MUKURWE-INI TTI

ELECTRICAL AND ELECTRONICS ENGINEERING

DEPARTMENT

DIPLOMA IN ELECTRICAL &

ELECTRONICS ENGINEERING (POWER
OPTION)
Topic 2. PLC Systems
INTRODUCTION

 PLC is a unit of hardware used to control and automate

industrial processes.
 It is a micro-computer based controller that uses stored
instructions in programmable memory to implement logic,
sequencing, timing, counting and arithmetic functions through
digital or analog input/output modules, for controlling
machines and processes.
 The term logic is used because programming is primarily
concerned with implementing logic and switching
operation.
 The PLC is designed as replacement for the hard-wired
relay and timer logic to be found in traditional control
panels, where PLC provides ease and flexibility of control
based on programming and executing logic instructions.
 A PLC has three main aspects: the inputs and
outputs and the control program.
 The input is anything that can sense the status of the
environment and then convert that information in to a
signal.
 Often the signal can simply be a voltage that is either on or
off.
 For example, input devices can be proximity switches,
photoelectric sensors, temperature sensors, push buttons, or
pressure sensors.
 The outputs are connected to the devices that need to
be controlled like motors, indicator lights, fans,
warning sirens or heating elements.
 Control processes need devices to monitor events
or measure needed values.
 These devices are generically called inputs to the PLC.
 The program uses a set of logical instructions that
drives the outputs based on the inputs.
THE NEED FOR PLCS

 Hardwired panels were very time consuming to wire,

debug and change.
 The PLCs eliminates much of the hard wiring that was
associated with conventional relay control circuits.
 PLCs have the great advantage that the same basic
controller can be used with a wide range of control
systems.
 PLCs require shorter installation and commissioning
times than do hard-wired systems .
 To modify a control system and the rules that are to be
used, all that is necessary is for an operator to key in a
different set of instructions. There is no need to rewire.
 The result is a flexible, cost effective, system which can be
used with control systems which vary quite widely in their
nature and complexity.
 PLCs are similar to computers but whereas computers are
optimized for calculation and display tasks, PLCs are
optimized for control tasks and the industrial environment.
 Thus PLCs have specific features suited for industrial
control :-
 Rugged and designed to withstand vibrations, temperature,
humidity and noise
 Modular plug-in construction, allowing easy replacement or
addition of units (e.g. input/output);
 Standard input/output connections and signal levels
 Have interfacing for inputs and outputs already inside the
controller.
  Easily understood programming language which is
primarily concerned with logic and switching
operations
 Ease of programming and reprogramming in-plant;
 Capable of communicating with other PLCs,
computers and intelligent devices;
 Competitive in both cost and space occupied with
relay and solid-state logic systems.
 These features make programmable controllers
highly desirable in a wide variety of industrial-
plant and process-control situations.
PLC ADVANTAGES
 Flexibility: One single PLC can easily run many machines.
 Correcting Errors: With PLC control, any change in circuit design
or sequence is as simple as retyping the logic. Correcting errors in
PLC is extremely short and cost effective.
 Space Efficient: Today's PLC memory is getting bigger and bigger
this means that we can generate more and more contacts, coils,
timers, sequencers, counters and so on. It is possible to have
thousands of contact timers and counters in a single PLC.
 Low Cost: Prices of PLC vary from few hundreds to few thousands.
 Testing: A PLC program can be tested and evaluated in a lab. The
program can be tested, validated and corrected saving very valuable
time.
 Visual observation: When running a PLC program a visual
operation can be seen on the screen. Hence troubleshooting a circuit
is really quick, easy and simple.
TYPICAL PLC APPLICATIONS
 PLCs are used to operate greenhouse irrigations systems.
 PLCs are used for sorting packages on a conveyor by
operating a diverter.
 PLCs are implemented in a variety of control operations
from large to small. Carwashes are a popular use for PLCs
because it involves intricate use of sensors and motors, but
also has the need for relatively complex logic.
 Lumber mills use PLCs to control the main saw and
loading of wood while various sensors ensure safe
operation so that people and equipment are not harmed
 PLCs can withstand the harsh condition desert conditions
while controlling an oil recovery process.
PLC ARCHITECTURE

