JPH0684761B2 - Positioning control hydraulic circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positioning control hydraulic circuit for feedback-controlling a hydraulic cylinder that drives a press machine or the like using a proportional flow rate control valve having a simple structure.

<Prior Art> Conventionally, as a positioning control hydraulic circuit of this type, for example, there is one that servo-controls a hydraulic cylinder that drives a press machine. This hydraulic circuit has a servo valve installed between the hydraulic cylinder and the hydraulic source, detects the displacement of the piston rod of the hydraulic cylinder with a detector, and inputs it as a feedback signal to the control unit. The difference between the target value of the displacement amount and the feedback signal value is obtained with a device, a DC current signal proportional to this difference is output to the servo valve, and the main spool that is displaced according to this signal changes the oil flow rate supplied to the cylinder. By controlling the supply direction, the piston rod is operated so as to follow the change in the target value.

<Problems to be Solved by the Invention> However, the servo valve of the conventional positioning control hydraulic circuit described above has a disadvantage that it is relatively expensive because of its complicated structure and high processing accuracy is required. Since it controls the oil flow rate and direction from a relatively high pressure hydraulic source at a considerable response speed, if the nozzle, fixed orifice or filter is clogged with dust, the performance will be extremely reduced. There is a drawback that maintenance such as maintenance is not easy. Also, when the hydraulic cylinder is started by the servo valve or the drive direction is switched, the response speed of the servo valve is fast, so that a sudden pressure change occurs in the hydraulic cylinder, an impact occurs, and smooth operation cannot be performed. This drawback cannot be solved unless the individual elements of the complex electric control circuit that drives the servo valve are individually optimized. Further, in the above-mentioned conventional hydraulic circuit, since the rod side port of the hydraulic cylinder is directly switched and connected to the tank when the piston rod moves forward, if the solenoid of the servo valve does not work for some reason at this time, the piston There is a drawback that the rod descends to the stroke end and the punch at the tip of the rod collides with the die and damages it.

Therefore, an object of the present invention is to replace a conventional servo valve by using a proportional type flow rate direction control valve which is inexpensive and has a simple structure and is not easily affected by dust, so that maintenance such as maintenance and inspection is easy, and the control valve described above is used. The pressure difference between the front and rear of the flow rate control unit is kept constant by adjusting the output of the hydraulic power source by the power match method, and it is possible to accurately control the position of the rod regardless of the size of the load while saving energy, and the above control. It is an object of the present invention to provide a positioning control hydraulic circuit that does not damage a die or the like of a processing machine even if a valve malfunctions.

<Means for Solving Problems> In order to achieve the above object, the positioning control hydraulic circuit of the present invention includes a cylinder 1 and a position for detecting a rod displacement amount of the cylinder 1 as illustrated in FIG. A detector 12 and a feedback device for outputting a control signal indicating a deviation between a detection signal from the position detector 12 and a signal indicating a target value.
13, 14 and one port of the cylinder 1 which is proportionally controlled by the control signals from the feedback devices 13, 14.
A proportional flow direction control valve 2 for switching and connecting a load port A connected to Ph to a pressure port P and a tank port T;
A counter balance valve 4, which is connected to the other port Pr of the cylinder 1 and is opened by receiving a constant pressure from the other port Pr and has a free flow to the other port Pr,
18, a switching valve 10 for switching and connecting the main line 7 of the variable displacement pump 6 to the pressure port P of the proportional flow rate control valve 2 and the counter balance valves 4 and 18, and the proportional flow rate control valve. A pressure compensator 29 for maintaining a constant differential pressure between the pressure port P and the load port A of No. 2 is provided. The pressure compensator 29 includes the variable displacement pump 6, the pressure port P and the load port A. In response to the differential pressure, the swash plate control cylinder chamber 6a of the variable displacement pump 6 is connected to the main line 7
And a throttle switching valve 23 that is switch-connected to the tank 24.

