Control types

There are three ways to move the spool, cartridge, or slider within a hydraulic control valve to affect the flow through it:

In full-pressure control, manually-operated levers connect directly to the spool and are subject to the full pressure of the system. Pilot-operated controls use hydraulic pressure, modified by a valve, usually lever operated, to shift the spool of the main control valve. In electro-hydraulic controls, an electrical solenoid moves the spool. The signal to the solenoid comes from relays, PLCs, or more commonly today, a microprocessor.