Servo valve bodies are machined with three common port shapes as shown in Fig. 11.13. The effect of port shape on flow is shown in Fig. 11.14. A full annulus port gives the highest flow per unit of spool displacement or, correspondingly, per unit of current input to the torque motor. The valve flow characteristics will be important when the valve transfer function is defined in a later section.
The jet pipe design (Fig. 11.12) is less sensitive to contamination than the flapper nozzle. This design uses fluidics technology. A nozzle, attached to the armature, directs a stream of fluid (pressure Ps) at two receivers. When the nozzle is centered (no current through the armature coils to cause it to turn), the two receivers are hit by the stream of fluid in like manner, and the resultant pressure on the ends of the spool is equal. When the armature turns, the stream of fluid from the nozzle impinges more directly on one nozzle, and the resulting pressure imbalance shifts the second-stage spool. There is a feedback spring for this design that works the same as for the double flapper nozzle. The spool shifts until the spring force produces a torque equal to the electromagnetic torque. At this point, the armature is moved back to the center position, the nozzle is centered, pressure at the two receivers is equal, the forces on the spool are equal, and it is held in position. A change in coil current changes the spool to a position that corresponds to the new level of current.