Basic of Programable Logic Controller PLC

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Basic PLC

2 I Copyright © 2024 TPS. All rights reserved
Day 5 Agenda
 Overview of typical distributed Control system
architecture
 DCS vs. SCADA vs. PLC’s Comparison
 The smart instrument
 Component in a DCS system

CPU
CPU CPU
CPU
RIO
RIO
Intra-PLC communications link
PLC station 1 PLC station 2
Local I/O chassis
Remote I/O chassis
I/O
rack
communications
link
Overview of Typical Distributed
Control System Architecture

VME/PC
4 –20 mA
4 –20 mA
4 –20 mA
PLC PLC
4 –20 mA
PLC
SCADA
TCP/IP (Ethernet)
Traditional System

VME/PC
4 –20 mA
4 –20 mA
4 –20 mA
PLC PLC
4 –20 mA
PLC
SCADA
TCP/IP (Ethernet)
e.g. Modbus Plus (if all Modicon PLCs)
Improving the System

VME/PC
4 –20 mA
4 –20 mA
4 –20 mA
PLC PLC
4 –20 mA
PLC
SCADA
TCP/IP (Ethernet)
or DH Plus (if all Allen Bradley PLCs)
Improving the system
Overview of typical distributed Control system
architecture

VME/PC
4 –20 mA
4 –20 mA
4 –20 mA
PLC PLC
4 –20 mA
PLC
SCADA
TCP/IP (Ethernet)
TCP/IP (Ethernet)
A universal bus?
architecture

VME/PC
PLC PLC PLC
SCADA
TCP/IP (Ethernet)
H2 HSE
H0
4 –20 mA
4 –20 mA
4 –20 mA
A universal bus?
architecture

 DCSs have a direct connection to the data source and are primarily
focused on real-time states – concentrating on past and present
process variables.
 With direct connection to the hardware device I/O, a very reliable
network connection to equipment is required.
 Redundancy is usually handled by parallel equipment – not by
diffusion of information around a distributed database.
 A DCS is sequential in nature with alarms generated not when a
point changes but when a process is run.
DCS vs. SCADA vs. PLC’s
– Comparison

 A DCS operator station is closely connected with its I/O signals
through local wiring and communication buses (e.g. Fieldbus).
 When DCS operators wish to see information they would
usually make a request directly to the field I/O and gets a
response.
 Field events can directly interrupt the system and advise the
operator.
 A DCS typically has correspondingly more complexity in its
process-control functionality.
– Comparison

 In many instances DCSs are not aware of a change in state, but simply
report the current real-time state at the instant the hardware is polled.
 Events and alarms (both central concepts in SCADA) are secondary to
process displays.
 Therefore, it is often necessary to set up alarms by hand on each point,
and many DCSs have limited alarm filtering.
 Because DCSs must have a continuous connection to the hardware,
they are generally deployed only when the system has a very small
geographical footprint, such as a factory or plant.
– Comparison

 Historically, SCADA systems have focused on control of remote
equipment where telecommunications may be regularly
interrupted.
 Consequently, SCADA systems are indirectly connected via a
database to the field equipment and can continue operating even
when telecommunications are temporarily lost.
 Further more, SCADA systems typically incorporate complex
polling capabilities to help address telecommunication restrictions.
 SCADA systems are thus said to be data-centric and have a
database driven architecture.
– Comparison

 Unlike DCSs that are state driven, SCADA systems are event
driven, with change of state (COS) driving the functionality of the
system
 COS is also the main criteria in data gathering and presentation.
 COS also focuses SCADA functionality on alarms, which are
typically automatically set on all points and event logs.
 Consequently, a large part of a SCADA system's functionality is
centred on complex filtering for alarms and event logs.
– Comparison

