Process Control

&
Experimental Design for Engineers
(MIEng 6331)

Introduction to Programmable
Logic Controllers
(PLCs)
 Structure – Introduction to PLC
Design and PLC information and Programming Programming
operational communication methods techniques
characteristics techniques

2.1. Introduction 2.3. Digital systems 2.5. Ladder and 2.7. Internal relays
functional block
2.2. Input-output 2.4. I/O processing programming 2.8. Jump and call
devices
2.6. IL, SFC and 2.9. Timers
ST programming
2.10. Counters
methods
2.11. Shift registers

2.12. Data handling

2.13. Designing programs

2.14. Programs
Ch 1
Programmable Logic
Controllers (PLCs):
An Overview
 Learning objectives
After completing this chapter, you will be able to:
 Define what a programmable logic controller
(PLC) is and list its advantages over relay
systems
 Identify the main parts of a PLC and describe
their functions
 Outline the basic sequence of operation for a PLC
 Identify the general classifications of PLCs
 1.1. Programmable Logic Controllers
 Most widely used
industrial process control
technology.
 A programmable logic
controller (PLC) is an
industrial grade computer
PLC
that is capable of being
 designed for multiple input and output
arrangements, programmed to perform
 extended temperature ranges, control functions.
 immunity to electrical noise, and  The programmable
 resistance to vibration and impact. controller has eliminated
 an example of a real-time system since much of the hardwiring
the output of the system controlled by associated with
the PLC depends on the input
conventional relay control
conditions.
circuits.
 1.1. Programmable Logic Controllers (Cont…)
PLC
 basically a digital computer designed for use in machine control.
 designed to operate in the industrial environment and is equipped
with special input/output interfaces and a control programming
language.
 offers several advantages over a conventional relay type of
control
 small and inexpensive compared to equivalent relay-based
process control systems
Relays have to be hardwired to perform a specific function
 When the system requirements change, the relay wiring has to be
changed or modified.
 THE RELAYS
A very important component in an Electrical Circuit.

A relay is a simple electromechanical switch made up of an

electromagnet and a set of contacts. Relays are found hidden in
all sorts of devices. In fact, some of the first computers ever built
used relays to implement.
Relay Construction
Relays are amazingly simple devices.
There are four parts in every relay:
1. The Electromagnet
2. The Armature that is attracted by the
electromagnet
3. A Spring
4. A Set of electrical contacts
 Relay Operation
 A relay may also be
called an
“electromagnetic switch”
 Relays use a “low
amperage circuit”
 The low amperage circuit
controls an
electromagnetic device
 The electromagnetic
device “closes/opens” the
high amperage circuit
 Relay Operation (Cont…)
 Open and close
electrical contacts to
turn a load off and
on.
 Most
electromechanical
relays contain a
moving component
called an armature
that is attracted by
the magnetic field the
coil generates.
 Relays are often used
because they cost less
than corresponding
electronic switches.
1.1. Programmable Logic Controllers (Cont…)
PLCs provide many other benefits
 Increased Reliability: Once a
program has been written and tested,
it can be easily downloaded to other
PLCs.
 More Flexibility: It is easier to create
and change a program in a PLC than
to wire and rewire a circuit.
 Lower Cost: Generally, if an
application has more than about a
half-dozen control relays, it will
probably be less expensive to install
a PLC
1.1. Programmable Logic Controllers (Cont…)
 Communications Capability: A PLC can
communicate with other controllers or
computer equipment to perform such
functions as supervisory control, data
gathering, monitoring devices and process
parameters, and download and upload of
programs.
 Faster Response Time: PLCs are designed
for high-speed and real-time applications.
 Easier to Troubleshoot: To find and fix
problems, users can display the control
program on a monitor and watch it in real
time as it executes.
1.2. Parts of a PLC
1.2. Parts of a PLC (Cont…)
I/Os (Inputs/Outputs)
are incorporated into
the PLC:
 Fixed I/O : typical of
small PLCs that
come in one package
with no separate,
removable units.
 Modular I/O: divided
by compartments into
which separate
modules can be
plugged.
1.2. Parts of a PLC (Cont…scan)
 Power supply: supplies DC
power to other modules that
plug into the rack. With larger
systems, power to field
devices is provided by
external AC or DC supplies.
 CPU :
 “brain” of the PLC. A typical
processor usually consists of a
microprocessor for
implementing the logic and
The PLC program is controlling the communications
executed as part of a among modules.
repetitive process  Controls all PLC activity and is
referred to as a scan. designed so that the user can
enter the desired program in
relay ladder logic.
 1.2. Parts of a PLC (Cont…)
I/O system:
 forms the interface by which
field devices are connected to
the controller.
 Condition the various signals
received from or sent to
external field devices.
 Input devices: pushbuttons,
limit switches, and sensors
are hardwired to the input
 PLCs employ an optical isolator, terminals.
which uses light to couple the  Output devices: small
circuits together. motors, motor starters,
 External devices are also referred to solenoid valves, and indicator
lights are hardwired to the
as “field” or “real-world” inputs and
output terminals.
outputs.
1.2. Parts of a PLC
(Cont…)

