Hydraulic accumulator fluid charge estimation system and method

A method for estimating a fluid charge of a hydraulic accumulator includes determining a first accumulator pressure at a first time with a pressure sensor, the first time during accumulator charging; determining a second accumulator pressure at a second time with the pressure sensor, the second time during accumulator charging; determining a first fan speed at the first time; determining a second fan speed at the second time; and estimating the fluid charge of the hydraulic accumulator as a function of the first accumulator pressure, the second accumulator pressure, the first fan speed, and the second fan speed.

TECHNICAL FIELD

The present disclosure relates generally to hydraulic circuits with accumulators. Specifically, an embodiment of the present invention relates to a method to estimate the fluid charge of an accumulator.

BACKGROUND

Power source-driven machines such as, for example, excavators, dozers, loaders, motor graders, and other types of heavy equipment typically include a cooling system that cools the associated power source and/or other machine components below a threshold that provides for longevity of the machines. The cooling system may include one or more air-to-air and/or liquid-to-air heat exchangers that may chill coolant circulated through the power source and/or machine components, and/or combustion air directed into the power source. Heat from the coolant or combustion air is transferred to air from a fan that is speed controlled based on a temperature of the power source, the temperature of machine component(s), and/or based on a temperature of an associated hydraulic system. The fan may also aid in circulating air in a machine compartment or space to increase the rate of heat dissipation.

The cooling system fan may be hydraulically powered. That is, a pump driven by the power source draws in low-pressure fluid and discharges the fluid at elevated pressures to a motor that is connected to the fan. When a temperature of the power source, machine component(s), and/or machine space is higher than desired, the pump and motor may work together to increase the speed of the fan. When the temperature of the power source, machine component(s), and/or machine space is low, the pump and motor work together to decrease the speed of the fan and, in some situations, even stop the fan altogether. Under some conditions, fan rotation can even be reversed such that airflow through a heat exchanger is also reversed to help dislodge any debris that has collected in the heat exchanger.

In some machine operating conditions, a hydraulic circuit driving the cooling fan described above and/or other hydraulic circuits of the same machine may have excess energy capacity and may store at least a part of this excess energy capacity in one or more accumulators. Energy from one or more of the accumulators may later be used to supplement prime mover, engine, and or other energy producing or storing devices.

An energy management system may be used to ensure that machine power is sufficient to meet the needs of all machine components and to release stored power when needed. The energy management system may monitor and control the storage and release of energy from one or more hydraulic accumulators associated with a hydraulic fan circuit to provide needed power to machine components based at least partially on an estimate of the fluid charge of the one or more hydraulic accumulators.

US Patent Application Publication US20080174174 A1 filed by Burns et al. discloses that the amount of energy stored in an accumulator is a function of the accumulator pressure and the volume of fluid stored in the accumulator. The temperature of the system, the type of gas used to pre-charge the system, and the initial pressure of the pre-charge gas can impact the amount of energy stored at a given accumulator pressure. The equation to calculate the energy stored in an accumulator is: E=(Pc*Vc−(P*Vc*((Pc/P)^(⊥/k))))/(1−k); where: E is the energy stored in the accumulator; Pc is the pre-charge pressure of the accumulator; Vc is the volume of gas in the accumulator at pre-charge; P is the current accumulator pressure; and k is ratio of specific heats (Boltzmann constant) for the pre-charge gas. The value of k for a gas varies with pressure at high pressures. Values of 1.3 to 1.8 may be used for typical gases and pressures. The pre-charge gas, pre-charge pressure, and volume of gas in the accumulator will not vary on a trailer during operation. Thus, the State Of Charge (SOC) of a hydraulic accumulator is a function only of its pressure. Although the accumulator pressure will vary with charge gas temperature, the SOC can be determined with acceptable accuracy even if this term is ignored.

SUMMARY OF THE INVENTION

One aspect of the disclosure includes a method for estimating a fluid charge of a hydraulic accumulator in a hydraulic circuit including a primary pump, a motor selectively fluidly connected to the pump, and drivingly connected to a fan. The method includes determining a first accumulator pressure at a first time with a pressure sensor, the first time during accumulator charging; determining a second accumulator pressure at a second time with the pressure sensor, the second time during accumulator charging; determining a first fan speed at the first time; determining a second fan speed at the second time; and estimating the fluid charge of the hydraulic accumulator as a function of the first accumulator pressure, the second accumulator pressure, the first fan speed, and the second fan speed.

Another aspect of the disclosure includes an alternative method for estimating a fluid charge of a hydraulic accumulator in a hydraulic circuit including a primary pump, a motor selectively fluidly connected to the pump, and drivingly connected to a fan. The method includes determining periodic accumulator pressures with a pressure sensor during an accumulator charging time period; determining periodic fan speeds during the accumulator charging time period; and estimating the fluid charge of the hydraulic accumulator as a function of the periodic accumulator pressures and the periodic fan speeds.

Another aspect of the disclosure includes an accumulator fluid charge estimation system having a hydraulic fan circuit and a controller. The hydraulic fan circuit includes a primary pump, a primary accumulator selectively fluidly connected to the pump, a fan, a motor selectively fluidly connected to the pump, and drivingly connected to the fan, a fan speed sensor configured to generate periodic fan speed signals indicative of periodic speeds of the fan, and a pressure sensor configured to produce periodic accumulator pressure signals indicative of periodic accumulator pressures at the primary accumulator. The controller is configured to estimate a fluid charge of the primary accumulator as a function of the periodic fan speed signals, and the periodic accumulator pressure signals during an accumulator charging period.

Another aspect of the disclosure includes an accumulator fluid charge estimation system having a hydraulic fan circuit and a controller. The hydraulic fan circuit includes a primary pump, a primary accumulator selectively fluidly connected to the pump, a fan, a motor selectively fluidly connected to the pump, and drivingly connected to the fan, a fan speed sensor configured to generate a first fan speed signal indicative of a first fan speed at a first time during an accumulator charging period, and a second fan speed signal indicative of a second fan speed at a second time during an accumulator charging period, and a pressure sensor configured to produce a first accumulator pressure signal indicative of a first accumulator pressure at the primary accumulator at the first time, and a second accumulator pressure signal indicative of a second accumulator pressure at the primary accumulator at the second time. The controller is configured to estimate a fluid charge of the primary accumulator as a function of the first fan speed signal, the second fan speed signal, the first accumulator pressure signal, and the second accumulator sensor signal.

Another aspect of the disclosure includes a machine having an engine, a hydraulic fan circuit, and a controller. The hydraulic fan circuit includes a primary pump drivingly connected to the engine, a primary accumulator selectively fluidly connected to the pump, a fan positioned to cool the engine, a motor selectively fluidly connected to the pump, and drivingly connected to the fan, a fan speed sensor configured to generate periodic fan speed signals indicative of periodic speeds of the fan, and a pressure sensor configured to produce periodic accumulator pressure signals indicative of periodic accumulator pressures at the primary accumulator. The controller is configured to estimate a fluid charge of the primary accumulator as a function of the periodic fan speed signals, and the periodic accumulator pressure signals during an accumulator charging period.

