Specifications

SERVOJET
®
PILOT STAGE
➣ The ServoJet
®
pilot stage consists mainly of torque motor, jet
pipe, and receiver.
➣ A current through the coil displaces the jet pipe from its
neutral position.This displacement, combined with the special
shape of the nozzle, directs a focused fluid jet from both
receivers towards one receiver.
➣ The jet now produces a pressure difference in the control
ports.
➣ This pressure difference results in a pilot flow, which in turn
causes a spool displacement.The pilot stage drain is through
the annular area around the nozzle to tank T.
OPERATION
➣ An electrical command signal (flow rate set point) is applied to
the integrated position controller which drives the valve coil.
➣ The current through the coil displaces the jet pipe from its
neutral position.
➣ The displacement of the jet directs the flow to one end of
the spool.
➣ Spool moves and opens P to one control port, while the
other control port is open to tank T.
➣ The position transducer (LVDT), which is excited via an
oscillator, measures the position of the main spool (actual
position voltage).
➣ The signal for the actual position of the spool is then
demodulated and fed back to the controller, where it is
compared with the command signal.
➣ The controller drives the pilot valve until the error between
command signal and spool position feedback signal is zero.
➣ Thus, the position of the main spool is proportional to the
electrical command signal.
SERVOJET
®
SERVO-PROPORTIONAL VALVE OPERATION
VALVE SPOOL
➣ Spool slides in bushing (sleeve) or directly in body bore.
➣ Bushing contains rectangular holes (slots) or annular grooves
that connect to supply pressure P
S
and tank T.
➣ At “null,” spool is centered in bushing; spool lobes (lands)
just cover P
S
and T openings.
➣ Spool motion to either side of null allows fluid to flow from
P
S
to one control port, and from other control port to T.
XT A P B T
2
Y
Annular
Area
Nozzle
Receiver
Jet
Pipe
18