 The essential difference is that a SCADA system transfers data
and control signals over a potentially slow, unreliable
communications medium – with redundancy usually handled in
a distributed manner.
 Thus, a SCADA system must continue to operate when field
communications have failed and the ‘quality’ of the data shown
to the operator is an important facet of system operation.
 The system thus needs to maintain a database of ‘last known
good values’ for prompt operator display.
– Comparison

 Because SCADA is oriented towards data gathering its focus is on
the control centre and operators.
 SCADA systems thus usually provide special ‘event’ processing
mechanisms to handle conditions that occur between data
acquisition periods.
 It frequently needs to carry out event processing and data quality
validation.
– Comparison

 The roles of a SCADA and DCS are quite different and it is rarely
possible for a single system to perform both functions.
 Many industries use both DCS and SCADA products.
 The operator stations for each product line can use the same
UNIX workstations.
 Further, the systems share data and thus form a composite
SCADA/DCS system.
 Today, several manufacturers offer hybrid systems that combine
the best features of both the traditional heritage DCS with that of
a modern SCADA-based systems.
– Comparison

Typical Small Hybrid System

VME/PC
PLC PLC PLC
SCADA
TCP/IP (Ethernet)
H2 HSE
H0
H1
A universal bus?
The smart instrument

Process Control Selection
 Large facilities with 1000’s of control loops will still use
traditional DCS or current derivative hybrid
 PLC/SCADA systems are preferred for smaller facilities
(100-200 loops)
 Hybrid systems (Delta V, Experion PKS) are acquiring
traits of traditional DCS
• Redundancy
• Security
• Database management
• Function blocks and algorithms
 Major DCS vendors are migrating toward the hybrid
systems due to user demands and open systems
 Hardware cost is less, but system integration costs may
offset

 A DCS is a proprietary control system comprising a number of functionally and/or
geographically distributed controllers linked by a redundant data highway. The
controllers communicate with the process field devices via input/output (I/O)
modules that can be either integral with the controller or located remotely via a field
network.
 In older traditional ‘Heritage’ systems the field network linking the I/O modules to
the field devices was either analog (4 to 20 mA) or part-digital (HART) carried on a
twisted-pair shielded cable.
 In more modern ‘Hybrid’ systems both bit-based (on/off) and message-based (up
to 256 bytes) field data is carried on a single-cable multidrop open fieldbus
networks (typically Foundation Fieldbus or Profibus).
 In addition to basic PID control, modern DCS controllers have extensive advanced
process control (APC) and computational capabilities and can generally incorporate
logic and sequential control.
Component in a DCS system

Typical Heritage DCS
Field Interface
modules in
cabinet
Field
Device
s
Ethern
et
Redundant
Network
Corporate
Computer
Plant
Compute
r
Operator
Workplace
Gatewa
y

Distributed Control Systems
 In a DCS, the data acquisition and control functions are
performed by a number of distributed microprocessor-based
units, situated near to the field devices
Data
Highway
Operator
stations
M
M M
M M
M
Dual redundant
highway
Remote
Field
Controllers
Overview
display
Detailed area
display
Alarming Trending

Distributed Control Systems
 Typically referred to as Heritage systems
 Control functions, simulation and optimization
routines are ‘distributed’ to field controllers
mounted in remote field locations
 Field Controllers are microprocessors capable of
performing a variety of algorithms on the control
signal
 These include:
• classic PID functions
• ratio,
• linearization,
• cascade,
• alarm and shutdown

 Review & questions

What’s gone wrong?
Exercise
Valve jammed open
 Level transmitter giving false low-level signal
Control loop left on manual with valve open
 Leaking control valve and pump out stage shut
down
– Alarm & shutdown logic diagrams
PLC applications in Automations

Simple Shutdown System Solution
LT
1
PSV
LC
1
I/P
FC
Fluid
Feed FC
Logic Solver
LT
2
LAHH
2
AS
HS
2
Reset
LI
2
Tripped Alarm
PLC Applications in Automations
Alarm & Shutdown Logic Diagrams (Cont.)