DETAILED DESCRIPTION

Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Generally, corresponding reference numbers will be used throughout the drawings to refer to the same or corresponding parts.

Referring now toFIG. 1, an exemplary embodiment of machine100is illustrated. In the embodiment illustrated, the machine100is depicted as a vehicle102, and in particular an excavator104. In other embodiments, the machine100may include any system or device for doing work. The machine100may include both vehicles102or stationary machines (not shown) such as, but not limited to, electric power generating devices, crushers, conveyors, tunnel borers or any other stationary machine that would be known to an ordinary person skilled in the art now or in the future. The vehicle102may include but is not limited to vehicles that perform some type of operation associated with a particular industry such as mining, construction, farming, transportation, etc. and operate between or within work environments (e.g. construction site, mine site, power plants, on-highway applications, marine applications, etc.). Non-limiting examples of vehicle102include trucks, cranes, earthmoving vehicles, mining vehicles, backhoes, loaders, material handling equipment, farming equipment, locomotives and other vehicles which travel on tracks, and any type of movable machine that would be known by an ordinary person skilled in the art now or in the future. Vehicle102may include mobile machines which operate on land, in water, in the earth's atmosphere, or in space. Land vehicles may include mobile machines with tires, tracks, or other ground engaging devices.

Machine100may be equipped with multiple systems that facilitate the operation of machine100at worksite110, for example a tool system120, a drive system130, and a power system140that provides power to the tool system120and drive system130. During the performance of most tasks, power from power system140may be disproportionately split between tool system120and drive system130. That is, machine100may generally be either traveling between excavation sites and primarily supplying power to drive system130, or parked at an excavation site and actively moving material by primarily supplying power to tool system120.

Machine100generally will not be traveling at high speeds and actively moving large loads of material with tool system120at the same time. Accordingly, power system140may be sized to provide enough power to satisfy a maximum demand of either tool system120or of drive system130, but not both at the same time. Although sufficient for most situations, there may be times when the total power demand from machine systems (e.g., from tool system120and/or drive system130) exceeds a power supply capacity of power system140. Power system140may be configured to consume stored energy during these times to temporarily increase its supply capacity. This additional supply capacity may also or alternatively be used to reduce a fuel consumption of power system140by allowing for selective reductions in the power production of power system140, if desired.

As illustrated inFIGS. 1 and 2, one embodiment of power system140includes an engine212, for example an internal combustion engine. In alternative embodiments the power system140may include other power sources such as electric motors (not shown), fuel cells, (not shown), batteries (not shown), ultra-capacitors (not shown), electric generators (not shown), and/or any power source that would be known by an ordinary person skilled in the art now or in the future.

In the embodiment depicted, the engine212is drivingly connected to a primary pump214in a hydraulic fan circuit210. A fan220in the hydraulic fan circuit210may be positioned on the machine100to cool the engine212, or in alternative embodiments, other power sources, machine100component(s), and/or machine100space(s) or compartment(s).

Hydraulic fan circuit210may include a collection of components that are powered by engine212to cool engine212. In the depicted embodiment, the hydraulic fan circuit210includes a primary pump214connected directly to a mechanical output216of engine212, a motor218fluidly connected to primary pump214by a closed-loop circuit222, and the fan220connected to the motor218. Engine212may drive primary pump214via mechanical output216to draw in low-pressure fluid and discharge the fluid at an elevated pressure. The motor218may receive and convert the pressurized fluid to mechanical power that drives fan220to generate a flow of air. The flow of air may be used to cool engine212and/or other power sources, machine100components, and/or machine100space(s) or compartments(s) directly and/or indirectly by way of a heat exchanger (not shown).

Primary pump214may be an over-center, variable-displacement or variable-delivery pump driven by engine212to pressurize fluid. For example, primary pump214may embody a rotary or piston-driven pump having a crankshaft (not shown) connected to engine212via mechanical output216such that an output rotation of engine212results in a corresponding pumping motion of primary pump214. The pumping motion of the primary pump214may function to draw in low-pressure fluid expelled from the motor218via a low-pressure passage224, and discharge the fluid at an elevated pressure to the motor218via a high-pressure passage226. Low- and high-pressure passages224,226together may form closed circuit222.

Primary pump214may be dedicated to supplying pressurized fluid to only motor218via high-pressure passage226or, alternatively, may also supply pressurized fluid to other hydraulic circuits associated with machine100(e.g., to hydraulic circuits associated with tool system120, drive system130, etc.), if desired. Similarly, primary pump214may be dedicated to drawing low-pressure fluid from only motor218via low-pressure passage224or, alternatively, may also draw in low-pressure fluid from other hydraulic circuits of machine100, if desired. It should be noted that, in some situations, primary pump214and motor218may be operated in reverse direction and, in these situations; the pressures within low- and high-pressure passages224,226may be reversed.

Motor218may include a fixed displacement, rotary- or piston-type hydraulic motor movable by an imbalance of pressure acting on a driven element (not shown), for example an impeller or a piston. Fluid pressurized by primary pump214may be directed into motor218via high-pressure passage226and returned from motor218via low-pressure passage224. The direction of pressurized fluid to one side of the driven element and the draining of fluid from an opposing side of the driven element may create a pressure differential across the driven element that causes the driven element to move or rotate. The pressure differential is the difference between the motor218input pressure PIand the motor218output pressure PO. The direction and rate of fluid flow through motor218may determine the rotational direction and speed of motor218and fan220. The pressure differential and fluid flow of motor218may determine the torque output.

Fan220may be disposed proximate a liquid-to-air or air-to-air heat exchanger (not shown) and configured to produce a flow of air directed through channels of the exchanger for heat transfer with coolant or combustion air therein. In other embodiments fan220may be configured and positioned to provide a flow of air directed to increase the rate of heat dissipation of machine100component(s) or in machine100spaces(s) or compartment(s). Fan220may include a plurality of blades connected to motor218and be driven by motor218at a speed corresponding to a desired flow rate of air and/or a desired coolant and/or air temperature.

Hydraulic fan circuit210includes a fan speed sensor202configured to generate a fan speed signal indicative of fan220speed. Fan speed sensors202are known in the art. Fan speed sensor202may include any fan speed sensor202that would be known by an ordinary person skilled in the art now or in the future. Fan speed sensor202may be communicatively connected to controller262to transmit the fan speed signal to the controller262.

