Hydraulic machine

Abstract

A hydraulic machine comprising: an actuator; a high pressure accumulator; a high pressure line that supplies pressure oil to the actuator and to which the high pressure accumulator is connected; a pump that supplies pressure oil to the high pressure line; a drive unit that drives the pump; and a control unit that controls the pump and the drive unit, wherein the control unit performs control such that, when the pressure in the high pressure line is more than or equal to a preset threshold pressure, the rotational speed of the drive unit is maintained at a preset first rotation speed, and when the pressure in the high pressure line is less than the threshold pressure, the rotational speed of the drive unit is made to be a second rotational speed that is greater than the first rotational speed.

Description

FIELD OF THE INVENTION

[0001] The present disclosure relates to a hydraulic machine, and more particularly, to a hydraulic machine having a Common Pressure Rail (CPR) that supplies pressurized fluid to actuators.BACKGROUND OF THE INVENTION

[0002] A hydraulic machine performs work by obtaining power from the pressure of a fluid. Such a hydraulic machine includes, for example, heavy equipment such as an excavator. Recently, Common Pressure Rail (CPR) hydraulic machines have been known, which are equipped with high pressure lines connected to an accumulator and supplying pressurized fluid to actuators.SUMMARY OF THE INVENTIONTechnical Solution

[0003] According to a first aspect of the present disclosure, a hydraulic machine is provided, comprising: an actuator, a high-pressure accumulator, a high-pressure line for supplying pressurized fluid to the actuator and to which the high-pressure accumulator is connected; a pump for supplying pressurized fluid to the high-pressure line; a driving unit for driving the pump; and a control unit for controlling the pump and the driving unit, wherein the control unit controls the rotational speed of the driving unit to be maintained at a first rotational speed if the pressure in the high-pressure line is higher than a preset threshold pressure, and controls the rotational speed of the driving unit to be a second rotational speed greater than the first rotational speed if the pressure in the high-pressure line is lower than the threshold pressure.

[0004] According to the first aspect of the present disclosure, there is an advantage in that a CPR hydraulic machine having excellent fuel consumption efficiency can be provided.

[0005] In some examples, the first rotational speed may be a rotational speed within a region representing an optimal braking fuel consumption ratio.

[0006] In some examples, the second rotational speed may be determined based on a pressure within the high pressure line.

[0007] In some examples, the control unit may, if the pressure in the high pressure line is higher than the threshold pressure, control the rotational speed of the driving unit to be maintained at the first rotational speed and control the torque of the driving unit to be maintained at a preset first torque; and if the pressure in the high pressure line is lower than the threshold pressure, control the rotational speed of the driving unit to be a second rotational speed greater than the first rotational speed and control the torque of the driving unit to be the second torque.

[0008] In some examples, the first rotational speed and the first torque may be rotational speed and torque within a range representing an optimal braking fuel consumption ratio of the hydraulic machine.

[0009] In some examples, the second rotational speed may be determined according to the pressure in the high pressure line, and the second torque may be a torque that represents an optimal braking fuel consumption ratio at the second rotational speed.

[0010] In some examples, the control unit may control the volume of the pump according to an integral value obtained by integrating values obtained by multiplying the difference between the second torque and the current torque (Treq−Tcur) by the gain value and dividing the values by the inlet and outlet pressure difference values of the pump.

[0011] In some examples, the control unit may increase the volume of the pump when the integral value increases, and may decrease the volume of the pump when the integral value decreases.

[0012] In some examples, the control unit may prevent the integral value from increasing any further when the integral value reaches a preset maximum value.

[0013] In some examples, the control unit may calculate the torque of the pump from the pressure and volume of the pump, and rotate the driving unit at a rotational speed greater than the second rotational speed if the instantaneous change rate of the torque of the pump is positive, and rotate the driving unit at a rotational speed less than the second rotational speed if the instantaneous change rate of the torque of the pump is negative.

