AIR COMPRESSOR GP-SEAT 1312875 - Caterpillar

Introduction
The EDC is a two stage electrohydraulic control unit. Mechanical feedback and ported oil control make a closed loop swashplate control circuit.A coil controls the swashplate angle. The current passed through the coil is proportional to the swashplate angle. The PCP (1) valve is the first stage of the electrical displacement control (EDC) unit. The hydraulic portion of the PCP valve is a closed loop that uses internal hydraulic feedback from the pump's swashplate piston. The PCP electrical section receives a direct current (DC) input in its torque motor stage. The torque motor stage consists of an armature (10) mounted on a torsional pivot (14) suspended in a magnetic field air gap. Magnets (12) and (13) are permanent magnets of parallel polarity constructed to form a magnetic bridge.When the EDC is null, the armature is centered in the air gap. Equal magnetic forces and the centering springs (9) hold the armature in the centered position. While the armature (10) is centered, flapper (15) is centered between the nozzles (16) and (17). Upstream from the nozzles are orifices (19) and (20). Between each nozzle and orifice is a control port ((21) and (23).In the null position, charge oil from the charge pump enters oil supply port (18) and passes through the two orifices. Because the flapper is centered between the two nozzles, there is no pressure difference and an equal amount of oil flows through each nozzle, past the flapper and out of the oil return port (22).As current is increased, the armature becomes polarized. The armature moves toward the opposite magnetic field. The amount of movement is dependent upon the amperage of the control current introduced to pole pieces (8) and (11).When current is applied, causing the armature to move towards magnet (13) a torque shift occurs at pivot (14). The torque shift causes the flapper to move closer to nozzle (16). As the flapper moves closer to the nozzle (16), the pressure rises between the nozzle (16) and orifice (20). This pressure increase causes the oil to flow through control port 1 (21).Initially, due to a step input from the control unit, the flapper closes off the nozzle sending all the oil through the control port causing the pump to go to full stroke. As oil pressure rises in the passageway between the nozzle and the orifice, it moves the flapper back toward the null position. When the torque outputs of the torque motor stage and the pressure feedback are equal, the pilot control system is equalized. When the pilot control system is equalized the flapper is positioned to maintain a desired speed setting. The control oil pressure in passage (21) is then proportional to the control current plus the control oil pressure in passage (23).The second stage of the EDC routes the control oil pressure from passage (21) to one end of actuator spool (6). When the control oil pressure reaches 85 kPa (12 psi it overcomes modulation spring (7) and shifts the actuator spool. When