JP2003343504A - Water pressure driving apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water pressure driving apparatus having a water pressure cylinder driven by working water sent from a water pressure pump, which controls the water pressure pump and cylinder with small control operating force, and improves a control response property, and also maintains high operating performance of the water pressure cylinder by regularly monitoring water pressure leakage within the water pressure cylinder. <P>SOLUTION: The water pressure driving apparatus is provided so as to drive a load so that working water fed from the water pressure pump directly driven by a variable speed motor is applied to the water pressure cylinder. The water pressure driving apparatus comprises a piston stroke detector detecting a moved position of a piston of the water pressure cylinder, and a motor control device controlling running speed of the variable speed motor so that a discharged amount of the water pressure pump becomes equivalent to a discharged amount corresponding to a targeted piston speed of the water pressure cylinder on the basis of a detected value for the moved position of the piston. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to a drive device for construction machines, a drive device for steering gears for ships, and the like, and causes working water from a hydraulic pump directly connected to a variable speed motor to act on a hydraulic cylinder. And a hydraulic drive unit configured to drive a load by the hydraulic cylinder.

[0002]

2. Description of the Related Art FIG. 5 shows a block diagram of a hydraulic control system of a hydraulic drive system for a construction machine disclosed in Japanese Patent Laid-Open No. 10-96251. In the figure, 101
Is a hydraulic pump directly connected to the engine 102, 10
Reference numeral 3 is an actuator, 104 is a directional control valve connected to the actuator, and the hydraulic fluid sent from the hydraulic pump 101 is controlled by the directional control valve 104 to be supplied to and discharged from the actuator 103. 1
Reference numeral 05 is a remote control device for remotely operating the directional control valve 103.

Reference numeral 8 is a swash plate, and 107 is a swash plate angle adjusting device for controlling the swash plate angle (inclination angle) of the swash plate 8. Reference numeral 108 denotes a rotation detector that detects the engine speed of the engine 102,
A controller 109 inputs a swash plate angle control signal to the swash plate angle adjusting device 107 based on the detected value of the engine speed in the controller 109. Also, 11
Reference numeral 1 denotes a pressure detector that detects the discharge pressure of the hydraulic pump 101 and inputs it to the swash plate angle adjusting device 107. The swash plate angle adjusting device 107 sets the swash plate angle of the swash plate 8 to the engine speed. The corresponding swash plate angle is corrected and adjusted by the discharge pressure.

FIG. 6 is a cross-sectional view of the upper half of the swash plate type hydraulic pump 101 in the vicinity of the connecting portion between the piston and the swash plate along the centerline of the pump rotation axis. In the figure, 1 is an engine 10.
A pump shaft connected to 2 and driven to rotate, 01 is a rotation axis of the pump shaft, 2 is a casing, and 3 is a base plate. Reference numeral 5 denotes a cylinder block fitted to the pump shaft 1 via a spline 18 and rotationally driven in synchronization with the pump shaft 1. A plurality of cylinders 6 are bored in the cylinder block 5 at equal intervals in the circumferential direction. Reference numeral 7 denotes a piston fitted in each of the cylinders 6 so as to be able to slide back and forth. Reference numeral 8 denotes a swash plate attached to the base plate 3, and the swash plate angle can be changed by a swash plate angle adjusting device 107 (see FIG. 5).

A slipper 1 is provided at the base of each piston 7.
5 is slidably mounted along a spherical surface in the form of a spherical bearing 015, each slipper 15 being attached to the swash plate 8 by a roller 14;
The sliding surface 23 is in sliding contact with the thrust plate 14 that is supported so as to be movable in the circumferential direction. Reference numeral 17 is an annular retainer ring that supports the slippers 15. Reference numeral 11 denotes a valve plate which is arranged on the oil supply / discharge hole 011 (see FIG. 4) side of each cylinder 6 and fixed to the end plate 4.

When the pump shaft 1 is rotationally driven by the engine 102 during the operation of the hydraulic pump 102, the cylinder block 5 rotates. A slipper 15 connected to the head of a piston 7 that is reciprocally fitted in a cylinder 6 by the rotation of the cylinder block 5.
Slides on the sliding surface 17 with the thrust plate 14 forming the inclined surface of the swash plate 8, whereby the piston 7 reciprocates in the cylinder 6 while rotating with the cylinder block 5, and the valve plate 11 The oil sucked from the suction hole is pressurized and sent to the discharge hole of the valve plate 11.

