JP5890040B2 - Pressure overshoot prevention system of electrohydraulic pump in hydraulic system - Google Patents

Description

  The present invention relates to a pressure overshoot prevention system for an electro-hydraulic pump in a hydraulic system, and in particular, when an actuator is in a state where it no longer operates, the pressure command to the electro-hydraulic pump is controlled in the hydraulic system. The present invention relates to a pressure overshoot prevention system for an electronic hydraulic pump in a hydraulic system that can prevent a chute.

Normally, in a hydraulic system, hydraulic oil is discharged from an electronic hydraulic pump, and the hydraulic oil stands by at the inlet of the main control valve. A plurality of spools are provided inside the main control valve, and a plurality of actuators are connected to the outside. A pilot pressure is generated at a flow rate requesting unit such as a joystick or a pedal, and the pilot pressure is supplied to the main control valve. In the main control valve, a specific spool is opened and closed by the pilot pressure, and hydraulic oil is supplied to an actuator linked to the spool by the opening and closing operation of the spool.
That is, by operating the joystick, the hydraulic oil discharged from the electrohydraulic pump is supplied to the actuator via the main control valve, whereby the actuator is operated.

The actuator has a configuration in which a piston rod is provided in a cylinder, and the piston rod operates in a direction in which the piston rod is expanded or contracted by the pressure of hydraulic oil. The piston rod may reach an end point where it cannot extend any more, or it may reach a situation where it cannot be further extended or contracted by a large external load. A situation where the piston rod cannot be actuated any more when a physical resistance is applied to the piston rod is defined as a stall.
Even if a stall occurs as described above, a joystick or a pedal may be continuously operated regardless of the operator's intention. Since the required flow rate is constantly required, the electrohydraulic pump may continue to discharge the hydraulic oil so as to supply the hydraulic oil to the actuator, so that the hydraulic pressure in the hydraulic system may reach a dangerous level.
In order to prepare for the above-mentioned danger, a safety device is provided in the hydraulic system. As an example of the safety device, a variable relief valve that is opened and discharges hydraulic oil when a pressure higher than an allowable pressure is set. Is mentioned.

In addition, among hydraulic systems, there is a hydraulic system including a main control valve with the center bypass line cut off and a pressure control type electrohydraulic pump. In such a hydraulic system, when the actuator stalls, the electrohydraulic There is a technique for reducing the flow rate by minimizing the swash plate angle of the pump.
However, under the actuator stall condition as described above, even when the hydraulic pressure is lowered to a safe level by opening the variable relief valve or reducing the swash plate angle to the minimum level, it is at a stable level. However, hydraulic oil is continuously discharged from the electrohydraulic pump, and a pressure peak is instantaneously generated by the flow rate of the hydraulic oil discharged at this time. Such a pressure peak has a problem of reducing the durability of the hydraulic system.

Hereinafter, a hydraulic system including an electronic hydraulic pump will be described with reference to FIG.
FIG. 1 attached herewith is a view for explaining a hydraulic system including a pressure control type electrohydraulic pump.
As shown in FIG. 1, the hydraulic system supplies a flow rate requesting unit 10 including a joystick, a pedal, and the like, an electronic hydraulic pump 50 that discharges hydraulic oil, and the hydraulic oil to the actuator 70 by opening and closing a spool. And a main control valve 60.
When the operator operates the joystick or the pedal, the flow rate request unit 10 generates a pilot pressure, and the pilot pressure is supplied to the main control valve 60.
The main control valve 60 has a plurality of spools therein. The spool operates at the pilot pressure, and allows the hydraulic oil to pass when the spool is opened, and interrupts the flow of the hydraulic oil when the spool is closed.
The actuator 70 has a configuration in which a piston rod is provided in a cylinder. The main control valve 60 is connected to the head side and the tail side of the piston to supply hydraulic oil. The piston rod is extended or contracted according to the direction of supplying hydraulic oil and the direction of discharging hydraulic oil. Stall occurs when the piston rod does not advance any further. That is, when the piston rod does not advance any more, the actuator 70 enters a stalled state.
The electronic hydraulic pump 50 discharges hydraulic oil in which hydraulic pressure is formed. The hydraulic oil pressure may be determined by the swash plate angle. For example, assuming that the shaft of the electrohydraulic pump 50 rotates at the same rotational speed, the pressure increases and the flow rate increases as the swash plate angle increases, and the pressure decreases and the flow rate decreases as the swash plate angle decreases. When the swash plate angle changes, it takes time to set a desired swash plate angle because there is a physical dynamic characteristic.
The swash plate angle of the electronic hydraulic pump 50 is adjusted by the pump regulator 40, and the pump regulator 40 is operated by the electronic proportional control valve 30.
The electronic proportional control valve 30 is operated by a pressure command, and the pressure command is received from the pump control unit 20.

