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3.0
INTRODUCTION TO PLC SYSTEM
3.1
Basic Characteristics Of PLC
Introduction
Programmable Logic Controllers (PLCs), also referred to as programmable
controllers, are in the computer family. They are used in commercial and
industrial applications. A PLC monitors inputs, makes decisions based on its
program, and controls outputs to automate a process or machine.
The National Electrical Manufacturers Association (NEMA) as defines a
programmable logic controller:
A digitally operating electronic apparatus which uses a programmable
memory for the internal storage of instructions for implementing
specific functions such as logic, sequencing, timing, counting, and
arithmetic to control, through digital or analog input/output modules,
various types of machines or processes.
In essence, the programmable logic controller consists of computer
hardware, which is programmed to simulate the operation of the individual
logic and sequence elements that might be contained in a bank of relays,
timers, counters, and other hard-wired components.
3.1.1 PLC Terminologies
a.
Sensor
A sensor is a device that converts a physical condition into an
electrical signal for use by the PLC. Sensors are connected to
the input of a PLC.
b.
Actuator
Actuators convert an electrical signal from the PLC into a
physical condition. Actuators are connected to the PLC output.
c.
Digital Input
A discrete input, also referred to as a digital input, is an input
that is either in an ON or OFF condition. Pushbuttons, toggle
switches, limit switches, proximity switches, and contact
closures are examples of discrete sensors which are
connected to the PLCs discrete or digital inputs. In the ON
condition a discrete input may be referred to as a logic 1 or a
logic high. In the OFF condition a discrete input may be
referred to as a logic 0 or a logic low.
d.
Analog Input
An analog input is an input signal that has a continuous signal.
Typical analog inputs may vary from 0 to 20 milliamps, 4 to 20
milliamps, or 0 to 10 volts. In the following example, a level
transmitter monitors the level of liquid in a tank. Depending on
the level transmitter, the signal to the PLC can either increase
or decrease as the level increases or decreases.
e.
Digital Output
A discrete output is an output that is either in an ON or OFF
condition. Solenoids, contactor coils, and lamps are examples
of actuator devices connected to discrete outputs. Discrete
outputs may also be referred to as digital outputs.
f.
Analog Output
An analog output is an output signal that has a continuous
signal. The output may be as simple as a 0-10 VDC level that
drives an analog meter. Examples of analog meter outputs are
speed, weight, and temperature.
g.
CPU
The central processor unit (CPU) is a microprocessor system
that contains the system memory and is the PLC
decisionmaking unit. The CPU monitors the inputs and makes
decisions based on instructions held in the program memory.
The CPU performs relay, counting, timing, data comparison,
and sequential operations.
h.
PLC Programming
A program consists of one or more instructions that
accomplish a task. Programming a PLC is simply constructing
a set of instructions. There are several ways to look at a
program such as ladder logic, statement lists, or function block
diagrams.
3.1.2 PLC Background
The first Programmable Logic Controller (PLC) was developed by a
group of engineers at General Motors in 1968, when the company
were looking for an alternative to replace complex relay control
systems.
The new control system had to meet the following requirements:




Simple programming
Program changes without system intervention (no internal
rewiring)
Smaller, cheaper and more reliable than corresponding relay
control systems
Simple, low cost maintenance
Since then, three decades have passed, during which the enormous
progress made in the development of micro electronics did not stop
short of programmable logic controllers. For instance, even if
program optimization and thus a reduction of required memory
capacity initially still represented an important key task for the
programmer, nowadays this is hardly of any significance.
Moreover, the range of functions has grown considerably. 15 years
ago, process visualisation, analogue processing or even the use of a
PLC as a controller, were considered as Utopian. Nowadays, the
support of these functions forms an integral part of many PLCs.
3.1.3 PLC Functions
The Functions of PLC can be divided into 3 main functions which is,
sequential control, advanced control, and supervisory control.
