Hydraulic systems, hydraulic units

Abstract

To provide a hydraulic unit that efficiently achieves energy savings and operates multiple actuators. [Solution] In the first state, the control unit drives the electric motor under high-speed conditions, which are relatively high speeds. In the second state, the control unit acquires information on the load factor of the electric motor at that time. The load factor of the electric motor can be calculated from the motor's rated output and the motor output at that time. Next, the control unit calculates the rotational speed according to the load factor of the electric motor and drives the electric motor under low-speed conditions, which are relatively low speeds (rotational speeds less than or equal to the rotational speed under high-speed conditions). More specifically, the control unit decreases the rotational speed ω2(L) of the electric motor as the load factor of the variable displacement pump decreases.

Description

Technical Field

[0001] The present invention relates to a hydraulic system or the like that operates a plurality of actuators.

Background Art

[0002] Conventionally, from the viewpoint of energy saving, a system for controlling a hydraulic pump with an inverter has been devised. For example, a system has been proposed in which the rotational speed of a fixed displacement pump is controlled with an inverter according to the discharge flow rate required for the operation of a hydraulic actuator (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, when trying to operate a plurality of actuators with one pump that performs such inverter control, when one actuator operates, the pressure of the hydraulic oil supplied to the other actuator may temporarily decrease and the operation may become unstable. In addition, inverter control considering the operations of a plurality of actuators is required, the control becomes complicated, and it is difficult to obtain a good energy-saving effect efficiently.

[0005] The present invention has been made in view of such problems, and an object thereof is to provide a hydraulic unit that efficiently obtains an energy-saving effect and operates a plurality of actuators.

Means for Solving the Problems

[0006] (1) In order to solve the above problems, the hydraulic system of the present invention comprises an electric motor, a plurality of variable displacement pumps driven by the electric motor, a plurality of actuators connected to the plurality of variable displacement pumps via a hydraulic fluid supply line, a detection unit for detecting the non-operating state and the operating state of each actuator, and a control unit for inverter control of the rotational speed of the electric motor. The control unit determines, using the plurality of detection units, a first state in which at least one of the actuators is in an operating state and a second state in which all of the actuators are in a non-operating state. In the first state, the electric motor is driven at a high speed condition, which is a relatively high speed. In the second state, information on the load factor of the electric motor is acquired, and the rotational speed of the electric motor is controlled according to the fluctuation in the load factor of the electric motor to drive the electric motor at a low speed condition, which is less than or equal to the rotational speed of the high speed condition. (2) The detection unit is a pressure detector, and the pressure detector may be placed in each of the hydraulic fluid supply lines connected to the actuator. (3) The control unit may determine that the operating state is when the pressure is below the cut-off point pressure of each of the variable displacement pumps is set as the pressure threshold, and that the operating state is when the pressure is below the pressure threshold, and that the operating state is when the pressure is above the pressure threshold. (4) In the second state, the control unit may set the same rotational speed as the high-speed condition if the load factor of the electric motor is equal to or greater than a predetermined value, and may reduce the rotational speed in accordance with the decrease in the load factor if the load factor of the electric motor falls below the predetermined value. (5) In addition, in order to solve the above problems, the hydraulic unit of the present invention comprises an electric motor, a plurality of variable displacement pumps driven by the electric motor, hydraulic fluid supply lines connected to each of the plurality of variable displacement pumps and which are passages for hydraulic fluid discharged from the variable displacement pumps, a pressure detector for detecting the pressure in each of the hydraulic fluid supply lines, and a control unit for inverter control of the rotational speed of the electric motor, wherein the control unit determines, using the plurality of pressure detectors, whether at least one of the hydraulic fluid supply lines is in a first state where the pressure is below a set pressure threshold, or whether all of the hydraulic fluid supply lines are in a second state where the pressure is above the pressure threshold, and in the first state, drives the electric motor at a high speed condition where the electric motor is relatively high speed, and in the second state, acquires information on the load factor of the electric motor and controls the rotational speed of the electric motor based on the fluctuation of the load factor of the electric motor to drive the electric motor at a low speed condition where the rotational speed is below the rotational speed of the high speed condition. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a hydraulic system, etc., that efficiently achieves energy-saving effects and operates multiple actuators. [Brief explanation of the drawing]