Low- and high-pressure passages224,226may be interconnected via multiple different crossover passages. In the exemplary embodiment depicted inFIG. 2, two different crossover passages interconnect low- and high-pressure passages224,226, including a makeup/relief passage230and a pressure-limiting passage232. Makeup/relief passage230may provide makeup fluid to low- and/or high-pressure passages224,226to help ensure that hydraulic fan circuit210remains full of fluid, and also provide a leak path for high-pressure fluid within low- and/or high-pressure passages224,226such that damage to the components of hydraulic fan circuit210may be avoided. Pressure-limiting passage232may provide for pilot pressure control associated with a displacement of primary pump214.

One or more makeup valves234, for example check valves, may be located within makeup/relief passage230to selectively connect the output from a charge pump236with low- and/or high-pressure passages224,226based on pressures of fluid in the different passages. That is, when a pressure within low- and/or high-pressure passage224,226falls below a pressure of fluid discharged by charge pump236, makeup valve(s)234may open and allow fluid to pass from charge pump236into the respective passage(s). Charge pump236may be driven by engine212to rotate with primary pump214and draw in fluid from a low-pressure sump238via a tank passage240, and discharge the fluid into makeup/relief passage230via a valve passage242.

One or more relief valves244may also be located within makeup/relief passage230. Relief valves244may be spring-biased and movable in response to a pressure of low- and/or high-pressure passages224,226to selectively connect the respective passages with a low-pressure passage246, thereby relieving excessive fluid pressures within low- and high-pressure passages224,226. An additional spring-biased pressure relief valve248may be located within low-pressure passage246and selectively moved by a pressure within low-pressure passage246between flow-passing and flow-blocking positions such that a desired pressure within low-pressure passage246may be maintained.

A resolver250may be disposed within pressure-limiting passage232and associated with a pilot pressure limiter252. Resolver250may be configured to connect fluid from one of low- and high-pressure passages224,226having the greater pressure with pilot pressure limiter252. In most instances, resolver250connects the pressure from high-pressure passage226with pilot pressure limiter252. However, when primary pump214and motor218are operating in the reverse flow direction or during an overrunning condition of motor218, it may be possible for the pressure within low-pressure passage224to exceed the pressure within high-pressure passage226. Under these conditions, resolver250may move to connect the pressure from low-pressure passage224with pilot pressure limiter252. When the pressure of fluid passing through resolver250exceeds a threshold limit, pilot pressure limiter252may move from a flow-blocking position toward a flow-passing position. It is contemplated that the threshold limit of pilot pressure limiter252may be tunable, if desired, to adjust a responsiveness or performance of hydraulic fan circuit210.

Pilot pressure limiter252may be in fluid communication with a pilot passage254that extends between charge pump236and a displacement actuator256of primary pump214. Specifically, pilot pressure limiter252may be connected to pilot passage254via a passage258. When pilot pressure limiter252moves toward the flow-passing position described above, pilot fluid from within pilot passage254may be allowed to drain to low-pressure sump238. The draining of pilot fluid from pilot passage254may reduce a pressure of fluid within pilot passage254.

The pilot fluid in passage254may be selectively communicated with displacement actuator256to affect a displacement change of primary pump214. Displacement actuator256may embody a double-acting, spring-biased cylinder connected to move a swashplate, a spill valve, or another displacement-adjusting mechanism of primary pump214. When pilot fluid of a sufficient pressure is introduced into one end of displacement actuator256, displacement actuator256may move the displacement-adjusting mechanism of primary pump214by an amount corresponding to the pressure of the fluid. Pilot pressure limiter252may limit the pressure within pilot passage254based on a pressure of fluid within low- and high-pressure passages224,226and, accordingly, also limit the displacement of primary pump214.

Pump displacement sensor298may be configured to generate a swashplate position signal indicative of the displacement of primary pump214. Pump displacement sensor298may be communicatively connected to controller262. Controller262may be configured to receive the swashplate position signal and control hydraulic fan circuit210to produce a desired fluid flow and/or pressure. For example, controller262may control valves260and266to move the primary pump214swashplate to a desired position in order to provide the necessary flow and pressure to motor218to produce a desired speed of fan220.

A directional control valve260may be associated with displacement actuator256to control what end of displacement actuator256receives the pressurized pilot fluid and, accordingly, in which direction (i.e., which of a displacement-increasing and a displacement-decreasing direction) the displacement-adjusting mechanism of primary pump214is moved by displacement actuator256. Directional control valve260may be a spring-biased, solenoid-actuated control valve that is movable based on a command from controller262. Directional control valve260may move between a first position at which a first end of displacement actuator256receives pressurized pilot fluid, and a second position at which a second opposing end of displacement actuator256receives pressurized pilot fluid. When the first end of displacement actuator256is receiving pressurized pilot fluid (i.e., when directional control valve60is in the first position), the second end of displacement actuator256may be simultaneously connected to low-pressure sump238via directional control valve260. Similarly, when the second end of displacement actuator256is receiving pressurized pilot fluid (i.e., when directional control valve260is in the second position), the first end of displacement actuator256may be simultaneously connected to low-pressure sump238via directional control valve260. One or more restrictive orifices264may be associated with pilot passage254to reduce pressure fluctuations in the pilot fluid entering and exiting the ends of displacement actuator256and, thereby, stabilize fluctuations in a speed of pump displacement changes.

A pressure control valve266may also be associated with pilot passage254and displacement actuator256and configured to control movement of displacement actuator256by varying a pressure of pilot passage254. Pressure control valve266may be movable from a first position at which full charge pressure is passed through directional control valve260, toward a second position at which some of the charge pressure is vented to low-pressure sump238before reaching directional control valve260and displacement actuator256. Pressure control valve266may be movable from the first position against a spring bias toward the second position based on a command from controller262. It is contemplated that pressure control valve266may be directly controlled via a solenoid or, alternatively, pilot operated via a separate solenoid valve (not shown), as desired. By selectively moving pressure control valve266to any position between the first and second positions, a pressure of the pilot fluid in communication with displacement actuator256and, hence, a displacement of primary pump214, may be controlled.

At least one accumulator may be associated with closed circuit222. In the embodiment ofFIG. 2, two accumulators are illustrated, including a low-pressure accumulator268and a high-pressure accumulator270. A low-pressure accumulator passage272and a high-pressure accumulator passage274may extend from low- and high-pressure accumulators268,270, respectively, to a discharge control valve276. Discharge control valve276may be fluidly connected to low- and high-pressure passages224,226by way of passages280and282respectively.

Discharge control valve276may be a double-acting, spring-biased, solenoid-controlled valve that is movable between three distinct positions based on a command from controller262. In the first position (shown inFIG. 2), fluid flow through discharge control valve276may be inhibited. In the second position, fluid may be allowed to pass between low-pressure accumulator268and low-pressure passage224and between high-pressure accumulator270and high-pressure passage226. In the third position, fluid may be allowed to pass between low-pressure accumulator268and high-pressure passage226and between high-pressure accumulator270and low-pressure passage224. Discharge control valve276may be spring-biased to the first position.

