Patent Description:
Excavators are an example of construction machines that use multiple hydraulic actuators to accomplish a variety of tasks. These actuators are fluidly connected to a pump that provides pressurized fluid to chambers within the actuators. This pressurized fluid force acting on the actuator surface causes movement of actuators and connected work tool. During operation of an excavator, the implement or load may be raised to an elevated position at which the implement gains potential energy. As the implement is released from the elevated position, the potential energy may be converted to heat when pressurized hydraulic fluid is forced out of the hydraulic actuator and is throttled across a hydraulic valve and returned to a tank. Recovering the wasted potential energy for reuse will improve the efficiency of the excavator. As the excavator starts to work, the boom cylinder piston can expand and contract twice during a work period as well as the arm cylinder and the bucket cylinder. Based on an analysis, the excess energy of the boom system accounts for around <NUM>% of input energy among the three cylinder systems: boom, arm, and bucket cylinder systems. There remains a need in the art for a system that recovers the energy in a cost effective and efficient manner.

<CIT> discloses a working device energy recovery system, including energy converters, a pressure energy retaining valve, a switchover valve, an energy collector, a logical control member, a main valve, a boom and corresponding connections. With the system, dropping potential energy of the boom can be controllably converted to hydraulic energy to be stored in the energy collector, and the potential energy can also be directly recycled to do work on the boom in a balancing manner.

<CIT> discloses a hydraulic unit adapted for connection to master and slave actuator systems including three valves, the first configured for selective fluid passage between cap ends, the second configured for selective fluid passage between a slave cap end and an accumulator, and the third fluidly coupled for selective fluid passage between each of a single pump and the accumulator, and the slave cap end. During actuator retraction, the valves permit pressurized fluid in the slave cap end to be delivered to accumulator for storage; during extension, the valves permit pressurized fluid from pump and accumulator to be delivered to the slave cap end.

<CIT> discloses a hydraulic system which includes a hydraulic actuator, an accumulator, an accumulator charge valve, and a hydraulic transformer fluidly connected between the accumulator charge valve and the accumulator. The hydraulic transformer includes a transformer motor mechanically coupled to a transformer pump. The accumulator charge valve is fluidly connected between the transformer motor and the hydraulic actuator. The transformer pump is sized to permit a maximum flow therethrough of no more than three-quarters of a flow permitted through the transformer motor.

<CIT> discloses a hydraulic system which shares a flow between a hydraulic transformer and one or more hydraulic pumps. The hydraulic transformer is in selective fluid communication with the pumps and actuates a first load. A second load is driven by an actuator in selective fluid communication with the pumps and the hydraulic transformer. The hydraulic system includes a controller to reduce dynamic responses in the system by causing flow sharing between the hydraulic transformer and a directional flow control valve.

The following documents may also provide technical background to the present disclosure: <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>.

The present invention is directed to a hydraulic system that recovers and stores energy and reuses the energy to power system components, thereby reducing the power demand on the engine and enabling the engine to be reduced in size.

The present invention provides a hydraulic system for a mobile construction vehicle as defined in the appended claims.

According to a first embodiment the present invention provides, a hydraulic system for a mobile construction vehicle including a variable displacement track motor configured to be coupled to a track of the mobile construction vehicle to drive the track; an accumulator for storing pressurized hydraulic fluid for use as a power supply to a non-track load implement; a pump dedicated to the track motor; and control valving that is operable between a first position at which the control valving directs hydraulic fluid from the dedicated pump to the variable displacement track motor to drive the variable displacement track motor, and a second position at which the control valving directs hydraulic fluid from the dedicated pump to the accumulator.

The control valving may include a proportional valve that diverts flow from the track motor to the accumulator in a proportional manner.

The control valving may be configured such that when the control valving is not operating in the first position to direct hydraulic fluid to the track motor the control valving is operating in the second position to direct hydraulic fluid to the accumulator.

The non-track load implement may include a swing motor for driving a swing of the mobile construction vehicle, and the accumulator may be configured to provide the stored pressurized hydraulic fluid to the swing motor to drive the swing motor.

The non-track load implement may include a swing motor for driving a swing of the mobile construction vehicle, and in the second position the control valving may direct hydraulic fluid from the pump to the swing motor to drive the swing motor.

According to a second embodiment the present invention provides, a hydraulic system for storing pressurized hydraulic fluid from a pump of a mobile construction vehicle and using the stored hydraulic fluid to power a track motor of the mobile construction vehicle, the hydraulic system including an accumulator configured to be coupled to the pump to receive and store the pressurized hydraulic fluid from the pump; and control valving that is operable between a first position at which the control valving directs hydraulic fluid from the pump to the accumulator to charge the accumulator, and a second position at which the control valving directs hydraulic fluid from the accumulator to the track motor to power the track motor.

The hydraulic system may further include the track motor, and the track motor may be a bidirectional overcenter track motor.

The accumulator may be stored within the track.

The control valving may include a proportional valve between the accumulator and the track motor that is configured, when the accumulator is pressurized with hydraulic fluid, to open to allow the accumulator to provide the pressurized hydraulic fluid to the track motor to drive the track motor.

The control valving may include a directional valve that, when the control valving is in the second position, the directional valve directs hydraulic fluid from the pump to the track motor to assist the accumulator in driving the track motor.

When the accumulator is depleted of pressurized hydraulic fluid the directional valve may continue to direct hydraulic fluid from the pump to the track motor to drive the track motor without the accumulator.

Embodiments of this invention will now be described in further detail with reference to the accompanying drawings, in which:.

The hydraulic system of the claimed invention is shown in <FIG> and <FIG> of the drawings.

While the present invention can take many different forms, for the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. Any alterations and further modifications of the described embodiments, and any further applications of the principles of the invention as described herein, are contemplated as would normally occur to one skilled in the art to which the invention relates.

This invention relates generally to a hydraulic system that provides energy recovery for a machine such as shown in <FIG>. Although not limited to such, the illustrated machine <NUM> is an excavator that includes a work implement <NUM> that may include a boom <NUM>, a stick <NUM>, and a bucket <NUM>. Operations performed by the implement <NUM> may include, for example, lifting, lowering, and otherwise moving a load (not shown). While the hydraulic system and method is illustrated and described in connection with an excavator, the system and method disclosed herein has universal applicability in various other types of machines as well. The term "machine" may refer to any machine with a hydraulically powered work implement 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 <NUM> may be an earth-moving machine, such as a wheel loader, excavator, dump truck, backhoe, motor grader, material handler or the like. The implement <NUM> may be moved to perform its various functions by one or more hydraulic actuators <NUM> that may be connected between the machine frame and the moving parts of the implement. In the illustrated embodiment, two hydraulic actuators <NUM> (the first actuator designated <NUM> and the second actuator designated <NUM>) are provided with each being configured as a double acting hydraulic cylinder with a housing <NUM> and a piston <NUM>. A swing drive <NUM> rotates the machine frame relative to the undercarriage, which is equipped with wheels or tracks <NUM> to move the excavator.