Relationships
between the inputs
and outputs are
determined by the
user program
1.2. Parts of a PLC (Cont…)
 Programming device:
 used to enter the desired program
into the memory of the processor.
 program can be entered using relay
ladder logic- most popular
programming languages
 hand-held programming device &
PC
 most PLCs have software available
so that a PC can be used as the
programming device.
 software allows users to create,
edit, document, store, and
troubleshoot ladder logic
programs.
 1.2. Parts of a PLC (Cont…)
 Program:
 user-developed series of instructions that directs the
PLC to execute actions.
 Programming language provides rules for combining
the instructions so that they produce the desired
actions.
 Relay ladder logic (RLL): standard programming
language used with PLCs. Its origin is based on
electromechanical relay control.
 Relay ladder logic program graphically represents
rungs of contacts, coils, and special instruction blocks.
 1.3. Principles of Operation

 A mixer motor is to be used to automatically Motor starter coil (M) is

stir the liquid in a vat when the temperature energized when both the
and pressure reach preset values. pressure and temperature
 Direct manual operation of the motor is switches are closed or
provided by means of a separate pushbutton when the manual
station. pushbutton is pressed.
 1.3. Principles of Operation (Cont…)
Use of PLC for the above process problem
 Same input devices: pressure switch,
temperature switch, and pushbutton
 Input and output devices would be hardwired
to an appropriate input/output module
according to the manufacturer’s addressing
location scheme.
 Same output field device (motor starter coil)
would also be used.
 PLC ladder logic program would be
constructed and entered into the memory of the
CPU.

Typical wiring connections for a 120 VAC

modular configured input and output module
is shown in Figure 1-19 & Figure 1-20.
 1.3. Principles of Operation (Cont…)
 Format used is similar
to the layout of the
hardwired relay ladder
circuit.
 Symbols represent
instructions, whereas the
numbers represent the
instruction location
addresses.
 To program the
controller, enter these
instructions one by one
into the processor
memory from the  I/O address format will differ, depending on
programming device. the PLC model and manufacturer.
 For the program to operate, the controller is
placed in the RUN mode, or operating cycle.
 1.3. Principles of Operation (RUN..Cont…)
RUN operation for the process control scheme
 1st Step: pressure switch, temperature switch, and pushbutton inputs are
examined and their status is recorded in the controller’s memory.
 closed contact is recorded in memory as logic 1 and an open
contact as logic 0.
 2nd Step: ladder diagram is evaluated,
 with each internal contact given an OPEN or CLOSED status
according to its recorded 1 or 0 state.
 3rd Step: When the states of the input contacts provide logic continuity from
left to right across the rung,
 output coil memory location is given a logic 1 value and the
output module interface contacts will close.
 When there is no logic continuity of the program rung, the
output coil memory location is set to logic 0 and the output
module interface contacts will be open.
 Completion of one cycle of this sequence by the controller is called a scan.
 1.3. Principles of Operation (Cont…)
 Figure 1-22
shows the
typical wiring
required to
implement the
process control
scheme using a
fixed PLC
controller.
 Allen-Bradley
Pico controller
equipped with
8 inputs and 4
outputs is used
to control and
monitor the
process.
 1.4. Modifying the Operation
Assume that the original process control circuit Relay system: require some
for the mixing operation must be modified as rewiring – to achieve the
shown in the relay ladder diagram of desired change
Figure 1-23
 the change requires that the manual
pushbutton control be permitted to operate
at any pressure, but not unless the specified
temperature setting has been reached.
PLC system:
require no
rewiring – to
achieve the
desired change:
the inputs and
outputs are still
the same
 1.5. PLCs versus Computers
 The architecture of a PLC Important Characteristics distinguishing
is basically the same as PLC from PC
that of a PC.  PLC is designed to operate in the
industrial environment with wide ranges
 PC can be made to operate
of ambient temperature and humidity.
as a PLC if  A well-designed industrial PLC
 Provide some way for installation is not usually affected by the
the computer to receive electrical noise inherent in most
information from industrial locations.
devices such as  Unlike PC, the PLC is programmed in
pushbuttons or relay ladder logic or other easily learned
languages.
switches.
 PLC comes with its program language
 Program to process the built into its memory and has no
inputs and decide permanently attached keyboard, CD drive,
means of turning load or monitor.
devices off and on.  PLCs come equipped with terminals for
input and output field devices as well as
communication ports.
1.5. PLCs versus Computers (Cont…)
Software associated with a PLC
 PLC software that allows the user to
program and document gives the user the
tools to write a PLC program – using ladder
logic or another programming language –
and document or explain the program in as
much detail as is necessary.
 PLC software that allows the user to
monitor and control the process is also
called a human machine interface (HMI).
 1.6. PLC Size and Application
Criteria used in categorizing PLCs Types of PLC applications
include:  Single ended or stand-alone
 Functionality PLC
 number of inputs and outputs,  Multitask PLC
 Control Management PLC
 cost, and
 physical size