[0014] In some examples, the control unit may calculate the torque of the pump from the pressure and volume of the pump, and if the instantaneous change rate of the torque of the pump is positive, rotate the driving unit at ‘the second rotational speed+D*dT / dt’ greater than the second rotational speed, and if the instantaneous change rate of the torque of the pump is negative, rotate the driving unit at ‘the second rotational speed+D*dT / dt’ less than the second rotational speed, wherein, D may be a gain value, and dT / dt may be an instantaneous change rate of the torque of the pump.

[0015] The above aspects, the appended claims and the examples disclosed herein either previously or hereinafter, may be suitably combined with each other, as will be apparent to one of ordinary skilled in the art.

[0016] Additional features and advantages are set forth in the following description, claims and drawings, and will in part be apparent to those skilled in the art or may be understood by practicing the disclosure described herein. Additionally, control units, computer-readable media and computer program products related to the aforementioned technical advantages are disclosed herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Aspects of the present disclosure, cited as examples, are described in detail below with reference to the accompanying drawings.

[0018] FIG. 1 is a drawing showing a schematic configuration of a hydraulic machine according to an example of the present disclosure.

[0019] FIG. 2 is a BSFC (Brake-specific fuel consumption) map of a hydraulic machine according to an example of the present disclosure.

[0020] FIG. 3 is a block diagram schematically showing the configuration of a driving unit torque controller of a control unit of a hydraulic machine according to an example of the present disclosure.

[0021] FIG. 4 is a graph showing that a driving unit torque controller adjusts the volume of the pump to control the torque of the driving unit.

[0022] FIG. 5 is a block diagram schematically showing the configuration of a driving unit rotational speed compensator of a control unit of a hydraulic machine according to an example of the present disclosure.

[0023] FIG. 6 is a graph showing that the driving unit rotational speed compensator performs compensation.

[0024] FIG. 7 is a block diagram schematically showing the overall configuration of a control unit of a hydraulic machine according to an example of the present disclosure.FORM FOR CARRYING OUT THE INVENTION

[0025] Aspects of the present disclosure, cited as examples, are described in detail with reference to the accompanying drawings below. The aspects described below provide information necessary to enable a person skilled in the art to practice the present disclosure.

[0026] FIG. 1 is a drawing showing a schematic configuration of a hydraulic machine according to an example of the present disclosure.

[0027] The hydraulic machine of the present disclosure typically relates to heavy equipment such as excavators, but may comprise, without limitation, any machine that performs work by obtaining power from the pressure of a fluid.

[0028] The hydraulic machine may comprise at least one actuator, a high pressure accumulator (310), a high pressure line (315), a pump (120), a driving unit (110), and a control unit (600). Additionally, in some examples, the hydraulic machine may comprise a low pressure accumulator (320) and a low pressure line (325). Additionally, in some examples, the hydraulic machine may comprise a tank. Additionally, in some examples, the hydraulic machine may comprise a valve (210) that allows or blocks the flow of fluid from the pump (120) to the high pressure line (315). Additionally, in some examples, the hydraulic machine may comprise a valve (210) that allows or blocks the flow of fluid from the low pressure line (325) to the tank. Additionally, in some examples, the hydraulic machine may comprise a valve (not shown) that allows or blocks the flow of fluid from the high pressure line (315) to the actuator, and a valve (not shown) that allows or blocks the flow of fluid from the actuator to the low pressure line (325).

[0029] The actuator is connected to a high pressure line (315) and receives high pressure pressurized fluid from the high pressure line (315), and uses the power from the pressure of the pressurized fluid to enable the hydraulic machine to perform work. Additionally, the actuator may be connected to the low pressure line (325) to discharge operation fluid into the low pressure line (325). The actuator may comprise, for example, a boom actuator (410), an arm actuator (420), a bucket actuator (430), a swing actuator (440), a travel actuator (451, 453), and the like. The boom actuator (410), the arm actuator (420) and the bucket actuator (430) may be hydraulic cylinders. The swing actuator (440) and the travel actuator (451, 453) may be hydraulic motors. The actuator may comprise an inlet port through which the actuator is connected to the high pressure line (315) and an outlet port through which the actuator is connected to the low pressure line (325).