[0008] [Figure 1] This is a circuit diagram showing a hydraulic system. [Figure 2] This is a flowchart showing the control method for a hydraulic system. [Figure 3] (a) is a conceptual diagram showing the relationship between the pump discharge rate and motor load factor with respect to pressure for a typical variable displacement pump, and (b) is a conceptual diagram showing the characteristics of each variable displacement pump. [Figure 4] (a) is a conceptual diagram showing the fluctuation of the load factor of an electric motor, and (b) is a conceptual diagram showing the rotational speed control of an electric pump. [Figure 5] (a) and (b) are conceptual diagrams illustrating the rotational speed control of an electric pump. [Figure 6](a) is a conceptual diagram showing the fluctuation of the load factor of an electric motor, and (b) is a conceptual diagram showing the rotational speed control of another electric pump. [Figure 7] (a) is a diagram showing the fluctuation of the load factor of an electric motor, and (b) is a conceptual diagram showing the rotational speed control of another electric pump. [Modes for carrying out the invention]

[0009] The embodiments will be described in detail below with reference to the attached drawings. Figure 1 is a circuit diagram showing the hydraulic system 100 according to this embodiment. The hydraulic system 100 includes an electric motor 10, variable displacement pumps 20a and 20b, actuators 31a and 31b, pressure detectors 40a and 40b, a control unit 50, directional control valves 60a and 60b, etc. In the hydraulic system 100, the components excluding the actuators 31a and 31b and the directional control valves 60a and 60b will be referred to as the hydraulic unit 200.

[0010] In this embodiment, the hydraulic system 100 is shown as having two actuators 31a and 31b, but it is not limited to this. The number of actuators in the hydraulic system 100 is not particularly limited as long as it has multiple actuators. Furthermore, the application of the hydraulic system 100 is not particularly limited, but it can be applied to a machining center that automatically performs machining by chucking or clamping a tool or workpiece with multiple actuators 31a and 31b.

[0011] The electric motor 10 can be any motor that can be controlled by an inverter, such as an AC synchronous motor, AC induction motor, or DC servo motor.

[0012] The electric motor 10 is connected to variable displacement pumps 20a and 20b coaxially. In other words, multiple variable displacement pumps 20a and 20b are driven by a single electric motor 10. The variable displacement pumps 20a and 20b are pumps that can vary the discharge oil volume (pump capacity) in relation to pressure by variable control above a predetermined pressure, and for example, known variable displacement vane pumps and variable displacement piston pumps can be applied.

[0013] Variable displacement pumps 20a and 20b are respectively connected to actuators 31a and 31b via working oil supply lines 30a and 30b. That is, the working oil supply lines 30a and 30b are respectively connected to the plurality of variable displacement pumps 20a and 20b, and are the flow paths of the working oil discharged from the variable displacement pumps 20a and 20b.

[0014] Actuator 31a has a cylinder 70a and a piston 73a disposed so as to be able to advance and retreat with respect to the cylinder 70a. By the piston 73a, the inside of the cylinder 70a is divided into pressure chambers 71a and 72a. Similarly, actuator 31b has a cylinder 70b and a piston 73b disposed so as to be able to advance and retreat with respect to the cylinder 70b. By the piston 73b, the inside of the cylinder 70b is divided into pressure chambers 71b and 72b.

[0015] Direction control valves 60a and 60b for controlling the operations of actuators 31a and 31b are respectively disposed in the working oil supply lines 30a and 30b. That is, the working oil from each of the variable displacement pumps 20a and 20b is respectively supplied to the direction control valves 60a and 60b via the working oil supply lines 30a and 30b. In the state shown in FIG. 1, the P port and the A port, and the T port and the B port of each of the direction control valves 60a and 60b are respectively in communication. For this reason, the working oil from the variable displacement pumps 20a and 20b is respectively supplied to the pressure chambers 71a and 71b of the cylinders 70a and 70b, and the pistons 73a and 73b move in the direction of protruding from the cylinders 70a and 70b (right side in the figure). At this time, the working oil in the pressure chambers 72a and 72b is drained to the working oil tank 80 via the direction control valves 60a and 60b. Note that the state in which the pistons 73a and 73b are moving with respect to the cylinders 70a and 70b like this is defined as the "operating state" of the actuator.

