Table of Contents
What is a PLC?
A PLC or Programmable Logic Controller is a ruggedized digital computer used in industries that have been adapted for controlling manufacturing processes. These manufacturing processes included robotic devices, assembly lines, or any activity requiring ease of programming, high reliability, and process fault diagnosis.
A Programmable logic controller may range from small modular types to a large rack-mounted modular device which is mostly networked to other SCADA and PLC systems. Small modular devices may be with tens of inputs and outputs or I/O in a housing integral with the processor while a large rack-mounted have thousands of I/O.
PLCs are generally designed for numerous arrangements of digital and analog I/O which have extended temperature ranges, resistance to vibration and impact, and immunity to electrical noise. These programs controlling machine operations are typically stored in a battery that is either backed up or has a non-volatile memory.
PLCs were first developed in the automobile industry for providing rugged, easily programmable and flexible controllers to replace hard-wired relay logic systems. Since then, they have widely adopted as high-reliability automation controllers suited for harsh environments.
Output results must be produced in response to the input conditions within a limited time in the case of PLC and therefore it is an example of a hard real-time system. The origin of PLC happened in the late 1960s in the US in the automotive industry. It was designed to replace the relay logic systems.
In earlier times, control logic used for manufacturing was mostly composed of cam timers, dedicated closed-loop controllers, drum sequencers, and relays. It was difficult for the design engineers to alter the process since these had a hardwire nature.
Changes required careful updating and rewiring of the documentation because if one wire were out of place, the whole system tends to become faulty. Most technicians spend a lot of hours troubleshooting by comparing them to existing wiring and examining the schematics.
Once the general-purpose computers became available, they were soon applied to the control logic in industrial processes. These early computers required strict control of working conditions such as power quality, temperature, cleanliness, and specialist programmers as they were unreliable.
The PLCs were considered to provide several advantages over earlier automation systems. They were known for tolerating the industrial environment much better than the computers. They were reliable, required less maintenance than the relay systems, and were compact.
Basics of Programmable Logic Controllers (PLCs)
Easily extensible with I/O modules, relay systems usually required complicated hardware changes in case of reconfiguration. Due to this, it became easy to iterate over the manufacturing process design. It was considered more user-friendly than computers using different programming languages for general-purpose as simple language programming mainly focused on switching operations and logic.
The controllers permit its operations to be monitored. PLCs were earlier programmed in ladder logic which resembled relay logic’s schematic diagram. To reduce the training demands of existing technicians, the program notation was chosen while other PLCs made use of a form of instruction list programming which was based on a stack-based solver.
Initially, many PLCs were not capable enough to graphically represent the logic so they represented it as a series of logical expressions in a kind of Boolean format which was similar to Boolean algebra. With time the programming terminals evolved and the ladder logic to be used became common because it had a familiar format that was used for electro-mechanical control panels.
PLC includes basic elements such as a CPU or a Central Processing Unit, input modules or point, programming device, and output modules. The types of input points or modules used by a PLC usually depend on the input devices used. Some input points or modules called discrete inputs to respond to the digital inputs which are either on or off. The other inputs or modules respond to the analog signals.
Converting the signals provided by various switches and sensors into logic signals which can be easily used by the CPU is the primary function of a PLC’s input circuitry. The CPU evaluates the status of outputs, inputs, and other variables because it executes the stored program.
The programming device is used to change or monitor stored values by either entering or changing the PLC’s program. The associated variables and programs are stored in the CPU once the PLC program is entered. In addition, the PLC system may also incorporate some sort of operator interface device to simplify the monitoring of the process or machine.
There are pushbuttons or sensors which are connected to the PLC inputs which are used to start and stop a motor. This motor is connected to the PLC output through an actuator or a motor starter. There is no need for an operator interface or a programming device.
Many control tasks were performed by the control relays, contactors, and various electromechanical devices prior to PLCs which is commonly known as hard-wired control. The circuit had to be designed in such a way that the electrical components were installed and specified and the wiring lists were created.
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After this, the electricians would wire the components which was necessary to perform a specific task. The wires were required to be reconnected correctly, if an error was found. Any change in the system expansion or function needed extensive rewiring and component changing.
Most of the industrial tasks were executed by using the contractors and relay controls before the PLCs got introduced. The hard-wired control relied on the circuit diagrams, wiring lists, and electrical components to get the job completed.
PLCs have a hard-wired control and therefore are only capable of performing the same tasks. In addition to this, electronic communication lines and the PLC program replace the interconnecting wires to huge extent which are required by the hard-wired control.