[0007]

In a hydraulic drive system having a swash plate type hydraulic pump 101 as shown in FIGS. 5 and 6, the cylinder block 5 rotates to reciprocate and slide in the cylinder 6. Spherical bearing 0 on the head of piston 7
The slipper 15 connected via 15 slides on the sliding surface 23 between the slipper 15 and the thrust plate 14 fixed to the swash plate 8 while periodically receiving a large reaction force from the piston 7. Therefore, the sliding surface 23 between the slipper 15 and the thrust plate 14 is periodically subjected to a large reaction force from the piston 7 that pumps oil.

Therefore, in such a conventional technique, a sliding surface 23 between the slipper 15 and the thrust plate 14 is formed.
Since the swash plate angle adjusting device 107 changes the inclination angle (swash plate angle) of the swash plate 8 while the large piston reaction force acts on the piston reaction force, The swash plate angle adjusting device 107 must be operated against the swash plate angle adjusting device 107.
Requires a large operating force, and the swash plate angle adjusting device 107 has a large operating resistance, so that the response of the swash plate angle control is not good.

When the hydraulic pump 101 is used as a hydraulic pump using water as a working fluid and the actuator 103 is operated as a hydraulic cylinder, since the viscosity of water is smaller than that of oil, a partition is formed by a piston. The leakage of fluid (water) between the oil chambers in the hydraulic cylinder tends to increase, and if the amount of leakage increases, the hydraulic cylinder may not function.

In view of the above problems of the prior art, the present invention provides a hydraulic drive system including a hydraulic cylinder driven by operating water from a hydraulic pump, which controls the hydraulic pump and the hydraulic cylinder with a small control operation force. It is an object of the present invention to improve the control responsiveness as well as to maintain the operating performance of the hydraulic cylinder at a high level by constantly monitoring the hydraulic pressure leak in the hydraulic cylinder.

[0011]

In order to solve the above problems, the present invention provides an invention as set forth in claim 1, which introduces working water sent from a hydraulic pump directly connected to a variable speed motor to a hydraulic cylinder. In a hydraulic drive device configured to actuate working water on a hydraulic cylinder to drive a load by the hydraulic cylinder, a piston stroke detector for detecting a piston displacement of the load driving end of the hydraulic cylinder, and the piston stroke A detection signal of a piston displacement is input from a detector, and the rotation speed of the variable speed motor is adjusted based on the detection value of the piston displacement so that the discharge amount of the hydraulic pump becomes a discharge amount corresponding to a target piston speed of the hydraulic cylinder. A hydraulic drive device is proposed, which comprises a motor control device for controlling.

The invention according to claim 2 relates to a specific configuration of the motor control device, and in the motor control device according to claim 1, the motor control device has a target piston speed and a piston displacement set based on a target output of the hydraulic cylinder. Speed deviation calculating means for calculating a speed deviation from the detected piston speed calculated on the basis of the detected value, and discharge amount correction for calculating a discharge amount correction value of the hydraulic pump corresponding to the speed deviation from the speed deviation calculating means. It is characterized by comprising a value calculation means and a motor rotation speed calculation section for calculating the rotation speed of the variable speed motor based on the discharge quantity correction value from the discharge quantity correction value calculation means.

According to this invention, the piston displacement at the load driving end of the hydraulic cylinder is detected and input to the motor control device, and in the motor control device, the target piston speed corresponding to the target output of the hydraulic cylinder and the detected piston displacement. Based on the speed deviation and the discharge amount correction value of the hydraulic pump based on the speed deviation, and based on the discharge amount correction value, the discharge amount of the hydraulic pump corresponds to the target piston speed of the hydraulic cylinder. Since it is possible to control the rotation speed of the variable speed motor so as to be equal to the amount, the discharge of the hydraulic pump is controlled by controlling the rotation speed of the variable speed motor based on the detected value of the piston displacement at the load drive end of the hydraulic cylinder. The quantity control and the output control of the hydraulic cylinder can be performed with high accuracy.

