Flowserve 3400IQ Digital Positioner - Manual (Page 16)

Logix 3400IQ Digital Positioner FCD LGENIM3402-00 – 0/07

One important parameter should be discussed at this point: Inner loop offset. Referring to Figure
1.1, a number called inner loop offset (IL_OFFSET) is added to the output of the control algorithm.
In order for the spool to remain in its null or balanced position, the control algorithm must output a
non-zero spool command. This is the purpose of the inner loop offset. The value of this number is
equivalent to the signal that must be sent to spool position control to bring it to a null position with
zero stem deviation. This parameter is important for proper control and will be discussed further in
the Control and Tuning section.
1.3 Fieldbus Overview
Understanding Fieldbus
Fieldbus is an all-digital, serial, two-way communication system which interconnects industrial ‘field’
equipment such as sensors, actuators, and controllers. Fieldbus is a Local Area Network (LAN) for
field instruments with built-in capability to distribute the control application across the network. See
Figure 1.4.
Figure 1.4 Fieldbus Connecting Control Room and Field Devices
Open System Design
The Fieldbus Foundation has defined standards to which field devices and operator/control stations
communicate with one another. The communications protocol is an open system to allow all field
devices and control equipment which are built to the FOUNDATION fieldbus standard to be integrated
into a control system, regardless of the device manufacturer. This inter operability of devices using
fieldbus technology is becoming the industry standard for automation and distributed control
systems.
Hardware Architecture
The physical architecture of fieldbus allows installation of fieldbus devices using a twisted-pair cable.
Often, existing wiring from analog devices can be used to wire up digital fieldbus devices. Multiple
field devices can be connected on one cable (a multi-drop link), rather than conventional point-to-
point wiring used for analog devices. See Wiring the Logix 3400IQ Digital Positioner to a Fieldbus
Network on page 27.
Software Architecture
Fieldbus software architecture provides for more control functions to be available in the micropro-
cessor-based field device. Since fieldbus is a digital communication system, more data is available
to operators for process monitoring, trend analysis, report generation, and trouble analysis. Device
software changes can be downloaded to field devices remotely from the operator station (or PC) in
the control room.
Fieldbus LAN
ST 3000 FF Logix 3400IQ
Digital Positioner
Fieldbus
Device
Fieldbus
Device
Control Room Device
(Operator Interface)