The pump control unit 20 receives the pilot pressure value formed by the flow rate requesting unit 10 and the swash plate angle value of the electrohydraulic pump 50 and calculates a pressure command.
A pressure command is applied as an electrical signal from the pump control unit 20 to the electronic proportional control valve 30, the electronic proportional control valve 30 operates the pump regulator 40, and the pump regulator 40 adjusts the swash plate angle of the electronic hydraulic pump 50. Then, discharge the hydraulic oil at a flow rate corresponding to the required flow rate.
On the other hand, in the hydraulic system, an allowable pressure can be set, and when a pressure higher than the allowable pressure is formed, the variable relief valve 80 is opened and the hydraulic oil can be maintained at the set pressure. Further, since the allowable pressure set in the hydraulic system is variable, it can be set by variable control according to the capacity of the hydraulic system.

The operation of the pump controller 20 will be described in more detail with reference to FIG.
FIG. 2 attached herewith is a diagram for explaining the control logic of the pressure-controlled electrohydraulic pump in the conventional hydraulic system.
The pump control unit 20 receives the pilot pressure value formed by the flow rate requesting unit 10 and the swash plate angle value of the electrohydraulic pump 50 and calculates a pressure command.
When the flow rate request unit 10 is operated, a pilot pressure is formed, and the pressure value of the pilot pressure can be regarded as the required pressure value.
When the required pressure value is input, a flow rate command is generated at a ratio set in the flow rate command generation unit 21. The flow rate command generation unit 21 may be data input in advance by the manufacturer of the hydraulic system. That is, a current signal corresponding to the required pressure value is generated, and the current signal becomes a flow rate command.
If the swash plate angle value of the electrohydraulic pump 50 is known, the current discharge flow rate can be known.
In the flow rate command calculation unit 23, when the flow rate command is added (+) and the discharge flow rate is subtracted (−), the displacement flow rate delta Q is calculated.
The displacement flow rate delta Q is converted into a pressure command by the flow rate control unit 24. The pressure command is used to control the electronic proportional control valve 30 as described above.

When the pressure command is changed, the pressure changes accordingly. This will be described with reference to FIG.
FIG. 3 attached herewith is a diagram for explaining the mapping between the pressure and the pressure command in the control logic of the conventional pressure control type electrohydraulic pump.
As shown in FIG. 3, when the pressure command secondary pressure Px changes, the hydraulic oil pressure changes by a displacement pressure delta P.
That is, when the pressure command is changed by the electronic proportional control valve 30, the pressure transmitted to the pump regulator 40 changes following the pressure command of the electronic proportional control valve 30. At this time, since there is a physical dynamic characteristic, there is a time difference until the pressure of the pump regulator 40 actually follows and changes after the electronic proportional control valve 30 is actuated.

The above-described time difference causes a delay in the flow rate change, and an abnormal peak P occurs in the stall situation. This will be described with reference to FIG.
FIG. 4 attached is a diagram showing the flow rate change with time change for explaining an example in which the discharge flow rate has a peak by the pressure control type electrohydraulic pump of the prior art.
As shown in FIG. 4, when the joystick of the flow rate requesting unit 10 is operated, the required flow rate and the required hydraulic pressure increase from the joystick operation time point t0. At this time, the pump pressure command also increases, and the discharge flow rate of the hydraulic oil from the electrohydraulic pump 50 increases.
When the operation state of the joystick is continuously maintained, the actuator 70 is expanded or contracted.
At a certain moment, when the time point t1 at which the actuator 70 is stalled is reached, the piston rod of the actuator 70 does not move any more, and from this point on, the actuator 70 does not accept any more hydraulic oil, so that the hydraulic system operates. Oil pressure increases.
In addition, from the time t1 when the stall occurs, a flow rate variation occurs as a displacement flow rate delta Q.
When the pressure rises, in the pump pressure command, the pressure command of the electronic proportional control valve 30 rises with a gradient a until time t2 when the movement to the minimum level of the swash plate angle is completed. Further, in the pump regulator 40, the pressure rises steeply with a gradient of b1 larger than the gradient of a to form a peak p, and thereafter falls with a gradient of b2 to follow the pressure of the electronic proportional control valve 30.
That is, under the situation where the stall occurs, the flow rate of the hydraulic oil substantially discharged from the electrohydraulic pump 50 is excessively discharged by the area c indicated by hatching in FIG. Therefore, there is a problem that the durability of the hydraulic system is lowered by such overshooted hydraulic oil.