Low- and high-pressure accumulators268,270may be in fluid communication with pilot passage254. Specifically, a fill passage281may fluidly connect each of low- and high-pressure accumulator passages272,274to pilot passage254. One or more check valves283may be disposed within fill passage281between pilot passage254and each of low- and high-pressure accumulators268,270to help ensure a unidirectional flow of fluid from charge pump236through restrictive orifices290,292into low- and high-pressure accumulators268,270when law- and high-pressure accumulators268,270are charging. The one or more check valves283may also facilitate flow of fluid bypassing the restrictive orifices290,292when the low- and high-pressure accumulators268,270are discharging. The restrictive orifices290,292may have a uniform known cross section which may be useful in estimating the fluid charge of the low- and high-pressure accumulators268,270, as is explained in more detail below in relation toFIG. 4.

High-pressure accumulator270may also be in fluid communication with another hydraulic circuit228that forms a portion of for example, tool system120, drive system130, or another system of machine100. In particular, an auxiliary supply passage278may fluidly connect hydraulic circuit228to high-pressure accumulator270to fill high-pressure accumulator270with waste or excess fluid having an elevated pressure. A check valve288and a restrictive orifice294may be disposed within auxiliary supply passage278to help provide for a unidirectional flow of fluid with damped oscillations from hydraulic circuit228into high-pressure accumulator270. Hydraulic circuit228may include a tool actuation circuit, a transmission circuit, a brake circuit, a steering circuit, or any other machine circuit known in the art.

Fan circuit210includes a pressure sensor296configured to generate an accumulator pressure signal indicative of the fluid pressure at the high-pressure accumulator270positioned appropriately in the hydraulic fan circuit210. For example, the pressure sensor may be in high-pressure accumulator passage274as depicted inFIG. 2. The pressure sensor296is communicatively connected to controller262to transmit the accumulator pressure signal to controller262.

During accumulator discharge, it may be beneficial to substantially isolate motor218from low- and high-pressure passages224,226(i.e., to substantially block direct fluid flow to motor218via low- and high-pressure passages224,226). For this reason, a fan isolation valve284may be fluidly connected to low- and high-pressure passages224,226, between motor218and low- and high-pressure accumulators268,270. Fan isolation valve284may be a spring-biased, solenoid-controlled valve that is movable between two distinct positions based on a command from controller262. In the first position (shown inFIG. 2), fluid may be allowed to flow through fan isolation valve284to motor218via low- and high-pressure passages224,226. In the second position, fluid flow through fan isolation valve284may be inhibited. Fan isolation valve284may be spring-biased to the first position.

When motor218is isolated by fan isolation valve284(i.e., when fan isolation valve284is in the second position), fluid may still circulate through motor218, and fan220may still be spinning. To help control fluid temperatures during this time, hydraulic fan circuit210may include a motor flushing valve286. Motor flushing valve286may be in fluid communication with isolated portions of low- and high-pressure passages224,226, and configured to move between three positions based on the pressures of fluid within these passages. In the first position (shown inFIG. 2), fluid flow from low- and high-pressure passages224,226to low-pressure sump38may be inhibited. When a pressure difference occurs between low- and high-pressure passages224,226, motor flushing valve286may move to the second or third positions to remove a small volume of high-temperature fluid to be replaced with low-temperature oil.

The controller262may include a processor (not shown) and a memory component (not shown). The processor may include microprocessors or other processors as known in the art. In some embodiments the processor may include multiple processors. The processor may execute instructions for implementing a method, as described below and in relation toFIGS. 3,4,5, and6for estimating a fluid charge of high-pressure accumulator270. In the depicted embodiment, the processor may execute instructions for estimating a fluid charge of high-pressure accumulator270as a function of the fan speed signal and the accumulator pressure signal.

Such instructions may be read into or incorporated into a computer readable medium, such as the memory component or provided external to processor. The instructions may include multiple lines or divisions of code. The lines or divisions of code may not be consecutive order, and may not be located in the same section of code. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions to generate the machine function control signal and implement the method for estimating a fluid charge of accumulator270. Thus embodiments are not limited to any specific combination of hardware circuitry and software.

The term “computer-readable medium” as used herein refers to any medium or combination of media that participates in providing instructions to processor for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks. Volatile media includes dynamic memory. Transmission media includes coaxial cables, copper wire and fiber optics.

The memory component may include any form of computer-readable media as described above or which would be known to an ordinary person skilled in the art now or in the future. The memory component may include multiple memory components.

The controller262may be enclosed in a single housing. In alternative embodiments, the controller262may include a plurality of components operably connected and enclosed in a plurality of housings. The controller262may be located on-board the machine100, or may be located off-board or remotely.

Controller262is configured to receive the fan speed signal and the accumulator pressure signal. Controller262may be communicatively connected to valves260,266,276, and284to control operations of hydraulic fan circuit210.

Industrial Applicability

The hydraulic fan circuit210may operate in multiple modes. During an exemplary first mode of operation, engine212may drive primary pump214to rotate and pressurize fluid. The pressurized fluid may be discharged from primary pump214into high-pressure passage226and directed into motor218. As the pressurized fluid passes through motor218, hydraulic power in the fluid may be converted to mechanical power used to rotate fan220. As fan220rotates, a flow of air may be generated that facilitates cooling of engine212, or other machine100component(s), space(s), and/or compartment(s). Fluid exiting motor218, having been reduced in pressure, may be directed back to primary pump214via low-pressure passage224to repeat the cycle.

The fluid discharge direction and displacement of pump214during this first exemplary mode of operation may be regulated based at least partially on the fan speed signal and the swashplate position sensor. Controller262may receive signals, and execute code, that indicates a desired fan220speed. Controller262may then generate appropriate commands to be sent to directional control valve260and pressure control valve266to affect corresponding adjustments to the displacement of primary pump214.

Low- and/or high-pressure accumulators268,270may be charged during the exemplary first mode of operation in a least three different ways. For example, when primary pump214is driven to pressurize fluid, any excess fluid not consumed by motor218may fill high-pressure accumulator270via discharge control valve276, when discharge control valve276is in the second position. Similarly, fluid exiting motor218may till low-pressure accumulator268. Low- or high-pressure accumulators26S,270may only be filled while discharge control valve276is in the second position and pressures within low- or high-pressure passages224,226are greater than pressures within low- or high-pressure accumulators268,270, respectively. Otherwise, low- or high-pressure accumulators268,270may discharge fluid into low- or high-pressure passages224,226when discharge control valve276is moved to the second position. The movement of discharge control valve276may be closely regulated based at least in part on the signal provided by pressure sensor296, such that low- and high-pressure accumulators268,270may be charged and discharged at the appropriate times. It should be noted that only one of low- and high-pressure accumulators268,270may be filled at a time, while the other of low- and high-pressure accumulators268,270will be discharging, and vice versa.

