and 204 indicates the nozzle 205 indicates the lines taking the jet to the control ports 206 for controlling the spool 207 and finally 208 indicates the pressure transducer which measures the position of the spool 207 and sends the signal to the position control loop 209 an electric current by means of the coil 201 moves the armature 202 from its neutral position thus moving the nozzle of the conduit 203 and directing a fluid flow towards a side of the hydraulic circuit 205 thus creating a pressure difference in the port 206 and moving the spool 207 the position of which is measured by the position traducer 208 fig7 depicts a diagram of a three stage four way servo valve 300 the pilot stage 301 moves the spool 302 of the pilot servo valve the position of which is controlled by the control loop 305 which in turn moves the spool 303 of the main servo valve the position of which is controlled by the control loop 304 this type of two or more stage servo valve is indeed necessary in large sized servo valves which operate in high pressure systems by using a four way servo valve during the tube waiting steps the balance condition which is established between the two chambers of the hydraulic capsule will be very different from that described for the three way servo valve because the pressure in the main chamber will be higher than 100 bars while that in the annular chamber will be higher than 220 bars when the tube is taken the oil is already pressurized in the main chamber and in all cases a movement of the piston in the direction in the sense of yielding i e of the.