Hydraulic drive system for electrically-operated hydraulic work machine

Abstract

An electrically-operated hydraulic work machine drives an actuator with a hydraulic pump driven by an electric motor and exercises load sensing control by controlling the rotation speed of the electric motor. The useful life of an electrical storage device, which is an electrical power source for the electric motor, is increased by suppressing the horsepower consumption of the hydraulic pump. This prolongs the operating time of the electrically-operated hydraulic work machine, and reduces the size of the electric motor. A controller exercises load sensing control over a variable displacement main pump by controlling the rotation speed of the electric motor, and provides the main pump with a torque control device that reduces the delivery rate of the main pump when the delivery pressure of the main pump increases, or provides the controller with a control algorithm that performs the same function as the torque control device.

1 . A hydraulic drive system for an electrically-operated hydraulic work machine, the work machine having
 an electric motor ( 1 ),   a hydraulic pump ( 2 ) driven by the electric motor,   a plurality of actuators ( 3   a - 3   c ) driven by a hydraulic fluid discharged from the hydraulic pump,   a plurality of flow control valves ( 6   a - 6   c ) for controlling the flow rate of the hydraulic fluid supplied from the hydraulic pump to the actuators, and   an electrical storage device ( 20 ) for supplying electrical power to the electric motor, the hydraulic drive system comprising:   an electric motor rotation speed control system ( 40 ,  41 ,  50 ,  51 ,  60 ,  61 ) that exercises load sensing control to control the rotation speed of the hydraulic pump in such a manner that the delivery pressure of the hydraulic pump is higher than the highest load pressure of the actuators by a target differential pressure; and   a torque control device ( 17 ;  50   r ,  50   s ) that exercises control to prevent an absorption torque of the hydraulic pump from exceeding a predefined maximum torque by decreasing the delivery rate of the hydraulic pump when the delivery pressure of the hydraulic pump increases.   
     
     
         2 . The hydraulic drive system according to  claim 1 ,
 wherein the electric motor rotation speed control system includes
 a first pressure sensor ( 40 ) for detecting the delivery pressure of the hydraulic pump, 
 a second pressure sensor ( 41 ) for detecting the highest load pressure, 
 an inverter ( 60 ) for controlling the rotation speed of the electric motor, and 
 a controller ( 50 ;  50 A); 
   wherein the controller includes a load sensing control computation section ( 50   a - 50   c ,  50   f - 50   h ) that computes a virtual displacement (q*) of the hydraulic pump, which increases or decreases depending on whether a differential pressure deviation (ΔP) between the difference (PLS) between the delivery pressure (Pps) of the hydraulic pump ( 2 ;  2 A) and the highest load pressure (PPLmax) and a target LS differential pressure (PGR) is positive or negative, in accordance with the delivery pressure of the hydraulic pump and the highest load pressure, which are detected by the first and second pressure sensors, and with the target LS differential pressure, computes a target flow rate (Qd) of the hydraulic pump by multiplying the virtual displacement by a reference rotation speed (N 0 ), and outputs a control command (Vinv) to the inverter for the purpose of controlling the rotation speed of the electric motor ( 1 ) in such a manner that the delivery rate of the hydraulic pump agrees with the target flow rate.   
     
     
         3 . The hydraulic drive system according to  claim 1 ,
 wherein the hydraulic pump is a variable displacement hydraulic pump ( 2 ); and   wherein the torque control device is a regulator ( 17 ) incorporated in the hydraulic pump ( 2 ).   
     
     
         4 . The hydraulic drive system according to  claim 2 ,
 wherein the hydraulic pump is a fixed displacement hydraulic pump ( 2 A);   wherein the torque control device is configured to exercise one function of the controller ( 50 A) incorporated herein; and   wherein the controller further includes a torque limit control computation section ( 50   r ,  50   s ) that, in accordance with the delivery pressure (Pps) of the hydraulic pump, which is detected by the first pressure sensor ( 40 ), computes a virtual displacement limit value (q*limit) that decreases with an increase in the delivery pressure of the hydraulic pump, and determines a new virtual displacement (q**) by selecting either the virtual displacement (q*) computed by the load sensing control computation section ( 50   a - 50   c ,  50   f - 50   h ) or the virtual displacement limit value, whichever is smaller, and computes the target flow rate (Qd) of the hydraulic pump by multiplying the new virtual displacement by the reference rotation speed (N 0 ).   
     
     
         5 . The hydraulic drive system according to  claim 2 , further comprising:
 an operating device ( 51 ) that designates the reference rotation speed (N 0 );   wherein the controller ( 50 ,  50 A) sets the reference rotation speed in accordance with a designation signal from the operating device, and computes the target LS differential pressure (PGR) and the target flow rate (Qd) in accordance with the reference rotation speed.   
     
     
         6 . The hydraulic drive system according to  claim 2 ,
 wherein the hydraulic pump is a variable displacement hydraulic pump ( 2 ); and   wherein the torque control device is a regulator ( 17 ) incorporated in the hydraulic pump ( 2 ).   
     
     
         7 . The hydraulic drive system according to  claim 4 , further comprising:
 an operating device ( 51 ) that designates the reference rotation speed (N 0 );   wherein the controller ( 50 ,  50 A) sets the reference rotation speed in accordance with a designation signal from the operating device, and computes the target LS differential pressure (PGR) and the target flow rate (Qd) in accordance with the reference rotation speed.