Therefore, the technical problem to be solved by the present invention is to stabilize the hydraulic system by reducing the discharge flow rate of the electronic hydraulic pump more rapidly when a stall occurs in which the hydraulic oil cannot be received in the actuator. It is to provide a pressure overshoot prevention system of an electrohydraulic pump in a hydraulic system.
The technical problem to be solved by the present invention is not limited to the technical problem described above, and other technical problems that are not mentioned are usually described in the technical field to which the present invention belongs from the contents described later. Those who have knowledge of the above can clearly understand.

A pressure overshoot prevention system for an electrohydraulic pump in a hydraulic system according to the present invention for solving the above technical problem adds (+) a flow rate command corresponding to a required pressure and discharges from the electrohydraulic pump 50. The flow rate command calculation unit 23 that calculates the displacement flow rate delta Q by subtracting (−) the discharged discharge flow rate; the flow rate control unit 24 that generates the first pressure command corresponding to the displacement flow rate delta Q; A stall determination unit 114 that determines a stall based on the first change rate of the discharge flow rate and the second change rate of the discharge flow rate; a flow rate pressure generation unit 115 that generates a hydraulic oil pressure value corresponding to the discharge flow rate; A gradient limiting unit that generates a limiting pressure command so as to limit an increasing gradient of the hydraulic oil pressure value; if the stall determining unit 114 determines that the stall is present, the limiting pressure is generated. A selection unit 117 that sets a command as a second pressure command; and a small value of the first pressure command and the second pressure command is selected as a final pressure command so as to control the electronic proportional control valve 30. A minimum pressure setting unit 120;
In the pressure overshoot prevention system for the electrohydraulic pump in the hydraulic system according to the present invention, the stall determination unit 114 determines that the stall is caused when the second change rate is greater than the first change rate. Also good.
In the pressure overshoot prevention system for the electrohydraulic pump in the hydraulic system according to the present invention, the limit pressure command is determined to be a stall rather than the first pressure command gradient a1 before time t4 when it is determined to be a stall. The second pressure command gradient a2 after the time point t4 may be smaller.
In the pressure overshoot prevention system for the electrohydraulic pump in the hydraulic system according to the present invention, if the selection unit 117 determines that the stall determination unit 114 has released the stall, the system pressure command is transmitted to the second pressure command. May be set as
Details of the embodiments are shown in the detailed description of the invention and the drawings.

  The pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system according to the present invention configured as described above changes the pressure command used for controlling the electrohydraulic pump more quickly when a stall occurs. The swash plate angle can be moved more rapidly in the hydraulic pump, which can significantly reduce the flow rate of hydraulic oil overshooted in the electrohydraulic pump. That is, it is possible to improve the durability of the hydraulic system by reducing the overshooting hydraulic oil.

It is a figure for demonstrating the hydraulic system provided with the pressure control type | mold electrohydraulic pump. It is a figure for demonstrating the control logic of the pressure control type | mold electrohydraulic pump in the hydraulic system of a prior art. It is a diagram for demonstrating mapping of a pressure and a pressure command in the control logic of the pressure control type electrohydraulic pump of a prior art. It is a diagram which shows the flow volume change accompanying the time change for demonstrating the example which a peak arises in a discharge flow volume with the pressure control type electrohydraulic pump of a prior art. It is a figure for demonstrating the control logic of a pressure control type electrohydraulic pump in the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system which concerns on one Example of this invention. It is a figure for demonstrating the logic of the maximum pressure restriction | limiting part in the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system which concerns on one Example of this invention. It is a diagram which shows the flow volume change accompanying the time change for demonstrating the example which prevents the peak in discharge flow volume in the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system which concerns on one Example of this invention.