[0016] Further, when the pistons 73a and 73b move to the moving ends (the right ends in the figure) of the cylinders 70a and 70b and the movement is completed, the supply amount of the hydraulic oil to the pressure chambers 71a and 71b decreases, and the pressure in the hydraulic oil supply lines 30a and 30b increases. In this state, the pistons 73a and 73b stop without further movement, and the pistons 73a and 73b maintain this state. Note that the state in which the pistons 73a and 73b stop with respect to the cylinders 70a and 70b and this state is maintained is defined as the "non-operating state" of the actuator. That is, the "non-operating state" is, for example, a pressure holding state or an intermediate stop state of the actuator. Note that the above pressure increase is just an example. For example, the pressure may increase in the hydraulic oil supply line up to the valve, and the examples of pressure increase are not limited to the above.

[0017] Also, when the direction control valves 60a and 60b are switched, the P ports and B ports, and the T ports and A ports of the respective direction control valves 60a and 60b communicate with each other. Therefore, the hydraulic oil from the variable displacement pumps 20a and 20b is respectively supplied to the pressure chambers 72a and 72b of the cylinders 70a and 70b, and the pistons 73a and 73b move in the direction of being pulled back into the cylinders 70a and 70b (the left side in the figure). At this time, the hydraulic oil in the pressure chambers 71a and 71b is drained to the hydraulic oil tank 80 through the direction control valves 60a and 60b. Note that, as described above, since the pistons 73a and 73b are moving with respect to the cylinders 70a and 70b, this state is the operating state of the actuator.

[0018] Further, when the pistons 73a and 73b move to the moving ends (the left ends in the figure) of the cylinders 70a and 70b and the movement is completed, the supply amount of the hydraulic oil to the pressure chambers 72a and 72b decreases, and the pressure in the hydraulic oil supply lines 30a and 30b increases. In this state, the pistons 73a and 73b stop without further movement, and the pistons 73a and 73b maintain this state. Note that, as described above, since this state is the state in which the pistons 73a and 73b stop and are held with respect to the cylinders 70a and 70b, this state is the non-operating state of the actuator.

[0019] In the above explanation, the operation of actuators 31a and 31b was described simultaneously, but it goes without saying that the operation of actuators 31a and 31b can be controlled individually. For example, it is not limited to the case where both actuators 31a and 31b are in an operating state or a non-operating state; there may be cases where one is in an operating state and the other is non-operating. Furthermore, the flow path configuration of the directional control valves 60a and 60b is not limited to the example described above, and the configuration is not particularly limited as long as it is possible to switch the operation of actuators 31a and 31b.

[0020] Pressure detectors 40a and 40b are installed in the hydraulic fluid supply lines 30a and 30b, respectively. Pressure detectors 40a and 40b are pressure sensors or pressure switches. In other words, pressure detectors 40a and 40b can detect whether the pressure in the hydraulic fluid supply lines 30a and 30b is above a predetermined pressure threshold. The pressure thresholds detected by pressure detectors 40a and 40b will be described in detail later.

[0021] The pressure detectors 40a and 40b are connected to the control unit 50. That is, the pressure information output from the pressure detectors 40a and 40b (including pressure measurements taken by the pressure sensors and signals output by the pressure switch when the pressure exceeds a set level) is input to the control unit 50. The control unit 50 includes, for example, a CPU that performs various calculations, an input / output unit that inputs and outputs various information, a storage unit that stores various conditions, and an inverter that controls the rotational speed (frequency) of the electric motor 10.

[0022] The control unit 50 controls the rotational speed of the electric motor 10 using an inverter based on the acquired pressure information. More specifically, the control unit 50 drives the electric motor 10 by switching between a high-speed condition, which is relatively high speed, and a low-speed condition, which is a rotational speed below the high-speed condition and relatively low speed. The control methods for the electric motor 10 under the high-speed and low-speed conditions will be described in detail later.

[0023] Next, the control method for the hydraulic system 100 will be described. Figure 2 is a flowchart showing the control method for the hydraulic system 100 (control method by the control unit 50). As mentioned above, the pressure in the hydraulic fluid supply lines 30a and 30b is detected by the pressure detectors 40a and 40b. The control unit 50 first acquires the pressure information output from the pressure detectors 40a and 40b (step S1).