In an exemplary second mode, low- or high-pressure accumulators268,270may be continuously charged via charge pump236. Specifically, at any time during normal operation, when a pressure of fluid from charge pump236is greater than pressures within low- or high-pressure accumulators268,270, fluid may be passed from charge pump236, through fill passage281, and past check valves283into the respective low- and high-pressure accumulator268,270. During this second mode charge valve276may be in the first closed position.

High-pressure accumulator270may also be charged by hydraulic circuit228. That is, at any time during normal operations, when a pressure of fluid from hydraulic circuit228is greater than a pressure within high-pressure accumulator270, fluid may be passed from circuit228, through auxiliary supply passage278, and past check valve288into high-pressure accumulator270.

When the controller262determines through sensor signals, and code execution that the demand for cooling air flow has been reduced, fan220may be slowed or even stopped, and controller262may implement an exemplary third mode of operation. During the exemplary third mode of operation, controller262may command fan isolation valve284to isolate motor218from primary pump214, and then command discharge control valve276to move to one of the second and third positions depending on the desired flow direction of primary pump214. At about this same time, controller262may command pressure control valve266to begin destroking primary pump214. When the appropriate valve commands have been issued, fluid from within one of low- or high-pressure accumulators68,70may discharge into low- or high-pressure passages224,226, respectively, via passages272,274, discharge control valve276, and passages280,282, thereby driving primary pump214as a motor. By driving primary pump214, hydraulic power from the accumulated fluid may be converted to mechanical power directed into engine212via mechanical output216. This power assist may help to increase a power supply capacity and/or decrease a fuel consumption of engine212during the exemplary third mode of operation.

During discharge of one of low- or high-pressure accumulators268,270, while motor218is isolated from primary pump214, fan220may continue to spin. Fan220, may for exampled be equipped with a flywheel (not shown) or oversized to integrate the mass of a flywheel, and may spin for an extended period of time without being driven by motor218. In one example, the extended period of time may be at least 4 seconds. In this manner, a significant amount of cooling may still be possible during discharge of low- or high-pressure accumulators268,270, and the speed of motor218may be substantially unaffected by the changing fluid pressures within the accumulators low- and high-pressure268,270. In addition, energy from the accumulated fluid may not be wasted on unnecessarily driving motor218.

It is contemplated that accumulator discharge could alternatively occur without complete motor isolation, if desired. Specifically, fan isolation valve284could be controlled to move to any position between the first and second positions described above such that a desired amount of pressurized fluid from high-pressure accumulator270passes through and drives motor218, while the remainder of the accumulated fluid passes through and drives primary pump214. In order to provide for a desired motor/fan speed during accumulator discharge, however, while pressures within high-pressure accumulator270are changing (i.e., decreasing), the displacement of pump214may be selectively adjusted based on the fluid pressure signal from sensor296and/or based on fan speed signal.

The controller262may implement an energy management strategy to ensure that the machine100provides adequate energy to all loads. This may include controlling multiple energy producing and energy storing devices to provide energy to meet load demands. Energy producing and storing devices may include engine212, fuel cells, electric motors, batteries, ultra-capacitors, accumulators268,270, and/or any energy producing or storing device that would be known to an ordinary person skilled in the art now or in the future. The controller262may determine how to meet load demands partially as a function of calculated or stored energy capacities of the energy producing or storing devices. The controller262may determine when to discharge the high-pressure accumulator to meet a load demand at least partially as a function of an estimated fluid charge of high-pressure accumulator270.

Referring now toFIG. 3, a method300for estimating a fluid charge of a hydraulic accumulator270in a hydraulic circuit210is depicted. The hydraulic circuit210includes a primary pump214; and a motor218selectively fluidly connected to the pump214, and drivingly connected to a fan220. The method includes determining an accumulator pressure; determining a fan speed; and estimating the fluid charge of the accumulator as a function of the accumulator pressure, and the fan speed.

The method300for estimating a fluid charge begins at step302and proceeds to step304. Step304includes determining the beginning volume of accumulator270. Method300may be used at any point in time to determine the fluid charge of accumulator270. At machine100start-up, the charge pump236may charge accumulator270to a beginning pressure while discharge control valve276is in the first position as shown inFIG. 2. The beginning fluid volume may be the fluid volume in the accumulator270after being charged to the beginning pressure at machine100start-up. In other embodiments the beginning fluid volume may be a calculated value based on accumulator270charging and discharging volumes. In other embodiments, the beginning fluid volume may be a known fluid volume based on the physical and operating characteristics of machine100and hydraulic fan circuit210.

The controller262may calculate the fluid volume in the accumulator270after being charged to the beginning pressure at machine100start-up. Accumulator270may be discharged at machine shut-down and assumed to be at zero. Since volume is equal to flow rate multiplied by time, beginning fluid volume may be determined as a function of flow to the accumulator270and time as expressed in equation 1:
VB=QB*tBequation 1
where VBis the beginning fluid volume in accumulator270, QBis the beginning flow rate to accumulator270, and tBis the beginning accumulator charge time. The beginning fluid volume in accumulator270may be expressed as an integral function in which yields equation 2:
VB=∫QBdtequation 2
where VBis the beginning fluid volume in accumulator270, and QBis the flow rate to accumulator270. The flow rate may be integrated over the beginning accumulator270charge time.

It is well known by ordinary persons skilled in the art that flow rate is equal to a constant times the area of a restriction times the square root of the quotient of the differential pressure times two and the density of the fluid. Restrictive orifice290includes a substantially constant cross sectional area which will be known by system designers. The pressure on one side of the restrictive orifice290is PR. The pressure on the other side of the restrictive orifice290is PA. PAmay be determined by the accumulator pressure signal from the pressure sensor296. Pressure relief valve248is configured to open at a known pressure. During machine100start-up, when discharge control valve276is in the first position, PRwill be equal to approximately the known pressure that pressure relief valve248is configured to open at. Therefore, flow rate during start-up may be estimated by equation 3:
QB=C*A(√[2(PV−PA)/ρ])  equation 3
where QBis a flow rate during start-up, C is a constant, A is the known cross sectional area of the restrictive orifice290, PVis the known pressure that pressure relief valve248is configured to open at, PAis the accumulator270pressure as indicated by the accumulator pressure signal, and ρ is the density of the fluid.

Since the beginning fluid volume of the accumulator270may be estimated by integrating the flow rate over the beginning charge time, it may be calculated and/or estimated by equation 4:
VB=∫[C*A(√[2(PV−PA)/ρ])]dtequation 4
where VBis the beginning fluid volume of accumulator270, QBis a flow rate during start-up, C is a constant, A is the known cross sectional area of the restrictive orifice290, PVis the pressure that pressure relief valve248is configured to open at, PAis the accumulator270pressure as indicated by the accumulator pressure signal, and ρ is the density of the fluid.