Advantages and features of the present invention, and methods for achieving the same, can be clearly understood with reference to the accompanying drawings and embodiments described below.
In the specification, the same reference numerals indicate the same components, and the same components as those in the conventional technology are denoted by the same reference numerals, and detailed description thereof is omitted.
The terminology described below takes into account the function in the present invention, which can be changed by the manufacturer's intention or conventional means. Therefore, the term should be defined based on the contents of the entire specification.
In the present specification, a stall refers to an actuator when the piston rod reaches the end point when the piston rod of the actuator 70 is extended or contracted, or when the piston rod does not move any more due to an external load. 70 means the phenomenon of stopping.
Further, in the present specification, overshoot means that when the pump regulator 40 responds to a pressure command issued from the electronic proportional control valve 30, while the time is physically delayed due to dynamic characteristics, It means a phenomenon in which hydraulic oil is discharged.

Hereinafter, a pressure overshoot prevention system of an electrohydraulic pump and a control logic of a pressure control type electrohydraulic pump in a hydraulic system according to an embodiment of the present invention will be described with reference to FIGS. 1 and 5.
FIG. 1 attached herewith is a view for explaining a hydraulic system including a pressure control type electrohydraulic pump. FIG. 5 attached herewith is a diagram for explaining the control logic of the pressure-controlled electrohydraulic pump in the system for preventing pressure overshoot of the electrohydraulic pump in the hydraulic system according to one embodiment of the present invention.

As shown in FIG. 5, the pump control unit 100 is operated by the control logic of the electrohydraulic pump.
The pump control unit 100 controls the electrohydraulic pump 50 so as to match the required flow rate by calculating the required flow rate and the flow rate of the hydraulic oil discharged from the electrohydraulic pump 50.
The required flow rate is generated by operating the flow rate requesting unit 10. Specifically, when the flow rate request unit 10 is operated, a required pressure is generated, and the required flow rate is determined at a ratio set by the flow rate command generation unit 21 according to the required pressure. The required flow rate is used to control the flow rate control unit 24 as a required flow rate command.
The flow control unit 24 controls the electronic proportional control valve 30 when the pressure command corresponding to the flow control is converted.
The electrohydraulic pump 50 may output a swash plate angle value, and the swash plate angle value is given to the discharge flow rate calculation unit 22 so that the flow rate currently discharged from the electrohydraulic pump can be calculated.
The flow rate command calculation unit 23 receives information about the flow rate command and the discharge flow rate. When the flow rate command calculation unit 23 adds (+) the request command and subtracts (−) the discharge flow rate, a displacement flow rate delta Q indicating the degree of change in the flow rate is calculated.
The displacement flow rate delta Q is converted into a pressure command by the flow rate control unit 24. The pressure command is used to control the electronic proportional control valve 30 as described above.
The hydraulic system according to one embodiment of the present invention further includes a minimum pressure setting unit 120 between the flow rate control unit 24 and the electronic proportional control valve 30.
In addition, the minimum pressure setting unit 120 receives a pressure command from the maximum pressure limiting unit 110.
That is, the minimum pressure setting unit 120 selects the smaller one of the first pressure command input from the flow rate control unit 24 and the second pressure command input from the maximum pressure limiting unit 110. Thus, the electronic proportional control valve 30 is controlled.

The maximum pressure limiting unit 110 will be described with reference to FIG.
FIG. 6 attached herewith is a diagram for explaining the logic of the maximum pressure limiting unit in the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system according to one embodiment of the present invention.
As shown in FIG. 6, the maximum pressure limiter 110 receives the pump discharge flow rate and the flow rate command, and generates a second pressure command.
The second pressure command is generated by limiting the gradient of the pump pressure command that rises according to the map of the gradient limiting unit 116 that can set the maximum pressure according to the current discharge flow rate.
This will be described in more detail.
The flow rate calculation unit 111 receives the flow rate command and the pump discharge flow rate value, and calculates the required flow rate by adjusting them. The required flow rate may change according to the operation amount of the flow rate requesting unit 10, and the required flow rate may change rapidly or slowly, and such a change level is calculated by the first flow rate change calculation. The change rate of the required flow rate is calculated by the unit 112.
Further, the second flow rate change calculation unit 113 receives the value of the pump discharge flow rate, and calculates the second rate of change of the discharge flow rate of the hydraulic oil actually discharged from the electrohydraulic pump 50.
The stall determination unit 114 compares the first change rate of the flow rate command with the second change rate of the discharge flow rate to determine whether or not the actuator 70 has stalled. That is, when the second rate of change is greater than the first rate of change, it is determined that the stall state has occurred.