[0024] Next, the control unit 50 compares a preset pressure threshold with the pressures in the respective hydraulic fluid supply lines 30a and 30b. That is, the control unit 50 determines whether the pressures in the hydraulic fluid supply lines 30a and 30b are below or above the pressure threshold (step S2).

[0025] Figure 3(a) shows the characteristics of a typical variable displacement pump, illustrating the relationship between the pump discharge volume V(P) and the discharge pressure P, and the relationship between the load factor L(P) of the electric motor used to drive the variable displacement pump and the discharge pressure P.

[0026] As shown in Figure 3(a), the pump discharge rate V(P) does not change much in the pressure range below pressure Pc, but in the pressure range above pressure Pc, the pump discharge rate V(P) decreases significantly with increasing pressure due to variable control. Hereafter, in the correlation diagram between pressure and pump discharge rate, the point (pressure Pc) where the rate of change of the pump discharge rate changes with pressure fluctuations will be referred to as the cutoff point pressure (the point at which cutoff begins).

[0027] Furthermore, the motor load factor L(P) has the characteristic that, in the pressure range lower than the cut-off point pressure Pc, it increases almost linearly with increasing pressure, and in the pressure range higher than the cut-off point pressure Pc, it decreases almost linearly with increasing pressure.

[0028] Figure 3(b) shows the characteristics of the variable displacement pumps 20a and 20b, and an example of the load factor of the electric motor 10 when the variable displacement pumps 20a and 20b are driven individually. In the example shown, the characteristics of the variable displacement pumps 20a and 20b are different, but they may have the same characteristics. Figure AV(P) This indicates the pump discharge rate of the variable displacement pump 20a, A L(P) This indicates the load factor of the electric motor 10 when the variable displacement pump 20a is driven by the electric motor 10. Also, in the figure B V(P) This indicates the pump discharge rate of the variable displacement pump 20b, and B L(P) This indicates the load factor of the electric motor 10 when the variable displacement pump 20b is driven by the electric motor 10.

[0029] As mentioned above, each of the variable displacement pumps 20a and 20b has a cut-off point pressure Pac and Pbc, and the load factor of the electric motor 10 is maximum at the cut-off point pressures Pac and Pbc. In this embodiment, since multiple variable displacement pumps 20a and 20b are driven by one electric motor 10, the load factor of the electric motor 10 is approximately the sum of the load factors corresponding to the state of each of the variable displacement pumps 20a and 20b.

[0030] In this embodiment, the cut-off point pressures Pac and Pbc of the variable displacement pumps 20a and 20b are used as the pressure thresholds mentioned above. That is, the control unit 50 determines, based on the pressure information from the pressure detectors 40a and 40b, whether the pressure in the hydraulic fluid supply lines 30a and 30b is less than or equal to the cut-off point pressures Pac and Pbc, or whether it is higher than the cut-off point pressures Pac and Pbc, respectively.

[0031] As described above, when actuators 31a and 31b are in a non-operating state, the flow rate of hydraulic fluid in the hydraulic fluid supply lines 30a and 30b decreases and the pressure increases compared to the operating state. Conversely, when actuators 31a and 31b are in an operating state, the flow rate of hydraulic fluid in the hydraulic fluid supply lines 30a and 30b increases and the pressure decreases compared to the non-operating state. Therefore, the control unit 50 can determine that actuators 31a and 31b are in a non-operating state when the hydraulic fluid pressure in the hydraulic fluid supply lines 30a and 30b is higher than their respective pressure thresholds. Similarly, the control unit 50 can determine that actuators 31a and 31b are in an operating state when the hydraulic fluid pressure in the hydraulic fluid supply lines 30a and 30b is below their respective pressure thresholds.

[0032] In this way, the control unit 50 acquires pressure information output from the pressure detectors 40a and 40b and can determine whether the actuators 31a and 31b are operating or not. In other words, the pressure detectors 40a and 40b function as detection units for detecting whether the actuators 31a and 31b are operating or not.

