Fluid pressure circuit

A fluid pressure circuit includes a directional switching valve arranged between a fixed displacement pump and a fluid pressure actuator and configured to switch a flow passage for a pressurized fluid, an accumulator arranged in a branch flow passage branched from a connection flow passage that connects the fluid pressure actuator and the directional switching valve, an accumulator flow control valve arranged between the connection flow passage and the accumulator, and a pump flow control valve arranged between the fluid pressure actuator and the fixed displacement pump and configured to variably divert a flow rate of the pressurized fluid discharged from the fixed displacement pump into a first system including the tank and a second system including the fluid pressure actuator.

TECHNICAL FIELD

The present invention relates to a fluid pressure circuit that controls a fluid pressure actuator according to an operation command.

BACKGROUND ART

A fluid pressure circuit that drives a fluid pressure pump according to an operation command to control a fluid pressure actuator such as a cylinder device is generally used in a work machine, a construction machine, a cargo handling vehicle, an automobile, and the like. As a fluid supply source suitable for the fluid pressure circuit, a fixed displacement fluid pressure pump has been frequently used in the fluid pressure circuit due to its simple structure and excellent maintainability. Further, there is a fluid pressure circuit in which the fluid discharged from a cylinder device is accumulated in an accumulator to effectively utilize energy.

For example, in a hydraulic circuit described in Patent Document 1, when an operating lever of an operating valve is operated in an extending direction, a directional switching valve is switched to an extended position, and pressure oil discharged from a fixed displacement hydraulic pump is introduced into a bottom chamber of a cylinder device to extend a rod outside, and on the other hand, when the operating lever is operated in a retracting direction, the directional switching valve is switched to a retracted position, and the pressure oil discharged from the fixed displacement hydraulic pump is introduced into a rod chamber to retract the rod into the cylinder device.

Further, a branch oil passage is branched from and connected to an oil passage connecting the directional switching valve and the tank. When the rod is retracted, the switching valve is brought into a pressure accumulation position such that part of the return oil discharged from the bottom chamber through the branched oil passage can be accumulated in an accumulator. The pressure oil accumulated in the accumulator is supplied to a regeneration pump motor to generate electricity, such that the energy is utilized effectively.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Here, in the hydraulic circuit described above, part of the oil discharged from the bottom chamber of the cylinder device is accumulated in the accumulator to be used, so that the energy use efficiency is high. Unfortunately, an impact is likely to occur when the directional switching valve is switched, because the fixed displacement hydraulic pump has a constant discharge amount.

The present invention has been made in order to solve the problems described above, and its object is to provide, at a low cost, a fluid pressure circuit capable of smoothly controlling a fluid pressure actuator according to an operation command and capable of effectively utilizing energy.

Solution to Problem

In order to solve the above problem, a fluid pressure circuit according to the present invention includes: a tank having a fluid stored therein; a fixed displacement pump configured to pressurize the fluid in the tank to generate a pressurized fluid; a fluid pressure actuator configured to be driven by the pressurized fluid discharged from the fixed displacement pump and to be controlled in accordance with an operation command; a directional switching valve arranged between the fixed displacement pump and the fluid pressure actuator and configured to switch flow passages for the pressurized fluid; an accumulator arranged in a branch flow passage branched from a connection flow passage that connects the fluid pressure actuator and the directional switching valve; an accumulator flow control valve arranged in the branch flow passage between the connection flow passage and the accumulator; and a pump flow control valve arranged between the fluid pressure actuator and the fixed displacement pump and configured to variably divert a flow rate of the pressurized fluid supplied from the fixed displacement pump into two systems consisting of a first system including the tank and a second system including the fluid pressure actuator. According to the feature, since the pump flow control valve variably outputs the flow rate of the input pressurized fluid to the two systems while using the fixed displacement pump having a simple structure, the fluid pressure actuator can be smoothly controlled according to the operation command, and the fluid pressure actuator can be driven by the fluid accumulated in the accumulator, so that energy can be effectively utilized. Further, the fluid pressure circuit mainly includes the fixed displacement pump, the directional switching valve, the accumulator flow control valve, and the pump flow switching valve, and therefore can be provided at a low cost.

It is preferable that the pump flow control valve may be a spool valve. According to this configuration, since the flow rate can be adjusted by controlling the stroke of the spool, the structure is simple.

