Flowserve 3400MD Digital Positioner - Manual (Page 9)

9
Logix 3400MD Digital Positioner FCD LGENIM3404-08-AQ – 5/15
flowserve.com
4.2 Positioner Operation
The Logix 3400MD positioner is an electric feedback instrument.
Figure 1 shows a Logix 3400MD positioner installed on a double-
acting linear actuator for air-to-open action.
The Logix 3400MD receives power from the two-wire, FF input signal.
This positioner utilizes FF communications for the command signal.
The command source can be accessed with the Rosemount 375
communicator or other host software.
0% is always defined as the valve closed position and 100% is always
defined as the valve open position. During stroke calibration, the
signals corresponding to 0% and 100% are defined.
The input signal in percent passes through a characterization/
limits modifier block. The positioner no longer uses CAMs or other
mechanical means to characterize the output of the positioner. This
function is done in software, which allows for in-the-field customer
adjustment. The positioner has four basic modes: Linear, Equal
Percent (=%), Quick Open (QO) and Custom characterization. In
Linear mode, the input signal is passed straight through to the control
algorithm in a
1:1 transfer. In Equal Percent (=%) mode, the input signal is mapped
to a standard 30:1 rangeability =% curve. In Quick Open the input
signal is mapped to an industry standard quick-open curve. If Custom
characterization is enabled, the input signal is mapped to either a
default =% output curve or a custom, user-defined 21-point output
curve. The custom user-defined 21-point output curve is defined
using a handheld or the Host configuration tool software. In addi-
tion, two user-defined features, Soft Limits and Final Value Cutoff,
may affect the final input signal. The actual command being used to
position the stem, after any characterization or user limits have been
evaluated, is called the Control Command.
The Logix 3400MD uses a two-stage, stem-positioning algorithm. The
two stages consist of an inner-loop, spool control and an outer-loop,
stem position control. Referring again to Figure 1, a stem position
sensor provides a measurement of the stem movement. The Control
Command is compared against the Stem Position. If any deviation
exists, the control algorithm sends a signal to the inner-loop control
to move the spool up or down, depending upon the deviation. The
inner- loop then quickly adjusts the spool position. The actuator
pressures change and the stem begins to move. The stem movement
reduces the deviation between Control Command and Stem Position.
This process continues until the deviation goes to zero.
The inner-loop controls the position of the spool valve by means
of a driver module. The driver module consists of a temperature-
compensated Hall Effect sensor and a piezo valve pressure modulator.
The piezo valve pressure modulator controls the air pressure under
a diaphragm by means of a piezo beam bender. The piezo beam
deflects in response to an applied voltage from the inner-loop
electronics. As the voltage to the piezo valve increases, the piezo
beam bends, closing off against a nozzle causing the pressure under
the diaphragm to increase. As the pressure under the diaphragm
increases or decreases, the spool valve moves up or down respec-
tively. The hall effect sensor transmits the position of the spool back
to the inner-loop electronics for control purposes.
Figure 3: Linear Mark One TM Control Valve Mounting