According to this invention, therefore, the discharge rate of the hydraulic pump is controlled by controlling the rotation speed of the variable speed motor, so that a pump discharge rate control apparatus such as a swash plate angle adjusting apparatus as in the prior art is large. It becomes unnecessary to control with the operating force, and the discharge amount of the hydraulic pump can be controlled with a small operating force, and the control response is improved.

The invention according to claim 3 is, in addition to claim 2,
A cylinder differential pressure detector that detects a differential pressure between two water chambers partitioned by the piston of the hydraulic cylinder and inputs the differential pressure to the motor control device is provided, and the motor control device is configured to detect the differential pressure from the cylinder differential pressure detector. Differential pressure correction value calculation means for calculating a correction value for the variable speed motor rotation speed based on a deviation between the differential pressure detection value and a preset reference differential pressure and inputting it to the motor rotation speed calculation unit. .

According to this invention, the cylinder differential pressure detector detects the differential pressure between the two water chambers of the hydraulic cylinder, and the motor control unit controls the variable speed motor rotation speed so that the differential pressure holds the reference value. Since the variable speed motor rotation speed is controlled by correcting the control signal of, the leakage amount of the fluid (water) between the water chambers in the hydraulic cylinder is controlled so that the differential pressure between the water chambers does not become less than the differential pressure reference value. Therefore, it is possible to constantly monitor the hydraulic pressure leak in the hydraulic cylinder and maintain the operating performance of the hydraulic cylinder high.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to the embodiments shown in the drawings. However, the dimensions, materials, shapes, relative positions, etc. of the components described in this embodiment are not intended to limit the scope of the present invention thereto, unless there is a specific description, and are merely illustrative examples. Nothing more.

FIG. 1 is an overall configuration diagram of a hydraulic drive system according to an embodiment of the present invention, and FIG. 2 is a control block diagram. FIG. 3 is a sectional view taken along the pump axis line showing the structure of the hydraulic pump in the above embodiment.

In FIG. 1, which shows the overall construction of a hydraulic drive system according to an embodiment of the present invention, 32 is a variable speed motor, 31 is a hydraulic pump directly connected to the variable speed motor 32, and 33 is a hydraulic pump.
Is a hydraulic cylinder, 34 is the output shaft 4 of the hydraulic cylinder 33.
4 is a load connected to the hydraulic cylinder 4 and driven by the hydraulic cylinder 33. The hydraulic cylinder 33 is a double-acting cylinder, and is a piston 4 that is reciprocally fitted in the cylinder 33a.
It is divided into two water chambers 42 and 43 by 1. 38
Reference numerals 39 denote working water pipes that connect the discharge passage on the side of the hydraulic pump 31 and the water chambers 42, 43 of the hydraulic cylinder 33.

Reference numeral 36 is a motor control device having the functions described later, 35 is a piston stroke detector for detecting the piston displacement of the drive end of the load 34 of the hydraulic cylinder 33, 37
Is a cylinder differential pressure detector for detecting the differential pressure between the two water chambers 42, 43 of the hydraulic cylinder 33. The piston displacement detection signal from the piston stroke detector 35 and the water chamber from the cylinder differential pressure detector. The detected value of the differential pressure between 42 and 43 is input to the motor control device 36.

In this embodiment, the hydraulic pump 31 uses the fixed swash plate type hydraulic pump shown in FIG. 3 and is constructed as follows. In FIG. 3, 1 is a pump shaft which is connected to the output shaft of the variable speed motor 2 and is driven to rotate, 2 is a casing, and 3 and 4 are base plates and end plates fixed to both front and rear ends of the casing 2. . Reference numeral 5 denotes a cylinder block fitted to the pump shaft 1 via a spline 18 and rotationally driven in synchronization with the pump shaft 1. A plurality of (6 in this example) cylinders 6 are bored in the cylinder block 5 at equal intervals in the circumferential direction. Reference numeral 7 denotes a piston fitted in each of the cylinders 6 so as to be able to slide back and forth. A bush (not shown) is inserted into the cylinder 6 and the piston 7 is inserted into the bush.
May be reciprocally slidably fitted. Reference numeral 8 denotes a swash plate fixed to the base plate 3 via a bracket 8a and fixed at a preset swash plate angle. A slipper 15 is slidably attached to the base of each piston 7 along a spherical surface in the form of a spherical bearing 015, and each slipper 15 is movable in the circumferential direction by meeting a roller 14a with the swash plate 8. A sliding surface 23 is in sliding contact with the supported thrust plate 14. Reference numeral 17 is an annular retainer ring that supports the slippers 15.

