Pump torque control system for hydraulic construction machine

A pump torque control system for a hydraulic construction machine, comprising:

• a prime mover (1);
• variable displacement hydraulic pumps (2, 3) that are rotationally driven by the prime mover;
• and
• hydraulic actuators (7, 8, 9, 10, 11, 12) that are driven by means of a hydraulic fluid delivered from the hydraulic pumps,
• wherein the pump torque control system comprises:
• pump absorption torque control means (31;
• 131) for controlling displacement volumes of the hydraulic pumps (2, 3) to ensure that a total absorption torque of the hydraulic pumps (2, 3) does not exceed a set maximum absorption torque;
• and
• speed sensing control means (33, 34, 35, 23, 41 to 57) for calculating a first torque reduction amount (ΔTs3) based on a deviation between a target engine speed of the prime mover (1) and an actual engine speed of the prime mover (1) and for performing a control to reduce the maximum absorption torque of the hydraulic pumps (2, 3), with an increase of an absolute value of the first torque reduction amount being made when the actual engine speed of the prime mover is lowered from the target engine seed, the maximum absorption torque being set in the pump absorption torque control means (31;
• 131), and
• wherein the speed sensing control means (33, 34, 35, 23, 41 to 57) includes:
• hydraulic fluid temperature detection means (34) for detecting the temperature of the hydraulic fluid;
• and
• first hydraulic fluid temperature modification means (45, 49) for modifying a control gain to be used to calculate the first torque reduction amount (ΔTs3) in order to ensure that the absolute value of the first torque reduction amount (ΔTs3) is reduced as the temperature of the hydraulic fluid detected by the hydraulic fluid temperature detection means (34) is reduced.