[0033] Next, the control unit 50 determines whether the pressures in all hydraulic fluid supply lines 30a and 30b are higher than their respective pressure thresholds (step S3). More specifically, the control unit 50 determines that the first state is present if the pressure in either hydraulic fluid supply line 30a or 30b is below the pressure threshold, and that the second state is present if the pressure in both hydraulic fluid supply lines 30a and 30b is higher than the pressure threshold. That is, the first state is when at least one of actuator 31a or actuator 31b is in operation, and the second state is when all actuators 31a and 31b are inactive.

[0034] When at least one actuator 31a or actuator 31b is operating, it is necessary to supply the hydraulic fluid required for operation to actuators 31a and 31b. For this reason, in the first state, the control unit 50 drives the electric motor 10 at a relatively high speed (for example, rotational speed ω1 = fixed at 1800 rpm) (step S4). For this reason, both variable displacement pumps 20a and 20b are driven at the same high speed.

[0035] Figure 4(a) is a diagram showing the relationship between motor load factor and pressure in Figure 3(a), but with the x and y axes reversed. Figure 4(b) is a conceptual diagram showing the rotational speed control of the electric motor 10, where the horizontal axis represents the load factor of the electric motor 10 and the vertical axis represents the rotational speed of the electric motor 10. As mentioned above, the load factor of the electric motor 10 is the sum of the load factors corresponding to the driving state of each of the multiple variable displacement pumps. However, for simplicity, Figure 4(a) shows the relationship between the pressure change in one variable displacement pump and the load factor change of the electric motor. For example, Figure 4(a) shows the relationship between the change in the discharge pressure of the other variable displacement pump 20a and the change in the load factor of the electric motor 10 when one variable displacement pump 20b is kept constant in a non-operating state. As mentioned above, the first state occurs when at least one of the pressures of the variable displacement pumps 20a and 20b is below the pressure threshold (Pt in the figure). Therefore, the control unit 50 rotates the electric motor 10 under constant high-speed conditions (rotational speed ω1).

[0036] When both actuator 31a and actuator 31b are operating, the hydraulic fluid necessary for the operation of actuator 31a and actuator 31b is discharged from either variable displacement pump 20a or 20b, respectively. Therefore, both variable displacement pumps 20a and 20b are driven in the pressure range below Pac and Pbc shown in Figure 3(b).

[0037] Also, for example, when one actuator 31a is in an operating state and the other actuator 31b is in a non-operating state, the variable displacement pump 20a in the operating state is driven in a pressure region below the cut-off point pressure Pac in Fig. 3(b). On the other hand, the variable displacement pump 20b in the non-operating state is driven in a pressure region higher than the cut-off point pressure Pbc in Fig. 3(b). That is, the variable displacement pump 20b reduces the pump discharge amount by variable control and supplies only the operating oil amount necessary to maintain the non-operating state of the actuator 31b to the actuator 31b.

[0038] Next, when the operations of all the actuators during operation are completed, since the pressures in the hydraulic oil supply lines 30a and 30b both become not less than the pressure threshold value, the control unit 50 determines that it is in the second state (step S3). For example, in Fig. 4(a), a region larger than the pressure threshold value Pt becomes the second state.

[0039] In this case, first, the control unit 50 acquires information on the load factor of the electric motor 10 at that time (step S5). The load factor of the electric motor 10 can be calculated from the rated output of the motor and the motor output at that time. Next, the control unit 50 calculates the rotational speed according to the load factor of the electric motor 10 and drives the electric motor 10 under a low-speed condition (a rotational speed not exceeding the rotational speed under the high-speed condition) (step S6). More specifically, as shown in Fig. 4(b), the control unit 50 decreases the rotational speed ω2(L) of the electric motor 10 as the load factor of the variable displacement pump decreases.

[0040] The calculation formula for ω2(L) is not particularly limited, but it can be set as ω2(L)=α×L(P)+β (where α(>0) and β are constants, and in the region where Pt<P (that is, the second state), ω2(L)≦ω1 is satisfied). As shown in the figure, it is desirable that ω1 = ω2(L)max.

[0041] When the system transitions from the first state to the second state, the control unit 50 immediately acquires the load factor information of the electric motor 10. Therefore, if inverter control of the electric motor 10 is performed immediately according to this load factor, the load factor will change even more significantly in accordance with the fluctuations in the rotational speed of the electric motor 10.