<Operation> In the above positioning control hydraulic circuit, the piston rod 1a
When moving forward, the switching valve 10 is switched so that the main line 7 of the variable displacement pump 6 is connected to the pressure port P of the proportional flow direction control valve 2, and the pressure compensating device 29 including the throttle switching valve 23 is provided. While operating, a signal representing a target value corresponding to the rod protrusion amount is input to the feedback devices 13 and 14. Then, the feedback devices 13 and 14 are
A control signal representing the deviation between the current position detection signal of the piston rod 1a input from the position detector 12 and the target value signal is output to the proportional flow rate control valve 2. The proportional flow rate directional control valve 2 receiving the control signal maintains the opening between the pressure port P and the load port A in proportion to the control signal.
Here, the differential pressure between both ports P and A increases due to load fluctuations,
When it decreases, the throttle switching valve 23 that responds to this changes to connect the swash plate control cylinder chamber 6a to the main line 7 and the tank 24, respectively, and the discharge amount of the variable displacement pump 6 decreases.
As a result, the differential pressure is kept constant. Therefore, the flow rate of the hydraulic oil discharged from the variable displacement pump 6 is controlled momentarily through the pressure port P and the load port A of the proportional flow rate direction control valve 2 which is maintained at a constant differential pressure and a predetermined opening. Meanwhile, the hydraulic oil supplied to one chamber of the cylinder 1 is discharged to the tank of the other chamber of the cylinder 1, and the piston rod 1a
Is moved forward so as to approach the target position x. At this time, since the pressure difference between the ports P and A is kept constant by the pressure compensator 29, the forward speed of the piston rod 1a is proportional to the control signal regardless of the load on the load port A. The variable displacement pump 6 is set to a pressure corresponding to the pressure of the load port A and a flow rate corresponding to the opening depending on the opening between both ports P and A, so that energy loss is involved. No, move the piston rod forward. When the piston rod 1a reaches the target position x, the control signals output from the feedback devices 13 and 14 become 0, and the pressure port P and the load port A of the proportional flow direction control valve 2 are closed to close the piston. The rod 1a stops.

Next, when returning the piston rod 1a, the main line 7 of the variable displacement pump 6 is connected to the other port of the cylinder 1.
The switching valve 10 is switched so as to be connected to Pr, and target values corresponding to the rod immersion amount are set in the feedback devices 13 and 14. Then, the feedback devices 13 and 14 output the same control signal as described above, and the proportional flow rate directional control valve 2 receiving this control signal controls the above-mentioned control between the load port A and the tank port T, contrary to the above. Maintain the opening proportional to the signal. In this way, the hydraulic oil discharged from the variable displacement pump 6 is supplied to the other port Pr of the cylinder 1, and the hydraulic oil in one chamber of the cylinder 1 is momentarily controlled in flow rate between the ports A and T. While being discharged to the tank. At this time,
Similarly to the above, the pressure compensator 29 causes the piston rod 1a to move back so as to approach the target position x ', while having an energy saving effect. And the piston rod 1a
When reaches the target position, the control signal becomes 0, the ports A and T are closed, the discharge of hydraulic oil is stopped, and the piston rod 1a is stopped.

Moreover, since the counter balance valves 4 and 18 are provided between the switching valve 10 of the above circuit and the port Pr of the cylinder 1, the following action is added to the above action. That is, when the piston rod 1a of the cylinder 1 moves forward, the counter balance valve 4
The other port Pr of the cylinder is opened by receiving the hydraulic pressure of the variable displacement pump 6 acting through the ports P and A of the proportional flow direction control valve 2. Therefore, this opening pressure acts as back pressure on the piston rod 1a, so that overrun of the rod is prevented, and when the failure of the switching valve 10 causes the ports A and T to communicate with each other, the counterbalance valve 4 is closed. Stop the piston rod 1a. Therefore, the runaway of the rod 1a can be prevented. On the other hand, when the piston rod 1a returns, the pressure oil that has passed through the switching valve 10 from the variable displacement pump 6 becomes a free flow and is supplied to the other port Pr of the cylinder 1 through the counter balance valve 18, The return movement of the rod is smoothed.

<Examples> Hereinafter, the present invention will be described in detail with reference to illustrated examples.

FIG. 1 shows an example of a positioning control hydraulic circuit of a press machine. This hydraulic circuit is provided with a punch 11 mounted on the tip of the press machine and has a piston rod 1a that moves up and down. The main line 7 is connected to the load port A of the flow rate control valve 2 by the line 3 and the rod side port Pr of the cylinder 1 is connected to the counter balance valve 4 by the line 5 and is connected to the variable displacement hydraulic pump 6. In addition, the pump port P is connected to a secondary port B communicating with the pressure port P of the flow direction control valve 2 via a line 8 and a secondary port A communicating with the counter balance valve 4 via a line 9. An electromagnetic switching valve 10 for switching connection is provided.

