a charge rate limiting orifice  230 , an electronic solenoid  232 , and an inverse shuttle valve  234 . In embodiments including one or more integrated hydraulic systems, the charge supply valve  210  includes the priority valve  224 . In such embodiments, the functions of the integrated hydraulic system and the brake system  116  are integrated into one subsystem supplied by the load-sensing pump  202 . In some embodiments, the charge supply valve  210  further includes the relief valve  226 . 
     The priority valve  224  is in fluid communication with the load-sensing pump  202 . The priority valve  224  may be adapted to provide fluid flow to the first accumulator  212 , the second accumulator  214 , and to the other hydraulic system. The priority valve  224  ensures that pressure is continuously available to the first accumulator  212  and the second accumulator  214  for brake charging, thus ensuring that charge of the first accumulator  212  and the second accumulator  214  has priority over the other hydraulic system. The fluid may be directed to the first accumulator  212  and the second accumulator  214  regardless of demand at the other hydraulic system. The fluid exiting the priority valve  224  flows through the screen  228  and the charge rate limiting orifice  230 . The fluid undergoes a reduction in pressure, when it flows across the charge rate limiting orifice  230 . 
     The charge supply valve  210  includes the electronic solenoid  232 , which may be electrically driven to direct the fluid from the priority valve  224  to the inverse shuttle valve  234 . The inverse shuttle valve  234  is adapted to proportion the flow of fluid to the first accumulator  212  and the second accumulator  214 . The inverse shuttle valve  234  is piloted by the pressure between the first accumulator  212  and the second accumulator  214 . For example, if the first accumulator  212  has the higher pressure, the first accumulator  212  will bias the inverse shuttle valve  234  to provide fluid flow to the second accumulator  214  with the lower pressure such that the first accumulator  212  and the second accumulator  214  are pressurized or charged evenly. 
     The sensor  216  may be associated with the first accumulator  212  and the second accumulator  214 . The sensor  216  may be adapted to sense and to communicate signals indicative of the pressure within or between the first accumulator  212  and the second accumulator  214 . The pressure of the first accumulator  212  and the second accumulator  214  may be controlled within a range of pressures. In an embodiment, a lower threshold defines a pre-determined cut-in pressure and an upper threshold defines a pre-determined cut-out pressure. The first accumulator  212  and the second accumulator  214  are charged via the margin valve  204 , at the margin valve output pressure, in order to achieve the cut-out pressure level. In the illustrated embodiment, the controller  118  may communicate with the sensor  216  to determine the pressure by use of a pressure-sensor arbitration. The first accumulator  212  and the second accumulator  214  may be adapted to hold a supply of pressurized fluid at a desired pressure and to provide the desired fluid to slow, decelerate, or stop movement of the machine  100 . For example, the first accumulator  212  and the second accumulator  214  may be maintained above a predetermined threshold to provide brake pressure when desired by the operator. In other words, the first accumulator  212  and the second accumulator  214  are adapted to store fluid pressure for brake control. In addition, the relief valve  226  may protect the first accumulator  212  and the second accumulator  214  from being over-charged or over-pressurized. 
     Further, the brake system  116  includes the controller  118 , which is operably coupled to the electronic solenoid  232  and the sensor  216 . The controller  118  evaluates signals from the sensor  216 . Further, the controller  118  uses the signals to generate control signals for electrical actuation of the charge supply valve  210 , via the electronic solenoid  232 , to maintain desired pressure in the first accumulator  212  and the second accumulator  214 . The controller  118  monitors the accumulator pressure in the first accumulator  212  and the second accumulator  214 , to determine if the accumulator pressure is equal to the pre-determined cut-out pressure. The controller  118  initiates the counter-timer to delay the cut-out event, on determination that the accumulator pressure is equal to the pre-determined cut-out pressure. On initiation of the counter-timer, the controller  118  compares the counter-timer with a pre-determined time threshold, which is determined by the controller  118  and is required to delay the cut-out event. The controller  118  may monitor the overall pressure of the brake system  116  and report the brake status to the operator and other systems. By control of the charge supply valve  210 , the controller  118  controls the input fluid flow to the first accumulator  212  and the second accumulator  214 . 