It is preferable that the fluid pressure circuit may include a control unit configured to relevantly control the pump flow control valve when the fluid pressure actuator is operated by the accumulator. According to this configuration, the fluid pressure actuator can be smoothly controlled and the load of the fixed displacement pump during the regeneration operation can be reduced.

It is preferable that the accumulator flow control valve may be a proportional valve configured to variably control a flow rate, and the control unit may output a complementary operation command to the accumulator flow control valve and the pump flow control valve. According to this configuration, the characteristics of the operation of the fluid pressure actuator with respect to the operation command during normal control can coincide with that during regeneration control.

It is preferable that the fluid pressure circuit may further includes a pressure sensor configured to detect a pressure of the fluid in the accumulator. According to this configuration, since an actual pressure of the fluid accumulated in the accumulator can be reflected, the control can be performed more smoothly.

It is preferable that the pump flow control valve may be arranged between the directional switching valve and the fixed displacement pump. According to this configuration, since the pump flow control valve is separate from the directional switching valve, the structure of the directional switching valve is not complicated.

DESCRIPTION OF EMBODIMENTS

Modes for implementing a fluid pressure circuit according to the present invention will be described below based on embodiments.

First Embodiment

A hydraulic circuit130as a fluid pressure circuit according to a first embodiment of the present invention will be described with reference toFIGS. 1 to 8. A hydraulic circuit as the fluid pressure circuit according to the first embodiment is a hydraulic circuit that controls a stroke of a cylinder device according to an operation command in a work machine, a construction machine, a cargo handling vehicle, an automobile, and the like, and is incorporated in, for example, a power train of a wheel loader100shown inFIG. 1. The wheel loader100mainly includes a vehicle body101, drive wheels102, a working arm103, a hydraulic cylinder104, and a bucket105in which gravel or the like is loaded. The vehicle body101is provided with a machine110such as an engine, a drive fluid circuit120, the hydraulic cylinder104, and the working hydraulic circuit130for driving a hydraulic cylinder5, which is a cylinder device, etc.

As shown inFIG. 2, the hydraulic circuit130mainly includes a main hydraulic pump2as a pump of fixed displacement type or a fixed displacement pump configured to be driven by a drive mechanism1such as an engine or an electric motor, a pilot hydraulic pump3, and a directional switching valve4, the hydraulic cylinder5as a fluid pressure actuator, a tank11, an electromagnetic proportional flow control valve26as an accumulator flow control valve for an accumulator27, the accumulator27, a controller28, a pressure sensor33, and an electromagnetic proportional flow control valve40as a pump flow control valve for the main hydraulic pump2.

The main hydraulic pump2is connected to the drive mechanism1such as an internal combustion engine, and is driven to rotate by power from the drive mechanism1to supply pressure oil downstream through an oil passage12.

The pressure oil discharged from the main hydraulic pump2flows through the oil passage12and an oil passage13into the directional switching valve4. The directional switching valve4is a six-port three-position type open center switching valve. In a state where a spool is in a neutral position, the entire amount of pressure oil discharged from the main hydraulic pump2flows through an oil passage14into the tank11.

Further, a relief valve7is arranged in a main circuit including the main hydraulic pump2in order to prevent an oil machine in the circuit from being damaged when a rod5aof the hydraulic cylinder5has reached an extension end or a retraction end, or when a load is suddenly applied to the hydraulic cylinder5, and therefore the inside of the circuit has an abnormally high pressure. The high-pressure oil discharged from the relief valve7is allowed to be discharged through the oil passage17to the tank11.

The pilot hydraulic pump3is connected to the drive mechanism1in the same way as the main hydraulic pump2and is driven to rotate by the power from the drive mechanism1to supply pressure oil through an oil passage18to a remote control valve6located downstream.

Further, a relief valve8is arranged in a pilot circuit including the pilot hydraulic pump3, and when the remote control valve6is in a neutral position where an operating lever6-1is not operated, the pressure oil is discharged through oil passages19,20and the relief valve8to the tank11.

The remote control valve6is a variable pressure reducing valve. When the operating lever6-1is operated back and forth, the pressure oil at a secondary pressure, which increases in proportion to the lever operation amount as shown inFIG. 3, is supplied through signal oil passages21and22to signal ports4A and4B of the directional switching valve4. Thus, the directional switching valve4is switched to an “extended” or “retracted” position of the hydraulic cylinder5.