[0030] The high pressure accumulator (310) is connected to the high pressure line (315). Additionally, as described above, the high pressure line (315) may be connected to the actuator to supply pressurized fluid to the actuator.

[0031] The low pressure accumulator (320) is connected to the low pressure line (325). Additionally, as described above, the low pressure line (325) may be connected to the actuator and the operation fluid may be discharged from the actuator through the low pressure line (325).

[0032] The pump (120) pressurizes the operation fluid to generate pressurized fluid and sends the pressurized fluid to the high-pressure accumulator (310) and the actuator through the high pressure line (315). The pump (120) may be a variable displacement pump in which the volume (volume of pressurized fluid discharged per rotation) is variable. The volume of the pump (120) may be controlled by the control unit (600). In some examples, the pump (120) has a swash plate, and the control unit (600) may control the volume of the pump (120) by sending a signal which changes the inclination angle of the swash plate of the pump (120) to the pump (120). The hydraulic machine may comprise at least one pump (120).

[0033] The driving unit (110) is a component that drives the pump (120) and may typically comprise an engine. However, the present disclosure is not limited thereto, and may be other types of driving units capable of driving the pump (120), such as an electric motor.

[0034] The control unit (600) can control each component of the hydraulic machine, particularly the volume of the pump (120) and the rotational speed of the driving unit (110). The control unit (600) may comprise an electronic control unit that receives a command input through an operator interface and / or reads values from various sensors, interprets the command and / or data, and then generates and outputs a control signal.

[0035] The tank provides operating fluid to the hydraulic pump (120) and may store operating fluid returned from the actuator through the low pressure line (325).

[0036] FIG. 2 is a BSFC (Brake-specific fuel consumption) map of a hydraulic machine according to an example of the present disclosure.

[0037] In a conventional hydraulic machine without an accumulator, the rotational speed of the driving unit (110) and the volume of the pump (120) are variable to maintain performance. In contrast, in a CPR hydraulic machine according to an example of the present disclosure, in a normal state where the actuator consumes a flow rate lower than the maximum flow rate that the pump (120) can supply, the rotational speed of the driving unit (110) can be controlled to be maintained at a preset first rotational speed. Additionally, at this time, the torque of the driving unit (110) may be controlled to be maintained at a preset first torque.

[0038] The first rotational speed and the first torque may be the rotational speed and the torque within a range that represents the optimal braking fuel consumption ratio of the hydraulic machine, the so-called sweet spot. A low braking fuel consumption ratio means that the same power can be achieved with relatively low fuel consumption.

[0039] In some examples, the first rotational speed and the first torque may be provided as preset values. Referring to FIG. 2, the first rotational speed may be, for example, 1400 rpm and the first torque may be preset to, for example, 700 Nm, and at these settings, the pump (120) may supply the maximum flow rate that the pump (120) can supply, for example, 220 lpm, to the high pressure line (315).

[0040] The rotational speed of the driving unit (110) may be directly controlled by a control signal. However, the torque of the driving unit (110) is controlled according to the volume of the pump (120) and the pressure difference between the inlet and outlet of the pump (120) (i.e., the outlet pressure of the pump (120)—the inlet pressure of the pump (120)) (assuming that most of the torque is consumed by the pump (120)). When the driving unit (110) is operated at a rotational speed of 1400 rpm, the pump (120) may be controlled to have, for example, a maximum volume, to match a torque of 700 Nm. However, these settings may have the following limitations:

[0041] If the actuator consumes a flow rate exceeding the maximum flow rate that the pump (120) can supply, the pressure in the high pressure line (315) drops rapidly and the performance of the actuator deteriorates. Therefore, the supply flow rate of the pump (120) supplied to the high pressure line (315) must be increased to recharge the high pressure accumulator (310) and also to send the required flow rate to the actuator. However, fixed rotational speed and torque cannot cope with these situations.