<FIG> illustrates a configuration of energy recovery where the gravitational potential energy of a mass is recovered. During operation the boom will be extended to complete useful work. When the mass is lowered, flow will be generated on the piston side of the cylinder <NUM> and can be recovered to some sort of device such as an accumulator <NUM> or motor <NUM>. In the typical situation the fluid is exhausted from the piston side through a directional control valve <NUM> to a tank <NUM> and all of the energy is dissipated through the directional control valve <NUM>. In the typical baseline situation the directional control valve is used to create a specific orifice size to create a controlled amount of metering to result in a controlled descent. However, in <FIG> either an accumulator <NUM>, a motor <NUM>, or a combination of these two may be used to create the required back pressure for the controlled descent of the boom cylinder <NUM>.

The basic control strategy for recovering energy and generating the required back pressure will differ between the accumulator <NUM> and the pump <NUM>. When using the accumulator <NUM> to control the pressure of the piston side on the boom cylinder <NUM>, a proportional valve may be used to generate a pressure drop between the pressure in the accumulator and pressure desired in the piston chamber of the boom cylinder <NUM>. The metering orifice size may be based on the desired or actual speed of descent of the cylinder as well as for example the current pressure in the accumulator and the desired pressure in the piston chamber of the cylinder which is a function of the mass of the cylinder as well as the pressure on the rod side. When using the pump <NUM> for energy recovery the speed may be controlled by the amount of fluid consumed by the pump. The amount of fluid consumed by the pump can be altered by either changing the speed of the pump (the engine in this configuration) or adjusting the displacement of the pump. Adjusting the displacement of the pump can be accomplished via a variable displacement pump and adjusting the speed of the pump could be accomplished via some system that decouples the speed of the pump from the prime mover such as an EHA (Electro-Hydrostatic Actuator) system.

<FIG> illustrates a configuration where multiple actuators can be connected to the same pressure source for both powering and recovery. In <FIG>, two linear actuators are illustrated <NUM>, <NUM>, but one of those actuators could also be a motor such as one for the swing drive <NUM>. A series of logic elements 40a, 40b may be used to control the directionality of the fluid from the pump <NUM> to the work ports to minimize metering losses. To ensure proper distribution of the flow between the actuators <NUM>, <NUM>, proportional valves 42a, 42b may be used to create the proper pressure drops. Position or speed sensors (not shown) on the actuators <NUM>, <NUM> which can be used as a measurement of flow, together with the pressure sensor <NUM> at the pump <NUM> and a detailed understanding of the flow characteristics of the proportional valves 42a, 42b, the pressures and flows throughout the circuit can then be known. These control inputs will be sufficient to control the speed of each actuator <NUM>, <NUM>. This system offers an advantage over a typical system as minimal amount of metering can be applied to flow directed towards the higher pressure actuator and this will decrease the amount of metering required to other functions as well as the overall pressure.

The system developed in <FIG> is particularly suited to powering a function, but is not particularly suited to recovering energy, as only the pump <NUM> is available to absorb the flow. In an alternative embodiment, an accumulator may be provided that is used to recover energy as described above with the description of <FIG>. To the desired speeds of the cylinders the proportional valves will have to meter the flow from the cylinder back to the pump or accumulator to ensure the proper flow rate from each is maintained.

The proportional valves 40a, 40b illustrated in <FIG> can be controlled to obtain the desired flow to each actuator <NUM>, <NUM>. The proportional valves 42a, 42b should be large enough to handle the flow and to react to sudden changes in pressure. In addition, using an electronically controlled proportional valve may result in sluggishness on the actual machine due to delays in response from the valve and sensors.

<FIG> illustrates a configuration where compensators 46a, 46b are used to control the back pressure from the cylinders <NUM>, <NUM> and ensure the pressure from each actuator is the same when it reaches the main pressure line where the pump <NUM> and accumulator <NUM> are located for energy recovery purposes. The pressure compensators 46a, 46b illustrated in <FIG> are controlled passively by the upstream flow coming from the actuators <NUM>, <NUM> and the downstream flow headed towards either a directional control valve or the pump <NUM> and accumulator <NUM>. The upstream pressure causes the compensator 46a, 46b to open and the downstream pressure causes the compensator to close along with a spring; this arrangement of actuation forces will cause the compensator to maintain a pressure drop and ensure a consistent meter out pressure from all of the functions. Proportional valve 43a is selectively connectable to the tank and proportional valve 43b is selectively connectable to the accumulator <NUM>. When combined with a four position proportional spool valve (not shown) this will allow a controlled descent of the cylinders.

<FIG> illustrates an alternative arrangement where the upstream pressure attempts to the open pressure compensators 46a, 46b but a controlled pressure is used in place of the downstream flow to close the compensator. The externally controlled pressure from pump <NUM> will allow each compensator 46a, 46b to have a different pressure from each function to the main pressure line, but also allow each function to travel at different rates. A pressure reducing valve could be used to generate the controlled pressure, but other means may be possible. In some situations, it may be desirable to be able to regulate the pressure in both directions.

The typical speed for the swing function is less than half of the maximum speed which means that the typical flow is less than half of possible maximum flow. The speed of the swing drive <NUM> may be directly coupled with the amount of high pressure flow when powering the function as well as the amount of high pressure flow exiting the swing drive <NUM> when braking. Therefore, if a pump is sized to be able to provide the maximum amount of flow required it will be typically oversized to provide the average amount of flow required, which may potentially lead to inefficient operation due to the properties of variable displacement pumps (which is a typical method for providing flow to the swing motor). However, utilizing more than one pump such as illustrated in <FIG> may improve the efficiency of the swing drive <NUM> as during low speed for example, and therefore low flow operations. Only one pump may be utilized unloading the second pump <NUM>, and therefore a high displacement and efficiency can be obtained. This stems from the fact that the leakage of a pump is typically directly related to the pressure the pump is operating and therefore at lower displacements the leakage contributes a higher percentage to the overall pumps theoretical output. This will also assist in improving efficiency when recovering high pressure flow as well. <FIG> illustrates a configuration where two variable displacement pumps <NUM>, <NUM> are used and <FIG> illustrates a configuration where one variable displacement pump <NUM> and one fixed displacement pump <NUM> are used. Other pump combinations are possible and the selection of these combinations depends on the desired efficiency, controllability, cost, etc. Control complexities may be required to ensure smooth transitions between one pump and two pumps, but utilizing two pumps offers other possibilities for combining other functions such as boom recovery and track. While using this on the swing drive <NUM> may make the most sense efficiency wise because of the operating modes, it can also be applied to other functions, multi-functions, using over center units for recovery, and even with an accumulator in parallel.

<FIG> illustrates a system where two variable displacement pumps <NUM>, <NUM> are installed for the swing function, but is also connected in such a manner that the boom potential energy can be recovered. The energy from the boom function or the swing function can be stored in the accumulator <NUM> or recovered to the pump-motor <NUM> and sent back to the engine shaft to be utilized immediately. The back pressure on the boom function can be controlled either by a proportional valve <NUM> or by a combination of a proportional valve and the pump-motor. One problem that may arise is the difference in pressure between the braking of the swing drive and the boom down (descent) pressure of the boom; this difference in pressure may result in inefficient recovery and poor control and performance of the swing and boom function. Embodiments described later herein discuss how to correct the difference in pressures between the various functions.

