INTRODUCTION TO INDUSTRIAL PLC

Every aspect of industry—from power generation to automobile painting to food packaging—uses

programmable controllers to expand and enhance production.

What type of task might a control system have?

It might be required to control a sequence of events or maintain some variable constant or follow some
prescribed change.

For example, the control system for an automatic drilling machine (Figure above (a)), might be required to
start lowering the drill when the workpiece is in position, start drilling when the drill reaches the
workpiece, stop drilling when the drill has produced the required depth of hole, retract the drill and then
switch off and wait for the next workpiece to be put in position before repeating the operation.

Another control system (Figure above (b)) might be used to control the number of items moving along a
conveyor belt and direct them into a packing case. The inputs to such control systems might be from
switches being closed or opened, e.g. the presence of the workpiece might be indicated by it moving
against a switch and closing it, or other sensors such as those used for temperature or flow rates. The
controller might be required to run a motor to move an object to some position, or to turn a valve, or
perhaps a heater, on or off.

Microprocessor controlled system

Instead of hardwiring each control circuit for each control situation we can use the same basic system for
all situations if we use a microprocessor-based system and write a program to instruct the microprocessor
how to react to each input signal from, say, switches and give the required outputs to, say, motors and
valves. Thus we might have a program of the form:

If switch A closes

Output to motor circuit

If switch B closes

Output to valve circuit

By changing the instructions in the program we can use the same microprocessor system to control a wide
variety of situations. As an illustration, the modern domestic washing machine uses a microprocessor
system. Inputs to it arise from the dials used to select the required wash cycle, a switch to determine that
the machine door is closed, a temperature sensor to determine the temperature of the water and a switch
to detect the level of the water. On the basis of these inputs the microprocessor is programmed to give
outputs which switch on the drum motor and control its speed, open or close cold and hot water valves,
switch on the drain pump, control the water heater and control the door lock so that the machine cannot
be opened until the washing cycle is completed.

Process Control
Process control consists of monitoring the state of a critical parameter, detecting when it varies from
desired state, and taking action to restore it. It involves the process variable, set points and manipulated
variable.

Two of the most important signals used in process control are called:

• Process Variable or PV

• Manipulated Variable or MV

In industrial process control, the Process Variable (PV) is measured by an instrument in the field and acts
as an input to an automatic controller which takes action based on the value of it. The PV is the parameter
that is to be controlled. To be controlled, the PV must be capable of being measured and that
measurement converted into a signal that can be acted on by the controller. Devices that measure PV are
transducers or sensors. In many cases, the PV sensor consists of a direct measurement device called an
element and a separate signal processor called a transmitter.

The set-point is the desired value of the PV, normally preset into the control system by an operator, or
derived as an output of another control calculation. The error signal is the difference between the PV and
the set-point, and is the basis for control action. The controller is the device that processes the error signal,
determines the required control action and provides a control output Manipulated Variable (MV) to the
process. The device that converts the control output into control action is the actuator.
 Control Modes
- In control, there are control systems which can be discrete or analog, manual or automated, periodic or
continuous. There are five basic forms of control available in Process Control:

1. On-Off control: The oldest strategy for control is to use a switch giving simple on-off control. This is a
discontinuous form of control action, and is also referred to as two-position control. A perfect on-off
controller is 'on' when the measurement is below the set-point (SP) and the manipulated variable (MV) is
at its maximum value. Above the SP, the controller is 'off' and the MV is at a minimum.

2. Modulating control: If the output of a controller can move through a range of values, this is modulating
control. Modulation Control takes place within a defined operating range only. That is, it must have upper
and lower limits. Modulating control is a smoother form of control than step control. It can be used in both
open loop and closed loop control systems.

3. Open loop control: Open loop control is thus called because the control action (Controller Output
Signal) is not a function of the PV (Process Variable) or load changes. The open loop control does not self-
correct, when these PV’s drift.

4. Feed forward control: Feed forward control is a form of control based on anticipating the correct
manipulated variables required to deliver the required output variable. It is seen as a form of open loop
control as the PV is not used directly in the control action.

5. Closed loop or feedback control: If the PV, the objective of control, is used to determine the control
action it is called closed loop control system.

- Discuss about industrial control system (ICS)

PROGRAMMABLE LOGIC CONTROLLER (PLC)

Introduction

A programmable logic controller (PLC) is a special form of microprocessor-based controller that uses a
programmable memory to store instructions and to implement functions such as logic, sequencing, timing,
counting and arithmetic in order to control machines and processes and are designed to be operated by
engineers with perhaps a limited knowledge of computers and computing languages.

The term logic is used because programming is primarily concerned with implementing logic and switching
operations, e.g. if A or B occurs switch on C, if A and B occurs switch on D. Input devices, e.g. sensors such
as switches, and output devices in the system being controlled, e.g. motors, valves, etc., are connected to
the PLC. The operator then enters a sequence of instructions, i.e. a program, into the memory of the PLC.
The controller then monitors the inputs and outputs according to this program and carries out the control
rules for which it has been programmed.

Programmable logic controllers, also called programmable controllers or PLCs, are solid-state members of
the computer family, using integrated circuits instead of electromechanical devices to implement control
functions. They are capable of storing instructions, such as sequencing, timing, counting, arithmetic, data
manipulation, and communication, to control industrial machines and processes.
Programmable controllers have many definitions. However, PLCs can be thought of in simple terms as
industrial computers with specially designed architecture in both their central units (the PLC itself) and
their interfacing circuitry to field devices (input/output connections to the real world).