 There are two types:

 Open architecture design allows the system to be
connected easily to devices and programs made by
other manufacturers.
 Closed architecture or proprietary system is one
whose design makes it more difficult to connect
devices and programs made by other manufacturers.
 NOTE: When working with PLC systems that
are proprietary in nature you must be sure that
any generic hardware or software you use is
compatible with your particular PLC.
PLC HARDWARE
 The structure of a PLC can be divided several
parts/components.
 The main parts are input/output modules, central processing
unit, memory and programming terminal.
 Processor unit or central processing unit
(CPU) is the unit containing the microprocessor
and this interprets the input signals and carries
out the control actions, according to the program
stored in its memory, communicating the
decisions as action signals to the outputs.
 Memory unit is where the program is stored that
is to be used for the control action to be exercised
by the microprocessor and data stored from the
input for processing and for the output for
outputting
 Input and output (I/O) modules – are where the
processor receives information from external devices
and communicates information to external devices.
 The I/O unit provides the interface between the system
and the outside world, allowing for connections to be
made through I/O channels to input devices such as
sensors and output devices such as motors and solenoids.
 It is also through the I/O unit that programs are entered
from a program panel.
 Every I/O point has a unique address which can be used
 Input and output (I/O) devices - is collection of
physical elements of the control system that either
provide or use I/O data.
 Programming device / terminal are used to enter
the required program into the memory of the
processor.
 The program is developed in the device and then
transferred to the memory unit of the PLC.
 Rack Assembly: Most medium to large PLC
systems are assembled such that the individual
components - CPU, I/O, power supply - are
modules that are held together within a rack.
 In smaller PLC systems - all of these components may
be contained in a single housing or "brick" - these
smaller systems are sometimes referred to as "bricks"
or "shoebox" PLCs.
 Power supply unit is needed to convert the
mains A.C voltage to low d.c. voltages necessary
for the processor and the circuits in the input end
output interface modules.
 Communication interface is used to receive and
transmit data on communication network from or
to other remote PLC.
 It is concurred with such actions as device
verification, data acquisition, synchronization
between user applications and connection
management.
PLC CPU ARCHITECTURE
 The CPU controls and supervises all operations
within the PLC, carrying out programmed
instructions stored in the memory.
 An internal communications highway, or bus
system, carries information to end from the CPU,
memory and I/O units, under control of the CPU.
 The CPU controls and processes all the operation
within the PLC. It is supplied with a clock with a
frequency of between 1 and 8 MHz.
 This frequency determines the operating speed of
the PLC and provides the timing and
 The information within the PLC is carried by
means of digital signals.
 The internal paths along digital signal flow are
called buses.
 A bus is just a number of conductors along which
electrical signals can flow.
 The internal structure of the CPU depends on the
microprocessor concerned
 The simplified model consist of five parts ALU,
CU, Registers, Buses, and memory.
 Arithmetic and Logic Unit (ALU) Which is
responsible for data manipulation and carrying out
arithmetic operations of addition and subtraction
and logic operations of AND, OR, NOT and
EXCLUSIVE – OR(X-OR).
 It receives control signals from the control unit telling it
to carry out these operations
 Control Unit – This controls the movement of
instruction in and out of the processor and also
controls the operation of ALU.
 It consists of a decoder, controls logic circuit and a
clock to ensure everything happens at the correct time.
 It is also responsible for performing the instruction
execution cycle.
 Registers – located within the microprocessor and
used to store information involved in program
execution.
 It is a small amount of internal memory that is used for the
quick storage and retrieval of data and instructions.
 All processors include some common registers used for
specific functions, namely the program counter, instruction
register, accumulator, memory address register and stack
pointer.
 Bus - Buses are the paths used for communication
within the PLC.
 The information is transmitted in binary form i.e. as a
group of bits with a bit being a binary digit of 0 or 1.
 System bus is used for communication between the I/O ports and
I/O unit.
 It is a cable which carries data communication between the major
components of the computer including the microprocessor.
 Control bus carries the signals relating to the control and co-ordination
of the various activities across the computer which can be sent from the
control unit within the CPU.
 It informs memory devices whether they are to receive data from an input or
output data and to carry out timing signals used to synchronize actions.
 Data bus carries the data used in the process carried out by the CPU.
 It is used for the exchange of data between the processor, memory and
peripherals, and is bidirectional.
 A micro processor termed as being 8-bit has an internal data bus which can
handle 8-bit number.
 Address bus is used to carry the addresses of memory location.
 It contains the connection between the microprocessor and memory that carry
the signals relating to the addresses which the CPU is processing at that time,
such as the locations that the CPU is reading from or writing to.