PLCs have a smaller physical size than the commonly used hard-wired solutions. They are easier and faster in making changes and have integrated diagnostics. They have the capability to override functions and the diagnostics are easily available. Applications can be duplicated faster at a low cost and can be immediately documented.
How Does a PLC Works?
Small industrial computers with modular components that are designed to automate control processes are PLCs or Programmable Logic Controllers. The controllers which are behind all the modern industrial automation are PLCs. Although there are many components that PLCs have, mostly they are divided into three categories.
These three categories are inputs, outputs, and a processor or Central Processing Unit (CPU). PLCs are highly powerful and complex computers; their functions can be described in simple terms. It takes the inputs and uses the Central Processing Unit to perform logical actions on it to deliver output.
Let’s understand this with the help of an example; dishwashers used in our homes have microprocessors that are similar to the PLCs. The dishwashers have inputs, CPU, and of course the output. Input is usually done through the buttons given on the front, the door switch, and the sensors. The dishwasher outputs include the water valves, the pumps, and the heat elements.
Inputs and outputs which are abbreviated as I/O were considered as the digital or discrete signal in case of the dishwasher example given above. Discreet signals are the most common and simple type of signals which are either on or off.
There may or may not be the usage of the analog signals in a dishwasher. While the digital signal consists of ON and OFF buttons, the analog signals may have 1-100%, 4-20mA, and 0-100 degrees Celsius.
The main difference that lies between computing devices and PLC is that PLCs are intended to and capable of tolerating severe conditions such as water, heat, cold, and moisture. It offers extensive I/O or input and output to connect the programmable logic controllers to actuators and sensors.
The function of PLC has evolved over the years to include motion control, relay control, sequential relay control, networking, distributed control systems, and process control. The storage, data handling, communication, and processing power capabilities of a few modern PLCs are approximately equivalent to desktop computers.
A PLC is a kind of specialized computer which is used to control processes and machines. It shares common terms with desktop computers like a CPU or central processing unit, software, communications, and memory.
The best way to understand the working of a programmable logic controller is through a cyclic scanning method called the scan cycle. The scan process consists of many steps which begin with the monitoring of time and cycling started by the operating system. CPU reads the data and checks the status of inputs.
The CPU executes the application program that is written in the relay-ladder logic and performs the communication tasks and the internal diagnosis. As per the program results, the data is written into the output module by the CPU to keep the outputs updated. This process is repeated as long as the programmable logic controller is in run mode.
Structure of Physical PLC
The input values are continuously monitored by programmable logic controllers received from different input sensing devices. Examples of such devices are weight scale, accelerometer, hardwired signals, etc. According to the nature of industry and production, PLC keeps producing corresponding output.
A block diagram of programmable logic controllers consists of five parts which are Chassis or Rack, Power Supply Module, CPU or Central Processing Unit, Communication Interface Module, and Input and Output Module. Let’s read about these five parts of PLC’s five parts in detail.
Chassis or Rack
Chassis or rack form the backbone of the PLC system as it is a very important part of it. Programmable logic controllers are available in various shapes and sizes depending on the requirements. Larger PLC racks are required where there is a high number of complex control systems.
The small-sized PLC available in the market is equipped with a fixed input and output pin configuration. This is the reason that the module type rack PLCs have opted as they accept various types of input and output modules and are fit and sliding concept-wise. All the modules that are present reside inside the chassis or the rack.
Power Supply Module
The use of a power supply module is to provide power to the PLC system as per the requirement. The AC and DC power available is required by the input or output module and the Central Processing Unit. The programmable logic controllers usually require a 24DC supply but few PLCs use the supply of isolated power.
CPU Module and Memory
The Central Processing Unit consists of ROM (read-only memory), RAM (random access memory), and the processor. While RAM is used for storing the data and programs, ROM comprises drivers, application programs, and the drivers. Basically, the CPU is the brain of PLC which includes a hexagonal or an octal microprocessor.
Since it has a CPU that is based on a microprocessor, it tends to replace the counters, time, and relays. Two different types of processors as a word processor or a single bit may be incorporated with a PLC. While the one-bit processor is used to do the logical functions, the word processor processes numerical data, text, records and controls the data.
The input data is read by the CPU from the sensors and is processed. Once these functions take place, the commands are sent to the controlling devices. As has been mentioned in the prior discussions, the DC power source is required for transmitting voltage signals.
A Central Processing Unit or CPU also consists of various electrical parts that are used to connect the cables being used by the other units.
Input and Output Module
Did you know that various physical parameters such as pressure flow and temperature can be used for sensing using PLC? Programmable logic Controllers consist of an exclusive module called the input and output module or the I/O module for interfacing inputs and outputs.
