WHAT IS THE DIFFERENCE BETWEEN PLC AND DCS
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Answer Posted / utpal
You must automate a process, but you can't decide between a
DCS and a PLC. Are these systems really all that different?
The answers depend on a slew of other questions.
Turn the clock back 10-15 years: 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.
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). Relational databases and spreadsheet
software enhance the ability of DCSs to store and
manipulate data. Artificial intelligence (AI) technology
gives us "smart" alarming. Today's DCS architecturally
looks much like the DCS of 20 years ago, but tomorrow's DCS
may control through networked "smart" devices-with no I/O
hardware of its own.
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." 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
controller normally expects an operator interface and
communications, so it can send alarms, messages, trend
updates, and display updates.
Many PLC installations use interface software from third-
party vendors for improved graphics and various levels of
alarming, trending, and reporting. The PLC and MMI software
normally interact by sitting on the network and using the
register exchange mechanism to get data from and to the
various PLCs. This type of communication presumes you have
preassigned data registers and can fetch data on an
absolute address basis. This can lead to data processing
errors (e.g., from the wrong input) you won't encounter
with the central database of 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.
| Is This Answer Correct ? | 82 Yes | 6 No |
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