Basic Operation; Hart; Position Definition; Command Input And Final Command - Flowserve Logix 420 User Instructions

Between actuator and valve there are moving parts. To avoid injury
FLOWSERVE provides pinch-point-protection in the form of cover
plates, especially where side-mounted positioners are fitted. If these
plates are removed for inspection, service or repair special attention is
required. After completing work the cover plates must be refitted.
Logix 420 positioner repair is limited to the replacement of sub-assem-
blies and circuit boards with FLOWSERVE-manufactured replacements
as outlined in this manual.
1
CAUTION: Before products are returned to FLOWSERVE for repair
or service, FLOWSERVE must be provided with a certificate
which confirms that the product has been decontaminated and is
clean. FLOWSERVE will not accept deliveries if a certificate has
not been provided (a form can be obtained from FLOWSERVE).
Apart from the operating instructions and the obligatory accident
prevention directives valid in the country of use, all recognized regula-
tions for safety and good engineering practices must be followed.
PRINCIPLES OF OPERATION

1.8 Basic Operation

The Logix 420 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 3.8 mA. The positioner
is configurable through the local user interface, hand-held or DTM. The
Logix 420 positioner can control single-acting pneumatic actuators with
linear or rotary mountings.
Analog
Digital
Comman
Command
d Input
Input
(4-20 mA)
Command
Input
Signal
Characterization,
Soft Limits,
Tight Shutoff
4
Command
P
+
I
_
Final
D
Figure 1: Principle of Operation of Logix 420
Figure 1: Principles of Operation of Logix 420
Logix 420 Digital Positioner FCD LGENIM0106-07-AQ – 10/15
®
The Logix 420 digital positioner is an electronic and pneumatic closed-
loop feedback instrument. Figure 1 shows a schematic of a Logix 420
positioner installed on a single-acting linear actuator for air-to-open
action.

1.9 HART

The Logix 420 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 375 communicator,
or other host software. See section 11 HART COMMUNICATION
HART COMMUNICATION for more information.

1.10 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.

1.11 Command Input and Final Command

The Command Input signal (in percent) passes through a characteriza-
tion/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
Air Supply
Piezo Kill
Circuit
Piezo
Voltage
Inner
Loop
Spool
Control
+
_
+
+
+
Output
Percentage
Inner-Loop
Offset
Single Acting
Pilot Relay
Poppet
Valve
Actuator
Hall
Sensor
Piezo
Valve
Inner-Loop
Output
Position
Feedback
Control
Valve