The amount of displacement of the piston rod 1a in the protruding direction is detected by a position detector 12 such as an encoder or a potentiometer, converted into an electric signal and input to a comparator 13 as a feedback device, and the comparator 13 is given in advance. The deviation between the signal represented by the target value of the projected amount and the electric signal is obtained, and a control signal proportional to this deviation is output to the solenoid 15 of the flow direction control valve 2 via the amplifier 14. The flow direction control valve 2 has three ports of a pressure port P, a tank port T, and a load port A, one end of a spool 16 is provided with the solenoid 15 for operating the same, and the other end is provided with a spring 17. Then, while the solenoid 15 is demagnetized, the spring 17 positions the spool 16 in the neutral position where all three ports are closed, and when a positive control signal (target value> detection value) is applied to the solenoid 15, the spring The spool 16 is operated against 17 to maintain the opening between the pressure port P and the load port A in proportion to the control signal,
Further, when a negative control signal (target value <detection value) is applied to the solenoid 15, the spool 16 is operated in the reverse direction to maintain the opening between the load port A and the tank port T in proportion to the control signal. Controls the flow rate of hydraulic oil. An annular groove (not shown) and radial through holes communicating with the annular groove are provided at both ends of the three lands 16a of the spool 16 so as to eliminate the axial thrust acting on the spool 16 from the hydraulic oil. I have to.

The solenoid operated directional control valve 10 is a spring center type closed center three-position directional control valve operated by electromagnetic operation. The pump port P is the secondary port B and the tank port T is the secondary port A at the symbol position on the right side of the drawing. And the pump port P is connected to the secondary ports A and B at the left symbol position.
The counter balance valve 4 is opened by receiving cracking pressure from the rod side port Pr of the cylinder 1 and has a check valve 18 in parallel which allows a free flow to the rod side port Pr.

The variable displacement hydraulic pump 6 hydraulically operates the piston 6b in the cylinder chamber 6a against the spring 6c to tilt the swash plate in the neutral direction to reduce the discharge amount, while the hydraulic pressure in the cylinder chamber 6a is reduced. Is released to the atmosphere, the swash plate is returned to the maximum inclination to obtain the maximum discharge amount, and the hydraulic pressure for operating the piston 6b is controlled by the 3-port throttle switching valve 19. That is, the 3-port throttle switching valve 19 connects the port P to the cylinder chamber and the port B to the main line 7, respectively, and directly to one control port X and to the other control port Y on the spring 20 side. The pilot pressure is guided from the main line 7 via the throttle 21, respectively, and is provided with an electromagnetic proportional relief valve 22 which opens upon receiving the relief pressure set on the control port Y side to release the pilot pressure. The port P is continuously switched and connected to the port A and the port B according to the relief pressure. The hydraulic pressure supplied to the port A of the 3-port throttle switching valve 19 is controlled by the 3-port throttle switching valve 23. That is, the 3-port throttle switching valve
The port 23 connects the port A to the port A of the throttle switching valve 19, the port P to the main line 7, the port T to the tank 24, and the one control port X to the main line 7.
The other control port Y on the side of the spring 25 is connected to the port P of the 4-port 2-position switching valve 26, respectively, and the balance position of the pilot pressure of the port X and the spring force of the spring 25 and the pilot pressure of the port Y is set. According to the above, the port A is switched and connected to the port T and the port P. The spring-offset 4-port 2-position switching valve 26 is for selecting the hydraulic pressure to be supplied to the control port Y of the throttle switching valve 19, and the port P is connected to the main line in accordance with the demagnetization and excitation of the solenoid. Port A communicating with 7 via pilot line 27
Alternatively, the load port A of the flow rate direction control valve 2 is switched and connected to the port B communicating with the pilot line 28. In this way, the pressure compensator for maintaining the differential pressure between the pressure port P and the load port A of the flow direction control valve 2 constant by the throttle switching valve 19, the relief valve 22, the throttle switching valve 23 and the switching valve 26.
Make up 29.

The operation of the positioning control hydraulic circuit having the above configuration will be described below.