 Memory: - There are several memory elements in a PLC
system.
 Executive memory or operating system memory which is read only
memory (ROM) to give permanent storage for operating system and
fixed data used by the CPU.
 It is the one that actually does the scanning in the PLC.
 System memory – in order for the operating system to function, a
section of the memory is allotted for system administration.
 As the executive program performs its duties, it often requires a place to
store intermediate results and information.
 A section of RAM (Random Access Memory) is installed for this purpose.
 Data memory – This is a RAM where information is stored on the
status of input and output devices and the values of timers and
counters and other internal devices.
 Data RAM is sometimes referred to as data table or register table.
 User program memory – The final area of memory in a PLC is
allocated to the storage of the user program.
 It is this memory area that the executive program instructs the micro-
 I/O status memory or I/O image table. A
portion of RAM is allocated for the storage of
current I/O status.
 Every single I/O module has been assigned to a
particular location within the I/O image table.
 The location within the input and output image
table/map are identified by addresses, each location
has its own unique address.
MEMORY ORGANIZATION
 This refers to how certain areas of memory in a PLC
are utilized.
 Physical addressing is the ability to read data from a
specific module terminal or write information to a
specific module terminal.
 During the execution of user program, the micro
processor scans the user program and interprets the
user command, when information is read from a
contact or input, it is stored in memory.
 This portion of memory is the input image table/map
which is designated to store this input information.
 Each input typically has at a minimum, a single bit
 Data resulting from logical analysis by the CPU
i.e. various output device status generated during
the execution of user program is stored in
memory labeled as the output image table/map
 From this point, the information is transferred to
a designated output module and then to a
particular field device.
BASIC PLC OPERATION
 A PLC works by continuously running a program
that checks the inputs and then updates the outputs.
 The process of the PLC running throughout its
program is called scanning.
 Scanning speed depends on the program size and
execution time.
 The total time for a PLC to check the inputs, run
the program and update the outputs is called the
cycle time.
 Typical cycle times are 10 ms to 100 ms.
 Every cycle the inputs are check and saved to
memory.
 Then the program is run using the status of the
saved inputs.
 After the program is done the outputs are updated
and the cycle starts again.
SCANNING PROCESSES

 The PLC’s CPU monitors the status of all inputs.

 It takes these values and energizes or de-
energizes the outputs according to the ladder
diagram / user program.
 This is referred to as Scanning.
 The CPU of the PLC executes the user program
over and over again when it is in the run mode.
 A scan does not consist of a PLC executing
ladder diagram rung by rung, but instead the PLC
performs an I/O and program scan.
 The I/O scans transfers data to and from the output and
input modules respectively.
 The information is transferred in the form of bits and
stored in image tables (image maps) are block of
memory designated to store the input and output bit
state)
 The input and output is the portion of the PLC that
interfaces with the outside world.
 The actual bridge between the physical world and
internal world of the PLC is the optical isolation
circuitry.
 There are four basic steps in the operation of all
PLCs; input scan, program scan, output scan,
and house keeping.
 These steps continually take place in a repeating
loop.
 Input scan: During the input scan, the current status
of every input module is stored in the input image
(memory) table, bringing it up-to-date.
 Thus all the status of the input devices (which in turn is
connected to the input module) is updated in the input
memory table.
 Program scan: Following the input scan, the CPU enters its
user program execution, or program scan.
 The execution involves starting at the program's first instruction,
then moving on to the second instruction and carrying out its
execution sequence.
 This continues to the last program instruction.
 Throughout the user-program execution, the CPU continually
keeps its output image (memory) table up-to-date.
 Output scan: During program scan, the output modules
themselves are not kept continually up to date.
 Instead, the entire output image table is transferred to the output
modules during the output scan which comes after the program
execution.
 Thus the output devices are activated accordingly during the
output scan
 Housekeeping – these steps includes
communication with programming, internal
diagnostic activities etc.
PLC INPUT AND OUTPUT (I/O) DEVICES