The electromagnetic proportional flow control valve26is a two-port three-position type normally closed electromagnetic proportional flow control valve, and incorporates, at an input position26a, a check valve which allows only the flow toward the accumulator27and, at an output position26b, a check valve which allows only the flow toward the hydraulic cylinder5.

The electromagnetic proportional flow control valve40is a three-port two-position type normally open electromagnetic proportional flow control valve, and is a spool valve that variably diverts the pressure oil discharged from the main hydraulic pump2to the oil passage12into two systems, the oil passage13and an oil passage42. The electromagnetic proportional flow control valve40has opening characteristics shown inFIG. 5, and communicates the oil passage12and the oil passage13and closes the oil passage42when the valve is in a neutral position40a. When an electric signal from the controller28is input to a solenoid unit40-1via an electric signal line41, the electromagnetic proportional flow control valve40is variably and gradually switched to a switching position40baccording to the amount of change in electric signal, for example, electric energy. When the amount of change becomes equal to or more than a predetermined amount, the electromagnetic proportional flow control valve40is completely switched to the switching position40b, the oil passage12and the oil passage13are closed, and the oil passage12is communicated with the tank11via the oil passage42.

(1) Normal Extension Operation Will be Described.

The relationship between the amount of operation of the operation lever6-1and the extension speed of the rod of the hydraulic cylinder5when the lever6-1is operated in an extending direction A has a characteristics curve as shown inFIG. 4. The directional switching valve4is configured such that the spool strokes substantially in proportion to a pilot secondary pressure of the remote control valve6, and the valve has opening characteristics in which the amount of opening increases in accordance with the spool stroke. Accordingly, as the amount of opening increases, the amount of pressure oil supplied to the hydraulic cylinder5increases, and therefore the operation speed of the rod5aof the hydraulic cylinder5increases. That is, the rod speed can be controlled according to the amount of operation of the operating lever6-1.

When the operating lever6-1is operated in the extending direction A to switch the directional switching valve4to an extended position, the pressure oil from the main hydraulic pump2flows through the oil passages12,13,15, and23to a bottom chamber5A of the hydraulic cylinder5, and the oil in a rod chamber5B flows through an oil passage24and is then discharged via the directional switching valve4through an oil passage25to the tank11. Thus, the rod5aof the hydraulic cylinder5moves in an extending direction.

(2) Normal Retraction Operation Will be Described.

When the operating lever6-1is operated in a retracting direction B to switch the directional switching valve4to a retracted position, the pressure oil from the main hydraulic pump2flows through the oil passages12,13,15, and24to the rod chamber5B of the hydraulic cylinder5, and the oil in the bottom chamber5A flows through the oil passage23as a connection flow passage, and is then discharged via the directional switching valve4through the oil passage25to the tank11. Thus, the rod5aof the hydraulic cylinder5moves in a retracting direction.

(3) Retraction Operation Involving Pressure Accumulation Will be Described.

When the operating lever6-1of the remote control valve6is operated in the retracting direction B, the controller28determines that the pressure accumulation in the accumulator27is possible if the pressure in the accumulator27is less than a predetermined high value PH, and performs the following operation. If the pressure in the accumulator27is equal to or more than the predetermined high value PH, the controller28determines that the pressure accumulation is unnecessary, and does not perform the pressure accumulation.

Referring toFIG. 6, when the operating lever6-1of the remote control valve6is operated in the retracting direction B to switch the directional switching valve4to the retracted position, the pressure oil from the main hydraulic pump2flows through the oil passages12,13, an oil passage of the directional switching valve4, and the oil passage24into the rod chamber5B of the hydraulic cylinder, and the oil in the bottom chamber5A flows through the oil passage23and is discharged via a throttle flow passage of the directional switching valve4through the oil passage25to the tank11.