Now, it is assumed that the piston rod 1a of the cylinder 1 is lowered and the punch 31 at the tip of the rod presses the work 31 on the die 30. The operator switches the switching valve 10 to the right symbol position in the drawing, switches the switching valve 26 of the pressure compensating device 29 to the right symbol position in the drawing, and sends a signal representing the target value corresponding to the rod target position x. Input to one input terminal of the comparator 13. The rod target position x is set to be large according to the width L of the work 31 to be pressed.
Thus, the hydraulic oil from the hydraulic pump 6 is supplied to the pressure port P of the flow direction control valve 2 through the pressure port P of the switching valve 10 and the secondary port B, while the piston rod detected by the position detector 12 is supplied. The current position of 1a is input to the other input terminal of the comparator 13 as a position detection signal. Comparator
Reference numeral 13 denotes a deviation of both signals by subtracting the position detection signal from the signal representing the target value, and a control signal proportional to the deviation, in this case, a positive control signal is sent via an amplifier 14 to the flow direction control valve. Output to the second solenoid 15. Upon receipt of the control signal, the solenoid 15 operates the spool 16 against the spring 17 to maintain the opening between the pressure port P and the load port A in proportion to the control signal.

Then, the counterbalance valve 4 connected to the rod side port Pr is opened by the discharge pressure of the hydraulic pump 6 acting on the head side port Ph of the cylinder 1 via the load port A and the line 3, while the pressure of the load port A is opened. Is guided to the control port Y of the throttle switching valve 23 via the pilot line 28, the port B of the switching valve 26, and the port P. In this way, the hydraulic fluid discharged from the hydraulic pump 6 is supplied to the head side chamber of the cylinder 1 through the pressure port P and the load port A of the flow rate direction control valve 2 which is controlled to a predetermined opening degree while the flow rate is controlled moment by moment. , The hydraulic oil in the rod side chamber is discharged to the tank through the opened counterbalance valve 4, the secondary port A of the switching valve 10 and the tank port T, and the piston rod 1a is lowered so as to approach the target position x. To be The closer the piston rod 1a is to the target position, the smaller the deviation between the above two signals,
Since the control signal also becomes small and the flow rate of the hydraulic oil is reduced between the ports P and A, the descending speed of the piston rod 1a becomes slow. At this time, since the cracking pressure of the counterbalance valve 4 always acts as back pressure on the descending piston rod 1a via the piston 1b, overrun of the piston rod 1a is prevented and the solenoid 15 fails. Even when the flow direction control valve 2 is in the neutral position where the pressure port P and the load port A are closed, the counterbalance valve 4 is immediately closed to stop the piston rod 1a.
There is no risk of the piston rod running out of control and damaging the die 30.

On the other hand, as the pilot pressure, the pressure of the load port A of the flow direction control valve 2 is introduced to the control port Y on the spring 25 side, and the pressure port P of the control valve 2, that is, the pressure of the main line 7 is introduced to the control port X on the opposite side. The throttle switching valve 25 that slides slides to a position where the pressure of the port X balances the pressure of the port Y and the spring force of the spring 24, and connects the port A communicating with the port A of the throttle switching valve 19 to the tank 24 and the main line 7. And switch continuously to and. The throttle switching valve 19 is located at the symbol position on the right side of the drawing by the biasing force of the spring 20 and communicates with the port A and the port P unless the relief valve 22 is opened. As the pressure difference of the load port A exceeds the spring force of the spring 25, the throttle switching valve 25 shifts to the symbol position on the left side in the figure, and the main line 7 moves to port P and port A.
Also, the working oil is supplied to the cylinder chamber 6a of the hydraulic pump 6 through the ports A and P of the throttle switching valve 19, and the working oil causes the swash plate to tilt in the neutral direction, thereby reducing the discharge amount of the hydraulic pump 6. . On the contrary, as the pressure difference between the pressure port P of the control valve 2 and the load port A becomes smaller than the spring force of the spring 25, the throttle switching valve 25 shifts to the right symbol position in the figure, and the cylinder chamber 6a.
Is opened to the tank 24, the swash plate is returned to the direction of maximum inclination, and the discharge amount of the hydraulic pump 6 increases. Thus
The pressure compensator 29 including the switching valve 26, the throttle switching valves 23 and 19, and the relief valve 22 increases or decreases the discharge amount of the hydraulic pump 6 in accordance with the increase or decrease in the pressure of the load port A of the flow direction control valve 2 to increase the pressure. Throttle switching valve 23 for differential pressure between port P and load port A
The pressure corresponding to the spring force of the spring 25 (for example, 6 kg / cm ² ) is maintained.

