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Before the invention of solid state logical circuits, logical control systems are built around electro-mechanical relays. A control panel of relays consists of thousands of relays in it. Relays played a major role in logic level control devices that includes high voltage and current switching. In order to fulfill the need of ON-OFF switching, still logic gates and electromechanical relays do not serve the purpose. A digital computer is needed along with programming and a logical control of it (E. A. Parr (1999)).
Programmable logical controllers
A programmable logical controller is a solid state electronic device that can be programmed and it executes logical functions. PLCs can store instructions to perform arithmetic, logical, counting, sequencing communication, manipulation of data, etc., to control machines and processors as well. PLC is the heart of a control system. By various input devices, it gets signals and performs logical functions by also receiving feedbacks (M. A. Laughton, D. J. Warne 2003).
Fig 1- conceptual diagram of PLC
Characteristic functions of PLC
It can be programmed on field by the user
It has programmed functions
The memory, inputs and outputs are scanned in a deterministic manner (Manufacturing Automation 2008)
It also has error checking and diagnostics
It can be monitored
It has a general purpose suitability
Advantages of PLC control systems
They are flexible
It has a faster response time
There are no moving parts
It has a modular design
There are many sophisticated instruction sets
It is less expensive
It has lesser or simple wiring (Harms, Toni M. &Kinner, Russell H. P.E 1989)
Major Types of Industrial Control Systems
Some of the major types of industrial control systems are
PLCs are based on Boolean expression. Many use timers. They are used to control processes that are computer based. PLCs are the primary components of SCADA and DCS.
All processes of decentralized elements are controlled by DCS. Labor costs and human intervention is minimized.
Network and control is used to attach small components of any industrial control system.
Supervisory Control And Data Acquisition.
It refers to a centralized system. It has many subsystemslike PLC, HMI and remote telemetry units.
These are more compact and they are adaptable. They are connected to the machine they control. It consists of CPU, input and output controls, etc. a unitary module will always be designed for a single purpose and they cannot be changed after constructing it. These are applicable for industries that do not change their manufacturing techniques. It is not good if the company needs any adaptation. The major advantage is that they are cheaper and compact.
Unlike unitary PLCs modular PLCs can be adapted or changed whenever necessary. Modular PLCs has a cabinet in which a standard system is placed. It consists of CPU, power supply unit, output control and input controls. This can be multifunctional. Specialized or extra modules can be added to different sections. For safety, these systems are mounted on rails or cabinets. These PLCs are adaptive. The functions can be easily changed. They are expensive.
Rack Mounted PLC
Rack mounted PLCs are also multi adaptive. These can be modified easily. The major difference is that here PLCs are mounted on racks. These PLCs are the most practical ones because they are multi adaptive and compact to fit in small places and they are used when space matters. The major drawback of these is that they are expensive.
How the PLC operates
It reads analog and digital signals from the sensors and execute a program. The result is then sent to output devices.
testing integrity of I/O modules,
to verify the logic of programs have changed,
These communications has traffic in PLC port an HMIs and remote I/O racks.
Communications can include traffics over the PLC programmer port, other external devices such as HMIs and remote I/O racks.
It is a scanning of values at input cards. They are saved to input memory tables.
Then program is scanned, and executed line by line in a format.
In this, the output memory values are written to modules of output.
True -logic 1, it means input is energized.
False -logic 0 -it means input is NOT energized.
True is logic 0. It implies: input NOT energized
False is logic 1. It implies: input energized.
XIC means Examine If Closed.
When hardware input energizes it is 1.
(XIO) - Examine If Open.
When hardware input does not energises it is 1.
OTE means Output Enable.
It mimics the actions of any relay coils.
TON means Timer ON. If input line is true it starts the timer and it resets if false. If the timer set point is reached, output becomes true.
(TOF) - Timer OFF.
They continue timings till the preceding logic becomes false.
(RTO) - Retentive Timer On.