In other embodiments, the beginning fluid volume in accumulator270may be calculated and/or estimated based on other methods that would be known by an ordinary person skilled in the art now or in the future. The method proceeds from step304to step306.

Step306includes determining the volume of fluid that entered the accumulator270during charging (referred to inFIG. 3as accumulator charging fluid volume). Referring toFIG. 4, a method400to estimate an accumulator charging fluid volume is depicted. The method400includes determining the accumulator charging fluid volume as a function of an accumulator pressure and a fan speed. Method400begins at step402and proceeds to step404.

Step404includes determining a pressure differential across the motor218, ΔPMK, and a corresponding fan220speed, NK. To calculate and/or estimate the accumulator270fluid charge, it may be necessary to know one corresponding motor218pressure differential and corresponding fan220speed. This one motor218pressure differential and corresponding fan220speed may be at any hydraulic fan circuit210operating point when discharge control valve276is in the second position, and the primary pump214is fluidly and drivingly connected to fan220; and may be calculated and/or estimated in any way that would be known by an ordinary person skilled in the art now or in the future.

In one embodiment, the controller262may control hydraulic fan circuit210with the discharge control valve276in the second position to charge accumulator270, until the accumulator pressure signal from the pressure sensor296stays substantially constant for a short time period. When the accumulator pressure signal is substantially constant for a short time period, it may indicate that the accumulator270is no longer charging and the pressures PR, PAon both sides of the restrictive orifice290are substantially equal. When discharge control valve276is in the second position, PRmay be substantially equal to the motor218inlet pressure P. The controller262may control the hydraulic fan circuit210in such a way that the motor218outlet pressure POis substantially zero, or another known constant value at all times. Since during the time period when PAremains constant, PIequals PA, and POis a known constant, the pressure differential across the motor218, ΔPMK, may be estimated. A corresponding fan220speed, NFK, may be determined through the fan speed signal. ΔPMKand NFKat this operating point may be stored by the controller262in the memory component.

The fan speed sensor202may be configured to produce a periodic fan speed signal indicative of the fan220speed. Controller262may be configured to receive the periodic fan speed signal and infer the periodic fan220speed.

It is known in the art: that the power to drive a fan changes by the ratio of fan speeds to the third power as shown in equation 6:
hpF2=hpF1*(NF2/NF1)3equation 6
where NF1is a first fan220speed, NF2is a second fan220speed, hpF1is the power needed to drive the fan220at a first speed, and hpF2is the power needed to drive the fan220at a second speed.

It is also known in the art: that power is equal to torque multiplied by speed as shown in equation 7:
hp=T*Nequation 7
where hp is power, T is torque, and N is speed. In the context of hydraulic fan circuit210, equation 7 may be interpreted as the power needed to drive fan220at a particular speed is equal to the motor218torque multiplied by the fan220speed.

Substituting equation 7 into equation 6 yields equation 8:
TF2*NF2=TF1*NF1*(NF2/NF1)3equation 8
where NF1is a first fan220speed, NF2is a second fan220speed, TF1is the motor218torque at the first speed, and TF2is the motor218torque at the second speed. Simplifying equation 8 yields equation 9:
TF2=TF1*(NF2/NF1)2equation 9
where NF1is a first fan220speed, NF2is a second fan220speed, TF1is the motor218torque at the first speed, and TF2is the motor218torque at the second speed.

It is well known to ordinary persons skilled in the art that motor torque is equal to pressure times displacement as expressed in equation 10:
TM=ΔPM*VMDequation 10
where TMis motor torque, ΔPMis the pressure differential across the motor, and VMDis the displacement of the motor.

Equation 10 may be substituted into equation 9 to yield equation 11:
ΔPM2*VMD2=ΔPM1*VMD1*(NF2/NF1)2equation 11
where NF1is a first fan220speed, NF2is a second fan220speed, ΔPM1is the pressure differential across the motor218at the first speed, ΔPM2is the pressure differential across the motor218at the second speed, VMD1is the displacement of the motor218at the first speed, and VMD2is the displacement of the motor218at the second speed.

In an embodiment where the motor218is a fixed displacement motor, or is operated at a substantially fixed displacement, the displacement terms are constants and cancel each other to yield equation 12:
ΔPM2=ΔPM1*(NF2/NF1)2equation 12
where NF1is a first fan220speed, NF2is a second fan220speed, ΔPM1is the pressure differential across the motor218at the first speed, and ΔPM2is the pressure differential across the motor218at the second speed.

The motor218pressure differential and the fan220speed for one hydraulic fan circuit210operating point are known from step404and may be substituted into equation 12 to yield equation 13:
ΔPM2=ΔPMK*(NF2/NFK)2equation 13
where NFKis a known fan220speed, NF2is a second fan220speed, ΔPMKis a known pressure differential across the motor218at the known speed, and ΔPM2is the pressure differential across the motor218at the second speed.

In an embodiment where the hydraulic fan circuit210is operated such that the motor218output POis close to, if not equal to, zero, the inlet motor218pressure PIis equal to the pressure differential ΔPMacross the motor.
PIn=ΔPMK*(NFn/NFK)2equation 14
where PInis the periodic motor218input pressure, ΔPMKis the known pressure differential, NFKis the known fan speed, and NFnis the periodic fan speed.

In other embodiments where the hydraulic fan circuit210is operated such that the motor218output POis close to a known non-zero value, the inlet motor218pressure PIis equal to the pressure differential ΔPMacross the motor minus the known non-zero value. Assuming that POis equal to a known outlet pressure yields equation 15:
PIn=[ΔPMK*(NFn/NFK)2]+POequation 15
where PInis the periodic motor218input pressure, ΔPMKis the known pressure differential, NFKis the known fan speed, POis the known outlet pressure, and Nnis the periodic fan speed.

The controller262may calculate and/or estimate the periodic fan220speed as a function of the fan speed signal received from the fan speed sensor202. Since all other terms are known, the periodic motor218inlet pressure may be calculated and/or estimated by controller262. The method400proceeds from step406to step408.

In step408the controller262may determine a periodic fluid flow to the accumulator270during charging as a function of the cross sectional area of restrictive orifice290, the fluid density, a periodic accumulator270pressure, and the periodic motor218inlet pressure, as expressed in equation 16:
Qn=f(A, ρ, PAn, PIn)  equation 16
where Qn(n=1, 2, 3 . . . ) is a periodic flow rate during charging, A is the known cross sectional area of the restrictive orifice290, ρ is the density of the fluid, PAn(n=1, 2, 3 . . . ) is the periodic accumulator270pressure as indicated by the accumulator pressure signal from sensor296, and PInis the periodic motor218input pressure.