[0042] Therefore, in a hydraulic machine according to an example of the present disclosure, in a normal state where the actuator consumes a flow rate lower than the maximum flow rate that the pump (120) can supply, as described above, the control unit (600) may control the rotational speed of the driving unit (110) to be maintained at the first rotational speed and the torque of the driving unit (110) to be maintained at the first torque. However, if the actuator consumes a flow rate exceeding the maximum flow rate that the pump (120) can supply, the control unit (600) may control the rotational speed of the driving unit (110) to be a second rotational speed that is greater than the first rotational speed so that the pump (120) can supply more flow rate to the high pressure line (315).

[0043] Since the pressure in the high pressure line (315) drops when the actuator consumes a flow rate exceeding the maximum flow rate that the pump (120) can supply, it can be determined by measuring the pressure in the high pressure line (315) whether the actuator consumes a flow rate exceeding the maximum flow rate that the pump (120) can supply. Accordingly, the control unit (600) may control the rotational speed of the driving unit (110) to be maintained at a preset first rotational speed when the pressure in the high pressure line (315) is higher than a preset threshold pressure, and may control the rotational speed of the driving unit (110) to be a second rotational speed greater than the first rotational speed when the pressure in the high pressure line (315) is lower than the threshold pressure. Here, the second rotational speed may be determined according to the pressure in the high pressure line (315). In some examples, a lookup table in which the values of the pressure in the high pressure line (315) and the values of the second rotational speed are mapped may be stored in advance in the memory, and the second rotational speed may be determined by referring to this lookup table.

[0044] When the rotational speed of the driving unit (110) is controlled to be a second rotational speed greater than the first rotational speed, the rotational speed of the driving unit (110) deviates from the sweet spot described above. Accordingly, the torque of the driving unit (110) must be newly set, and the hydraulic machine according to an example of the present disclosure may control the torque of the driving unit (110) to be the second torque when the actuator consumes a flow rate exceeding the maximum flow rate that the pump (120) can supply, i.e., when the pressure in the high pressure line (315) becomes lower than the threshold pressure. The second torque is the torque that represents optimal fuel consumption efficiency at the second rotational speed, and the points of the second rotational speed and the second torque form a new spot line on the BSFC map, as shown in FIG. 2. In some examples, a lookup table in which the value of the second rotational speed and the value of the second torque are mapped (or, since the second rotational speed is determined depending on the pressure in the high-pressure line (315), a lookup table in which the value of the pressure in the high-pressure line (315) and the value of the second torque are mapped) may be stored in advance in the memory, and the second torque may be determined by referring to this lookup table.

[0045] Referring to FIG. 2, for example,

[0046] (1) when the safety lever is lowered, the driving unit (110) may maintain an idle state at 800 rpm.

[0047] (2) When the driver raises the safety lever but does not operate the actuator operating lever (not shown), the rotational speed of the driving unit (110) is maintained at 1400 rpm and the torque is maintained at the minimum torque.

[0048] (3) When the driver operates the actuator operating lever with the safety lever raised and the flow rate consumed by the actuator is lower than the flow rate that the pump (120) can supply at maximum volume at 1400 rpm, the rotational speed is maintained at 1400 rpm and the torque of the driving unit (110) repeatedly rises and falls within the sweet spot. In this state, the hydraulic machine operates at maximum efficiency.

[0049] (4) When the driver operates the actuator operating lever with the safety lever raised and the actuator consumes more than the maximum flow rate of the pump (120), the pressure in the high pressure line (315) drops because additional flow rate of the accumulator is used. In this case, the rotational speed of the driving unit (110) is controlled to increase to the second rotational speed in accordance with the amount of pressure drop in the high pressure line (315), thereby increasing the discharge flow rate of the pump (120). In addition, the second torque corresponding to the second rotational speed on the sweet line is controlled to be the torque of the driving unit (110). Referring to FIG. 2, the sweet line starts at, for example, 1400 rpm, 700 Nm, and along the sweet line, the second rotational speed increases and the second torque decreases slightly.