Table 3.1: PLC Functions
Type of Control
Functions
a. Replace of relay control system
b. Timer/Counter
Sequential
c. Replace of PCB card control
d. Controller of Automatic, semi automatic and
manual system
a. Solve mathematical problems
b. Information management
c. Analog control
Advanced
d. Servo motor control
e. Stepper motor control
f. PID control
a. Alarm and display process
b. Faulty display and diagnosis
c. Computer interface
d. Printer interface
e. Factory automation network
Supervisory
f. Local area network (LAN)
g. Wide area network (WAN)
h. Factory automation (FA), Flexible
Manufacturing System (FMS), Computer
integrated manufacturing (CIM). Etc.
3.1.4 Types of PLC
The PLCs currently on offer in the market place have been adapted
to customer requirements to such an extent that it has become
possible to purchase an eminently suitable PLC for virtually any
application. As such, miniature PLCs are now available with a
minimum number of inputs/outputs starting from just a few hundred
Pounds. Also available are larger PLCs with 28 or 256 inputs/outputs.
Depending on how the central control unit is connected to the input
and output modules, differentiation can be made between compact
PLCs (input module, central control unit and output module in one
housing) or modular PLCs. Fig. 3.2 shows the FX0 controller by
Mitsubishi representing a compact PLC as an example.
Modular PLCs may be configured individually. The modules required
for the practical application – apart from digital input/output modules
which can, for instance, include analogue, positioning and
communication modules – are inserted in a rack, where individual
modules are linked via a bus system. This type of design is also
known as series technology. The examples of modular PLCs are
shown in fig 3.1. These represent the familiar modular PLC series by
the new S7-300 series by Siemens.
The card format PLC is a special type of modular PLC, developed
during the last few years. With this type, individual or a number of
printed circuit board modules are in a standardised housing. The
Festo FPC 405 is representative of this type of design (Fig. 3.1).
Fig. 3.1: Compact PLC (Mitsubishi FX0),
modular PLC(Siemens S7-300),
PLC plug-in cards (Festo FPC 405)
3.1.5 Advantages of PLC
As well as more complex tasks, can be done with a PLC. Wiring
between devices and relay contacts is done in the PLC program.
Hard-wiring, though still required to connect field devices, is less
intensive. Modifying the application and correcting errors are easier
to handle. It is easier to create and change a program in a PLC than
it is to wire and rewire a circuit.
Following are just a few of the advantages of PLCs:
• Smaller physical size than hard-wire solutions.
• Easier and faster to make changes.
• PLCs have integrated diagnostics and override functions.
• Diagnostics are centrally available.
• Applications can be immediately documented.
• Applications can be duplicated faster and less expensively.
A PLC system provides many benefits to control solutions, from
reliability and repeatability to programmability. The benefits achieved
with programmable controllers will grow with the individual using
them—the more you learn about PLCs, the more you will be able to
solve other control problems.
Table 3.2 lists some of the many features and benefits obtained with
a programmable controller.
Table 3.2: Typical programmable controller features and benefits.
3.2
Building Structure of a PLC
With computer systems, differentiation is generally made between
hardware, firmware and software. The same applies for a PLC, which is
essentially based on a micro computer.
The hardware consists of the actual device technology, i.e. the printed
circuit boards, integrated modules, wires, battery, housing, etc. This
includes fundamental system routines, used for starting the processor after
the power has been switched on. Additionally, there is the operating system
in the case of programmable logic controllers, which is generally stored in a
ROM, a read-only memory, or in the EPROM.
Firmware is the software part, which is permanently installed and supplied
by the PLC manufacturer.
The software, which is the user program written by the PLC user. User
programs are usually installed in the RAM, a random access memory,
where they can be easily modified.
Fig. 3.2: Fundamental design of a microcomputer
3.2.1 Central Processing Unit
The central processing unit, or CPU, is the most important element of
a PLC. The CPU forms what can be considered to be the “brain” of
the system. The three components of the CPU are:
• the processor
• the memory system
• the power supply
Fig 3.3 illustrates a simplified block diagram of a CPU. CPU
architecture may differ from one manufacturer to another, but in
general, most CPUs follow this typical three-component organization.
Fig 3.3: CPU block diagram
A microprocessor consists in the main of an arithmetic unit, control
unit and a small number of internal memory units, so-called registers.
The task of the arithmetic unit – the ALU (arithmetic logic unit) – is
to execute arithmetic and logic operations with the data transmitted.