To improve fuel efficiency and reduce cost, the engine size may be reduced by load leveling or peak shaving. This in turn means that an engine can be downsized as less peak power is required; an energy storage device can provide bursts of power when required to meet the power demand and performance requirements. The techniques discussed so far and that will be de discussed hereafter load level and shave the peaks from the boom, arm, bucket, and swing functions on the excavator.

On a baseline excavator the track function is controlled by variable displacement motors and the fluid is supplied from the variable displacement pumps. On a typical excavator the track motors are capable of varying their displacements but only to a limited number of discrete positions; typically two positions. The track function is connected to supply flows at higher pressures so combinations of speeds and torques can be obtained for moving at the required speeds or climbing slopes. However, newer solutions are moving to an individualized approach for each function and therefore there may be a dedicated pump for the track function; and often these pumps <NUM>, <NUM> are directly connected to the engine M and may be constantly spinning. The track function on an excavator is typically used sparingly and therefore pumps installed just for the track function may be churning constantly and wasting energy. There are a number of ways to minimize this churning loss such as clutching the pumps out when not required or combining the track pumps with other functions.

<FIG> shows a hydraulic system that more efficiently implements the track function. The <FIG> system includes the engine M, the two variable displacement pumps <NUM>, <NUM>, two valves 58a, 58b, two track motors 60a, 60b, the swing function, the swing accumulator <NUM> and a proportional valve <NUM> between the accumulator <NUM> and the swing. The pumps <NUM>, <NUM> are dedicated to power the track motors 60a, 60b while a swing pump is dedicated to power the swing motor, although as described in greater detail below, in an alternate embodiment the pumps <NUM>, <NUM> can power both the track motors 60a, 60b and the swing motor, and/or power other functions such as the boom, among others. Further, as noted herein the pumps <NUM>, <NUM> can also be made available for boom down (descent) recovery. The energy recovery can be accomplished via either recovery to the accumulator <NUM> or recovery directly to the engine shaft. As the pumps <NUM>, <NUM> can be used for three different functions the use of the pumps <NUM>, <NUM> for one of those functions is reduced and therefore the perceived waste of energy is reduced. The variable nature of both the pumps <NUM>, <NUM> and the motors 60a, 60b can allow the excavator <NUM> to be more able to provide the speed and torque required.

The valves 58a, 58b of the system illustrated in <FIG> are proportional valves 58a, 58b, although the system need not be limited as such. It will be appreciated that the valves 58a, 58b can alternatively be digital valves; that is, the valves 58a, 58b can be capable of diverting flow from the variable displacement pumps <NUM>, <NUM> in either a proportional manner or a digital manner. The track motors 60a, 60b of the <FIG> system are fully variable displacement motors. The variable displacement nature of the motors 60a, 60b enables the motors to capture more of the operating range from which the efficiency and the torque requirement on the tracks can be increased. The track system may function similar to a hydrostatic mechanism.

Referring to <FIG>, the engine M drives the variable displacement pumps <NUM>, <NUM>, which draw hydraulic fluid from the tank. Referring to the right box flow pattern of the proportional valve 58a and the left box flow pattern of the proportional valve 58b, hydraulic fluid from the pumps <NUM>, <NUM> can be routed to the respective variable displacement track motors 60a, 60b to power the track motors 60a, 60b. With the proportional valve <NUM> at the top of <FIG> open, swing braking can be used to charge the swing accumulator <NUM>. Referring to the left box flow pattern of the proportional valve 58a and the right box flow pattern of the proportional valve 58b, hydraulic fluid from the pumps <NUM>, <NUM> can be diverted from the track motors 60a, 60b to the swing circuit. The pumps <NUM>, <NUM> are therefore no longer driving the track motors 60a, 60b. With the proportional valve <NUM> open, and with the connection to the swing movement disabled, the hydraulic fluid from the pumps <NUM>, <NUM> can be routed to the swing accumulator <NUM> to charge the swing accumulator <NUM>. This can be done for example to charge the accumulator <NUM> before, after, or as the swing braking charges the accumulator <NUM>. As will be appreciated, the swing accumulator <NUM> can be charged by the pumps <NUM>, <NUM> whenever there is available engine power. As will also be appreciated, the pumps <NUM>, <NUM> need not be idle when not driving the track motors 60a, 60b, and can instead be used to charge the accumulator <NUM>.

In the <FIG> hydraulic system, the swing function has its own pump to drive the swing motor, it being part of a pump controlled actuation architecture that dedicates pumps to respective functions. In an alternate embodiment, the dedicated swing pump can be omitted and the swing motor can instead be powered by the track function pumps <NUM>, <NUM>. It will be appreciated, of course, that the pumps <NUM>, <NUM> can also be prime movers that power still other functions such as the boom, among others. With respect to <FIG>, with the proportional valves 58a, 58b diverting hydraulic flow to the swing function, and with the proportional valve <NUM> choked off, the variable displacement pumps <NUM>, <NUM> can be used to power the swing motor. As will be appreciated, this can be an efficient use of the pumps <NUM>, <NUM> since it is rare that the swing function is used simultaneously as the track function is used. And in the rare event that the swing function is necessary or desired when the pumps <NUM>, <NUM> are powering the tracks, with the proportional valve <NUM> open, the swing accumulator <NUM> can provide power to the swing motor, at least for example to make minor adjustments in the swing movement. Of course, this embodiment also enables the pumps <NUM>, <NUM> to power both the track motors 60a, 60b and the swing motor at the same time, for example, where the proportional valve <NUM> at the top of <FIG> is choked off and the proportional valves 58a, 58b direct flow to both the track function and the swing function. To lower the power demand on the pumps <NUM>, <NUM> and consequently the engine size requirements as discussed above, the accumulator <NUM> can assist the pumps <NUM>, <NUM>. With the proportional valve <NUM> open, the pumps <NUM>, <NUM> and the accumulator <NUM> can power the swing function. In one mode, the accumulator <NUM> can power the swing motor, i.e. without the pumps <NUM>, <NUM>. In an intermediate mode, as the accumulator <NUM> starts to deplete then both the accumulator <NUM> and the pumps <NUM>, <NUM> (or one of the pumps <NUM>, <NUM> if desired) can power the swing motor, where the proportional valve <NUM> equalizes the pressure between the accumulator <NUM> and the pumps <NUM>, <NUM>. As the accumulator <NUM> continues to deplete in pressure, the pumps <NUM>, <NUM> can gradually provide a greater amount of power. Where the accumulator <NUM> is finally depleted, then in a third operating mode the proportional valve <NUM> can be choked off and the pumps <NUM>, <NUM> take over in the swing movement, in which case there is more of a flow controlled actuation than a pressure controlled actuation.

In another embodiment, the hydraulic system does not necessarily have to be tied to a pump controlled actuation architecture, and instead a conventional prime mover system can be used, for example, where two prime mover pumps power all of the functions. In such a system, the pumps could provide hydraulic fluid through the track spool of conventional excavator control valves (instead of diverting valves) that, in turn, route the hydraulic fluid to the variable displacement track motors 60a, 60b to drive the tracks. The variable displacement motors 60a, 60b would allow more efficient use of the hydraulic flow from the pumps even in such a conventional prime mover system.