[0050] FIG. 3 is a block diagram schematically showing the configuration of a driving unit torque controller of a control unit of a hydraulic machine according to an example of the present disclosure, and FIG. 4 is a graph showing that the driving unit torque controller adjusts the volume of a pump (120) to control the torque of the driving unit (110).

[0051] With reference to FIG. 2, it was previously seen that in some examples, the control unit (600) may control the torque of the driving unit (110) to be the second torque (Treq) on the sweet line. However, an engine torque is the sum of the torque of the main pump, the torque of other small pumps, and other torques. Although the main pump accounts for a large portion of the torque of the driving unit (110), the torque of the driving unit (110) may deviate from the second torque (Treq) due to other torques. Additionally, it is difficult to predict how much the torque of the driving unit (110) will deviate.

[0052] Therefore, in some examples, the control unit (600) may perform feedback control so that the current torque (Tcur) of the driving unit (110) becomes equal to the second torque (Treq).

[0053] Referring to FIG. 3, in some examples, the control unit (600) may adjust the volume (Vcom) of the pump (120) according to the integral value obtained by integrating the values obtained by multiplying the difference (Treq−Tcur) between the second torque (Treq) and the current torque (Tcur) by the gain value and dividing the values by the inlet and outlet pressure difference values (Ppump). The control unit (600) may increase the volume of the pump (120) when the integral value increases and may decrease the volume of the pump (120) when the integral value decreases. For example,Volume change [cc / rev]=20*π*torque difference [Nm] / pump inlet and outlet pressure difference [bar]*gain

[0054] can be given. Here, π represents the ratio of the circumference.

[0055] Meanwhile, the volume (Vcom) of the pump (120) has a maximum value that cannot be increased any further in terms of hardware. Therefore, an anti-windup function can be added to the integrator. The anti-windup function prevents the integral value from increasing any further once the integral value reaches a preset maximum value. As long as the integral value is within the maximum value, the anti-windup function does not work. Otherwise, the volume (Vcom) of the pump (120) reaches a maximum value while the integral value continues to increase, and even if and attempt is made to reduce the volume (Vcom) of the pump (120) at this time, the volume (Vcom) of the pump (120) cannot be reduced until the amount exceeding the maximum value of the integral value is resolved.

[0056] In some examples, the hydraulic machine may comprise an engine sensor (520) to determine engine torque.

[0057] FIG. 5 is a block diagram schematically showing the configuration of a driving unit rotational speed compensator of a control unit of a hydraulic machine according to an example of the present disclosure, and FIG. 6 is a graph showing that the driving unit rotational speed compensator performs compensation.

[0058] When a high torque is applied to the driving unit (110), the rotational speed of the driving unit (110) decreases. Therefore, there is a need to address this phenomenon. As a solution, one could first consider compensating for the rotational speed drop using the actual rotational speed value. However, this can cause fluctuations in rotational speed, which in turn may reduce fuel consumption efficiency. Therefore, other solutions are needed.

[0059] The main reason for the decrease in the rotational speed of the driving unit (110) is the change rate of the torque of the driving unit (110). A major portion of the torque of the driving unit (110) is related to the torque of the main pump, which is calculated by the following equation.T=V·dP2⁢π,wherein V is volumetric displacement.Therefore, in a hydraulic machine according to an example of the present disclosure, the control unit (600) may compensate for the rotational speed of the driving unit (110) by using the instantaneous change rate of the torque of the pump (120). For example, in some examples, the control unit (600) calculates the torque of the pump (120) from the inlet and outlet pressure difference (Ppump) and the volume (Vcom) of the pump (120), and if the instantaneous change rate of the torque of the pump (120) is positive, rotates the driving unit (110) at a rotational speed (ωcom) greater than the second rotational speed (ωreq), and if the instantaneous change rate of the torque of the pump (120) is negative, rotates the driving unit (110) at a rotational speed (ωcom) less than the second rotational speed (ωreq). Since a drop in rotational speed is expected when the instantaneous change rate of the torque of the pump (120) is positive, the driving unit (110) is rotated at a rotational speed (ωcom) greater than the second rotational speed (ωreq) to compensate for the drop in rotational speed, thereby ultimately controlling the rotational speed of the pump (120) to be maintained at the second rotational speed (ωreq).

