A command source supplies a dc current signal to the pilot stage
of the Electrical Displacement Control. See Internal Workings
Schematic. The input current commands the pilot’s torque motor
stage, a bridge network consisting of an armature mounted on
Two permanent magnets polarized in parallel and a connecting
plate form a frame for the magnetic bridge. At null the armature
is centered in the air gap between the magnet’s opposing poles
by the equivalence of their magnetic forces and the null-adjust
centering springs. As the input current increases, the end of the
armature becomes biased either north or south, depending on
the direction of the current. The resulting armature movement
is determined by the current, the spring constant, and the dif-
ferential pressure feedback forces, explained next.
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toward the other, a
erential control pressure results, the high
torque output from the motor equals the torque output from
the pressure feedback, the pilot system is equilibrium. It is this
pressure feedback that makes the pilot a stand-alone closed-
loop pressure control valve.
The second stage of the EDC uses a unique double spool ar-
rangement that serves to separate the null deadband from the
feedback, giving both safety against null drift and quick dynamic
response to command charges.
The second stage’s null adjust is set with the modulating spring
compressed to the equivalent of 12 psi, which is the amount of
erential pressure required to move the actuator spool one
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band is moved toward or away from the “A” control port.
As
erential control pressure (C1-C2) rises beyond the 12 psi
deadband, the actuator spool moves in one direction or the oth-
er, pivoting the crosslink about its center. The pivoting crosslink
pushes or pulls the porting spool in the opposite direction of the
actuator spool. When the porting spool has moved far enough
to open A or B to supply pressure, oil is ported to the pump servo-
cylinders to move the swashplate. As the swashplate moves, the
drag linkage follows, pivoting the crosslink about the stationary
end of the actuator spool, driving the porting spool back to its
neutral position. Because the feedback signal is entered into the
control loop after the command has been input, response time
and accuracy are enhanced.
Input phasing is such that, when facing the input shaft of the
pump and rotation is clockwise, current input to the Red (B)
lead or MS connector pin will produce a pressure rise at the “A”
pump.
When the MCV107A Hydraulic Displacement Control - MV is
used on a motor in conjunction with the pump-mounted EDC,
a single electrical command can be used to control both devices.
This is done by “staging” the HDC so that the motor swashplate
starts to destroke just as the pump reaches full stroke. For further
information, see technical literature for the MCV107A.
INTERNAL WORKINGS SCHEMATIC, WITH OPTIONAL PRESSURE OVERRIDE
THEORY OF OPERATION
Schematic of the Internal Workings of the MCV104A with Optional Pressure Override Valve.
Oil Paths Shown Externally for Clarity.
1302
© Danfoss, 2013
K07116
•
Rev A
B
• September 2013
7