Reference numeral 11 denotes a valve plate which is arranged on the supply / discharge hole 011 side of each cylinder 6 and fixed to the end plate 4. As shown in FIG. 4, the valve plate 11 is provided with cocoon-shaped suction holes 09 and discharge holes 010 facing both sides of the rotary shaft center 20 of the pump. A suction passage 9 and a discharge passage 10 are formed in the end plate 4, the suction passage 9 communicates with a suction hole 09 of the valve plate 11, and the discharge passage 10 communicates with the discharge hole 010. Reference numerals 12 and 13 denote a base plate side bearing and an end plate side bearing that pivotally support the pump shaft 1. 16 is an oil seal and 19 is a pump chamber.

The operation of the fixed swash plate type hydraulic pump will be described. As shown in FIGS. 3 to 4, when the pump shaft 1 is rotationally driven by the variable speed motor 2 (see FIG. 1), the cylinder block 5 is moved. Rotate. Due to the rotation of the cylinder block 5, the slipper 15 connected to the head of the piston 7, which is reciprocally fitted in the cylinder 6, via a spherical bearing 015, is fixed to the swash plate 8 as described later. Further, the piston 7 slides on the thrust plate 14, and the piston 7 reciprocates in the cylinder 6 while rotating together with the cylinder block 5.

When the piston 7 moves to the left in the suction stroke in FIG. 3, the supply / discharge hole 011 of the piston 7
Is communicated with the suction hole 09 of the valve plate 11, and the working water is sucked from the suction passage 9. On the other hand, when the piston 7 moves to the right in the discharge stroke in FIG. 3, the supply / discharge hole 011 of the piston 7 becomes the discharge hole 01 of the valve plate 11.
The hydraulic fluid in the cylinder 6 is delivered to the discharge passage 10 by the piston 7. The working water that has passed through the discharge passage 10 is supplied to and discharged from the two water chambers 42 and 43 of the hydraulic cylinder 33 by switching a directional control valve (not shown), and reciprocally drives the piston 41 of the hydraulic cylinder 33.

Next, referring to FIG. 2, the control operation of the hydraulic drive system having such a configuration will be described. The piston displacement at the drive end of the load 34 of the hydraulic cylinder 33 detected by the piston stroke detector 35 is input to the piston speed calculation unit 61 of the motor control device 36, where the differential operation is performed to calculate the piston speed, and the piston speed is calculated. It is input to the deviation calculator 63. Reference numeral 64 denotes a hydraulic cylinder target output setting unit, which sets a required drive output of the load 34 by the hydraulic cylinder 33, that is, a target output of the hydraulic cylinder 33. Reference numeral 62 denotes a target piston speed setting unit, which sets a target piston speed of the hydraulic cylinder 33 corresponding to the target output set in the hydraulic cylinder target output setting unit 64,
It is input to the piston speed deviation calculation unit 63.

In the piston speed deviation calculating section 63,
The pump discharge correction amount calculation unit 65 calculates the speed deviation between the target piston speed from the target piston speed setting unit 62 and the detected piston speed from the piston speed calculation unit 61.
To enter. The pump discharge correction amount calculation unit 65 calculates the discharge amount correction value of the hydraulic pump 31 corresponding to the speed deviation between the target piston speed and the detected piston speed from the piston speed deviation calculation unit 63 to calculate the motor rotation speed. Input to the section 66.

On the other hand, the differential pressure detection value between the two water chambers 42, 43 of the hydraulic cylinder 33 detected by the cylinder differential pressure detector 37 is input to the differential pressure deviation calculating section 69 of the motor control device 36. It Reference numeral 68 denotes a reference differential pressure setting unit, which sets an allowable value for lowering the differential pressure due to water pressure leakage between the two water chambers 42, 43 of the hydraulic cylinder 33, that is, an allowable minimum differential pressure (reference differential pressure) considering the water pressure leakage. Has been done. The reference differential pressure is input to the differential pressure deviation calculator 69. In the differential pressure deviation calculating section 69, the deviation or difference between the differential pressure detection value from the cylinder differential pressure detector 37 and the reference differential pressure (allowable minimum differential pressure) preset in the reference differential pressure setting section 68. The pressure deviation is calculated and input to the motor rotation speed correction amount calculation unit 70.