[0061] In some examples, the control unit (600) may rotate the driving unit (110) at a ‘second rotational speed (ωreq)+D*dT / dt’ that is greater than the second rotational speed (ωreq) when the instantaneous change rate of the torque of the pump (120) is positive, and may rotate the driving unit (110) at a ‘second rotational speed (ωreq)+D*dT / dt’ that is less than the second rotational speed (ωreq) when the instantaneous change rate of the torque of the pump (120) is negative. Here, D is the gain value, and dT / dt is the instantaneous change rate of the torque of the pump (120).

[0062] Referring to FIG. 5, in some examples, the control unit (600) multiplies the torque value of the pump (120) obtained by calculation by a gain value, and if the instantaneous change rate of the value is positive, the driving unit (110) may be rotated at ‘the second rotational speed (ωreq)+D*dT / dt’ which is greater than the second rotational speed (ωreq), and if the instantaneous change rate of the value is negative, the driving unit (110) may be rotated at ‘the second rotational speed (ωreq)+D*dT / dt’ which is smaller than the second rotational speed (ωreq).

[0063] FIG. 7 is a block diagram schematically showing the overall configuration of a control unit of a hydraulic machine according to an example of the present disclosure.

[0064] According to an example, the control of the rotational speed (ωcom) of the driving unit (110) and the volume (Vcom) of the pump (120) are shown below in sequence.

[0065] Process 1) Using a pressure sensor (510), the pressure in the high pressure line (315) is measured, and the rotational speed determining unit (630) determines the rotational speed of the driving unit (110). As described above with reference to FIG. 2, when the pressure in the high pressure line (315) is higher than the threshold pressure, the rotational speed is maintained at the preset first rotational speed, and when the pressure in the high pressure line (315) is lower than the threshold pressure, the rotational speed is increased to the second rotational speed (ox).

[0066] Process 2) The second rotational speed value (ωreq) is received from Process 1, and a torque determining unit (640) finds a second torque value (Treq) corresponding to the second rotational speed (ωreq) on the sweet line (i.e., a second torque value (Treq) that represents optimal fuel consumption efficiency at the second rotational speed).

[0067] Process 3) Receive the second torque value (Treq) from Process 2, and read the current torque value (Tcur) of the driving unit (110) using the engine sensor (520). As illustrated in FIG. 3, the driving unit torque controller (610) transmits a pump volume command (Vcom) that controls the volume of the pump (120) to the pump (120).

[0068] Process 4) The driving unit rotational speed compensator (620) receives a pump volume command value (Vcom) that controls the volume of the pump (120) from Process 3, and also receives a second rotational speed value (ωreq) from Process 1. And, using the pump volume command value (Vcom) and the second rotational speed value (ωreq), a rotational speed command (ωcom) regarding how much the second rotational speed (ωreq) should be increased or decreased to prevent the rotational speed droop phenomenon is transmitted to the driving unit (110).

[0069] The terms used herein are for the purpose of describing certain aspects only and are not intended to limit the present disclosure. Unless the context explicitly indicates otherwise, a singular form may comprise a plural form. Additionally, the term “and / or” comprises any and all combinations of one or more of the associated listed items. The term “comprising” specifies the presence of stated features, integers, steps, operations, elements and / or components, but does not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0070] Comparative terms such as “below,”“above,”“further up,”“further down,”“horizontal,” or “vertical” may be used herein to describe the relationship of any element depicted in the drawings to another element. These terms and the foregoing may comprise other orientations of the device in addition to the orientation depicted in the drawings. When an element is said to be connected or coupled to another element, this may mean not only a direct connection, but also other intervening elements. On the other hand, when it is said that an element is directly connected or coupled to another element, it implies that there are no intervening elements.