     Referring to  FIG. 3 , there is shown a flowchart depicting a method  300  to stabilize pressure in the brake charge system  130 . The method  300  is explained in conjunction with elements from  FIG. 1  and  FIG. 2 . The method  300  starts with step  302 . 
     At step  302 , the controller  118  sends signals to de-energize the electronic solenoid  232  to initiate a charge cycle for the first accumulator  212  and the second accumulator  214 . An event when the controller  118  de-energizes the electronic solenoid  232  may be referred to as cut-in event. The method  300  proceeds to step  304 . 
     At step  304 , a pilot fluid is supplied to the margin valve  204 . The pilot fluid is at an accumulator pressure that is within or between the first accumulator  212  and the second accumulator  214 . The accumulator pressure acts as a reference signal for the margin valve  204 . The method  300  proceeds to step  306 . 
     At step  306 , the margin valve  204  senses the accumulator pressure or the reference signal and generates an output signal for the cylinder  208 . The output signal corresponds to a margin valve output pressure, which is sum of the accumulator pressure and margin pressure created by the margin valve  204 . The method  300  proceeds to step  308 . 
     At step  308 , the cylinder  208  actuates the load-sensing pump  202 . The load-sensing pump  202  delivers fluid to at least one of the first accumulator  212  and the second accumulator  214 , at an output pressure, which is equal to the margin valve output pressure. In other words, the margin valve  204  operates the load-sensing pump  202  to deliver the fluid to the charge supply valve  210 . The method  300  proceeds to step  310 . 
     At step  310 , based on the signal from the sensor  216 , the controller  118  determines whether the accumulator pressure is equal to the pre-determined cut-out pressure. If the output pressure is equal to the pre-determined cut-out pressure, then the method  300  proceeds to step  312 . If the output pressure is less than the pre-determined cut-out pressure, then the method  300  returns to step  304 . 
     At step  312 , the controller  118  initiates the counter-timer. The method  300  proceeds to step  314 . 
     At step  314 , upon actuation of the counter-timer, the pilot fluid is delivered to the margin valve  204 . The method  300  proceeds to step  316 . 
     At step  316 , the margin valve output pressure is generated by the margin valve  204 , based on the reference signal corresponding to the pilot fluid delivered to the margin valve  204 . The method  300  proceeds to step  318 . 
     At step  318 , the load-sensing pump  202  senses if the margin valve output pressure is equal to the pre-determined cut-off pressure of the high pressure cut-off valve  206 . When the load-sensing pump  202  senses that the margin valve output pressure is equal to the pre-determined cut-off pressure, the method  300  proceeds to step  320 . When the load-sensing pump  202  senses that the margin valve output pressure is less than the pre-determined cut-off pressure, the method  300  proceeds to step  322 . 
     At step  320 , the cylinder  208  actuates the load-sensing pump  202 . The load-sensing pump  202  delivers fluid to at least one of the first accumulator  212  and the second accumulator  214 , at the output pressure, which is equal to the pre-determined cut-off pressure. The method  300  proceeds to step  324 . 
     At step  322 , the cylinder  208  actuates the load-sensing pump  202 . The load-sensing pump  202  delivers fluid to at least one of the first accumulator  212  and the second accumulator  214 , at the output pressure, which is equal to the margin valve output pressure. The method  300  proceeds to step  324 . 
     At step  324 , the counter-timer is incremented towards the pre-determined time threshold. The method  300  proceeds to the step  326 . 
     At step  326 , the controller  118  compares the counter-timer with the pre-determined time threshold. If the controller  118  determines that the counter-timer is equal to the pre-determined time threshold, then the method  300  proceeds to end step  328 . If the controller  118  determines that the counter-timer is less than or not equal to the pre-determined time threshold, then the method  300  returns to step  314 . 
