Positioner Operation - Flowserve Logix 3200MD Installation, Operation And Maintenance Manual

4.2

Positioner Operation

The Logix 3200MD positioner is an electric feedback instrument.
Figure 1 shows a Logix 3200MD positioner installed on a double-
acting linear actuator for air-to-open action.
The Logix 3200MD receives power from the two-wire, 4-20 mA
input signal. However, since this positioner utilizes HART com-
munications, two sources can be used for the command signal:
Analog and Digital. In Analog source, the 4-20 mA signal is used for
the command source. In Digital source, the level of the input 4-20
mA signal is ignored and a digital signal, sent via HART, is used as
the command source. The command source can be accessed with
ValveSight software, the HART 375 communicator, or other host
software.
Whether in Analog or Digital Source, 0% is always defined as the
valve closed position and 100% is always defined as the valve open
position. In Analog Source, the 4-20 mA signal is converted to a
percentage. During loop 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
Figure 1: Logix 3200MD Digital Positioner Schematic (air-to-open configuration)
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User instructions - Digital Positioner 3200MD LGENIM0059-01 10/08
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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 three basic modes: Linear, Equal
Percent (=%) 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. 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
ValveSight software. In addition, two user-defined features, Soft Lim-
its and MPC (Minimum Position 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 3200MD 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