It continues timing from last accumulated point.
OTL: OuTput Latch.
OUT: OuTput Unlatch.
Jump to Subroutine
JSR: Jump to Sub Routine
This command is used in order to jump from one of the rungs to another.
Discrete I/O modules
For every input we should find
1. Voltage level
2. Response speed
For every output, we must determine
The power needed to switch output
Programmable logic controllers: Hardware, software architecture
A PLC has a CPU which has the processor, memory that are executive and application, interfacing input and output modules that are connected to input output devices. When an input is given, an appropriate response is obtained by the program fed. Mostly, the responses are switching on or off any devices. The high level signals from input field devices are converted into logic level signals that is understood by the PLCs. Based on the logic of user program, the logic solver decides what it is needed as output. The high level signals needed for field devices are obtained by conversion of logic level signals from logic solver by the output modules. In order to enter the program to memory, program loader is used and it is also used to monitor the program execution.
Major units of PLC
Central processing unit
This is the brain of PLCs. It has a microprocessor with fixed and alterable memory. The fixed memory ROM has programs given by manufacturer and this cannot be altered when power to CPU is gone. The alterable memory has many IC chips that can be reprogrammed. It is stored in RAM. The CPU receives input from input devices and they executes the program and delivers output signals to load control devices like solenoids and relays.
It allows the users to enter desired program to the random access memory. The program is written by ladder logics which gives the sequence of operations of control. This can be connected to CPU only when we write a program. This is generally an LED, CRT with a desktop device.
It serves as an interface between input sensing and output loading devices and the CPU. The main purpose of I/O modules is for transforming various input output interfaces such as pressure switches, solenoids, limit switches, etc. these are hard wired to I/O module terminals.
Functions of input/output modules
The hardware design of PLCs focuses mainly on conversions like analog to digital and digital to analog, signal conditioning of input and output, isolation of modules from controllers. The resolution of input/output modules range between 12 to 16 bits. It has an accuracy of 0.1 percentage over industrial temperature range.
Output voltages( analog) - ±5 V, ±10 V, or 0 V - 5 V, 0 V - 10 V
Output currents- and 4 -20 mA or 0 - 20 mA
Setting time requirement- 10 µs to 100 ms for DACs.
In order to isolate the signal conditioning circuits, DAC and ADC on field side of system from controller opto couplers or digital isolators are used (Maher, Michael J 1989).
PLC Output Module
To control valves and actuators, the PLC system analog outputs takes a standard output ranges.
Voltages- as ±5 V, ±10 V, 0 V - 5 V, 0 V - 10 V
Currents- , 4 - 20 mA, or 0 - 20 mA
There will always be a digital isolation in which controllers digital output is isolated from analog signal conditioning and DACs. In these there are two architectures used.
A dedicated DAC in each channel
Bi polar output converters that require dual supplies.
Switches and Multiplexers
For data acquisition and sample hold, the ADG12xx/ADG13xx family of ±15V switches and multiplexers are used. In case of other applications, ADG1408 and ADG1409, ±15V, and the multiplexers at 9V over full signal range is used. For reliable and predictable performances, these are used.
Network Operating System
The network OS also known as dialoguer is a software that runs and enables the servers to manage all groups, applications and functions. In such typical LAN networks, we can share files and access of printers among many computers that are interconnected. Some of the popular network OS are linux, BSD and unix.
Common Networking Configurations
The following are the types of networking configurations available
Host Only Networking configuration
Bridged type Networking
Network address translation
Local area network connects systems in a small area or a building or between buildings. It is of less cost to transfer files between systems
Metropolitan area network is used to connect larger areas like a whole plant
Wide area network is used for connecting wide geographical areas by satellites.
Number systems used in PLCs
Deci means 10. Here there are 10 numbers from 0 to 9. It is base 10 number system
This system is used by computers PLCs, etc that are digital. Here there are two numbers 0 and 1. It is base 2 system
It is used by early PLCs. Here there are 8 numbers from 0 to 7. It is base 8 system.
