Specifications

LINEAR FORCE MOTOR
➣ A linear force motor is a permanent magnet differential motor.
➣ The motor consists of a coil, pair of high energy rare earth
magnets, armature, and centering springs.
➣ Without a current being applied to the coil, the magnets and
springs hold the armature at equilibrium.
➣ When current is applied to the coil with one polarity, the flux
in one of the air gaps surrounding the magnets is increased,
cancelling out the flux in the other.
➣ This dis-equilibrium allows the armature to move in the
direction of the stronger magnetic flux.
➣ The armature is moved in the opposite direction by changing
the polarity of the current in the coil.
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.
OPERATION
➣ An electrical signal corresponding to the desired spool position
is applied to the integrated electronics and produces a pulse
width modulated (PWM) current in the linear force motor coil.
➣ The current causes the armature to move which then directly
activates the spool.
➣ The spool moves and opens pressure P to one control port,
while the other control port is opened to tank T.
➣ The position transducer (LVDT), which is mechanically attached
to the spool, measures the position of the spool by creating an
electrical signal that is proportional to the spool position.
➣ The demodulated spool position signal is compared with
the command signal, and the resulting electrical error drives
current to the force motor coil.
➣ The spool moves to its commanded position and the spool
position error is reduced to zero.
➣ The resulting spool position is thus proportional to the
command signal.
DIRECT DRIVE SERVO-PROPORTIONAL VALVE OPERATION
S NN S
N S S N
Direction
of Armature
N S
S N
S NN S
Permanent Magnets
Centering Springs
Coil Armature
PATB X
19