At this time, when an electric signal corresponding to a pressure Py from a pressure sensor10arranged in a pilot signal oil passage22is input to the controller28, an electric signal Sy corresponding to the pressure Py is input to the electromagnetic proportional flow control valve26through an electric signal line by an arithmetic circuit preliminary integrated in the controller28. The electromagnetic proportional flow control valve26is gradually switched to a side of the input position26aaccording to the amount of change in electric signal Sy, and part of the oil discharged from the bottom chamber5A flows through an oil passage29as a branch flow passage, the check valve of the electromagnetic proportional flow control valve26, and an oil passage30as a branch flow passage, and is then accumulated in the accumulator27. When the retraction operation of the rod5ais completed, the controller28stops outputting the electric signal to the electric signal line31, and the electromagnetic proportional flow control valve26is brought into the neutral position shown inFIG. 2.

(4) Extension Operation by Regeneration Will be Described.

When the operating lever6-1of the remote control valve6is operated in the extending direction A, the controller28determines that the pressure oil accumulated in the accumulator27can be regenerated if the pressure in the accumulator27is equal to or more than a predetermined low value PL, and performs the following operation. If the pressure in the accumulator27is less than the predetermined low value PL, the regeneration is not performed. The predetermined high value PHis a pressure higher than the predetermined low value PL.

Referring toFIG. 7, when the operating lever6-1of the remote control valve6is operated in the extending direction A to switch the directional switching valve4to the extended position, the pressure oil from the main hydraulic pump2flows through the oil passages12,13, and15, an oil passage of the directional switching valve4, and the oil passage23into the bottom chamber5A of the hydraulic cylinder, and the oil in the rod chamber5B flows through the oil passage24and is discharged via an oil passage of the directional switching valve4through the oil passage25to the tank11.

At this time, when an electric signal corresponding to a pressure Px from a pressure sensor9and an electric signal corresponding to a pressure Pz from the pressure sensor33are input to the controller28, an electric signal Pxz corresponding to the pressures Px and Pz is input to the electromagnetic proportional flow control valve26through the electric signal line32by the arithmetic circuit preliminarily integrated on the controller28. The electromagnetic proportional flow control valve26is gradually switched to a side of the output position26baccording to the amount of change in electric signal Pxz, and the pressure oil accumulated in the accumulator27variably flows through the oil passage30, the check valve of the electromagnetic proportional flow control valve26, and the oil passage29and is then joined to the oil passage23, and is supplied to the bottom chamber5A of the hydraulic cylinder. Thus, the pressure oil accumulated in the accumulator27is regenerated.

At the same time, the electric signal Pxz is input from the controller28through the electric signal line41to the solenoid unit40-1of the electromagnetic proportional flow control valve40. The electromagnetic proportional flow control valve40is gradually switched to the switching position40baccording to the amount of change in electric signal Pxz, and an opening between the oil passage12and the oil passages13is variably gradually reduced, and an opening between the oil passages12and42is variably gradually increased. When the amount of change in electric signal Pxz is large and the electromagnetic proportional flow control valve40is completely switched to the switching position40b, the communication between the oil passage12and the oil passage13is shut off, and the oil passage12is completely communicated with the tank11via the oil passage42.

Here, the oil passage12of the main hydraulic pump2is branched into two systems of the oil passage13and the oil passage42by the electromagnetic proportional flow control valve40, and an oil amount Q12discharged from the oil passage12is variably divided into an oil amount Q13of the oil passage13and an oil amount Q42of the oil passage42to be output (Q12=Q13+Q42). An oil amount Q5A flowing into the bottom chamber5A of the hydraulic cylinder5is the sum of an oil amount Q29supplied from the accumulator27via the electromagnetic proportional flow control valve26to the oil passage23, and an oil amount Q23supplied from the main hydraulic pump2via the electromagnetic proportional flow control valve40and the directional switching valve4to the oil passage23(Q5A=Q29+Q23). Thus, the pressure oil of the oil amount Q29is regenerated from the accumulator27. The oil amount Q5A is the same as the amount of oil flowing into the bottom chamber5A during the normal extension operation, and the oil amount Q29and the oil amount Q42are complementary to each other. That is, the electromagnetic proportional flow control valve26and the electromagnetic proportional flow control valve40have characteristics complementary to each other with respect to the amount of change in the electric signal Pxz. For example, the oil amount Q29supplied from the accumulator27to the bottom chamber5A is the same as the oil amount Q42discharged from the oil passage12via the electromagnetic proportional flow control valve40to the oil passage42when the directional switching valve4is fully opened (i.e., Q29=Q42). That is, considering the amount of movement of the directional switching valve4according to the pressure Px corresponding to the amount of operation of the operating lever6-1, Q29=Q42×f (Px) may be used. Here, f (Px) is a function of the pressure corresponding to the amount of operation of the operating lever6-1, and is substantially proportional to the amount of operation and is 1 when the amount of operation exceeds a predetermined value. As a result, the characteristics curve of the relationship between the amount of operation of the lever and the rod speed during the regeneration operation has the same characteristics as that inFIG. 4during the normal operation.