The stall state will be described in more detail. The stall state is a state in which the piston rod of the actuator 70 is not moving even though the driver is operating the joystick. Therefore, although the flow rate command exists, the actuator 70 does not accept the flow rate of the hydraulic oil. This means that the flow of the hydraulic system is blocked, and the swash plate angle of the electrohydraulic pump 50 rapidly decreases. That is, the state where the fluctuation value with respect to the difference between the flow rate command and the pump discharge flow rate is larger than the set value and the fluctuation value of the discharge flow rate of the electrohydraulic pump 50 is smaller than the set value is determined as the stall state.
Further, the hydraulic pressure value corresponding to the current pump discharge flow rate is set by the flow rate pressure generation unit 115 that follows the pump discharge flow rate value. The hydraulic oil pressure value increases at a gradient value set by the gradient limiting unit 116.
The selection unit 117 receives a limit pressure command set from the gradient value and a system pressure command set by the hydraulic system, and when the stall determination unit 114 determines that a stall is detected, Outputs a pressure command, and if it is not stalled, outputs a system pressure command.
That is, when the stall situation is canceled, the selection unit 117 cancels the selection of the limit pressure command, so that the system pressure command is output.
The pressure command output from the selection unit 117 is set as the second pressure command.
Thereafter, the minimum pressure setting unit 120 finally outputs a smaller one of the first pressure command given from the flow rate control unit 24 and the second pressure command given from the selection unit 117. To do.
Thus, in the stalled state, the flow rate of the hydraulic oil discharged from the electronic hydraulic pump 50 is more rapidly reduced by giving the pressure electronic proportional control valve 30 a pressure command having a limited gradient. Can solve the problem of overshooting.

The operation of reducing the discharge peak of hydraulic oil using the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system according to one embodiment of the present invention will be described with reference to FIG.
FIG. 7 of the accompanying drawings shows a flow rate change with a time change for explaining an example of preventing a peak in the discharge flow rate in the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system according to one embodiment of the present invention. FIG.
As shown in FIG. 7, when the joystick of the flow rate requesting unit 10 is operated, the required flow rate and the required hydraulic pressure increase from the joystick operation time point t0. At this time, the pump pressure command also increases, and the discharge flow rate of the hydraulic oil from the electrohydraulic pump 50 increases.
When the operation state of the joystick is continuously maintained, the actuator 70 is expanded or contracted.

At a certain moment, when the time point t1 at which the actuator 70 is stalled is reached, the piston rod of the actuator 70 does not move any more, and after this point, the actuator 70 cannot accept any more hydraulic fluid, The hydraulic oil pressure increases.
Further, the flow rate fluctuation is generated as a displacement flow rate delta Q from the occurrence time t1 of the stall.
When the pressure increases, the pressure command of the electronic proportional control valve 30 changes with respect to the pump pressure command. The swash plate angle changes at the first pressure command gradient a1 corresponding to the first pressure command by the initial pressure command, and the second pressure command from the time t4 when the stall determination unit 114 determines that the stall is present. The swash plate angle changes at a second pressure command gradient a2 corresponding to.
As described above, since the second pressure command has a limited gradient as compared with the first pressure command, the second pressure command gradient a2 is smaller than the first pressure command gradient a1. Formed to be.
When the first pressure command is changed to the second pressure command, the electrohydraulic pump 50 follows the pressure command. Therefore, at the time t1 when the stall occurs, the initial first actual pressure line b1 is The first pressure command gradient a1 is followed, but immediately after changing at the second pressure command gradient a2, it is stabilized after being reduced as exemplified by the second actual pressure line b2.