Therefore, the flow rate of the hydraulic oil supplied to the cylinder 1 is determined by the spool 16 which is proportionally operated by the solenoid 15 which receives the control signal, regardless of the magnitude of the pressure at the load port A of the flow direction control valve 2, that is, the presence or absence of press working. It is uniquely determined by the opening between the ports P and A, and the descending speed of the piston rod 1a is not affected by the load applied to it. Further, since the pressure compensator 29 constitutes a so-called power match circuit for increasing or decreasing the discharge amount and discharge pressure of the variable displacement hydraulic pump 6 according to the magnitude of the load, a constant displacement pump and an unload valve are used. There is no energy loss like a pressure match circuit, and the energy saving effect is remarkable. When the switching valve 26 is switched to the left symbol position in the figure, the throttle switching valve
The pilot pressure of the main line 7 is applied to both control ports X and Y of 23, and the throttle switching valve 23 is always located at the right symbol position in the figure by the bias of the spring 25, and the port A and the throttle switching valve 19 The cylinder chamber 6a of the hydraulic pump 6 is opened to the tank 24 via the ports A and P of the hydraulic pump 6, whereby the discharge amount of the hydraulic pump 6 becomes maximum regardless of the pressure of the load port A of the flow direction control valve 2. The cylinder 1 can be driven at high output or at high speed. Further, although the switching valve 10 and the flow direction control valve 2 are located at the neutral position or the piston rod 1a is lowered to the stroke end, the hydraulic pump 6 continues to move for some reason and the pressure in the main line 7 is relieved. When the pressure becomes equal to or higher than the pressure, the relief valve 22 is opened, the pilot pressure of the control port Y of the throttle switching valve 19 is released, the throttle switching valve 19 is immediately switched to the left symbol position in the figure, and the cylinder chamber of the hydraulic pump 6 is opened. The pressure of the main line 7 is introduced into 6a, and the discharge amount of the hydraulic pump 6 is minimized. Therefore, the safety of the hydraulic circuit is ensured, and also in this case, there is no wasteful unloading unlike the pressure matching circuit, and the energy saving effect is exhibited.

Thus, if the piston rod 1a reaches the target position x,
The control signal output from the comparator 13 to the solenoid 15 via the amplifier 14 becomes 0, the pressure port P and the load port A of the flow direction control valve 2 are closed, and the counter balance valve 4
Is closed, the piston rod 1a is stopped, and press working is completed.

Next, when raising the piston rod 1a, the operator
The switching valve 10 is switched to the left symbol position in the drawing, and a signal representing a target value corresponding to the rod target position x'is input to the input terminal of the comparator 13. Then, the hydraulic oil from the hydraulic pump 6 passes from the pressure port P of the switching valve 10 through the secondary port A to the check valve 18 of the counter balance valve 4.
To the pressure port P of the flow direction control valve 2 via the secondary port B, while the comparator 13 outputs the signal indicating the target value and the position detection signal from the position detector 12 as described above. In contrast to the above, a negative control signal is output to the solenoid 15 of the flow direction control valve 2 via the amplifier 14. Then, with this solenoid 15, the spool 16
The pressure port P is closed by sliding on the side opposite to the spring 17, and the opening between the load port P and the tank port T is maintained in proportion to the control signal. Thus, the hydraulic fluid discharged from the hydraulic pump 6 is supplied to the rod side chamber of the cylinder 1 as a free flow through the check valve 18, and the hydraulic fluid in the head side chamber passes through the line 3 to the load port A and the tank port T. Is discharged to the tank while the flow rate is controlled every moment, and the piston rod 1a is raised so as to gradually approach the target position. When the piston rod 1a reaches the target position, the control signal becomes 0, the ports A and T are closed, the discharge of hydraulic oil to the tank is stopped, and the piston rod 1a is stopped.

In this case, the hydraulic oil from the hydraulic pump 6 that reaches the pressure port P of the flow direction control valve 2 via the secondary port B of the switching valve 10 does not flow to any of them because the pressure port P is closed by the spool 16. . Further, since the load port A has substantially the same pressure as the main line 7, the switching valve 26 of the pressure compensator 29 is
, The throttle switching valve 23 is always located at the right symbol position in the figure, the discharge amount of the hydraulic pump 6 becomes maximum, and the piston rod 1a rises at high speed. Further, similarly to the above, even if the flow direction control valve 2 is in the neutral position due to a failure of the solenoid 15 or the like while the piston rod 1a is rising, the closed counterbalance valve 4 prevents the piston rod 1a from falling. In addition, if the pressure in the main line 7 rises too much due to some trouble, the relief valve 22 works to minimize the discharge amount of the hydraulic pump 6, so that safety and energy saving can be achieved. When the spool 16 of the flow direction control valve 2 is at the position where the pressure port P and the load port A are opened and the piston rod 1a is lowered, if the switching valve 10 is switched to the symbol position on the left side in the drawing, The hydraulic oil discharged from the rod side chamber of the cylinder 1 can be merged with the hydraulic oil flowing from the hydraulic pump 6 to the pressure port P of the flow direction control valve 2 to form a differential circuit, and the piston rod 1a descends at high speed. Can be made.