<FIG> shows a hydraulic system similar to that of <FIG>, except further including track accumulators 30a, 30b for energy storage and re-use on demand. The components of the <FIG> system are in many respects substantially the same as the above-referenced <FIG> system, and consequently the same reference numerals are used to denote structures corresponding to similar structures. The foregoing description of the hydraulic system of <FIG> is equally applicable to the <FIG> hydraulic system, except as may be noted herein. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the <FIG> and <FIG> hydraulic systems may be substituted for one another or used in conjunction with one another where applicable.

The <FIG> hydraulic system includes the engine M, the two variable displacement pumps <NUM>, <NUM>, two check valves, two excavator control valves, two bidirectional over-center track motors 60a, 60b, the two track accumulators 30a, 30b, the proportional valves between the track accumulators 30a, 30b and the track motors 60a, <NUM>, the swing function, the swing accumulator <NUM>, and the proportional valve <NUM> between the accumulator <NUM> and the swing function. The track accumulators 30a, 30b and their respective proportional valves are disposed inside the tracks themselves, i.e. in the space within the tracks. As described in greater detail below, the accumulators 30a, 30b can provide an extra power source for the track motors 60a, 60b. The accumulators 30a, 30b can be charged for example when the prime mover pumps <NUM>, <NUM> are not being otherwise utilized, and this stored energy in the accumulators 30a, 30b can allow for less power demand from the track functions for a period of time until the accumulators 30a, 30b are depleted. With an appropriate duty cycle, the <FIG> system would enable downsizing of the engine.

Referring to the circuit of <FIG>, and more particularly to the center box flow pattern of the excavator control valves, hydraulic fluid from the pumps <NUM>, <NUM> is routed to the swing circuit. As with the <FIG> system, the pumps <NUM>, <NUM> can charge the swing accumulator <NUM>, power a swing motor where a swing pump is omitted, among other functions referred to with respect to the <FIG> embodiment. Referring to the left box flow pattern of the left excavator control valve and the right box flow pattern of the right excavator control valve, hydraulic fluid from the pumps <NUM>, <NUM> can be diverted from the swing function to the track motors 60a, 60b via the respective check valves to power the respective track motors 60a, 60b, for example in a forward direction. With the proportional valves between the track motors 60a, 60b and track accumulators 30a, 30b closed, hydraulic fluid downstream of the track motors 60a, 60b is routed to the tank. Referring to the right box flow pattern of the left excavator control valve and the left box flow pattern of the right excavator control valve, hydraulic fluid from the pumps <NUM>, <NUM> can be diverted from the swing function to the track motors 60a, 60b via the respective check valves to power the respective track motors 60a, 60b, for example in a reverse direction. With the proportional valves between the track motors 60a, 60b and track accumulators 30a, 30b open, and the track motors 60a, 60b operated at for example zero percent displacement, the pumps <NUM>, <NUM> can provide pressurized hydraulic flow to the respective track accumulators 30a, 30b, to charge the track accumulators 30a, 30b. The pumps <NUM>, <NUM> can charge the accumulators 30a, 30b until the accumulators <NUM>, <NUM> are charged to a predetermined pressure and/or until a not-shown relief valve opens.

As will be appreciated, the track accumulators 30a, 30b can be charged any time the tracks are not being used. When the tracks are used, i.e. powered by the track pumps <NUM>, <NUM>, and with the proportional valves between the track motors 60a, 60b and track accumulators 30a, 30b open, the charged accumulators 30a, 30b can serve as a boost system to provide additional power to the pumps <NUM>, <NUM> to drive the track motors 60a, 60b, for an amount of time time until the accumulators 30a, 30b are depleted. The accumulators 30a, 30b can thus aid the pumps <NUM>, <NUM> in driving the track function, thus reducing the power demand on the engine and, accordingly, enabling the size of the engine to be reduced if desired. As noted above, with a proper duty cycle the engine size can be downsized with little or no compromise to performance or functionality. A proper duty cycle can consider for example passively charging the accumulators 30a, 30b at any period of time where there is available engine power. Of course, in instances where the amount of time to deplete the accumulators 30a, 30b is exceeded, the reduced size engine would provide a decreased amount of movement power to the pumps <NUM>, <NUM> until the accumulators 30a, 30b are recharged sufficiently to power-assist the track motors 60a, 60b, although due to the variable displacement nature of the track motors 60a, 60b the performance decrease will be less significant than a stock system which utilizes motors that can only be in two different displacement modes.

In the illustrated embodiment, the track motors 60a, 60b are overcenter motors and, as such, the motors can travel in both directions; that is, the track motors 60a, 60b enable the vehicle to move forward or backwards. The <FIG> system need not be limited as such and other embodiments are contemplated. In an alternative embodiment, for example, a selector valve can be employed to route hydraulic fluid to either side of the motor, as will be appreciated.

The several embodiments herein enable energy recovery and pressure leveling. As discussed before, the braking pressure of the swing and the boom down pressure may be very different. For example, the braking pressure of the swing drive may be approximately <NUM> bar, while the boom down piston side pressure may vary between <NUM> bar and <NUM> bar. However, as will be appreciated the boom down pressure can vary well outside this range. On the acceleration side the swing drive <NUM> can accelerate around for example <NUM> bar, in line with the brake pressure, but the pressure required to raise the boom may be related to the load and vary dramatically; and in some cases can be quite high. In terms of flow, the swing drive <NUM> may exhibit a flow rate of for example approximately <NUM> - <NUM> liters per minute, while the boom function may exhibit flow rates of for example <NUM> liters per minute. As will be appreciated, in terms of efficiency for hydraulic machines, high flows and low pressures are typically less efficient than low flows and high pressures. The method described herein efficiently increases the pressure of the boom flow and decreases the flow rate to bring it more in line with that of the swing drive <NUM>, as well as works in the "sweet-spot" of hydraulic equipment.

<FIG> illustrates a system where a proportional valve <NUM> can selectively connect together the piston side and rod side of a cylinder during either a boom up or boom down operation. The piston side of the boom cylinder <NUM> may be connected to the proportional valve <NUM> as shown as well as to a means to recover energy such as the as-shown motor <NUM> and/or an accumulator and/or a system such as that described in <FIG>, <FIG> or <FIG>. Although it is shown as a single spool valve, this can be applied in an independent metering fashion as well with individual valves for each connection. When the proportional valve <NUM> is fully opened the pressure on the rod side and the piston side will be the same and effectively the rod area on the piston side will be supporting the weight of the cylinder and its load. The weight of the cylinder and its load will cause it to descend. The amount of flow leaving the cylinder will be equal to the speed of the cylinder descending multiplied by the rod area, but at a greatly increased pressure as the effective area supporting the load is greatly reduced. This system is able to increase the pressure and reduce the flow which will allow the hydraulic equipment to more efficiently capture the energy. An added control feature of this system is using the proportional valve <NUM> to limit the pressure on the rod side of the chamber by restricting the flow through the proportional valve <NUM>. This will decrease the pressure in the rod side of the cylinder as well as decrease the pressure on the piston side of the chamber. No additional flow is required to "back-fill" the rod side of the cylinders as this is accomplished in the natural lowering process. In instances where the boom cylinders <NUM> are not powered down, to prevent any discontinuity in feel when approaching the ground the "regen" system can be disconnected so the boom <NUM> can be powered into the ground for digging operations. One method that can be used to accomplish this is to have an optical, proximity, auditory, or some other type sensor to detect the oncoming ground and pre-emptively power the rod side while disabling regen. Another method, which will be described in greater detail below is to use a passive circuit using accumulator pressure as a standby pressure that does not require pump power to have at standby. This "regen style" circuit can provide a two-fold benefit. For example, the amount of flow required when lowering the boom can be reduced or eliminated and this may decrease the amount of installed flow required; i.e. the pump size can be potentially reduced. Secondly, the pressure filling the rod side of the cylinder is typically low, but a high flow is required, which is a poor operating point for a pump; by eliminating this flow requirement less flow will be metered and the pump will not operate as much at an inefficient point.