[0071] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Terms used herein should be interpreted to have a meaning consistent with their meaning in the context of this specification and the related art, and will not be interpreted in an idealized or overly formal sense, unless explicitly defined herein.

[0072] The present disclosure is not limited to the aspects described above and illustrated in the drawings, but rather, those skilled in the art will appreciate that various changes and modifications may be made therein within the scope of the present disclosure and the appended claims. While many aspects have been disclosed in the drawings and specification for purposes of illustration and not limitation, the scope of the present inventive concept is set forth in the claims that follow.

Claims

1. A hydraulic machine comprisingan actuator,a high pressure accumulator,a high pressure line that supplies pressurized fluid to the actuator and to which the high pressure accumulator is connected,a pump that supplies pressurized fluid to the high pressure line,a driving unit that drives the pump, anda control unit that controls the pump and the driving unit;wherein the control unitcontrols a rotational speed of the driving unit to be maintained at a preset first rotational speed when the pressure in the high pressure line is higher than a preset threshold pressure, andcontrols the rotational speed of the driving unit to be a second rotational speed greater than the first rotational speed when the pressure in the high pressure line becomes lower than the threshold pressure.

2. The hydraulic machine of claim 1,wherein the first rotational speed is a rotational speed within a range that represents an optimal braking fuel consumption ratio.

3. The hydraulic machine of claim 1,wherein the second rotational speed is determined according to the pressure within the high pressure line.

4. The hydraulic machine of claim 1,wherein the control unit controls the rotational speed of the driving unit to be maintained at the first rotational speed and torque of the driving unit to be maintained at preset first torque when the pressure in the high pressure line is higher than the threshold pressure, andcontrols the rotational speed of the driving unit to be a second rotational speed greater than the first rotational speed and the torque of the driving unit to be a second torque when the pressure in the high pressure line becomes lower than the threshold pressure.

5. The hydraulic machine of claim 4,wherein the first rotational speed and the first torque are rotational speed and torque within a range that represents an optimal braking fuel consumption ratio of the hydraulic machine.

6. The hydraulic machine of claim 4,wherein the second rotational speed is determined according to the pressure within the high pressure line, andthe second torque is the torque that represents an optimal braking fuel consumption ratio at the second rotational speed.

7. The hydraulic machine of claim 4,wherein the control unit controls A volume of the pump according to an integral value obtained by integrating values obtained by multiplying a difference between the second torque and a current torque (Treq−Tcur) by a gain value and dividing the values by an inlet and outlet pressure difference values of the pump.

8. The hydraulic machine of claim 7,wherein the control unitincreases the volume of the pump when the integral value increases, anddecreases the volume of the pump when the integral value decreases.

9. The hydraulic machine of claim 8,wherein the control unit prevents the integral value from increasing any further when the integral value reaches a preset maximum value.

10. The hydraulic machine of claim 1,wherein the control unitcalculates a torque of the pump from the pressure and a volume of the pump, androtates the driving unit at a rotational speed greater than the second rotational speed when an instantaneous change rate of the torque of the pump is positive, and rotates the driving unit at a rotational speed less than the second rotational speed when the instantaneous change rate of the torque of the pump is negative.

11. The hydraulic machine of claim 10,wherein the control unitcalculates the torque of the pump from the pressure and the volume of the pump, androtates the driving unit at ‘the second rotational speed+D*dT / dt’ which is greater than the second rotational speed when the instantaneous change rate of the torque of the pump is positive, and rotates the driving unit at ‘the second rotational speed+D*dT / dt’ which is less than the second rotational speed when the instantaneous change rate of the torque of the pump is negative;wherein D is a gain value and dT / dt is the instantaneous change rate of the torque of the pump.

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