That is, in the pressure overshoot prevention system according to one embodiment of the present invention, the flow rate peak p can be significantly reduced by reducing the pressure command more rapidly.
Thereby, the time point t3 at which the movement of the swash plate angle to the minimum level is completed in the electrohydraulic pump 50 can be advanced, so that the flow rate c discharged while the swash plate angle is set to the minimum level when a stall occurs. Can be reduced.
Therefore, the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system according to the embodiment of the present invention finally uses the smaller value of the output values of the maximum pressure limiting unit 110 and the flow rate control unit 24 as a pressure command. By outputting, the pressure peak can be reduced in the stall state.
As described above, in the pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system according to the embodiment of the present invention, when a stall occurs, the pressure command used for controlling the electrohydraulic pump 50 is changed more rapidly. The swash plate angle of the electrohydraulic pump 50 can be moved more rapidly, and the flow rate c of hydraulic oil overshooted in the electrohydraulic pump 50 can be significantly reduced.

The embodiments of the present invention have been described above with reference to the accompanying drawings. However, those skilled in the art to which the present invention pertains may make other specific examples without changing the technical idea and essential features of the present invention. It will be understood that the present invention can be implemented in a general form.
Therefore, it should be understood that the embodiments described above are merely examples in all aspects, and are not construed as being limited to these embodiments. The scope of the present invention is indicated by the scope of claims to be described later, and all changes or modified forms derived from the meaning and scope of the claims and equivalent concepts thereof are within the scope of the present invention. It should be construed to be included.

  The pressure overshoot prevention system of the electrohydraulic pump in the hydraulic system according to the present invention improves the durability of the hydraulic system by rapidly reducing the discharge flow rate of the electrohydraulic pump when a stall occurs where the actuator does not operate It is effective to make it.

DESCRIPTION OF SYMBOLS 10 Flow request | requirement part 20 Pump control part 21 Flow rate command production | generation part 22 Discharge flow rate calculation part 23 Flow rate command calculation part 24 Flow rate control part 30 Electronic proportional control valve 40 Pump regulator 50 Electrohydraulic pump 60 Main control valve 70 Actuator 80 Variable relief valve 100 Pump control unit 110 Maximum pressure limiting unit 111 Flow rate calculating unit 112 First flow rate change calculating unit 113 Second flow rate change calculating unit 114 Stall determining unit 115 Flow rate pressure setting unit 116 Gradient limiting unit 117 Flow rate selecting unit 120 Minimum pressure setting unit t0 Joystick operation time t0 to t1 Actuator operation (movement) section t1 Stall time t2 Time when the movement of the pump swash plate angle to the minimum level is completed in the prior art t3 Movement of the pump swash plate angle to the minimum level When it is completed t4 A1 first pressure command gradient a2 second pressure command gradient b1 first actual pressure line b2 second actual pressure line c discharge flow rate p peak point

Claims (4)

A pressure overshoot prevention system for an electrohydraulic pump in a hydraulic system,
A flow rate command calculation unit 23 that adds (+) the flow rate command corresponding to the required pressure and subtracts (−) the discharge flow rate discharged from the electrohydraulic pump 50 to calculate the displacement flow rate delta Q;
A flow rate control unit 24 for generating a first pressure command corresponding to the displacement flow rate delta Q;
A stall determination unit 114 configured to determine actuator stall based on the first change rate of the flow rate command and the second change rate of the discharge flow rate;
A flow rate pressure generator 115 for generating a hydraulic oil pressure value corresponding to the discharge flow rate;
A gradient limiting unit 116 that generates a limiting pressure command so as to limit the increasing gradient of the hydraulic oil pressure value;
When the stall determination unit 114 determines that the stall is present, the selection unit 117 that sets the limit pressure command as a second pressure command; and the first pressure command so as to control the electronic proportional control valve 30. And a minimum pressure setting unit 120 that selects a smaller value of the second pressure commands as a final pressure command;
A pressure overshoot prevention system comprising:   2. The pressure overshoot prevention system according to claim 1, wherein the stall determination unit 114 determines that a stall occurs when the second rate of change is greater than the first rate of change. The limit pressure command is
The second pressure command gradient a2 after the time t4 determined to be a stall is smaller than the first pressure command gradient a1 before the time t4 when it is determined to be a stall. The pressure overshoot prevention system according to 1. The selection unit 117 is
2. The pressure overshoot prevention system according to claim 1, wherein a system pressure command is set as the second pressure command when the stall determination unit 114 determines that the stall is released.