It is well known by ordinary persons skilled in the art that flow rate is equal to a constant times the area of a restriction times the square root of the quotient of the differential pressure times two and the density of the fluid. The flow rate to the accumulator270from the hydraulic fan circuit210may be substantially equal to the flow rate through the restrictive orifice290. The restrictive orifice290has a substantially constant cross sectional area which will be known by system designers. The pressure on the accumulator270side of the restrictive orifice290, PA, may be determined by the accumulator pressure signal generated by the pressure sensor296. While the discharge control valve76is in the second position, and the accumulator270is charging, the pressure on the input of the restrictive orifice290, PR, is substantially equal to the motor218inlet pressure PI. The periodic flow rate may then be calculated and/or estimated by equation 17:
Qn=C*A(√[2(PIn−PAn)/ρ])  equation 17
where Qnis a periodic flow rate during charging, C is a known constant determined through calculations from system geometry or experimentally, A is the known cross sectional area of the restrictive orifice290, ρ is the density of the fluid, PAnis the periodic accumulator270pressure, and PInis the periodic motor218input pressure.

In embodiments including the additional hydraulic circuit228, the flow rate to the accumulator270during charging from hydraulic circuit228may be calculated and/or estimated using similar methods or any other method which would be known to an ordinary person skilled in the art now or in the future. The method400proceeds from step408to step410.

Step410includes calculating and/or estimating the charging volume, VCas a function of the integration of the periodic flow rate of fluid to the accumulator270, Qn, during accumulator270charging. This yields equation 18:
VC=t1t2∫Qndtequation 18
where VCis the charging volume, Qnis the periodic flow rate through restrictive orifice290during accumulator270charging, t1 is the beginning of an accumulator270charging period, and t2 is the end of an accumulator270charging period.

The accumulator270charging volume may be calculated for each period that the hydraulic fan circuit210is operating in a mode where the accumulator270is being charged. Methods for integrating a periodic value, such as a flow rate, over a time period are well known by ordinary persons skilled in the art. These charging volumes may be added together along with any charging volumes from the other hydraulic circuit228to determine a total charging volume for a time period.

It is well known by ordinary persons skilled, in the art for a controller262to save periodic values in the memory component(s). When calculating a periodic value using other stored periodic values, the controller262may use the most recent periodic value in the calculation and/or estimation method.

Although the exemplary embodiment of the method to determine a charging volume of an accumulator270assumes periodic signals from the fan speed sensor202and the pressure sensor296, an ordinary person skilled in the art will recognize that the steps and equations in the method400may be easily adapted to use a constant or occasionally sporadic signal. A non-limiting example of a constant signal includes an analogue signal. Electronic devices to perform mathematical operations such as addition, multiplication, and integration of analogue signals are well known by ordinary persons skilled in the art.

The method400proceeds from step410to step412. The method ends at step412.

Referring back toFIG. 3, the method to determine an accumulator270fluid charge proceeds from step306to step308. Step308includes determining the discharging fluid volume of accumulator270. The discharging fluid volume of accumulator270may be the volume of fluid in the accumulator270at the end of a discharge period. An ordinary person skilled in the art will recognize that the end of a discharge period may be at any time during a discharge period that it is desired to calculate the volume of fluid in accumulator270.

Referring toFIG. 5, a flow chart of an exemplary embodiment of a method500to determine the discharging fluid volume of an accumulator270is depicted. The method500starts at step502. The method500proceeds from step502to step504. Step504includes determining the accumulator270fluid volume at the completion of the last charging period.

The controller262may store the accumulator270fluid volume at the completion of a charging period. In one embodiment the controller262may determine the accumulator270fluid volume at the completion of the last charging period by adding the accumulator270beginning fluid volume and any accumulator270charging fluid volumes. The method500proceeds from step504to step506.

Step506includes determining the accumulator270pressure at the completion of the last charging period. The controller262may determine the accumulator270pressure at the completion of the last charging period as a function of the accumulator pressure signal from the pressure sensor296, and store the value in the memory. The method500proceeds from step506to step508.

Step508includes determining the accumulator pressure at the completion of a discharge period. When the discharge control valve276is in the third position, the accumulator270may discharge. The controller262may determine the accumulator270pressure at the completion of the discharge period as a function of the accumulator pressure signal from the pressure sensor296, and store the value in the memory. The method500proceeds from step508to step510.

Step510includes determining the accumulator270fluid volume at the completion of a discharge period, VD, as a function of the accumulator270fluid volume at the completion of the last charging period, VLC, the accumulator270pressure at the completion of the last charging period, PALC, the accumulator270pressure at the completion of the discharge period, PACD, and the total accumulator270volume, VTA. This yields the function expressed in equation 19:
VD=f(VLC,PALC,PACD,VTA)  equation 19
where VDis the discharging fluid volume of accumulator270, VLCis the fluid volume of accumulator270at the completion of the last charging period, PALCis the accumulator270pressure at the completion of the last charging period, PACDis the accumulator270pressure at the completion of the discharge period, and VTAis the total volume of the accumulator270. In the depicted embodiment, the discharge period may include any period when the discharge control valve276is in the third position and the accumulator270is discharging fluid.

From knowledge of the design of the accumulator270, an ordinary person skilled in the art will know the total accumulator270volume, VTA. It is well known to ordinary persons skilled in the art that the accumulator270total volume, VTA, is equal to the sum of the volume of fluid in the accumulator270, VF, and the volume of gas in the accumulator270, VG, as represented in equation 20:
VTA=VF+VGequation 20
where VTAis the total accumulator270volume, VFis the volume of fluid in the accumulator270, and VGis the volume of gas in the accumulator270.

At the completion of a charging period, the volume of gas in the accumulator270, VGLC, will then equal the total volume of the accumulator270, VTA, less the volume of fluid in the accumulator270, VLC, at the completion of the last charging period, as represented by equation 21:
VGLC=VTA−VLCequation 21
where VGLCis the volume of gas in the accumulator270at the completion of the last charging period, VTAis the total accumulator270volume, and VLCis the volume of fluid in the accumulator270, at the completion of the last charging period.

Similarly, at the completion of a discharge period, the volume of gas in the accumulator270, VGD, will then equal the total volume of the accumulator270, VTA, less the volume of fluid in the accumulator270, VD, at the completion of the discharge period, as represented by equation 22:
VGD=VTA−VDequation 22
where VGDis the volume of gas in the accumulator270at the completion of the discharge period, VTAis the total accumulator270volume, and VDis the volume of fluid in the accumulator270, at the completion of the discharge period.

From gas laws, ordinary persons skilled in the art will know that the product of the pressure of a gas, PG1, and volume of the gas to a power, (VG1)n, at a first pressure equals the product of the pressure of the gas, PG2, and volume of the gas to a power, (VG2)n, of the gas at a second pressure, and that both equal a constant, as expressed in equation 23:
PG1*(VG1)n=PG2*(VG2)n=Kequation 23
where PG1is the pressure of a gas at a first pressure, PG2is the pressure of the gas at a second pressure, VG1is the volume of the gas at the first pressure, VG2is the volume of the gas at the second pressure, K is a constant, and n is a variable based upon the gas and other factors.