This system is used by all PLCs. Hexa means 16. There are 16 numbers from 0-9 and A-F. It is a base 16 system.
This is not really a numbering system. It is just to make human machine interface easier by a 4 digit code.
The base of a number can be changed by modern PLCs. "Radix" is used for that.
Programming Fundamentals: Ladder, Functional Block and Statement List Language
Ladder logics are used in PLCs. It has been developed for mimic relay logic. Relays are rarely used for logics.
It is used to primarily develop programs for PLC operations.it is named so because the programs resemble ladders with two vertical lines on either sides and inter connection between them where all devices are connected. There must be a proper understanding about ladder logics in order to write programs effectively.
We can consider a simple wiring layout. The diagram shows a circuit to on/off a motor. We can redraw this as shown. Both of these have motor with a switch connected in series and electric power is supplied when switch closes.
The ladder diagram shows a circuit for on/off a motor by using push buttons. The push button 1 is open, 2 is closed in normal state.
When 1 is pressed, motor starts. Power is maintained in the circuits till when button 1 is released. When button 2 is pressed, motor stops. Push button 1 is open always in normal condition.
PLC Ladder Programming
Certain rules are adopted while drawing ladder diagrams.
Power rails are represented by vertical lines
- Flow of power is taken from left hand and it is vertical across a rung.
- Each rung on the ladder defines one operation in a control process.
- A ladder diagram is always read from left to right and from top to bottom.
Fig: The scanning motion employed by the PLC
The continuity of power at different situations in a ladder network.
The AND logic in illustrated in Figure 5. The ladder diagram is also shown.
The ladder diagram is shown.
The output should not go even when input ceases. For, an eg- a motor started by push button. The motor should run continuously even though button is not pressed. Here latch circuits can be used.
In the shown example,
There will be output when input contacts A closes. These contact forms an OR logic gate system
When the programs are given as graphs, functional blocks are used. Here the data and signals are described as blocks. A functional block which is a program instruction, yields one or more outputs wen executed.
The figure below shows a OR gate.
1) If either the A or B input is 1, an output comes.
It is a textual programming language for the
Programs are given as a series of instructions on new lines. Each PLCs has one operator then one or many operand which are the operator's subjects.
AND logic ladder
OR logic ladder
NOR logic ladder
NAND logic ladder
XOR logic ladder
Simple ladder program to control lights
Applications of PLCs
- Automations, automobiles,etc
Future of PLCs
The future of PLCs are bright. PLCs will always continue to grow as robust, and all emerging economics created demand for more automations. Automation is going to be the future world's achievement. The market is demanding PLCs that may provide real time logics, process and motion control, etc. The PLC based PAC market is growing a lot. In future PLCs may change into a nano level rather than macro level.
Thus PLCs are greatly used in all types of industries. A complete knowledge of basics of PLCs, architecture and ladder programming are essential for working with PLCs. Unlike general purpose computers, PLCs are designed to operate in industrial environments and with high temperature and humidity levels. Thus PLCs provide a great advantage over conventional systems.
E. A. Parr, Industrial Control Handbook, Industrial Press Inc., 1999 ISBN 0-8311-3085-7
M. A. Laughton, D. J. Warne (ed), Electrical Engineer's Reference book, 16th edition, Newnes, 2003 Chapter 16 Programmable Controller
"The father of invention: Dick Morley looks back on the 40th anniversary of the PLC".Manufacturing Automation. 12 September 2008.
Harms, Toni M. &Kinner, Russell H. P.E., Enhancing PLC Performance with Vision Systems. 18th Annual ESD/HMI International Programmable Controllers Conference Proceedings, 1989, p. 387-399.
Maher, Michael J. Real-Time Control and Communications. 18th Annual ESD/SMI International Programmable Controllers Conference Proceedings, 1989, p. 431-436.