TODAY’S PROGRAMMABLE CONTROLLERS

Many technological advances in the programmable controller industry continue today. These advances
not only affect programmable controller design, but also the philosophical approach to control system
architecture. Changes include both hardware (physical components) and software (control program)
upgrades. The following list describes some recent PLC hardware enhancements:

- Faster scan times are being achieved using new, advanced microprocessor and electronic
technology.
- Small, low-cost PLCs which can replace four to ten relays, now have more power than their
predecessor, the simple relay replacer.
- High-density input/output (I/O) systems (see figure below) provide space-efficient interfaces at
low cost.
 - Intelligent, microprocessor-based I/O interfaces have expanded distributed processing. Typical
interfaces include PID (proportion integral-derivative), network, CANbus, fieldbus, ASCII
communication, positioning, host computer, and language modules (e.g., BASIC, Pascal).
- Mechanical design improvements have included rugged input/output enclosures and
input/output systems that have made the terminal an integral unit.
- Special interfaces have allowed certain devices to be connected directly to the controller. Typical
interfaces include thermocouples, strain gauges, and fast-response inputs.
- Peripheral equipment has improved operator interface techniques, and system documentation is
now a standard part of the system.

The PLC 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.

PLCs have the great advantage that the same basic controller can be used with a wide range of control
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 optimised for calculation and display tasks, PLCs
are optimised for control tasks and the industrial environment.

Thus PLCs have specific features suited for industrial control :-

1. Rugged and designed to withstand vibrations, temperature, humidity and noise

2. Modular plug-in construction, allowing easy replacement or addition of units (e.g.

input/output);

3. Standard input/output connections and signal levels

4. Have interfacing for inputs and outputs already inside the controller.

5. Easily understood programming language which is primarily concerned with logic and switching
operations
 6. Ease of programming and reprogramming in-plant;

7. Capable of communicating with other PLCs, computers and intelligent devices;

8. 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

1. Flexibility: One single PLC can easily run many machines.

2. 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.
3. 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.
4. Low Cost: Prices of PLC vary from few hundreds to few thousands.
5. Testing: A PLC program can be tested and evaluated in a lab. The program can be tested, validated
and corrected saving very valuable time.
6. 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.

Discuss disadvantages of PLCs

PLC Hardware
Typically a PLC system has the basic functional components of processor unit, memory, power supply unit,
input/output interface section, communications interface and the programming device. Figure below
shows the basic arrangement.

a) 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.
b) 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.

c) Input and output 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:

d) Input and output (I/O) devices - is collection of physical elements of the control system that either
provide or use I/O data.

e) 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.

f) 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.

g) 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.

h) 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.

A programmable controller, as illustrated in Figure below, consists of two basic sections:

• the central processing unit

• the input/output interface system

The central processing unit (CPU) governs all PLC activities. The following three components, shown in
Figure below, form the CPU:

• the processor

• the memory system

• the system power supply

The operation of a programmable controller is relatively simple. The input/ output (I/O) system is
physically connected to the field devices that are encountered in the machine or that are used in the
control of a process. These field devices may be discrete or analog input/output devices, such as limit
switches, pressure transducers, push buttons, motor starters, solenoids, etc. The I/O interfaces provide
the connection between the CPU and the information providers (inputs) and controllable devices
(outputs).
 PLC CPU architecture

There are two types:

1. Open architecture design allows the system to be connected easily to devices and programs made by
other manufacturers.

2. Closed architecture or proprietary system is one whose design makes it more difficult to connect
devices and programs made by other manufacturers.

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 synchronization for all
elements in the systems. 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.

1. 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
2. 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.

3. 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.

4. 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 coordination 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. Every memory location is given a unique address.

5. 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-
processor to examine or ‘scan’ to find the user instructions.

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 designated to store its information.

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.

(1) 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.
(2) 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.

(3) 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.

(4) Housekeeping – these steps includes communication with programming, internal diagnostic activities
etc.

The input/output system forms the interface by which field devices are connected to the controller (see
Figure below). The main purpose of the interface is to condition the various signals received from or sent
to external field devices. Incoming signals from sensors (e.g., push buttons, limit switches, analog sensors,
selector switches, and thumbwheel switches) are wired to terminals on the input interfaces. Devices that
will be controlled, like motor starters, solenoid valves, pilot lights, and position valves, are connected to
the terminals of the output interfaces. The system power supply provides all the voltages required for the
proper operation of the various central processing unit sections.

Although not generally considered a part of the controller, the programming device, usually a personal
computer or a manufacturer’s miniprogrammer unit, is required to enter the control program into memory
(see Figure below). The programming device must be connected to the controller when entering or
monitoring the control program.

When deciding whether to use a PLC-based system or a hardwired relay system, the designer must ask
several questions. Some of these questions are:

• Is there a need for flexibility in control logic changes?

• Is there a need for high reliability?

• Are space requirements important?

• Are increased capability and output required?

• Are there data collection requirements?

• Will there be frequent control logic changes?

• Will there be a need for rapid modification?

• Must similar control logic be used on different machines?

• Is there a need for future growth?

• What are the overall costs?

The merits of PLC systems make them especially suitable for applications in which the requirements listed
above are particularly important for the economic viability of the machine or process operation.
 TYPICAL AREAS OF PLC APPLICATIONS

PLCs perform a great variety of control tasks, from repetitive ON/OFF control of simple machines to
sophisticated manufacturing and process control. Table below lists a few of the major industries that use
programmable controllers, as well as some of their typical applications.

Table below provides a small sample of how PLCs are being used in industry.