PLC / DCS
The Programmable Logic Controller (PLC) is
king of machine control while the Distributed Control System (DCS)
dominates process control. If you manufacture plastic widgets, you speak PLC.
If you produce chemicals, you speak DCS.
Today, the two technologies share kingdoms as the functional
lines between them continue to blur. We now use each where the other used to
rule. However, PLCs still dominate high-speed machine control, and DCSs prevail
in complex continuous processes.
Distributed Control System:
The early DCS looked dramatically different from the early
PLC. Initially, the DCS performed the control functions of the analog panel
instruments it replaced, and its interface mimicked their panel displays. DCSs
then gained sequence logic capabilities to control batch processes as well as
continuous ones. DCSs performed hundreds of analog measurements and controlled
dozens of analog outputs, using multi-variable Proportional Integral Derivative
(PID) control. With the same 8-bit microprocessor technology that gave rise to
the DCS, PLCs began replacing conventional relay/solid-state logic in machine
control. PLCs dealt with contact input/output (I/O) and started/stopped motors
by performing Boolean logic calculations.
The big change in DCS over the past 20 years is its move
from proprietary hardware to the personal computer (PC) and standard LAN
technologies. With each advance in PC power, DCSs have moved up in power. PCs
gave us speedy, responsive, multi-media, windowed, operator-process interfaces
(OPI).
Most DCSs offer redundant controllers, networks, and I/Os.
Most give you "built-in" redundancy and diagnostic features, with no
need for user-written logic.
DCSs allow centralized configuration from the operator or
engineering console in the control room. You can change programming offline,
and download without restarting the system for the change to be effective.
DCSs allow inter-controller communications. You can do data
exchange in most DCS systems ad hoc (no need for predefined data point lists).
You access data by tag name, regardless of hardware or location.DCSs use
multi-tasking operating systems, so you can download and run applications aside
from the real-time control functions and still do fractional-second control.
DCSs now come in "micro" systems, to price-compete with PLCs-but with
full DCS features and capabilities.
The typical DCS has integrated diagnostics and standard
display templates that automatically extend/update when your database changes.
This database is central to the system-you don't have different databases
sitting in the controllers.
DCSs have user-friendly configuration tools, including
structured English, control block libraries, SFC (sequential function chart),
and even RLL (relay ladder logic).
Most DCSs allow graphical configuration, provide online
diagnostics, and are self-documenting. Most provide for user-defined control
blocks or customized strategies. The controllers execute control strategies as
independent tasks; thus, making changes to part of the control logic has no
impact on the rest.
An important difference between DCSs and PLCs is how vendors
market them. DCS vendors typically sell a complete, working, integrated, and
tested system; offering full application implementation. They offer many
services: training, installation, field service, and integration with your
Information Technology (IT) systems. A DCS vendor provides a server with a
relational database, a LAN with PCs for office automation, networking support
and integration of third-party applications and systems. The DCS vendor tries
to be your "one-stop shop."
Programmable Logic Controller:
The PLC is more of a "do-it-yourself" device,
which is sometimes simpler to execute.
Programmable Logic Controllers. When PLCs were solely
replacements for hard-wired relays, they had only digital I/O, with no operator
interface or communications. Simple operator interfaces appeared, then evolved
into increasingly complex interfaces as PLCs worked with increasingly complex
automation problems. We went from a panel of buttons and I/O-driven lamps to
PLC full-color customized graphic displays that run on SCADA software over a
network.
PLCs now have many DCS-like control functions (e.g., PID algorithms)
and analog I/O. They've moved past their birthplace: the digital world (switch
and binary sensor inputs and output contacts to run motors and trigger
solenoids).
PLCs are fast: They run an input-compute-output cycle in
milliseconds. On the other hand, DCSs offer fractional second (1/2 to 1/10)
control cycles. However, some DCSs provide interrupt/event-triggered logic for
high-speed applications.
PLCs are simple, rugged computers with minimal peripherals
and simple OSs. While increasing reliability, PLC simplicity is not conducive
to redundancy. Thus, fully redundant ("hot," automatic, bumpless)
variations of PLCs, with their added hardware and software, sometimes suffer
from a reduction in their reliability-a characteristic PLCs are famous for.
Data exchange typically requires you to preassign data
registers and hard code their addresses into the logic. If you add registers or
need to reassign data, you typically have to deal manually with the Domino
Effect.
Typical PLC Relay Ladder Logic (RLL) languages include
function blocks that can perform complex control and math functions (e.g., PID
algorithms). Complex multi-loop control functions (e.g., cascade management and
loop initialization) are not typical. For functions too messy to implement in
RLL, most PLCs provide a function block that calls a user-written program
(usually in BASIC or C).
PLCs typically operate as "state" machines: They
read all inputs, execute through the logic, and then drive the outputs. The
user-written logic is typically one big RLL program, which means you may have
to take the whole PLC off-line to make a change of any size. You also run into
database synchronization problems because of the separation of PLCs and the Man
Machine Interface (MMI) software packages, as opposed to the central databases
of DCSs.
A PLC will run in a stand-alone configuration. A DCS
Some PLCs use proprietary networks, and others can use LANs.
Either way, the communication functions are the same-fetch and put registers.
This can result in bottlenecking and timing problems if too many PCs try communicating
with too many PLCs over a network.
A PLC may have a third-party package for operator
interfaces, LAN interface to PCs and peripherals, PLC data highway or bus,
redundant controllers with local and distributed I/O, local MMI and local
programming capability. The PLC would have redundant media support, but not the
redundant communication hardware or I/O bus hardware you'd find in a DCS. A PLC
would have preprogrammed I/O cards for specific signal types and ranges.
Today, the decision between PLC and DCS often depends on
business issues rather than technical features. Questions to consider are those
involving:
The internal expertise to execute the project, Level of
support available from a vendor/integrator, Long-term maintainability, and
Life-cycle costs.
PLCs and DCSs overlap in their features, but also have
distinct strengths and weaknesses. When deciding between the two, know who will
deliver and support your system, and how they will do it.
0 comments:
Post a Comment