[0042] Figure 5(a) is a conceptual diagram showing the control of the rotational speed of the electric motor 10. In the first state, the rotational speed is constant, but in the second state, the rotational speed is immediately controlled, and the target value of the rotational speed changes immediately in accordance with the resulting change in the load factor. Therefore, there is a risk that the rotational speed will change drastically and become unstable.

[0043] Therefore, as shown in Figure 5(b), the control unit 50 may control the electric motor 10 so that the change in rotational speed becomes gradual. For example, the control unit 50 may compare the target rotational speed calculated based on the acquired load factor information with the current rotational speed and set the rate of change (amount of change) of rotational speed according to the difference. In other words, the rotational speed of the electric motor 10 may be controlled by lowering the gain of the load factor change or by setting an upper limit on the amount of rotational speed change per unit time (rate of change in rotational speed). By doing so, abrupt changes in rotational speed and the resulting abrupt fluctuations in the load factor can be suppressed, and the electric motor 10 can be controlled stably.

[0044] As described above, in this embodiment, multiple actuators 31a and 31b are operated by separate variable displacement pumps 20a and 20b, and pressure detectors 40a and 40b are installed for each of the hydraulic fluid supply lines 30a and 30b. Therefore, the influence of the operation of one actuator on the operation of the other actuator can be suppressed. In addition, when neither actuator 31a nor 31b is operating, energy saving can be achieved by reducing the rotational speed of the electric motor 10.

[0045] For example, in a machine tool or similar device, the time spent chucking or clamping a workpiece or tool is only a small fraction of the total time; most of the time is spent simply maintaining that state. In other words, the time spent in operation is very short compared to the time spent in non-operation. Therefore, by reducing the rotational speed of the electric motor 10 during the non-operation state, a significant energy saving effect can be achieved.

[0046] Furthermore, in the second state, the rotational speed of the electric motor 10 is controlled in accordance with the load factor fluctuation of the variable displacement pump. In this way, the rotational speed of the electric motor 10 is reduced as the load factor of the variable displacement pump decreases, enabling stable control.

[0047] For example, in the second state, if the operation is simply kept at a constant low speed (i.e., ω2 = constant (<ω1)), the behavior may become unstable due to hunting of the discharge volume of the variable displacement pump. In particular, when controlling multiple variable displacement pumps as in this embodiment, control becomes more difficult. In contrast, in this embodiment, instead of keeping the operation at a constant low speed in the second state, stable control becomes possible by correlating the load factor and rotational speed, simply by adjusting the gain of the speed command value.

[0048] In this embodiment, pressure detectors 40a and 40b were used to detect the state (non-operating or operating) of actuators 31a and 31b, but other methods are also applicable. For example, a detection unit that detects the switching signals of directional control valves 60a and 60b, or contact switches of cylinders 70a and 70b, can be used as a detection unit to detect the operating state of actuators 31a and 31b. However, if more actuators are added to the hydraulic system 100 of this embodiment, the control becomes more complex with each additional actuator if the switching signals of directional control valves 60a and 60b or contact switches of cylinders 70a and 70b are used. In contrast, the method of installing pressure detectors 40a and 40b on each hydraulic fluid supply line 30a and 30b is desirable because it allows for easy control and reliable understanding of the actuator's state, even when additional actuators are added.

[0049] Furthermore, in the embodiments described above, the cut-off point pressures Pac and Pbc of each variable displacement pump were used as pressure thresholds for determining whether the actuators 31a and 31b were in an inoperable or operating state. That is, the control unit 50 determined that the actuator was operating when the pressure was below the pressure threshold and that the actuator was in an inoperable state when the pressure was above the pressure threshold, but it is not limited to this.

[0050] Figure 6(a) is a diagram showing the relationship between the variable displacement pump and the load factor of the electric motor, similar to Figure 4(a), and Figure 6(b) is a conceptual diagram showing the rotational speed control of the electric motor 10, similar to Figure 4(b). As shown, a pressure Pt lower than the cut-off point pressure Pc of each variable displacement pump may be set as a pressure threshold for determining the state of the actuator (non-operating or operating state).

[0051] In this case, if the pressure of the variable displacement pump is less than or equal to pressure Pt, that is, if at least one of the pressures of the variable displacement pumps 20a and 20b is below the pressure threshold, the first state is reached and the electric motor 10 is driven at a constant high speed condition (rotational speed ω1).