The system in <FIG> behaves similarly to the "regen style" circuit in that it is designed to boost the pressure and reduce the flow returning from the boom pistons. This can increase the efficiency of the process if a pump is used for recovery and/or it can shrink the size of the required accumulator <NUM> if an accumulator method is used. Using a single cylinder <NUM> to recover halves the effective area, doubling the recovered pressure. A metering valve <NUM> can be used with the accumulator <NUM> since the pressures are not separated, and the accumulator pressure may need to match a common braking pressure.

<FIG> is a system similar to the <FIG> system except the <FIG> system includes a proportional valve <NUM> and a control valve <NUM> having different flow paths. Referring to the right box flow patterns of the control valve <NUM>, as the boom is lowered hydraulic fluid from the piston side of the actuator <NUM> is routed to the accumulator <NUM> and fluid from the piston side of the actuator <NUM> is routed to the proportional valve <NUM> and then to the rod sides of the cylinders <NUM>, <NUM>. In the case of zero input energy and all gravity driven, i.e. where it is desired to have all of the boom potential energy recovered, the pump <NUM> provides no input and the proportional valve <NUM> is fully open, so that the fluid pressure from the piston side of the left side cylinder <NUM> powers both of the rod sides of the cylinders <NUM>, <NUM>, and the fluid from the piston side of the right side cylinder <NUM> is entirely taken up by the accumulator <NUM>. As will be appreciated, the potential energy stored in the accumulator <NUM> comes from only the piston side of the right side cylinder <NUM>. The system thus employs uneven loading wherein, instead of two cylinders <NUM>, <NUM> resisting the load from the boom dropping, only the right side cylinder <NUM> provides such resistance. The right side cylinder <NUM> has double the amount of force and thus can create double the amount of pressure. In this sense, using the single cylinder <NUM> instead of two cylinders <NUM>, <NUM> is a form of hydraulic pressure transformer in that the single cylinder <NUM> provides half the flow but results in double the pressure.

Of course, there may be cases where it is desirable that the pump <NUM> provide input energy or a purely gravity driven drop of the boom is not desired, such that it is not possible to recover all of the boom potential energy. Still referring to the right box flow patterns of the control valve <NUM>, if an operator command is to drop the boom faster than what can be provided by gravity then the pump <NUM> can be used to add flow to aid in the dropping rate. With the proportional valve <NUM> fully open, the pump <NUM> provides pump flow through the proportional valve <NUM> to the rod sides of the cylinders <NUM>, <NUM> and to the piston side of the left side cylinder <NUM> to thereby urge the boom to drop faster. This can also facilitate smoother transition for powering into the ground. If the operator command is to power into the ground upon the boom hitting ground, the pump <NUM> can provide pump flow to the rod sides of the cylinders <NUM>, <NUM> prior to hitting the ground, so that the boom will have standby power to power into the ground. If the operator command is to lower the dropping rate of the boom, the proportional valve <NUM> can be choked as desired to effectively create more resistance to the rod side areas, thereby slowing down the rate of fall of the pistons and thus the rate of drop of the boom. With the standby pressure on the pump <NUM>, once the boom hits the ground the proportional valve <NUM> can be fully opened and digging can be started immediately. Of course, if the operator command is to slow the boom drop rate even further, the pump flow can be reduced accordingly, or to zero, and the proportional valve further choked.

Referring now to the left box flow patterns of the control valve <NUM>, to raise the boom, the pump <NUM> as well as the stored energy in the accumulator <NUM> pressure both of the piston sides of the actuators <NUM>, <NUM>. The accumulator <NUM> adds flow to the flow of the pump <NUM> at the same pressure. The proportional valve <NUM> at the accumulator <NUM> can equalize the pressure between the accumulator <NUM> and the pump <NUM>. As the accumulator <NUM> starts to deplete, the pump <NUM> can provide greater flow. The accumulator <NUM> can provide flow as it depletes until it meets a certain pressure for example the pressure required to actuate the boom. Once the accumulator <NUM> reaches such pressure, power can no longer be drawn from the accumulator <NUM>. As such, the proportional valve <NUM> can be choked off and drive can be provided from the pump <NUM>.

The hydraulic systems of <FIG> and <FIG> are preferably configured such that their actuators <NUM>, <NUM> are oriented vertically or generally close to vertical, rather than horizontally. As will be appreciated, such vertical orientation will more effectively gravity-assist the lowering of the boom. Of course, linkages could be provided to convert any horizontal movement to a more generally vertical movement.

Similar to <FIG> and <FIG> where an accumulator is used to recover from only one cylinder <NUM>, <FIG> shows powering from the accumulator <NUM> with only one cylinder as a way to re-use the energy. The second cylinder <NUM> would be powered by the stock pumps, which can be pump <NUM> in some embodiments, with the accumulator <NUM> to supplement, reducing the required input energy. Each cylinder <NUM>, <NUM> could have a different pressure, easing the valving setup and increasing the efficiency so long as the boom structure <NUM> is configured to withstand this differential force. This would ease controllability and allow use of the captured energy in an efficient manner. Initially as the pressure in the accumulator <NUM> is quite high, the amount of force generated by the other cylinder <NUM> would be relatively low. However, as the accumulator <NUM> assists in raising the boom <NUM>, the pressure would begin to decrease and therefore the stock pumps would need to provide additional pressure. If only a small amount of force is required from the non - accumulator cylinder <NUM> then potentially the cylinder <NUM> could operate in a regen type scenario where the amount of flow is reduced, but the pressure is increased. In the <FIG> configuration, the metering valve <NUM> (see <FIG>) to the accumulator <NUM> is removed, reducing the amount of losses that get implemented. This is available because the pressures are separated, and therefore, the accumulator <NUM> pressure does not need to match a common braking pressure. The decision to use regen or not can be based on for example the operating point and efficiency of the pumps. Re-using energy in this manner can reduce the amount of flow demand from the pump <NUM> as the accumulator <NUM> is able to provide up to half of the required flow. This results in less power from the pump <NUM> and allows a downsizing of the unit.

