Instruction Manual
D103785X012
Principle of Operation
June 2017
96
Figure A‐2. Typical FIELDVUE Instrument to Personal Computer Connections for ValveLink Software
FIELD TERM.
CONTROL SYSTEM
HART MODEM
E1568
DVC6000 HW2 digital valve controllers are loop‐powered instruments that provide a control valve position
proportional to an input signal from the control room. The following describes a double‐acting digital valve controller
mounted on a piston actuator.
The input signal is routed into the terminal box through a single twisted pair of wires and then to the printed wiring
board assembly submodule where it is read by the microprocessor, processed by a digital algorithm, and converted
into an analog I/P drive signal.
As the input signal increases, the drive signal to the I/P converter increases, increasing the I/P output pressure. The I/P
output pressure is routed to the pneumatic relay submodule. The relay is also connected to supply pressure and
amplifies the small pneumatic signal from the I/P converter. The relay accepts the amplified pneumatic signal and
provides two output pressures. With increasing input (4 to 20 mA signal), the output A pressure always increases and
the output B pressure decreases. The output A pressure is used for double‐acting and single‐acting direct applications.
The output B pressure is used for double‐acting and single‐acting reverse applications. As shown in figure A‐3 the
increased output A pressure causes the actuator stem to move downward. Stem position is sensed through the
feedback linkage by the travel sensor which is electrically connected to the printed wiring board assembly submodule.
The stem continues to move downward until the correct stem position is attained. At this point the printed wiring
board assembly stabilizes the I/P drive signal. This positions the flapper to prevent any further increase in nozzle
pressure.
As the input signal decreases, the drive signal to the I/P converter submodule decreases, decreasing the I/P output
pressure. The pneumatic relay decreases the output A pressure and increases the output B pressure. The stem moves
upward until the correct position is attained. At this point the printed wiring board assembly stabilizes the I/P drive
signal. This positions the flapper to prevent any further decrease in nozzle pressure.