[0052] Furthermore, if the pressure of each variable displacement pump is greater than the pressure Pt, that is, if the pressures of both variable displacement pumps 20a and 20b are greater than the pressure threshold, the system enters a second state. In this case, the control unit 50 reduces the rotational speed ω2(L)(≦ω1) of the electric motor 10 according to the load factor of the electric motor 10.

[0053] In this way, by setting the pressure threshold Pt to a pressure lower than the cut-off point pressure Pc, the control of the variable displacement pump can be made more stable. For example, the pressure higher than the cut-off point pressure is a region where the pump discharge volume fluctuates greatly with pressure changes. Therefore, when switching the rotation speed of the electric motor 10 in this region, if the fluctuation in rotation speed is too large, the variable displacement pump will try to increase the pump discharge volume instead. This behavior makes the control of pressure and pump discharge volume unstable and causes fluctuations in pressure and pump discharge volume. In contrast, by switching the rotation speed of the electric motor 10 in a region where the fluctuation in pump discharge volume with pressure changes is small, more stable control can be achieved.

[0054] In this case, as shown in Figure 6(a), after transitioning from the first state to the second state, the motor load factor increases up to the cut-off pressure as the pressure increases, and then decreases. That is, in the second state, even with the same motor load factor, there can be two states with different pump pressures. For this reason, the state of the variable displacement pump cannot be determined solely from the load factor information of the electric motor 10.

[0055] In contrast, in this embodiment, as shown in Figure 6(b), when the motor load factor is above a predetermined value (for example, above the load factor at the pressure threshold Pt), the control unit 50 controls the rotation speed to a constant value (=ω1) regardless of the change in the load factor. That is, a dead zone is provided in a predetermined pressure range (a load factor range above a predetermined value) in which the rotation speed is not changed in response to changes in the load factor. In this way, near the boundary between the region where the load factor increases and the region where it decreases, the control of the rotation speed due to frequent changes in the load factor is reversed, but stable control can be achieved by providing a dead zone. For this reason, even if the pressure threshold is set to a pressure lower than the cut-off point pressure, stable control of the rotation speed can be achieved.

[0056] Furthermore, if the pressure threshold Pa becomes significantly smaller than the cut-off pressure Pc, there is a risk that a sufficient pump discharge volume may not be obtained even when a sufficient pump discharge volume is required. In other words, the actuator may operate at a slow speed. For this reason, it is desirable to set the pressure threshold Pt to approximately 0.90 to 0.99 Pc.

[0057] It should be noted that the cut-off pressure of a variable displacement pump cannot always be determined externally. Furthermore, it may vary depending on the individual variable displacement pump. In contrast, generally, the cut-off pressure is often a predetermined pressure lower than the maximum discharge pressure of the variable displacement pump. For example, for a vane pump, the cut-off pressure is approximately -1 to -1.5 MPa of the maximum discharge pressure, and for a piston pump, it is approximately -0.1 to -0.3 MPa of the maximum discharge pressure. Therefore, the pressure threshold can be set to, for example, -1 to -2.5 MPa of the maximum discharge pressure for a vane pump, and more preferably, -1.3 to -2.0 MPa of the maximum discharge pressure. Similarly, the pressure threshold can be set to, for example, -0.1 to -1.0 MPa of the maximum discharge pressure for a piston pump, and more preferably, -0.2 to -0.5 MPa of the maximum discharge pressure. In this way, even if the exact cut-off pressure of each variable displacement pump cannot be determined, the pressure threshold can be set to be below the cut-off pressure.

[0058] Furthermore, for rotational speed control in the second state, it is not necessary to linearly correlate the load factor and rotational speed. Figure 7(a) is a diagram showing the relationship between the load factor of a variable displacement pump and an electric motor, similar to Figure 6(a), and Figure 7(b) is a conceptual diagram showing rotational speed control of the electric motor 10, similar to Figure 6(b). As shown, in the second state, the change in rotational speed in response to the change in motor load factor may be changed to form a curve.

