Emerson FIELDVUE DVC6200f, Инструкция по эксплуатации

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DVC6200f Digital Valve Controller
November 2010 264
F
OUNDATION
Fieldbus Communication
DVC6200f digital valve controllers use
the
F
OUNDATION
fieldbus to communicate with other fieldbus
instruments and the control system. Fieldbus is an all
digital, serial, two-way communication system which
interconnects “field” equipment such as transmitters,
digital valve controllers, and process controllers.
Fieldbus is a local-area network (LAN) for instruments
used in both process and manufacturing automation
with built-in capability to distribute the control
application across the network.
Function Block Overview
A fieldbus system is a distributed system composed of
field devices and control and monitoring equipment
integrated into the physical environment of a plant or
factory. Fieldbus devices work together to provide I/O
and control for automated processes and operations.
The Fieldbus Foundation provides a framework for
describing these systems as a collection of physical
devices interconnected by a fieldbus network. One of
the ways that the physical devices are used is to
perform their portion of the total system operation by
implementing one or more function blocks.
Function Blocks
Function blocks within the fieldbus device perform the
various functions required for process control.
Because each system is different, the mix and
configuration of functions are different. Therefore, the
Fieldbus Foundation has designed a range of function
blocks, each addressing a different need: Analog Input
Block (AI), Multiple Analog Input (MAI), Discrete Input
Block (DI), Manual Loader Block (ML), Bias/Gain
Station Block (BG), Control Selector Block (CS), P,
PD Controller Block (PD), PID, PI, I Controller Block
(PID), Ratio Station Block (RA), Analog Output Block
(AO) and Discrete Output Block (DO).
Function blocks perform process control functions,
such as analog input (AI) and analog output (AO)
functions as well as proportional-integral-derivative
(PID) functions. The standard function blocks provide
a common structure for defining function block inputs,
outputs, control parameters, events, alarms, and
modes, and combining them into a process that can
be implemented within a single device or over the
fieldbus network. This simplifies the identification of
characteristics that are common to function blocks.
The Fieldbus Foundation has established the function
blocks by defining a set of parameters used in all
function blocks called universal parameters. The
Fieldbus
Foundation has also defined a standard set
of function block classes, such as input, output,
control, and calculation blocks. Each of these classes
have a set of parameters established for it.
Additionally, they have published definitions for
transducer blocks commonly used with standard
function blocks. Examples include temperature,
pressure, level, and flow transducer blocks.
Fieldbus Foundation specifications and definitions
allow vendors to add their own, extended parameters,
as well as their own algorithms. This approach permits
extending function block definitions as new
requirements are discovered and as technology
advances.
Each block has a tag name. Service personnel need
only know the tag name of the block to access or
change the appropriate block parameters.
Input events may affect the operation of the algorithm.
An execution control function regulates the receipt of
input events and the generation of output events
during execution of the algorithm. Upon completion of
the algorithm, the data internal to the block is saved
for use in the next execution, and the output data is
snapped, releasing it for use by other function blocks.
Once the inputs are snapped, the algorithm operates
on them, generating outputs as it progresses.
Algorithm executions are controlled through the setting
of internal parameters. Internal parameters do not
appear as normal input and output parameters.
However, they may be accessed and modified
remotely, as specified by the function block.
Figure D-1 illustrates the internal structure of a
function block. When execution begins, input
parameter values from other blocks are snapped-in by
the block. The input snap process ensures that these
values do not change during the block execution. New
values received for these parameters do not affect the
snapped values and will not be used by the function
block during the current execution.
Function blocks are also capable of performing
short-term data collection and storage for reviewing
their behavior.
Instrument-Specific Blocks
In addition to function blocks, fieldbus devices contain
two other block types to support the function blocks.
These are the resource block and the transducer
block. The resource block contains the hardware
specific characteristics associated with a device.
Transducer blocks couple the function blocks to local
input/output functions.
D