It is well known by ordinary persons skilled in the art that as long as the accumulator270is charged with fluid, the fluid pressure, PF, in the accumulator270will be equal to the gas pressure, PG, in the accumulator270. Applying this knowledge to equation 23 yields equation 24:
PF1*(VG1)n=PF2*(VG2)n=Kequation 24
where PF1is the pressure of a fluid in the accumulator270at a first gas pressure, PF2is the pressure of a fluid in the accumulator270at a second gas pressure, VG1is the volume of the gas at the first gas pressure, VG2is the volume of the gas at the second gas pressure, K is a constant, and n is a variable based upon the gas and other factors.

Equation 24 may be rearranged to yield equation 25:
(PF1/PF2)1/n*VG1=VG2equation 25
where PF1is the pressure of a fluid in the accumulator270at a first gas pressure, PF2is the pressure of a fluid in the accumulator270at a second gas pressure, VG1is the volume of the gas at the first gas pressure, VG2is the volume of the gas at the second gas pressure, and n is a variable based upon the gas and other factors.

From equation 21, the volume of gas in the accumulator270, VGLC, at the completion of the last charging period may be expressed as a function of the total volume of the accumulator270, VTA, and the volume of fluid in the accumulator270, VLC, at the completion of the last charging period. The pressure of the fluid at the completion of the last charging period, PALC, may have been stored by the controller262as explained above in relation to step506. Substituting the pressure of the fluid at the completion of the last charging period, PALC, for the pressure of a fluid in the accumulator270at a first gas pressure; and the volume of gas in the accumulator270, VGLC, at the completion of the last charging period for the volume of a gas in the accumulator270at a first gas pressure; into equation 25, yields equation 26:
(PALC/PF2)1/n*(VTA−VLC)=VG2equation 26
where PALCis the pressure of the fluid in accumulator270at the completion of the last charging period, PF2is the pressure of a fluid in the accumulator270at a second gas pressure, VTAthe total volume of the accumulator270, VLCis the volume of fluid in the accumulator270at the completion of the last charging period, VG2is the volume of the gas at the second gas pressure, and n is a variable based upon the gas and other factors.

From equation 22, the volume of gas in the accumulator270, VGD, at the completion of the discharge period may be expressed as a function of the total volume of the accumulator270, VTA, and the volume of fluid in the accumulator270, VD, at the completion of the discharge period. The pressure of the fluid at the completion of the discharge period, PACD, may be calculated and/or estimated by the controller262from the accumulator pressure signal generated by the pressure sensor296as explained above in relation to step508. Substituting the pressure of the fluid at the completion of the discharge period, PACD, for the pressure of a fluid in the accumulator270at a second gas pressure; and the volume of gas in the accumulator270, VGD, at the completion of the discharge period for the volume of a gas in the accumulator270at a second gas pressure; into equation 26, yields equation 27:
(PALC/PACD)1/n*(VTA−VLC)=(VTA−VD)  equation 27
where PALCis the pressure of the fluid in accumulator270at the completion of the last charging period, PF2is the pressure of a fluid in the accumulator270at a second gas pressure, VTAthe total volume of the accumulator270, VLCis the volume of fluid in the accumulator270at the completion of the last charging period, VDis the volume of fluid in the accumulator270at the completion of a discharge period, and n is a variable based upon the gas and other factors.

Rearranging equation 27 to solve for the volume of fluid in the accumulator270, VD, at the completion of a discharge period yields equation 28:
VD=VTA−[(PALC/PACD)1/n*(VTA−VLC)]  equation 28
where PALCis the pressure of the fluid in accumulator270at the completion of the last charging period, PF2is the pressure of a fluid in the accumulator270at a second gas pressure, VTAthe total volume of the accumulator270, VLCis the volume of fluid in the accumulator270at the completion of the last charging period, VDis the volume of fluid in the accumulator270at the completion of a discharge period, and n is a variable based upon the gas and other factors.

The method500proceeds from step510to step512. The method500ends at step512.

Referring back toFIG. 3, the method300proceeds from step308to step310. At step310, the controller262may determine the current volume of fluid in accumulator270, VA, as a function of the beginning volume of fluid in accumulator270, VB, the charging volume of fluid in accumulator270, VC, and the discharging volume of fluid in accumulator270, VD. The current volume of fluid in accumulator270may be expressed by equation 29:
VA=f(VB, VC, VD)  equation 29
Where VAis the current volume of fluid in accumulator270, VBis the beginning volume of fluid in accumulator270, VCis the charging volume of fluid in accumulator270, and VDis the discharging volume of fluid in accumulator270.

If the accumulator270is charging, the current fluid volume in accumulator270, VA, may be calculated and/or estimated by the controller262through adding the charging volume, VC, to the beginning volume, VB. The beginning volume, VB, may be calculated and/or estimated through the method described in relation to the start-up of system200, or if the accumulator270has discharged prior to the current charging period, through the calculation of the volume of fluid in accumulator270at the end of the last discharge period, VD, as would be known by an ordinary person skilled in the art now or in the future.

If the accumulator270is discharging, the current fluid volume in accumulator270, VA, may be calculated and/or estimated by the controller262through calculating the discharge volume, VD. The method300proceeds from step310to step312.

Step312includes determining the accumulator270pressure, PA. The accumulator pressure, PA, may be determined by controller262through the accumulator pressure signal generated by the pressure sensor296. The method300proceeds from step312to step314.

Step314includes determining the fluid charge of the accumulator270as a function of the current fluid volume of accumulator270, VA, and the accumulator pressure, PA.

It is well known by ordinary persons skilled in the art that hydraulic energy, F, is equal to fluid volume, VF, multiplied by fluid pressure, PF, as expressed by equation 30:
E=PF*VFequation 30
where E is hydraulic energy, PFis fluid pressure, and VFis fluid volume.

Applying equation 30, the controller262may calculate and/or estimate the fluid energy in the accumulator270, AFE, by multiplying the accumulator270pressure, PA, by the current fluid volume in accumulator270, VA, as expressed in equation 31:
AFE=PA*VAequation 30
where AFEis the fluid energy in accumulator270, PAis the accumulator270pressure, and VAis the current fluid volume in accumulator270. The method300proceeds from step314to step316. The method300ends at step316.

Ordinary persons skilled in the art now or in the future will recognize that the calculations, estimations, and equations in the above described methods may be modified to take into account system losses and system geometry and design as would be known by ordinary persons skilled the art now or in the future.

From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications or variations may be made without deviating from the spirit or scope of inventive features claimed herein. Other embodiments will be apparent to those skilled in the art from consideration of the specification and figures and practice of the arrangements disclosed herein. It is intended that the specification and disclosed examples be considered as exemplary only, with a true inventive scope and spirit being indicated by the following claims and their equivalents.