In the motor rotation speed correction amount calculation section 70, the variable speed motor 3 is calculated based on the calculated value of the differential pressure deviation.
The correction value of the rotation speed of 2 is calculated and input to the motor rotation speed calculation unit 66. That is, in the motor rotation speed correction amount calculation unit 70, when the differential pressure detection value between the two water chambers 42 and 43 partitioned by the piston 41 becomes smaller than the reference differential pressure, the two Water chamber 4
It is determined that the leakage of the working water between the Nos. 2 and 43 becomes larger than the allowable value, and a correction value for increasing the rotation speed of the variable speed motor 32 is calculated and input to the motor rotation speed calculation unit 66.

In the motor rotation speed calculation section 66,
Based on the discharge amount correction value of the hydraulic pump 31 input from the pump discharge correction amount calculation unit 65, the correction value of the rotation speed of the variable speed motor 32 corresponding to the discharge amount correction value is calculated. Further, in the motor rotation speed calculation unit 66, a correction value for the rotation speed of the variable speed motor is corrected by using the differential pressure deviation from the motor rotation speed correction amount calculation unit 70 as the correction value for the rotation speed of the variable speed motor. Variable speed motor 3
The rotation speed correction value of 2 is calculated and output to the variable speed motor 32.

As a result, the variable speed motor 32 has two water chambers 42 of the hydraulic cylinder 33, in which the amount of discharge of the hydraulic pump 31 corresponds to the target piston speed of the hydraulic cylinder 33. The operation is performed at a rotational speed such that the pressure difference between 43 is maintained above the reference pressure difference.

Therefore, according to this embodiment, the piston displacement at the driving end of the load 34 of the hydraulic cylinder 33 is detected and input to the motor control device 36, and the motor control device 36 corresponds to the target output of the hydraulic cylinder 33. A speed deviation between the target piston speed and the detected piston speed based on the piston displacement detected by the piston stroke detector 35 and a discharge amount correction value of the hydraulic pump 31 based on the speed deviation are calculated, and based on the discharge amount correction value Since the rotation speed of the variable speed motor 32 can be controlled so that the discharge amount of the hydraulic pump 31 becomes the discharge amount corresponding to the target piston speed of the hydraulic cylinder 33, the load 3 of the hydraulic cylinder 33 can be controlled.
By controlling the rotation speed of the variable speed motor 32 based on the detected value of the piston displacement at the four drive ends, the discharge amount control of the hydraulic pump 31 and the output control of the hydraulic cylinder 33 can be performed with high accuracy.

Further, since the discharge amount of the hydraulic pump 31 is controlled by controlling the rotation speed of the variable speed motor 32, a pump discharge amount control device such as a swash plate angle adjusting device as in the prior art can be operated with a large operating force. It ’s no longer necessary to control
The discharge amount of the hydraulic pump 31 can be controlled with a small operating force, and the control response is improved.

Further, according to this embodiment, the two water chambers 4 of the hydraulic cylinder 33 are controlled by the cylinder differential pressure detector 37.
The differential pressure between the variable speed motor 32 is detected by the motor controller 36 so as to maintain the reference differential pressure value.
Since the rotational speed of the variable speed motor 32 is controlled by correcting the control signal of the rotational speed, the leakage amount of the operating water between the water chambers in the hydraulic cylinder 33 is determined by the differential pressure between the water chambers being the differential pressure reference value. It is possible to suppress the following.

[0034]

As described above, according to the first and second aspects of the invention, the target piston speed corresponding to the target output of the hydraulic cylinder and the detection piston based on the detected value of the piston displacement at the load driving end of the hydraulic cylinder. A velocity deviation from the detected piston velocity based on the displacement and a discharge amount correction value of the hydraulic pump based on the velocity deviation are calculated, and the discharge amount of the hydraulic pump corresponds to the target piston velocity of the hydraulic cylinder based on the discharge amount correction value. Since the rotation speed of the variable speed motor can be controlled so that the discharge amount is obtained, the rotation speed of the variable speed motor is controlled by controlling the rotation speed of the variable speed motor based on the detected value of the piston displacement at the load driving end of the hydraulic cylinder. Discharge amount control and hydraulic cylinder output control can be performed with high accuracy.