In the above embodiment, the port Pr of the cylinder 1 and the switching valve
Although the circuit in which the counter balance valves 4 and 18 are provided between 10 has been described, the power match type control by the pressure compensator 29 including the throttle switching valve 23 and the variable displacement pump 6 is possible even if this valve is omitted. Needless to say, the energy saving effect is exhibited.

<Effects of the Invention> As is apparent from the above description, the positioning control hydraulic circuit of the present invention includes the cylinder 1, the position detector 12 for detecting the rod displacement amount of the cylinder 1, and the position detector 12 for detecting the position. Feedback devices 13 and 14 that output a control signal that represents a deviation between a detection signal and a signal that represents a target value, and are proportionally controlled by control signals from the feedback devices 13 and 14, and are connected to one port Ph of the cylinder 1. The load port A is connected to the proportional flow direction control valve 2 for switching and connecting the pressure port P and the tank port T, and the other port Pr of the cylinder 1 to receive a constant pressure from the other port Pr. The counterbalance valves 4 and 18 which are opened at the same time as the free flow to the other port Pr and the main line 7 of the variable displacement pump 6 are connected to the proportional flow direction control valve 2 described above.
Of the pressure port P and the counter balance valves 4 and 18, and a pressure compensator 29 for keeping the differential pressure between the pressure port P and the load port A of the proportional flow direction control valve 2 constant. The pressure compensator 29 includes the variable displacement pump 6 and the swash plate control cylinder chamber 6a of the variable displacement pump 6 in response to the differential pressure between the pressure port P and the load port A. Since the throttle switching valve 23 is connected to the main line 7 and the tank 24 for switching connection, it is less expensive and simpler in structure than a hydraulic circuit using a conventional servo valve and less susceptible to dust, and maintenance and inspection. Maintenance of the cylinder rod is facilitated, and the position of the cylinder rod can be controlled accurately regardless of the load, while saving energy, and the cylinder can be started and reciprocally switched smoothly without causing an impact. You can

Moreover, by providing a counterbalance valve between the switching valve and the cylinder, this valve can prevent overrun of the cylinder rod and runaway at the time of failure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing an embodiment of a positioning control hydraulic circuit of the present invention applied to a press machine. 1 ... Cylinder, 1a ... Piston rod, 2 ... Electromagnetic proportional flow rate control valve, 4 ... Counterbalance valve, 6 ... Variable displacement hydraulic pump, 7 ... Main line, 10 ... Electromagnetic switching Valve, 12 ... Position detector, 13 ... Comparator, 15 ... Solenoid, 18 ... Check valve, 19,23 ... Throttle switching valve, 22 ... Electromagnetic proportional relief valve, 26 ... Switching valve, 29 …… Pressure compensator.

Continuation of front page Judgment panel Judgment chief Judge Tetsuo Sekiguchi Judge Jitsumi Kajisawa Judge Masayuki Ogura (56) References JP 59-8681 (JP, B2)

Claims (1)

[Claims] 1. A cylinder (1), a position detector (12) for detecting a rod displacement amount of the cylinder (1), a detection signal from the position detector (12) and a signal representing a target value. A feedback device (13, 14) that outputs a control signal representing a deviation and a proportional control by a control signal from the feedback device (13, 14) are connected to one port (Ph) of the cylinder (1). Load port (A)
A proportional flow direction control valve (2) for switching connection between the pressure port (P) and the tank port (T), and the cylinder (1)
Connected to the other port (Pr) of the other port (Pr)
Counterbalance valve (4, 4) that opens by receiving a constant pressure from the valve and makes the flow to the other port (Pr) a free flow
18) and the main line (7) of the variable displacement pump (6)
Is connected to the pressure port (P) of the proportional flow rate control valve (2) and the counter balance valve (4, 18), and the proportional flow rate control valve (2).
Pressure compensator (29) for maintaining a constant differential pressure between the pressure port (P) and the load port (A), the pressure compensator (29) including the variable displacement pump (6) and The swash plate control cylinder chamber (6a) of the variable displacement pump (6) responding to the differential pressure between the pressure port (P) and the load port (A)
And a throttle switching valve (23) for switching and connecting to the main line (7) and the tank (24).