In the hydraulic systems of <FIG> and <FIG>, during boom raising the control valve <NUM> directs hydraulic fluid from the accumulator <NUM> to both the actuators <NUM>, <NUM> to power the actuators <NUM>, <NUM> to raise the boom. In the hydraulic system of <FIG>, during boom raising the control valve <NUM> directs hydraulic fluid from the accumulator <NUM> to the right side (as shown in <FIG>) actuator <NUM> to power the actuator <NUM> to raise the boom. With the <FIG> system, the control valve <NUM> can also, or alternatively, direct hydraulic fluid from the pump <NUM> to the other actuator <NUM> (left side as shown in <FIG>) to raise the boom. For example, the accumulator <NUM> can provide hydraulic fluid to the actuator <NUM> at a first pressure and flow, and the pump <NUM> can provide hydraulic fluid to the actuator <NUM> at a lower pressure and/or less flow. Thus, in <FIG>, the accumulator <NUM> and/or the pump <NUM> can be used to raise the boom.

In the hydraulic systems of <FIG> and <FIG> a metering valve <NUM> is disposed between the control valve <NUM> and the accumulator <NUM>. This metering valve <NUM> can also be included in the <FIG> hydraulic system between the control valve and accumulator <NUM> shown in that figure, as will be appreciated. During a boom lowering the metering valve <NUM> can be used to proportionately meter the hydraulic flow from the piston side(s) of the actuator(s) <NUM>, <NUM> to the accumulator to control the rate of lowering the boom and/or force on the boom. During a raising of the boom, the metering valve <NUM> can be used to proportionately meter the hydraulic flow from the accumulator <NUM> to the piston side(s) of the actuator(s) <NUM>, <NUM> to control the rate of raising the load implement and/or force on the load implement. In <FIG>, where the left side actuator <NUM> is being powered by the pump <NUM> and the right side actuator <NUM> is being powered by the accumulator <NUM>, if it desired not to use the entire pressure of the accumulator <NUM> then valve <NUM> can be used to reduce that pressure. The pump <NUM> can be used to control the velocity of the lifting of the boom, using whatever pressure is required.

There may be instances where one cylinder may not be capable of supporting the boom down load without flowing over the relief valves or potentially damaging the cylinder. Shown in <FIG>, valve <NUM> provides a connection between the unused cylinder and the rod side areas provides the regen flow as well as a proportional metering orifice <NUM> to tank. The proportional metering orifice <NUM> can be used to adjust the actuation pressure. The piston side of the cylinder <NUM> can be connected to a system to recover the energy such as an over center pump, an accumulator <NUM> with a metering valve <NUM> in series, or some combination. The energy recovery cylinder <NUM> can be used to support the majority of the load to maximize the energy recovery capability of the system. If possible the non-energy recovery cylinder <NUM> can be in a regen type configuration as this would not require a pump to back fill the rod chamber and the proportional valve connecting the boom and rod side can be used for additional controllability. Using both cylinders <NUM>, <NUM> to lower the boom <NUM> in this fashion will increase the range of operating conditions where energy recovery can be accomplished as well as reduce the stress on the boom structure as a smaller moment will be generated. In a further embodiment, a system may be provided where both cylinders <NUM>, <NUM> are used to recover energy back to the accumulator <NUM>, pump <NUM>, etc., but one could be used as a non-energy recovery cylinder with or without regen when necessary or desired. To operate efficiently, when the accumulator <NUM> pressure is low, it may not be desirable to boost pressure of the boom cylinders as much (so less metering is required), but as the pressure of the accumulator rises, to continue recovering energy the pressure of the boom can be increased. Many other combinations of the above described methods and configurations are contemplated, as will be appreciated by those skilled in the art.

<FIG> illustrates a system which is able to convert the high flow, low pressure exhaust flow to a higher pressure and lower flow rate using a reciprocating linear actuator <NUM>. The low pressure flow will be passively diverted to one of the larger area chambers and the high pressure flow will be exhausted from a chamber with a smaller area. In this sense, the reciprocating linear actuator <NUM> operates as a hydraulic pressure transformer in that it transforms lower pressure and higher flow rate to higher pressure and lower flow rate. In one form, as shown in <FIG>, there may be a single shaft <NUM> with two pistons <NUM>, <NUM> located inside a cylindrical body <NUM>. A seal <NUM> can be provided in the center of the cylindrical body <NUM> to separate the center chamber into two distinct volumes, which forms four chambers <NUM>, <NUM>, <NUM>, <NUM> that will increase and decrease in volume along with the linear motion of the piston and rods. At the inlet to each of the large chambers is a selector valve <NUM> that connects the relatively low pressure flow to one of the larger area chambers and connects the other larger area chamber to the tank or zero pressure source. The position of the selector valve <NUM> can be determined by for example the velocity of the rod <NUM> and the position of the rod within the cylinder <NUM>. In the illustrated embodiment, for example, the pistons <NUM>, <NUM> each have a nub that corresponds to notches in the end walls of the cylindrical body <NUM>. When the rod <NUM> is nearing the end of its stroke, the seating of the nub of a piston with a corresponding notch can create a pilot signal to indicate to the selector valve <NUM> to switch positions, for example from right box flow patterns to left box flow patterns, thereby to change the direction of the linear actuator <NUM>. In this way, the reciprocating member <NUM>, <NUM>, <NUM> reciprocates back and forth in the cylindrical body <NUM> to provide a near continuous amount of flow. Check valves <NUM> can be provided for connecting each of the chambers to the tanks source so they can fill when the piston is moving in the opposite direction in an effort to be prepared for the next stroke.

The system includes an accumulator <NUM> on the common line of the low pressure ports of the selector valves. This can be used to minimize the changes in pressure in the exhaust flow from the piston side of the cylinder; without this feature, and depending on the application, the behavior of the cylinder may seem either erratic or uncontrollable.

Referring to the left box flow patterns of the control valve, with the metering valve to the left of the accumulator <NUM> closed and the metering valve to the right of the accumulator <NUM> open, as the boom is lowered, hydraulic fluid from the piston sides of the actuators <NUM>, <NUM> is routed through the lower check valve <NUM>, through the selector valve <NUM>, and to one of the larger area chambers <NUM>, <NUM> of the reciprocating linear actuator <NUM>. The reciprocating linear actuator <NUM>, in turn, exhausts hydraulic fluid at a relatively higher pressure from the corresponding smaller area chamber <NUM>, <NUM> through the open right side metering valve and to the accumulator <NUM>. The right side metering valve can be used to meter some of the accumulator pressure to get the desired pressure out of the reciprocating linear actuator <NUM>. Pump flow is routed to the rod sides of the cylinders <NUM>, <NUM> as back fill. As will be appreciated, the potential energy stored in the accumulator <NUM> comes from the piston sides of the cylinders <NUM>, <NUM> via the reciprocating linear actuator <NUM>, and the pressure in the accumulator <NUM> may be relatively higher or relatively lower than the piston side pressure. If the accumulator <NUM> is insufficiently charged to raise or assist in raising the boom, then an additional reciprocating linear actuator <NUM> cycle (or cycles) can be used to recover additional potential energy from another lowering of the boom until the accumulator <NUM> is sufficiently charged for use.

