The above price picture is for reference only, please consult in detail
The main valve of the servo valve is in generalReversing valve
The same is the structure of the slide valve. But not by reversing valve electromagnet driven by hydraulic pressure, but the preamplifier output valve to drive this point, and the hydraulic valve is quite similar, but the pre stage valve electro-hydraulic valve is an electromagnetic reversing valve, and the front valve servo valve dynamic characteristics good flapper valve or jet pipe valve.
That is to say, the main valve of the servo valve is controlled by the output pressure of the front valve, and the pressure of the front valve is from the inlet of the servo valve P. If the pressure of the P port is insufficient, the pre valve can not output enough pressure to drive the main spool.