Switches and start and stop pushbuttons are examples of input devices while the electric heater, relays, and valves are output devices. Input and output modules assist in interfacing the input and output devices with the help of a microprocessor.
The input module of programmable logic controllers does four main functions. The signal is received by the input module interface from the process devices at 220 V AC. After this, the input signals get converted to 5 V DC which can be used by PLC.
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Further, PLCs are isolated and prevented from undergoing fluctuation using the isolator block. Once this happens, the signal is transferred to the output end which is the PLC. Now, the input module has two main sections called the logical section and the power section.
Both of these sections are isolated electrically from one another.
The input and output module of PLC works similarly but in a reverse way. The output module is known to interface the processor and the output load. It has been noticed that when the program logic high signal is being generated from the processor, LED turns ON and let the light fall on the phototransistor.
Once the transistor reaches the conduction region, a pulse gets generated. The isolator block is mainly used to isolate the control section and the logic section. For transferring the information between the communication networks and CPU, the usage of intelligent input and output modules is done.
The communication modules are very useful as they help in connecting with the other programmable logic controllers and computers which are being placed at a remote or nearby location. The working of PLC is a vast topic, hope you have understood the basics of PLC. Coming to the types of programmable logic controllers, there are different types of PLCs; let’s read about them in detail.
Types of PLCs
Programmable logic controllers are of two types: fixed PLC or compact PLC and the modular PLC. Let’s read what compact PLC and modular PLC are in detail.
Compact PLC or Fixed PLC
It consists of numerous modules withing single ease. It has been termed as the fixed PLC as it has a fixed number of input & output modules and external input & output cards. Since it has a fixed number, it is not capable of expanding the modules further. Each and every I/O or input and output is being decided by the manufacturer.
Modular PLC
Modular PLC, unlike fixed or compact PLC, permits multiple expansion via “modules” and is therefore termed as the modular PLC. I/O or input and output components can be increased in modular PLC. It is much easier to use as its components are independent of each other.
The term modular refers to the number of modules that can be connected to the devices. When it comes to modular PLC, it means that several modules can be joined to a common bus or rack with extendable I/O or input or output capabilities.
It consists of power supply modules, a central processing unit, and some other input and output modules that can be plugged into same rack. These racks may or may not be from the same manufacturers.
PLCs are further divided into three types based on the output: Transistor output PLC, Relay Output PLC, and the Triac output PLC. The transistor output PLC is known for using switching operations and is generally used in microprocessors. The relay output type is best for AC and DC output devices.
PLCs are divided into Mini, Micro, and Nano PLC according to the physical size. They are also divided into small, medium, and large PLCs depending on the number of I/O features and program memory size.
Applications of PLC
Programmable logic controllers are used in various industries such as the automobile industry, chemical industry, steel industry, and the energy sector. The scope of programmable logic controllers increases as per the development which takes place where it is applied. PLCs are used in the travel industry to monitor various safety control systems and for operating escalators and lifts. Here are some more PLC applications.
Cement Manufacturing
The process of cement manufacturing includes mixing different raw materials in a kiln. The proportions and quality used have a huge effect on the final product. It becomes crucial to use the right quantity and quality of raw material and therefore the accuracy of data regarding this forms the base.
A PLC comprised distributed control system is used in the cement industry in its production process. This control system is used in its user-mode; PLC in particular is used for controlling ball milling, shaft kiln, and coal kiln.
Glass Industry
PRC controllers have been in use in this industry for controlling the material ratio and for processing flat glass. With the advancement in technology over the years, the demand has increased for programmable logic controllers in the glass industry.
Since the production of glass is a sophisticated and elaborate process, PLCs are used with the bus technology in the control mode. They are applied in the analog data recording, position control, and in digital quality.
Paper Industry
Applications of PLC take place in the paper industry in various forms. PLCs are used in different processes in the paper industry like controlling machines producing paper at a high speed. PLCs monitor and control the production of newspapers and book pages in offset web printing.
PLC programming applications are also used in the food industry’s filling machine control system, aerospace sector’s water tank quenching systems, closed-loop textile shrinkage system, and industrial batch washing machine control.
The coal-fired boiler fan change-over system in hospitals use PLC, other than this the silo feeding, corrugation machine control system as well as injection molding control system in the plastic industry use programmable logic controllers. These were some of the PLC applications.
PLC Programming
It is of utmost importance to design and implement concepts according to your case when you are using PLC. To successfully do it, you first need to have a deep understanding of the specifics of PLC programming.
