Logix
®
Digital Positioners FCD LGENIM0110-0 05/16
flowserve.com
8
15
PRINCIPLES OF OPERATION
15.1 Basic Operation
The Logix
®
digital positioner is a two-wire 4-20
mA input digital valve positioner which uses the HART
protocol to allow two-way remote communications. The
positioner is completely powered by the 4-20 mA input
signal. Start-up current must be at least 3.9 mA. The
positioner is configurable through the local user interface,
hand-held or DTM. The Logix
®
positioner can
control both double- and single-acting pneumatic actuators
with linear or rotary mountings.
The Logix
®
digital positioner is an electronic and
pneumatic closed-loop feedback instrument. Figure 1
shows a schematic of a Logix
®
positioner
installed on a double-acting linear actuator for air-to-open
action.
15.2 HART
The Logix
®
receives power from the two-wire, 4-
20 mA input signal. However, since this positioner utilizes
HART communications, 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
(used only for power) and a digital signal, sent via the
HART communication protocol, is used as the command
source. The command source can be accessed with
ValveSight
®
software, the HART 475 communicator, or
other host software. See Section 10 for more information.
15.3 Position Definition
Whether in Analog or Digital Source, The position at 0% is
always defined as the valve in a closed position and 100%
is always defined as the valve in an open position. In
Analog Source, the 4-20 mA signal is converted to a
position (in percent). During loop calibration, the signals
corresponding to 0% and 100% are defined.
15.4 Command Input and Final
Command
The Command Input signal (in percent) passes through a
characterization/limits modifier block. This function is done
in software, which allows for in-the-field customer
adjustment. The characterization block can apply no
adjustment
(Linear),
one
of
several
pre-defined
characterization curve adjustments (including several Equal
Percent), or a 21-point Custom Characterization curve
adjustment. 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 rangeability equal percent curve. If Custom
Characterization is enabled, the input signal is mapped to 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 Limits and Tight Shutoff may affect
the position. The actual command being used to position
the stem after the evaluation of characterization curves and
user limits is called the Final Command.
Figure 3: Principles of Operation of Logix