 Input/output (I/O) is information representing the

data that is received from senses elements / devices
and the commands that are sent to actuating and
indicating devices.
 The I/O system is collection of physical elements of
the control system that either provide or use I/O data.
 The term sensor is used for an input device that
provides a usable output in response to a specified
physical input.
 For example, a thermocouple is a sensor which converts a
temperature difference into an electrical output.
 The term transducer is generally used for a device that
converts a signal from one form to a different physical
form.
 Thus sensors are often transducers, but also other devices
can be transducers, e.g. a motor which converts an electrical
input into rotation.
 The number of I/O devices used within a control system
is called its point count.
 Thus the total number of digital and analog point is used to
give an indication of the size of a control system.
 PLC has input and output lines through which is
connected to a system it directs.
 Any electrical signal processing always requires a voltage
supply (an active part) and a load (passive part) or vice versa.
 I/O modules connect "real world" field devices to the
controller.
 They convert the electrical signals used in the field devices
into electronic signals that can be used by the control system,
and translate real world values to I/O table values.
 I/O modules communicate with PLC CPU in one of three
ways:
 Backplane - The I/O modules can be located in the same rack or
station. Communications then takes place within the rack or across
the backplane.
 Backplane extension - backplane extension modules allow I/O
modules to be located in racks or stations which are separated from
the controller.
 Device network - modules can communicate with a controller over
a network. Industrial networks are used to interconnect field level
devices with controllers. Common IO networks are FieldBus,
Profibus, and DeviceNet.
 There are major types of I/O
 Analog – continuous devices that sense and respond to a
range of values
 Digital – binary devices which must be in one of only two
states on or off.
ANALOG INPUT AND OUTPUT DEVICES

 Analog input devices senses continuous

parameters common analog inputs are pressure,
temperature, speed transducers etc.
 An analog input card converts a voltage by
current leg or signal that can be anywhere from 0
to 20mA) into digitally equivalent number that
can be understood by the CPU.
 To input an analog voltage (into a PLC or any
other computer) the continuous voltage value
must be sampled and then converted to a
numerical value by an A/D converter.
 The process of sampling the data is not instantaneous, so
each sample has a start and stop time.
 The time required to acquire the sample is called the sampling
time. A/D converters can only acquire a limited number of
samples per second.
 The time between samples is called the sampling period T, and the
inverse of the sampling period is the sampling frequency (also
called sampling rate).
 The sampling time is often much smaller than the sampling period
 Analog output devices respond to a range of output values
from the controller common analog output signals include
motor speed, valve position, air pressure etc.
 An analog output card will convert a digital number sent by
the CPU to its real world voltage or current.
 Analog device data requires significantly more manipulation
and processing then digital device data.
DIGITAL INPUT AND OUTPUT DEVICES

 Inputs come from sensors that translate physical

phenomena into digital signal.
 Thus digital input devices may be either on or
off, they may not hold any other value.
 Common digital field input devices include push
buttons, unit switches and photo eyes.
 Digital output devices are devices which give
either on or off.
 Common types are relays, motor starter, solenoid
valves etc.
EXAMPLES OF INPUTS AND OUTPUTS