A PLC program comprises a set of instructions that are present in textual or graphical form. These texts or graphics represent the logic that governs the entire process that programmable logic controllers are controlling.
There are basically two major classifications of PLC programming languages which are further classified into different parts. The two major programming languages are textual language and the graphical form.
Textual language consists of an instruction list and structured text whereas graphical form is further divided into ladder logic, function block diagram, and sequential function chart. All these PLC programming languages are capable of programming a PLC but mostly ladder logic is preferred.
Ladder Logic
It is the simplest and most used programming language used to program PLC. It is also known as relay logic. Ladder logic is used to represent the relay contacts that are used in relay-controlled systems.
Functional Block Diagrams
FBD or Functional Block Diagram is a very simple graphical method that is used for programming multiple functions in programmable logic controllers. A function block is known for yielding one or more output values by executing its program instruction unit.
The benefit of using FBD is that numerous inputs can be done to receive outputs. When the inputs and outputs are huge in number, the input of one function block can be connected to the output of another function block.
Structured Text Programming
Structured text programming is a textual programming language that uses various statements to determine what exactly is to be executed. More conventional programming protocols are followed in this type of programming.
There are a series of logical statements that are constituted of expressing relationships and assignments by using various operators. Let us understand how the programming languages are used practically with the help of an example.
Let’s assume there is a signal lamp. Now, when we use a signal lamp, we switch it on in case a pump is running and the pressure is also found to be satisfactory. The other case when we switch it on is when the lamp test switch is found closed.
In this application, inputs will be required from both the pump and pressure sensors, if one needs output. For this, AND logic gates are being used.
OR logic is used when there is a test input condition that is required to deliver output from the lamp regardless of the fact that there is a signal from the AND system or not. END and RET instructions are used in the ladder diagram to ensure that PLC has reached the program’s end.
Yet another example is that of a valve that is to be operated so that it can lift a load. The pump is required to be running when this action takes place. During this process, either the lift, the switch is being operated or a switch is operated which indicates that the load has not been lifted yet and is lying at the bottom.
PLC Trainers
A programmable logic controller training system is generally termed a PLC trainer. It contains programming software and is best for students who are studying technology in schools and colleges to start PLC programming. There are very limited PLC trainers available in the market. These are basically kits that you can use to learn how these functions.
Some of the PLC training kits available in the market are by Mitsubishi, Omron, Siemens, and Allen Bradley. The PLC trainer by Siemens is DLPLC-SIMGD which uses a modular structure, digital inputs, Siemens PLC module, experiment hanging box or color stereogram, and different simulation units.
These units are all hanging box style to which the experiment corresponds. These can be combined as per the project requirements and in case, the need increases, components can be added to the same. Optionally Mitsubishi analog module can also be used which is capable of completing analog experiments.
There is also a network module that can provide three-phase asynchronous motor and two different kinds of the network frequency inverter. In addition to these, it provides corresponding experiments’ motor speed’s complete frequency control.
Some other PLC trainer kits are DLPLC-X1 Portable PLC trainer, Elevator Trainer Teaching Equipment with PLC Program, Crossing Traffic Light Training Model, and DLPLC – T201 PLC trainer.
Why Tesca Technology is Best PLC Trainers Manufacturer, Suppliers, and Exporters?
Tesca Technology is a single-sourcing partner that fulfills all the educational and didactic needs by providing solutions for the same. It conducts quality control at multiple stages, provides service support, and hassle-free shipment ordering. This is what makes it the best PLC Trainer manufacturer, supplier, and exporter. Here is the list of a few PLC trainers by Tesca.
- PLC (Siemens) Trainer 8 Input – 6 Output with Different Application Software for Car Parking, Washing Machine, Vending Machine, Tank level Control & Elevator Control
- Universal PLC (Siemens) Trainer – Digital 8 Input / 6 Output
- Universal PLC (Siemens) Trainer With HMI
- PLC (Siemens) Trainer 14 Input – 10 Output with Different Application Software for Car Parking, Washing Machine, Vending Machine, Tank level Control & Elevator Control
- Universal PLC (Siemens) Trainer – Digital 12 Input / 8 Output
- Universal PLC (Siemens) Trainer With HMI
- PLC (Siemens) Trainer 24 Input – 16 Output
- Universal PLC (Siemens) Trainer – Digital 30 Input / 26 Output
- Universal PLC (Siemens) Trainer With HMI
- PLC (Delta) Trainer 8 input – 6 output Optional: Application Board for PLC
Tesca Technology is known for providing excellent technical support, installations, training and demonstrations. So, if you are looking forward to buying PLC Trainers then choose the ones as per your requirements from the huge range offered by Tesca!
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