     At end step  328 , the controller  118  sends a signal, to energize the electronic solenoid  232 , for the cut-out event. Energization of the electronic solenoid  232  results in the cut-out event, that is, stop of flow of the fluid to the first accumulator  212  and the second accumulator  214 . Hence, the charging is stopped. In an embodiment, the electronic solenoid  232  may be energized after the pre-determined time threshold corresponding to the counter-timer, has elapsed. 
     INDUSTRIAL APPLICABILITY 
     In operation, the controller  118  receives pressure signal for at least one of the first accumulator  212  and the second accumulator  214 , via the sensor  216 . When the pressure in at least one of the first accumulator  212  and the second accumulator  214  is below the lower threshold, the controller  118  de-energizes the electronic solenoid  232  for initiation for the charge cycle. Upon de-energization of the electronic solenoid  232 , the pilot fluid at the accumulator pressure is delivered to the margin valve  204 , via the load-sense line  222 . The margin valve  204  determines the accumulator pressure as the reference signal and delivers the margin valve  204  output to the cylinder  208 . The cylinder  208  actuates the load-sensing pump  202  to deliver the fluid to the charge supply valve  210  at the output pressure equal to the margin valve output pressure. The output pressure of the load-sensing pump  202  increases transiently during the charge cycle until the controller  118  determines that the output pressure has reached the pre-determined cut-out pressure. As the output pressure of the load-sensing pump  202  is determined to be equal to the pre-determined cut-out pressure of the high pressure cut-off valve  206 , the controller  118  initiates the counter-timer. After initiation of the counter-timer, the controller  118  compares the counter-timer with the pre-determined time threshold. In the meanwhile, the hydraulic fluid is delivered to the charge supply valve  210  at the margin valve output pressure, via the margin valve  204 . As the load-sensing pump  202  senses that the margin valve output pressure is equal to the pre-determined cut-off pressure, the hydraulic fluid is delivered to the charge supply valve  210  at the pre-determined cut-off pressure. Hence, the high pressure cut-off valve  206  now operates the load-sensing pump  202 , via the cylinder  208 , to deliver the fluid to the charge supply valve  210  constantly at the pre-determined cut-out pressure. The load-sensing pump  202  delivers the fluid to charge supply valve  210  at the output pressure (the pre-determined cut-out pressure), until the accumulator pressure reaches the output pressure (the pre-determined cut-out pressure). In other words, during the time when the counter-timer becomes equal to the pre-determined time threshold, the pressure of the brake charge system  130  becomes uniform. As the controller  118  determines that the counter-timer is equal to the pre-determined time threshold, the controller  118  signals for energization of the electronic solenoid  232  and the cut-out occurs, thereby stopping charge of the first accumulator  212  and the second accumulator  214 . In an embodiment, the controller  118  may determine and signal a delay of units of time corresponding to the counter-timer, to the electronic solenoid  232 , for the transition between the charging state and the non-charging state. 
     The disclosed method  300  of pressure stabilizing the brake charge system  130  uses the load-sensing pump  202 , operated for the pre-determined cut-out pressure setting to control maximum brake charge circuit pressure. In the disclosed method  300 , the transition between the charging state and the non-charging state is delayed until the brake charge system  130  pressure is uniform. At that time, the flow of fluid has stopped in the brake charge system  130  and the transition from charging to non-charging can be done without a step change in flow of the fluid. However, current methods commonly experience pressure spikes and instability at the end of a charge cycle, that is, at the cut-out event. Current methods face instability, which is caused by the step change in the fluid flow when transition occurs immediately between the charging state to the non-charging state. Hence, the proposed method  300  provides the benefits of electro-hydraulic control at the cut-in event with the benefits of constant charge control at the cut-out event, without creation of instability, pressure acceleration, and/or noise. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claim appended hereto as permitted by applicable law. Moreover, any combination of the described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein. 
     One skilled in the art will realize the disclosure may be embodied in other specific forms without departing from the disclosure or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting of the disclosure described herein. Scope of the invention is thus indicated by the appended claim, rather than the foregoing description, and all changes that come within the meaning and range of equivalence of the claim are therefore intended to be embraced therein.