 Inputs for a PLC come in a few basic varieties the

simplest are AC and DC inputs.
 Examples of input devices are:
 Proximity switches – use inductance, capacitance or
light to detect an object logically
 Switches – mechanical mechanisms will open or close
electrical contacts for a logical signal
 Potentiometer – measures angular position
continuously using resistance.
 LVDT (Linear variable differential transformer) –
measures linear displacement continuously using
magnetic coupling.
 Outputs to actuators allow a PLC to cause something to happen
in a process.
 Outputs from PLC are often relays, but they can also be solid
state electronics such as transistors for DC output or TRIACs
for AC outputs.
 Continuous output requires special output cards with digital to
analog converters.
 Examples are:
 Solenoid valves – logical output that can switch a hydraulic or
pneumatic flow
 Lights – logical output that can often be powered directly from PLC
output boards
 Motor starters – motors often draw a large amount of current when
started, so they require motor starters which are basically large relays.
 Servo motors – a continuous output from the PLC can command a
variable speed or position.
ACTIVE AND PASSIVE INPUTS/OUTPUTS
 Active I/O are those inputs or outputs which have the
power source and are referred to as having a current
source or voltage source (sourcing)
 Passive I/O are those inputs or outputs which do not
have power source and acts as the load or current sink
(sinking)
SOURCING AND SINKING
 Sourcing and sinking are used to describe the
way in which d.c devices are connected to a PLC
and uses d.c currents and voltages.
 Sourcing – When active, current flows from
supply, through the use a single supply voltage.
 With sourcing, using the conventional current flow
direction as from positive to negative, an input device
receives current from the input module i.e. the input
module is the source of the current (Fig a)
 If the current flows from the output module to an
output load then the output module is referred as to
sourcing (fig b)
 Sinking- when active the output allows current to
flow to a common ground.
 This is best selected when different voltages are
supplies.
 With sinking, using the conventional current flow direction
from positive to negative, our input device supplies current
to the input module i.e. the input module is the sink for the
current (fig a)
 If the current flows to the output module from an output
load then the output module is referred to as sinking (fig b)
TYPICAL CONNECTIONS OF PLC
TYPES OF PLC SYSTEM

 The PLC sizes are given in terms of program memory

size and the maximum number of I/O points the
system can support.
 However to evaluate properly any PLC, consideration
is taken for many additional features such as its
processor, cycle time, language facilities, functions
expansion capability etc.
PLC size Max I/O point User memory size
defined (No. of instructors)
Small 40/40 1k
Medium 128/128 4k
Large >128/>128 >4k
 Small PLC – small and mini PLCs are designed as robust, compact
units which can be mounted on or beside the equipment to be
controlled.
 They are mainly used to replace hard wired logic relays, timers, counters etc
that control individual items of plant or machinery, but can also be used to
co-ordinate several machines working in conjunction with each other.
 Programming is by way of logic instruction list (mnemonic) or relay ladder
diagrams.
 Medium-sized PLC: - In this range, modular construction
predominates with plug-in modules on rack mounting system or
Back plane system.
 This allows the simple upgrading or expansion of the system by fitting
additional 1/0 cards into the racks
 Large PLC - where control is very large numbers of input and output
points is necessary or complex control functions are required, a large
PLC is selected.
 It is designed for use in large plants or machines requiring continuous
control.
 They are also employed as supervisory controllers to monitor and control
 PLC STYLES OF CONSTRUCTION
 The main styles are unitary, modular and rack mounting.
 Unitary PLC - is the smallest and least expensive.
 It contains every feature of a basic system in one box and is attached to
the machine being controlled.
 They are not expandable so the application is limited to on-board I/O.
 Modular – These are a range of modules that slot together to
build up a system.
 Basic modules are the power supply, the main module containing the
CPU, the input module and the output module.
 Modular PLCs are used in applications where a higher I/O count is
needed or when using specialty modules such as quadrature encoders.
 They may be designed to be fixed direct to a back panel.
 Usually they are arranged on a rack or rail and mounted inside a large
cabinet for protection and security.
 The main advantage is that the number of input and output terminals
can be expanded to cope with changes to the hardware system.
 Rack mounting – are usually more expensive,
expandable and powerful than modular PLC.
 The rack provides a power and communication backplane that
greatly increases the communication rate between the
processor and the modules as well as allowing some specialty
modules to communicate with each other without the
processor.
 The number of available 1/0 points is also much higher in the
rack systems.
 END