Of course, if an operator command is to drop the boom faster than the pump <NUM> can be used to add additional flow to aid in the dropping rate. The pump <NUM> can provide pump flow to the rod sides of the cylinders <NUM>, <NUM> to thereby urge the boom to drop faster. This can also facilitate smoother transition for powering into the ground. If the operator command is to power into the ground upon the boom hitting ground, the pump <NUM> can provide additional pump flow to the rod sides of the cylinders <NUM>, <NUM> prior to hitting the ground, so that the boom will have standby power to power into the ground. With the standby pressure on the pump <NUM>, once the boom hits the ground digging can be started immediately. Of course, if the operator command is to slow the boom drop rate, the pump flow can be reduced accordingly.

Referring now to the right box flow patterns of the control valve, with the metering valve to the left of the accumulator <NUM> open and the metering valve to the right of the accumulator <NUM> closed, to raise the boom, the pump <NUM> as well as the stored energy in the accumulator <NUM> pressure both of the piston sides of the actuators <NUM>, <NUM>. The accumulator <NUM>, through the open left side metering valve, adds flow to the flow of the pump <NUM> at the same pressure. The left side metering valve can be used to meter some of the pump pressure to get the desired pressure out of the accumulator <NUM>. As the accumulator <NUM> starts to deplete, the pump <NUM> can provide greater flow. The accumulator <NUM> can provide flow as it depletes until it meets a certain pressure for example the pressure required to actuate the boom. Once the accumulator <NUM> reaches such pressure, power can no longer be drawn from the accumulator <NUM>. As such, the left side metering valve is closed and drive can be provided from the pump <NUM>.

<FIG> is a system similar to the <FIG> system except the <FIG> system replaces the linear reciprocating pressure transformer <NUM> with a rotary pressure transformer <NUM>. The rotary pressure transformer <NUM> includes a variable hydraulic pump motor <NUM> and a bidirectional hydraulic pump motor <NUM>. Depending on the displacement value (positive or negative) and the direction of rotation, the pump motor <NUM> can work either as a pump or a motor mode. As will be appreciated, the pump motor <NUM> can be a fixed or variable pump motor <NUM>. The pump motor <NUM> has an outlet port connected to the accumulator <NUM> with a proportional valve therebetween, and an inlet port for drawing hydraulic fluid from the tank or other source. The pump motor <NUM> is connected to the pump motor <NUM> via a shaft <NUM>. The pump motor <NUM> has an upper port (as viewed in <FIG>) that receives hydraulic fluid from the piston sides of the cylinders <NUM>, <NUM> during a boom lowering operation and expels pump flow during a boom raising operation. The pump motor <NUM> has a lower port (as viewed in <FIG>) that receives pump flow during a boom raising operation and expels hydraulic fluid during a boom lowering operation.

Referring to the right box flow patterns of the control valve shown in <FIG>, with the proportional valve connected to the accumulator <NUM> open, as the boom is lowered pressurized hydraulic fluid from the piston sides of the actuators <NUM>, <NUM> is routed to the pump motor <NUM>. The pump motor <NUM> uses the pressurized fluid to power the motor shaft <NUM> and then expels the hydraulic fluid through the outlet of the motor <NUM> to the tank. The motor shaft <NUM>, in turn, powers the pump motor <NUM> so that the pump motor <NUM> draws hydraulic fluid from the tank and pressurizes same. The pump motor <NUM> then supplies the pressurized fluid through the proportional valve and to the accumulator <NUM> to thereby charge the accumulator <NUM>. In other words, potential energy of the load, here provided by the boom descent, is transferred to the accumulator <NUM>. As in the <FIG> system, the variable displacement pump <NUM> can provide pump flow to the rod sides of the cylinders <NUM>, <NUM> as back fill. As will be appreciated, the potential energy stored in the accumulator <NUM> comes from the piston sides of the cylinders <NUM>, <NUM> via the rotary pressure transformer <NUM>, and the pressure in the accumulator <NUM> may be relatively higher or relatively lower than the piston side pressure. If the accumulator <NUM> is not sufficiently charged to raise or assist in raising the boom, then an additional rotary pressure transformer <NUM> cycle (or cycles) can be used to recover additional potential energy from another lowering of the boom until the accumulator <NUM> is sufficiently so charged.

Of course, if an operator command is to drop the boom faster then the pump <NUM> can be used to add additional flow to aid in the dropping rate. The pump <NUM> can provide pump flow to the rod sides of the cylinders <NUM>, <NUM> to thereby urge the boom to drop faster. This can also facilitate smoother transition for powering into the ground. If the operator command is to power into the ground upon the boom hitting ground, the pump <NUM> can provide additional pump flow to the rod sides of the cylinders <NUM>, <NUM> prior to hitting the ground, so that the boom will have standby power to power into the ground. With the standby pressure on the pump <NUM>, once the boom hits the ground digging can be started immediately. Of course, if the operator command is to slow the boom drop rate, the pump flow can be reduced accordingly.

Referring now to the left box flow patterns of the control valve, with the proportional valve connected to the accumulator <NUM> open, to raise the boom, the accumulator <NUM> provides pressurized hydraulic fluid to the pump motor <NUM>. The pump motor <NUM> uses the pressurized fluid to power the motor shaft <NUM> and then expels the hydraulic fluid through the outlet of the pump motor <NUM> to the tank. The motor shaft <NUM> drives the motor pump <NUM>. The motor pump <NUM>, in turn, draws hydraulic fluid from the tank via the pump <NUM> and control valve, pressurizes the flow, and provides the pressurized flow to the piston sides of the actuators <NUM>, <NUM> thereby raising the boom. The pump <NUM> can also provide pressurized flow to the piston sides of the cylinders <NUM>, <NUM> via the control valve and pump motor <NUM>, to raise or assist in raising the boom. In other words, both the pump <NUM> and the pump motor <NUM> can be used to lift the boom, in a manner similar to a two stage pump for example. The accumulator <NUM> can provide flow as it depletes until it meets a certain pressure for example the pressure required to actuate the boom. Once the accumulator <NUM> reaches such pressure, power can no longer be drawn from the accumulator <NUM>. As such, the accumulator proportional valve can be choked off and drive can be provided from the pump <NUM>.

The illustrated hydraulic systems of <FIG> and <FIG> each have two hydraulic actuators <NUM>, <NUM>. The hydraulic system need not be limited as such. As noted earlier, and as will be appreciated, one or more hydraulic actuators may be connected between the machine frame and the moving parts of an implement such as the boom <NUM>. Thus, in <FIG> and <FIG>, the hydraulic system may include a single actuator instead of two actuators.

The several embodiments herein enable utilizing recovered energy. The energy from the boom or swing can be reused in a number of different ways. If used immediately it can be directed towards a pump or if it is captured to an accumulator it can be directed to either the boom or the swing drive. It is also possible to combine one or more of the methods described herein to efficiently use the energy. In some cases, sacrifices to efficiency gains can be made to create smooth operation. Metering valves for example can be wasteful but very smooth in their operation and thus can facilitate this. A variable displacement pump/motor can also be used, but in certain displacement ranges, the volumetric efficiency of the motor for a means of energy transfer may be lower than a route using metering valves and an accumulator. Because of this, it will be appreciated that sizing of components can be done based on the most common operating modes for higher efficiencies, allowing for lower efficiencies at deviations from those averages.