The control unit regulates and controls the entire logic sequence of
the operations required for the execution of a command.
Fig 3.4: Design of a microprocessor
3.2.2 Memory Unit
They are several memory elements in a PLC system:
1. System read-only-memory (ROM) to give permanent storage for
the operating system and fixed data used by CPU
2. Random-access-memory (RAM) for the users program.
3. Random-access-memory (RAM) for data. This is where
information is stored on the status of input and output devices and
the values of timers and counters and other internal devices.
4. Possibly, as a bold-on extra module, erasable and programmable
read-only-memory (EPROM) for ROMs that can be programmed
and then the program made permanent.
The program and data in RAM can be changed by the user. All PLCs
will have some amount of RAM to store programs that have been
developed by the user and program data. However, to prevent the
loss of programs when the power supply is switched off, a battery is
used in the PLC to maintain the RAM for a period of time. After a
program has been developed in RAM it may be loaded into an
EPROM memory chip, often a bold-on module to PLC, and so made
permanent.
3.2.3 Display & Indicator Unit
The CPU status indicators reflect the current mode of CPU operation.
If, for example, the mode switch is set to the RUN position, the green
RUN indicator is lit. When the mode switch is set to the STOP
position, the yellow STOP indicator is lit.
The I/O status indicators represent the On or Off status of
corresponding inputs and outputs. When the CPU senses an input is
on, the corresponding green indicator is lit.
3.2.4 Input Interface Unit
The input module of a PLC is the module, which sensors or other
input devices are connected to. The sensor signals are to be passed
on to the central control unit. The important functions of an input
module (for the application) are as follows:




Reliable signal detection
Voltage adjustment of control voltage to logic voltage
Protection of sensitive electronics from external voltages
Screening of signals
The main component of today’s input modules which meets these
requirements is the optocoupler.
Fig. 3.5: Block diagram of an input module
Input devices, such as switches, pushbuttons, and other sensor
devices are connected to the terminal strip under the bottom cover of
the PLC.
3.2.5 Output Interface Unit
The output module of a PLC is the module, which output devices are
connected to. Output modules conduct the signals of the central
control unit to final control elements, which are actuated according to
the task. In the main, the function of an output – as seen from the
application of the PLC – therefore includes the following:
 Voltage adjustment of logic voltage to control voltage
 Protection of sensitive electronics from spurious voltages from
the controller
 Power amplification sufficient for the actuation of major final
control elements
 Short-circuit and overload protection of output modules
In the case of output modules, two fundamentally different methods
are available to achieve the above: Either the use of a relay or power
electronics.
Fig. 3.6: Block diagram of an output module
The optocoupler once again forms the basis for power electronics
and ensures the protection of the electronics and possibly also the
voltage adjustment.
If relays are used for the outputs, then the relay can assume
practically all the functions of an output module: The relay contact
and relay coil are electrically isolated from one another.
Relay outputs have the advantage that they can be used for different
output voltages. By contrast, electronic outputs have considerably
higher switching speeds and a longer service life than relays.
3.3
PLC Hardware Unit
Typically a PLC system has the basic function components of processor
unit, memory, power supply unit, input/output interface section,
communications interface and the programming devices. Fig. 3.7 shows the
basic arrangement.
Programmming
devices
Input
Interface
Program & data
memory
Communications
Interface
Processor
Output
Interface
Power supply
Fig. 3.7: PLC system
3.3.1 Housing Unit
Housing unit provides protection to the circuits and components - the
internal components of the PLC.
3.3.2 Programming Unit
The programming unit is 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. Programming
devices can be a hand-held device (Program console) a desktop
console, or a computer.
3.3.3 Secondary Storage
Secondary storage (also known as external memory or auxiliary
storage) (well known as external hard disk), differs from primary
storage in that it is not directly accessible by the CPU. The computer
usually uses its input/output channels to access secondary storage
and transfers the desired data using intermediate area in primary
storage. Secondary storage does not lose the data when the device
is powered down—it is non-volatile.
3.3.4 Power Supply Unit
The power supply unit is needed to convert the mains a.c. voltage to
the low d.c. voltage (5V) necessary for the processor and the circuits
in the input and output interface modules.