Of these, the I/O count is the most important

factor!
 1.7. Programs
 Programs for use with PLCs can be written in a number of
formats.
 To make it easier for engineers with no great knowledge of
programming to write programs for PLCs, ladder
programming was developed.
 Most PLC manufacturers adopted this method of writing
programs; however, each tended to develop its own versions
and so an international standard has been adopted for ladder
programming and indeed all the methods used for
programming PLCs.
 Standard: International Electrotechnical Commission(IEC)
1131-3 published in 1993, now referred to as IEC 61131-3-
latest edition dated 2013
 1.7. Programs (Cont…)
 IEC 61131-3 programming languages
 ladder diagrams (LAD),
 instruction list (IL),
 sequential function charts (SFC),
 structured text (ST), and
 function block diagrams (FBD).
 The standard includes a library of pre-programmed functions and function
blocks
 Function is the term used for a pre-programmed calculation
 function that gives the average value of two inputs,
 function block – used when inputs are evaluated and give a value to an
output
 IEC standard gives formal definitions for each input and output parameter so
that function blocks designed by different programmers can be readily
interconnected.
 Any PLC that is IEC compliant supports these functions as a library with the
code being written in a prom of flash ram on the device.
 1.7. Programs (Cont…)
 Structured text (ST) and instruction list (IL)
are graphical and are entered into the
programming device from a keyboard, one line
at a time.
 Ladder diagrams (LAD), sequential
function charts (SFC) and function block
diagrams (FBD) are graphical and so a
program can be build up with graphical
elements on the screen of the programming
device.
 The IEC Standard
The IEC 61131 standard covers the complete life cycle of PLCs;
Part 1: General definition of basic terminology and concepts.
Part 2: Electronic and mechanical equipment requirements and
verification tests for PLCs and associated equipment.
Part 3: Programming languages: Five languages are defined: ladder
diagram (LAD), sequential function charts (SFC), function block
diagram (FBD), structured text (ST), and instruction list (IL).
Part 4: Guidance on selection, installation, and maintenance of PLCs.
Part 5: Software facilities needed for communication with other devices
based on the Manufacturing Messaging Specification (MMS).
Part 6: Communications via fieldbus software facilities.
Part 7: Fuzzy control programming.
Part 8: Guidelines for the implementation of PLC programming
languages defined in Part 3.
 Programming PLCs
 PLC manufacturers have programming
software of their PLCs.
E.g. Mitsubishi has MELSOFT
Siemens has SIMATIC STEP 7
Rockell Automation have
RSLogix for the Allen-Bradley PLC-5
family of PLCs.