If the flow energy is to be used immediately the flow can be diverted back to the pump/motor or to a separate motor. If an over-center pump is used in the system, the power can be added back to the engine shaft to assist other functions. Alternatively, if the pumps are configured to handle it, the flow can be directed back to the inlet of the pump. This can reduce the increase in pressure required to obtain the working pressure at the outlet which will reduce the amount of torque required to spin the pump. The torque required for a pump may be proportional to the delta in pressures across the pump trying to be generated. Another option is to distribute the hydraulic energy immediately to another function that is demanding flow by incorporating suitable valving. As will be appreciated, such valving may be more significant than the afore described configurations, but the efficiency in general should be higher as there are less energy conversions required to get it to work. Efficiency losses from metering may not play a large role if the pressures are close together. However, if the energy from the boom and swing motor are stored to an accumulator the energy may be used in a different manner than described above.

If the energy stored in the accumulator <NUM> is used to power the swing drive <NUM> a system similar to that described in co-owned international published patent application <CIT> can be used. In this system the accumulator is connected in series with a proportional metering valve to the swing drive. The proportional metering valve can be used to generate the required pressure drop from the accumulator to the desired working pressure of the swing motor; from a basic perspective the opening of the proportional metering valve can be based upon the required pressured drop and the flow to the swing motor. In the referenced international patent application there is an additional dedicated swing pump added to the system to decouple the swing function from the stock system. However, it is also possible to power the swing drive from the accumulator and the stock pump on the excavator; if the boom and swing energies are recovered back to the accumulator then the efficiency of this system can be expected to not be much worse, if not better, than the system described in the referenced international patent application. If a dedicated swing pump is not included the accumulator can power the swing drive until the pressure in the accumulator is not sufficient to meet the performance requirements or the swing drive is operating at a lower pressure where it is inefficient to use the accumulator. When the accumulator is deemed to be unusable either for performance, efficiency, or other reasons the stock pump can operate as normal and provide flow to the swing drive. With a properly sized accumulator, the swing function can behave nearly as if it is decoupled from the other functions. An example of this configuration is shown in <FIG>.

The swing and boom recovery systems on the same vehicle can be combined. The two systems can have different actuation pressures, as well as pressure applied at different times. Recovery from the boom can be via use of an accumulator, where the pressure in lowering and the pressure to raise is usually the same. Accumulators can be charged to a higher pressure; to create a constant braking pressure, a metering orifice can be used to make the difference between the cylinder pressure and the accumulator pressure. The accumulator can be sized to make the end of recoverable energy be at a pressure equal to the braking pressure, decreasing the amount of metering required. Then to use the energy stored in the accumulator, it can be at a lower pressure with a metering orifice to create a constant pressure for accelerating and the accumulator can be considered near empty close to this acceleration pressure. Since boom recovery relies on gravity to push the boom down to create pressure which can be stored as potential energy, generally the rod side of the boom cylinders can be kept at low pressure and allow the gravity to directly create that without adding energy to the system. Because of this, when the bucket makes contact with the ground, a low pressure is seen there, so it will stop recovering, but it may not be able to dig until it is realized that pressure from the pumps needs to be supplied and then reaction time from valves and pumps also delays this an unsatisfactory amount. Two different methods can be used to alleviate this problem, one of which is to sense an oncoming ground reaction through optical or auditory sensors showing distance to the ground. Cylinder position sensors could potentially also be used, but this is less reliable and may have false negatives unnecessarily. Another way, which mimics what the stock vehicle does, is to provide standby fluid energy which can power the cylinders down once it makes contact. In the stock machine, the boom can be powered down at a low pressure and large flow, which equates to a relatively low pressure, but if another function is being used, then the pump requires a pressure to be generated, which then requires a pressure drop either through a meter out orifice or through a meter in orifice on the boom in order to provide the necessary or desired flow and pressure to the cylinder. This can be wasteful, and thus bypassing this may be desirable. Instead of using a pump, a standby pressure can be used for example from the high pressure accumulator, which is not wasteful. The pressure in the accumulator is static, so having pressure ready to be used when required does not waste energy. Once the accumulator pressure is used, pressure sensors can sense this shift and the pumps and valves can be actuated so that they provide the energy for digging. The accumulator can simply provide a transition high pressure while the pumps and valves get into position before they are able to contribute so that the transition from recovery lowering and powered digging is transparent to the user. This figure shows one such configuration using a shuttle valve and a priority valve to provide the passive circuit which will allow for this pressure transition to occur. <FIG> shows an additional configuration using an extra pump <NUM> to the stock pump system <NUM> allowing for higher efficiencies.

As will be appreciated, flow demand for the swing drive and energy loss due to metering or inefficient operating points may affect the suitability of a motor and/or cylinder. The swing drive is operated by a motor which can rotate an infinite number of rotations and therefore is unlimited in the amount of flow it can demand. This is dissimilar to a cylinder, which powers the boom, arm, and bucket functions, where the volume of flow is limited based on the working area of the cylinder and the length of stroke. Sizing an accumulator for the movement of a cylinder is in some respects less difficult than sizing it for a motor due to the bounding of the volume of flow. General operation of a swing drive usually does not exceed <NUM> degrees of rotation because the swing drive can rotate in the opposite direction over a shorter rotation to reach the same desired position. Most operations for an excavator are either a <NUM> degree operation or a <NUM> degree operation. Additionally, the average operating efficiency of a motor is in the range of <NUM>%(or even lower at some undesirable operating points) whereas the efficiency of a cylinder is ><NUM>% as there is virtually no volumetric loss and small amount of mechanical inefficiency due to friction. This difference in efficiency suggests a cylinder solution may be desirable.

One embodiment to reuse the stored energy in the accumulator with the boom system is to direct the stored accumulator energy towards both boom cylinders. The force in the vertical direction can be controlled by a suitable technique for example by metering the energy. Additionally, for such a system, if the motion stops the pressure in both cylinders may equal the accumulator pressure. When the pressure in the accumulator is depleted to such a level where the boom can no longer be lifted the stock pumps can be used to provide the required flow and pressure.

Claim 1:
A hydraulic system for a mobile construction vehicle (<NUM>), comprising:
a variable displacement track motor (60a, 60b) configured to be coupled to a track of the mobile construction vehicle (<NUM>) to drive the track;
an accumulator (<NUM>) for storing pressurized hydraulic fluid for use as a power supply to a non-track load implement (<NUM>);
a pump (<NUM>, <NUM>) dedicated to the track motor (60a, 60b); and
control valving (58a, 58b) that is operable between a first position at which the control valving (58a, 58b) directs hydraulic fluid from the dedicated pump (<NUM>, <NUM>) to the variable displacement track motor (60a, 60b) to drive the variable displacement track motor (60a, 60b), and a second position at which the control valving (58a, 58b) directs hydraulic fluid from the dedicated pump (<NUM>, <NUM>) to the accumulator (<NUM>).