3.3.5 Printer Unit
Printer unit is used to print the program of control system controlled
by a PLC either graphics or text.
3.4
Input and Output Device
The input devices considered include digital and analogue devices such as
mechanical switches for positioning detection, proximity switches,
photoelectric switches, encoders, temperature and pressure switches,
potentiometers, linear variable differential transformers, strain gauges,
thermistors, thermotransistors and thermocouples. Output devices
considered include relays, contactors, solenoid valves and motor.
3.4.1 Input Device
The term sensor is used for 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 convert
signal from one form to a difference physical form.
The following are examples of some of the commonly used PLC input
devices and their sensors.
a.
Mechanical Switch
A mechanical switch generates an on-off signal or signals as a result
of some mechanical input causing the switch to open or close.
Fig.3.8: Switch Sensor
Fig. 3.9 Limit switch
b.
Proximity switch
Proximity switches are used to detect the presence of an item without
making contact with it.
Fig 3.10: Proximity sensor
c.
Photoelectric sensor and switch
Photoelectric switch devices can operate as transmissive types
where the object being detected breaks a beam of light, usually
infrared radiation, and stop it reaching the detector (fig. 3.11)(a)) or
reflective types where the object being detected reflects a beam of
light onto the detector (fig. 3.11 (b)).
Fig. 3.11: Photoelectric sensor
d.
Encoder
The term encoder is used for a device that provides a digital output
as a result of angular or linear displacement.
Fig 3.12: Encoder
e.
Temperature Sensor
A temperature sensor is a device that gathers data concerning the
temperature from a source and converts it to a form that can be
understood either by an observer or another device. Several types of
temperature sensor as follows:





Bimetallic strip
Resistive temperature detector (RTD)
Thermodiodes and thermotransistor
Digital Temparature sensor LM3911, LM35 etc
Thermocouple
Fig 3.13: Bimetal thermostat sensor
Fig 3.14: RTD sensor
Fig3.15: LM 32 and LM3911 temperature sensor
f. Position/ Displacement Sensor
The term of position sensor is used for a sensor that gives a measure
of the distance between a reference point and the current location of
the target, a displacement sensor being one that gives a measure of
he distance between the present position of the target and previously
recoded position. Two types mostly used are, resistive and angular
position sensor and linear variable differential transformer
(LVDT).
Fig 3.16: LVDT
3.4.2 Output Device
The output of the ports of a PLC are of the relay type or optoisolator
with transistor or triac types depending on the devices connected to
them which are to be switched on or off. Generally, the digital signal
from an output channel of the PLC is used to control an actuator
which in turn control some process. The term actuator is used for the
device which transform the electrical signal into some more powerful
action which then results in the control of the process. The following
are some of examples:
a.
Relay
One example of output device is relay, the term contactor being used
when large current are involved. When the output from the PLC is
switched on, the solenoid magnetic field is produces and pulls on the
contacts and so closed a switch or switches (fig. 3.17). the result is
that much larger currents can be switched on. Thus relay might be
used to switch on the current to a motor.
Fig. 3.17: Relay used as an output device
b.
Directional Control Valves
Another example of an actuator is a solenoid operated valve. The
valve may be used to control the directions of flow of pressurised air
or oil and so used to operate other devices such as a piston moving
in a cylinder.
Fig 3.18: Directional control valve
c.
Motors
Motors are common actuators, but for logical control applications
their properties are not that important. Typically logical control of
motors consists of switching low current motors directly with a PLC,
or for more powerful motors using a relay or motor starter. Many
industrial processes only required the PLC to switch a d.c motor on
and off.
References:
1.
2.
3.
4.
R. Bliesener, F. Ebel, C. Löffler, B. Plagemann,H. Regber, E. v. Terzi, A.
Winter. Programmable logic controllers: Basic level TP301Textbook.
FESTO DEDATIC.
Basic Of PLC’s. SIEMENS (http://www3.sea.siemens.com/step/downloads.html.)
L.A Bryan, E.A Bryan. (1997). Programmable Controller : Theory &
Implementations 2nd Edtion.Industrial Text.
W. Bolton. (2006). Programmable Logic Controller. Newnes