Control system for construction machinery

A control system for construction machinery, comprising:

• a hydraulic cylinder operable by a working oil discharged from a hydraulic pump;
• a control valve arranged between the hydraulic pump and the hydraulic cylinder to control an operation of the hydraulic cylinder according to a position of a spool therein, the control valve having a first spool position for draining the working oil discharged from a rod side chamber of the hydraulic cylinder to a drain tank;
• and
• a pressure compensated valve installed in a return hydraulic line through which the working oil discharged from the control valve at the first spool position is drained to the drain tank, the pressure compensated valve being configured to control a flow rate of the working oil passing through the pressure compensated valve according to a pressure difference between a front end and a rear end of the control valve,
• wherein an opening amount of the pressure compensated valve is controlled according to a pressure difference between a first pilot pressure supplied through a first pilot line branched off from a rod hydraulic line connected to the rod side chamber and a second pilot pressure supplied through a second pilot line branched off from the return hydraulic line in front of the pressure compensated valve,
• wherein the pressure compensated valve includes a compensation spring that is connected to a compensation spool and is configured to compensate the pressure difference between the front end and the rear end of the control valve by a spring force,
• wherein the working oil is drained to the drain tank through a return passage, a tank passage and a tank port that are selectively in communication with each other by a valve spool of the control valve, and the return passage, the tank passage and the tank port constitute at least a portion of the return hydraulic line, and
• wherein an inlet of the pressure compensated valve is in communication with a second pilot line that is in communication with the return passage and an outlet of the pressure compensated valve is in communication with the tank passage.