The servo valve in series FF has a polarized torque motor. In the torque motor, two pieces of charged permanent magnets polarize polepieces, and both ends of its armature are respectively inserted into the gaps formed by upper and lower polepieces. A flexure tube is employed to play the role not only in a spring support for the armature-flapper assembly but also in a sealing between electromagnetic and hydraulic parts, the flapper of the first stage hydraulic amplifier is inserted between two nozzles, forming two variable orifices. A feedback spring extends from th e flapper inner and inserts its ball end in the small slot of the spool. Hydraulic fluid continuously flows from supply ports Ps, through an inner filter and both inlet orifices, past both variable orifices formed by the flapper and nozzles, and then through a drain orifice to return port Pr, pressure in both flapper-controlled chambers are respectively acted on both ends of the second stage four-way spool.
When electrical current input is applied to the torque motor coils, due to the interaction between controlling and permanent flux,a magnetic torque is created on the armature. This torque makes the armature-flapper assembly rotate about the flexure tube support, moving the flapper in one direction or in another direction. The moved flapper results in increasing the area of one flapper-nozzle orifice and decreasing the area of the other one, creating differential pressure in the two nozzle-controlled chambers. This differential pressure moves the spool pushing the ball end of feedback spring and creating a retoring torque on the armature-flapper assembly. The spool goes on moving until the feedback torque becomes equal to the magnetic torque caused by control current input. At that time, the flapper is moved back to a nearly centered position. As the magnetic torque is proportional to the spool position as well, therefore , the spool position is proportional to the current input while the mentioned torques are being balanced. Then, with constant valve flow output to load is proportional to current input when rectangular holes(slots) or annular grooves are employed in the valve bushing.