1. When performing the arm extension operation, the pilot pressure switches the arm spool and the converging spool twice, and at the same time, it is input to the low pressure pressure sensor (SE-8).
2. The output voltage of the low pressure pressure sensor is input to the electromechanical controller, which performs pilot signal processing internally, and outputs the command current corresponding to the input voltage to the PI cut-off proportional valve (PSV-D).
3. The PI cut-off proportional valve outputs the second pressure of the proportional valve corresponding to the command current of the electromechanical controller, and switches the PI cut-off spool. 4. Due to the switching of the arm spool and the PI cut-off spool, the negative pressure controls the pressure change Pressure is input to PI, P2 side negative pressure sensor (SE.26, SE-27) and back pressure sensor (SE-280PI, P2 side negative pressure sensor and back pressure sensor output voltage is input to the electromechanical controller, internally Negative pressure control signal processing, output to PI pump proportional valve (PSV-PI) and P2 spring proportional valve CPSV-P2).
5. The PI pump proportional valve and the P2 pump proportional valve output the secondary pressure of the proportional valve corresponding to the command current from the electromechanical controller, which changes the discharge volume of the PI pump and the P2 pump.
6. Switch the spool of the arm through the pure hydraulic command (secondary pressure of the pilot), and switch the PI cut-off spool through the command of current hydraulic pressure (the second pressure of the proportional valve). In this way, the discharge oil of the PI pump is merged into the discharge oil of the P2 pump.