Therefore, according to the present invention, since the discharge rate of the hydraulic pump is controlled by controlling the rotation speed of the variable speed motor, a pump discharge rate control apparatus such as a conventional swash plate angle adjusting apparatus is large. It becomes unnecessary to control with the operating force, and the discharge amount of the hydraulic pump can be controlled with a small operating force, and the control response is improved.

The invention according to claim 3 is, in addition to claim 2,
A cylinder differential pressure detector that detects a differential pressure between two water chambers partitioned by the piston of the hydraulic cylinder and inputs the differential pressure to the motor control device is provided, and the motor control device is configured to detect the differential pressure from the cylinder differential pressure detector. Differential pressure correction value calculation means for calculating a correction value for the variable speed motor rotation speed based on a deviation between the differential pressure detection value and a preset reference differential pressure and inputting it to the motor rotation speed calculation unit. .

According to the third aspect of the invention, the cylinder pressure difference detector detects the pressure difference between the two water chambers of the hydraulic cylinder, and the motor controller holds the pressure difference at the reference value. Since the variable speed motor rotation speed is controlled by correcting the control signal of the variable speed motor rotation speed, the differential pressure between the water chambers is used as a reference for the leakage amount of fluid (water) between the water chambers in the hydraulic cylinder. It can be suppressed so as not to fall below the value, and the hydraulic cylinder leakage can be constantly monitored to keep the operating performance of the hydraulic cylinder high at all times.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a hydraulic drive system according to an embodiment of the present invention.

FIG. 2 is a control block diagram in the embodiment.

FIG. 3 is a cross-sectional view taken along the pump axis line showing the structure of the hydraulic pump in the embodiment.

FIG. 4 is a view as seen from the arrow BB of FIG.

FIG. 5 is a block diagram of a hydraulic control system for a hydraulic drive system for a construction machine according to a conventional technique.

FIG. 6 is a sectional view of a main part of a hydraulic pump showing a conventional technique.

[Explanation of symbols]

1 pump shaft 5 cylinder block 6 cylinders 7 pistons 8 swash plate 14 Thrust plate 15 slippers 015 spherical bearing 31 Water pressure pump 32 variable speed motor 33 hydraulic cylinder 34 load 35 Piston stroke detector 37 Cylinder differential pressure detector 41 piston 42 water chamber 43 water chamber 36 Motor control device

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 3H001 AA03 AC02 AC03 AD04 AE11                 3H045 AA09 AA10 AA12 AA22 BA12                       BA19 CA05 CA23 DA04 EA13                       EA26 EA38

Claims (3)

[Claims] 1. A structure in which hydraulic water delivered from a hydraulic pump directly connected to a variable speed motor is guided to a hydraulic cylinder, and the hydraulic water acts on the hydraulic cylinder to drive a load by the hydraulic cylinder. In hydraulic drive,
A piston stroke detector that detects a piston displacement of the load drive end of the hydraulic cylinder, and a detection signal of the piston displacement is input from the piston stroke detector, and the discharge amount of the hydraulic pump is based on the detected value of the piston displacement. A hydraulic control device comprising: a motor control device that controls the rotation speed of the variable speed motor so that the discharge amount corresponds to the target piston speed of the hydraulic cylinder. 2. A speed deviation calculation for calculating a speed deviation between a target piston speed set based on a target output of the hydraulic cylinder and a detected piston speed calculated based on a detected value of the piston displacement. Means, a discharge amount correction value calculating means for calculating a discharge amount correction value of the hydraulic pump corresponding to the speed deviation from the speed deviation calculating means, and the discharge amount correction value calculating means for calculating the discharge amount correction value from the discharge amount correction value calculating means. The hydraulic drive system according to claim 1, further comprising a motor rotation speed calculation unit that calculates a rotation speed of the variable speed motor. 3. A cylinder differential pressure detector for detecting a differential pressure between two water chambers partitioned by a piston of the hydraulic cylinder and inputting the differential pressure to the motor control device, wherein the motor control device includes the cylinder Calculation of a correction value for the variable speed motor rotation speed based on the deviation between the differential pressure detection value from the differential pressure detector and a preset reference differential pressure, and input to the motor rotation speed calculation unit The hydraulic drive system according to claim 2, further comprising means.