Pump control unit for hydraulic system

A pump control unit for a hydraulic system of a hydraulic excavator, the hydraulic system comprising:

• first and second hydraulic pumps driven by a prime mover, the first and second hydraulic pumps being variable displacement type;
• a plurality of first actuators driven by a hydraulic fluid delivered from the first hydraulic pump, the first actuators including a boom cylinder for driving a boom of the hydraulic excavator;
• a plurality of second actuators driven by a hydraulic fluid delivered from the second hydraulic pump, the second actuators including a swing motor for driving an upper swing structure of the hydraulic excavator;
• a plurality of operating means including first and second operating means for operating the boom cylinder and the swing motor, respectively;
• and
• a relief valve for determining respective maximum pressures of the hydraulic fluids delivered from the first and second hydraulic pumps;
• the pump control unit comprising:
• a pressure detecting means for detecting a delivery pressure of the second hydraulic pump;
• a first pump torque control means for setting maximum absorption torque of the first hydraulic pump and controlling a displacement volume of the first hydraulic pump so that an absorption torque of the first hydraulic pump does not exceed the maximum absorption torque;
• and
• a second pump torque control means for setting maximum absorption torque of the second hydraulic pump and controlling a displacement volume of the second hydraulic pump so that an absorption torque of the second hydraulic pump does not exceed the maximum absorption torque,
• wherein the second pump torque control means has a preset maximum torque value consumable by the second hydraulic pump and a preset torque value less than the value of the maximum torque, and the second pump torque control means sets the maximum torque value as the maximum absorption torque of the second hydraulic pump when the delivery pressure of the second hydraulic pump detected by the pressure detecting means is lower than a predetermined pressure that is below the maximum pressure determined by the relief valve, and sets the torque value less than the maximum torque value as the maximum absorption torque of the second hydraulic pump when the delivery pressure of the second hydraulic pump detected by the pressure detecting means increases to reach the maximum pressure determined by the relief valve, and
• wherein the first pump torque control means sets, as the maximum absorption torque of the first hydraulic pump, the difference of the total pump torque consumable by the first and second hydraulic PUMPS and the maximum absorption torque of the second hydraulic pump set for the second pump torque control means.