[0059] Furthermore, in the embodiments described above, the rotational speed was controlled according to the motor load factor, but other methods may also be used. For example, the variable displacement pump with the highest pressure threshold or the variable displacement pump with the highest load factor may be used, and the discharge pressure of the variable displacement pump and its rotational speed may be correlated. For example, in the second state, the control unit 50 may decrease the rotational speed as the discharge pressure of the variable displacement pump increases.

[0060] [Other technological concepts 1] Electric motor and, Multiple variable displacement pumps driven by the aforementioned electric motors, Multiple actuators are connected to each of the multiple variable displacement pumps via a hydraulic fluid supply line, A detection unit for detecting the non-operating and operating states of each of the aforementioned actuators, A control unit that controls the rotation speed of the electric motor with an inverter, It is equipped with, The control unit determines, using a plurality of detection units, a first state in which at least one actuator is operating and a second state in which all actuators are not operating; in the first state, drives the electric motor at a relatively high speed; and in the second state, acquires information on the pressure of the variable displacement pump that has the maximum threshold among the variable volume pumps, and controls the rotational speed of the electric motor based on the fluctuation of the pressure of the variable displacement pump to drive the electric motor at a low speed below the rotational speed of the high speed condition.

[0061] Preferred embodiments of the present invention have been described above with reference to the attached drawings, but the present invention is not limited to these examples. It will be obvious to those skilled in the art that various modifications or alterations can be conceived within the scope of the technical idea disclosed herein, and these will naturally also fall within the technical scope of the present invention. [Explanation of symbols]

[0062] 10 Electric motors 20a, 20b Variable Displacement Pumps 30a, 30b Hydraulic fluid supply lines 31a, 31b Actuators 40a, 40b Pressure Detectors 50 Control Unit 60a, 60b Directional control valves 70a, 70b cylinders 71a, 71b, 72a, 72b Pressure chamber 73a, 73b Piston 80 Hydraulic oil tank 100 Hydraulic Systems 200 Hydraulic Units

Claims

1. Electric motor and, Multiple variable displacement pumps driven by the aforementioned electric motors, Multiple actuators are connected to each of the multiple variable displacement pumps via a hydraulic fluid supply line, A detection unit for detecting the non-operating and operating states of each of the aforementioned actuators, A control unit that controls the rotation speed of the electric motor with an inverter, It is equipped with, The control unit determines, using a plurality of detection units, a first state in which at least one actuator is operating and a second state in which all actuators are not operating; in the first state, drives the electric motor at a high speed; and in the second state, acquires information on the load factor of the electric motor and controls the rotational speed of the electric motor based on fluctuations in the load factor of the electric motor to drive the electric motor at a low speed below the rotational speed of the high speed condition.

2. The hydraulic system according to claim 1, wherein the detection unit is a pressure detector, and the pressure detector is arranged in each of the hydraulic fluid supply lines connected to the actuator.

3. The hydraulic system according to claim 2, characterized in that the control unit uses a pressure lower than the cut-off point pressure of each of the variable displacement pumps as a pressure threshold, determines that the state is operational when the pressure is below the pressure threshold, and determines that the state is non-operating when the pressure is higher than the pressure threshold.

4. In the second state, the control unit sets the same rotational speed as the high-speed condition when the load factor of the electric motor is above a predetermined value, and when the load factor of the electric motor falls below the predetermined value, it reduces the rotational speed in accordance with the decrease in the load factor, as described in claim 3.

5. Electric motor and, Multiple variable displacement pumps driven by the aforementioned electric motors, A hydraulic fluid supply line is connected to each of the multiple variable displacement pumps and is a passage for the hydraulic fluid discharged from the variable displacement pumps, A pressure detector for detecting the pressure in each of the aforementioned hydraulic fluid supply lines, A control unit that controls the rotation speed of the electric motor with an inverter, It is equipped with, The control unit determines, using a plurality of pressure detectors, whether at least one of the hydraulic fluid supply lines is in a first state where the pressure is below a set pressure threshold, or whether all of the hydraulic fluid supply lines are in a second state where the pressure is above the pressure threshold; in the first state, drives the electric motor under high-speed conditions where the motor is relatively fast; and in the second state, acquires information on the load factor of the electric motor, controls the rotational speed of the electric motor based on fluctuations in the load factor of the electric motor, and drives the electric motor under low-speed conditions where the rotational speed is below the rotational speed under the high-speed conditions.

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