Layers of Protection
Process
Plant Emergency Response *
Other Technologies (e.g. PRV)
Automatic SIS
Critical Alarms and Manual Response
Process Plant
* Mitigation layers
Community Emergency Response *
Containment Systems*
Process Control

 Layers of Protection
‒ A Protection Layer consists of a grouping of
equipment and/or administrative controls that
function in concert with other protection layers
to control or mitigate process risk.
‒ An independent protection layer (IPL):
 Reduces the identified risk by at least a factor
of 10.
 Has high availability (0.9)
 Designed for a specific event
 Independent of other protection layers
 Dependable and auditable

 Mean Time Between Failures
‒ MTBF is the average value of ‘T’ over the
operating life of the system.
T1 T2 T3
Up
Down

 MTBF = MTTF + MTTR
MTTF
MTBF
Up
Down
MTTR

Risk reduction achieved by all safety-related systems and external risk
reduction facilities
Residual
risk
Acceptable risk EUC risk
Necessary risk reduction
Actual risk reduction
Increasing
risk
Partial risk covered
by external risk
reduction facilities
by E/E/PE
safety-related systems
by other technology
safety-related systems
 Risk reduction: general concepts (IEC 61508 part 5 )

 Safety Integrity Levels: based on IEC Table 2
‒ Target failure measures (PFDavg) for a safety
function operating in a low demand mode of
operation
Safety
integrity
level
Low demand mode of operation
(average probability of failure to
perform its
design function on demand)
Risk Reduction
Factor (RRF)
4  10-5
to 10-4
10 000 to 100 000
3  10-4
to 10-3
1 000 to 10 000
2  10-3
to 10-2
100 to 1 000
1  10-2
to 10-1
10 to 100

 SIL determination
‒ Safety Integrity Level (SIL) is a basic concept
‒ SIL defines the level of safety performance for a
SIS
‒ SILs are defined as 1, 2, or 3. (and level 4 for
IEC61508 standards)
‒ The higher the SIL, the better the safety
performance of the SIS
‒ Better SIS performance is achieved by higher
availability of the safety function

 Sequential functional chart, or SFC, is a graphical
“language” that provides a diagrammatic
representation of control sequences in a program.
Basically, sequential function chart is a flowchart-like
framework that can organize the subprograms or
subroutines (programmed in LD, FBD, IL, and/or ST)
that form the control program. SFC is particularly
useful for sequential control operations, where a
program flows from one step to another once a
condition has been satisfied (TRUE or FALSE).
Sequence Control Program

 The SFC programming framework contains three
main elements that organize the control program:
‒ Steps
‒ Transitions
‒ Actions
Sequence Control Program (Cont.)

 A step is a stage in the control process. For
example, the mixing application has three steps—
the initial step, the mixing step, and the emptying
step. When the control program receives an input, it
will execute each of these steps starting with step 1.
Each step may or may not have an action associated
with it. An action is a set of control instructions
prompting the PLC to execute a certain control
function during that step. An action may be
programmed using any one of the four IEC 1131-3
languages.
Sequence Control Program (Cont.)

References

 Lessons In Industrial Instrumentation Rev1.2by
Tony R. Kuphaldt
 Programmable Controllers Second Edition by L. A.
Bryan E. A. Bryan
 Practical SCADA for industry. By David Bailey &
Edwin Wright
 Automation systems and PLCs. By Hugh Jack
 Fundamentals of PLCs. By Phil Melore
 Principles of User Interface Design. Article by Niall
Murphy
References

 SCADA System Security. Article By Abhishek
Bhattacharjee, Stephen Flannigan and Jens
Nasholm, Citect Inc.
 Alarm code in PLCs. Article by Allen Nelson,
PLCs.net
 InTouch® Reference Guide. By Invensys Systems,
Inc.
 Ethernet Communications Modules. By Automation
Direct, Inc.
 Fundamentals of Control 2006 by PAControl.com
References

Basic of Programable Logic Controller PLC

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