Referring toFIG. 8, the amount of change in electric signal Pxz output to the electromagnetic proportional flow control valve26is an amount ΔPx corresponding to only the pressure Px as in the case of retraction when the pressure Pz is equal to or more than the predetermined high value PH, and is an amount ΔPz corresponding to only the pressure Pz when the pressure Pz is equal to or more than the predetermined low value PLand less than the predetermined high value PH, and when the pressure Pz is less than the predetermined low value PL, the amount is zero. That is, when the pressure accumulated in the accumulator27is high, the regeneration operation is performed, and when the pressure is low, the regeneration operation is not performed. When the pressure Pz is equal to or more than the predetermined low value PLand less than the predetermined high value PH, which is a moderate pressure, the percentage of the regeneration oil amount Q29supplied from the accumulator27is set to be lower than that when the pressure Pz is equal to or more than the predetermined high value PH. Thus, even when the pressure accumulated in the accumulator27is relatively low, regeneration can be performed, which is excellent in energy efficiency. Regeneration may be performed only when the pressure in the accumulator27is equal to or more than the predetermined high value PH. Thus, the control of the electromagnetic proportional flow control valves26and40can be simplified.

Arranging the electromagnetic proportional flow control valve40, which is controlled by the electric signal from the controller, between the oil passage12and the oil passage13, causes the pressure oil accumulated in the accumulator27to be regenerated via the electromagnetic proportional flow control valve26to the bottom chamber5A of the hydraulic cylinder5while using the fixed displacement type main hydraulic pump2, and at the same time, causes the oil discharged from the main hydraulic pump2to be communicated with the low pressure tank11by the electromagnetic proportional flow control valve40, thereby reducing the discharge pressure of the main hydraulic pump2. The relationship between a pump output E, a pump discharge pressure P, and a discharge flow rate Q is as follows:
E∝P×Q
Therefore, the output (load) of the main hydraulic pump2is reduced, which allows energy saving of the system to be achieved.

Further, by adding the accumulator27, the electromagnetic proportional flow control valves26,40, etc. to a hydraulic circuit including a fixed displacement type main hydraulic pump, which has been frequently used, a regeneration function can be easily added at low cost.

As described above, the embodiments according to the present invention have been described with reference to the drawings. However, the specific configuration is not limited to these embodiments, and any changes and additions without departing from the scope of the present invention are included in the present invention.

For example, the case has been described in which part of the return oil from the bottom chamber5A is accumulated in the accumulator27when the rod5ais retracted, and the accumulated pressure oil is regenerated to the bottom chamber5A when the rod5ais extended. However, part of the return oil from the rod chamber5B may be accumulated in the accumulator27when the rod5ais extended. Furthermore, part of the return oil from the bottom chamber5A and the rod chamber5B may be accumulated in the accumulator27both when the rod5ais retracted and when the rod5ais extended.

Further, the fluid pressure actuator may be other than a hydraulic cylinder. The present invention can be applied to any circuit that accumulates oil in an accumulator and regenerates the accumulated oil in a hydraulic circuit including a fixed displacement type main hydraulic pump, for example, that accumulates part of the return oil at the time of braking of a hydraulic motor in the accumulator, and regenerates the accumulated pressure oil at the time of acceleration of the hydraulic motor.

Further, the case in which oil is used as fluid has been described as an example, however the present invention can be applied to any fluid such as water or air.

Further, the electromagnetic proportional flow control valves26and40are not limited to have the configuration in which the switching operation is performed by electricity, but may be hydraulically operated valves.

Further, the function of the electromagnetic proportional flow control valve40may be incorporated in the directional switching valve4. In this case, it is preferable that the directional switching valve4be controlled by both a pilot oil pressure and an electric signal.

REFERENCE SIGNS LIST