What Does PLC Stand For and What Does it Do?

What does PLC stand for?

In industrial automation terms, PLC stands for a Programmable Logic Controller. PLCs are specialized computers designed and built to automate a variety of industrial electromechanical processes. Automation of manufacturing processes began long before the invention of PLCs. From the early to the late 1900s, industrial automation was carried out by complicated hard-wired relay control systems.

However, these relay circuits’ required an enormous amount of wiring and more control-panel space to create even simple automation circuits. Such that thousands of relays could be needed to automate simple factory processes, with no provision for configuration changes within the logical circuits. In response to the challenges associated with hard-wired relay control systems, in 1968 Dick Morley invented and developed the first Programmable Logic Controller (PLC). Later on, in 1973, Modicon Company developed the first commercial PLCs to replace the complicated relay circuitry in automobile plants like Landis Auto Ltd and General Motors.

The newly developed PLCs drastically increased the functionality of industrial control systems with reduced cabinet space requirements to house the logic. Nevertheless, the 1970s PLC versions were complex, delicate, and massive. They required careful shielding from mechanical vibrations, noise, and heat, and they operated in climate-controlled manufacturing environments. Also, they were quite difficult to program. Then, just like the microprocessor invention revolutionized computers, so did the PLCs. More powerful, and resilient PLCs could now be designed to withstand and operate reliably in harsh industrial environments.

Today’s PLCs are enormously flexible providing a wide range of control functions including calculating, timing, comparing, counting, and processing a variety of discrete and analog signals. Also, modern PLC systems are designed with exceptionally stable operating systems, and backup power supplies to ensure uninterrupted process control.

Basic Hardware Components of a PLC 

A typical PLC control system consists of five primary components including:  

A) The Processor: This is a solid-state device that consists of the PLC memory and Central Processing Unit (CPU). As a microprocessor-based circuitry, the CPU includes an Arithmetic & Logic unit, a program memory, a process image memory, internal timers, and counters. Hence, it performs all the required data computations and processing operations by receiving inputs and producing corresponding outputs. The processor also hosts the operating system that provides software support for the creation of user-defined PLC programs.  

B) Memory: PLC memory includes Read Only Memory (ROM) and Random-Access Memory (RAM). ROM stores fixed data and operating programs used by the processor. While RAM stores timer values, counters, and information received from internal devices and field devices connected to the I/O modules. 

C) Input/output Modules: These modules connect the processor to input and output field devices. They can either be analog or digital. Input modules transmit digital or analog signals from input field devices to the PLC’s CPU. Examples of input field devices include transmitters, switches, pressure sensors, and encoders. On the other hand, the PLC processor uses the output modules to communicate to output field devices or processes being controlled. Typical output field devices used in PLC systems include motors, relays, lamps, proportional valves, etc. 

D) Power Supply: A PLC Power Supply is connected to AC mains (120/240 VAC) for the supply voltage. It then converts the power line AC voltage to the appropriate DC voltage required to power the primary components of the PLC. The most commonly used PLC power supplies are rated at 24V DC (Volts DC). Most PLC systems have separate power supplies which provide well-regulated DC power and protection for the other PLC components including CPU, memory, and I/O modules. Note: The PLC power supply does not provide power to input or output field devices. 

E) Programming Device: In today’s industrial applications, the various PLC programming devices used include laptops, hand-held devices, or desktop computers. They are used as programming terminals to enable users to create decision-making and user-defined PLC programs. Such devices run PLC programming software.

How do you Program a PLC? 

A PLC program is a set of instructions either in textual or graphical form.  It is written by the user using a programming device and then downloaded onto the PLC memory, this process is referred to as programming a PLC.  PLC programming languages are divided into two major categories namely: 

• Graphical Languages: (i) Ladder Diagrams (LD) (also known as Ladder Logic); (ii) Sequential Function Chart (SFC); (iii) Function Block Diagram (FBD).  
• Textual Language: (i) Instruction List (IL); (ii) Structured Text (ST or STX) 

The five PLC programming languages that are standardized by IEC 61131-3 include: 

A) Ladder Diagram (LD):  This was the first PLC programming language to be developed and is designed to mimic the physical layout of relay-based circuits. Ladder Logic programs utilize internal logic to replace physical electrical devices in a PLC control system, apart from the ones actuated by electrical signals. A typical PLC program written using Ladder Diagram would include two vertical power rails among a series of horizontal rungs. 

B) Function Block Diagram (FBD): FBD programs make use of a variety of Ladder Diagram commands, though they are much easier to read, interpret and conceptualize compared to Ladder Logic programs. A typical Function Block Diagram would describe a function that connects program blocks together to create a PLC program. The input and output blocks are then connected using connection lines. 

C) Sequential Function Chart (SFC): This graphical PLC programming language is similar to Ladder Diagram (LD) and Function Block Diagram (FBD). It uses the same logic as a flowchart and it consists of a number of steps, transitions and actions.  

D) Structured Text (STX, ST): This is a high-level textual-based PLC programming language that closely resembles basic C programming. With Structured Text the user has to write lines of code that execute sequentially by performing Boolean checks, evaluating specific functions, and actuating appropriate PLC outputs. A typical STX code would consist of statements separated by semicolons while making use of functions such as ‘IF’, ‘FOR’ ‘WHILE’, ‘ELSE’, and ‘ELSEIF’.  

E) Instruction List (IL): This is more of an assembly language that is textual based like Structured Text. It consists of multiple lines of code and a series of instructions, with each instruction being written on a new line and any necessary comments annotated at the end of each line. So, in terms of Instruction List program flow, each line of code specifies the instructions, conditions and the execution outcomes. Also, an IL program is read from top-to-bottom and left-to-right.  

How does a PLC operate?

PLCs operate in program scan cycles. There are four basic steps involved in every PLC program scan cycle, these are: 

A) Internal Checks: This is usually the first step in which the PLC processor performs internal diagnostics or self-test, by constantly checking the other PLC hardware and software components for faults. It also runs memory routines continuously to make sure that the program memory is not damaged. Still in this step, the processor also regularly communicates with programming terminals to eliminate and avoid any programming errors. If the processor detects no problems in this step, it will start the scan cycle.

B) Input Monitoring: In this step, the PLC processor monitors the state of all input field devices that are connected to it through the input module. It then collects all the necessary data and critical information on the condition of the inputs and process flow. It processes the received information as pre-programmed data and records it in a data table in the RAM (Random-Access Memory) to be used in the next step. 

C) Logic Execution: After the CPU has checked the status of all the connected inputs, it then executes the user program in its application memory. Logic execution simply means that the processor examines the PLC program line by line; one instruction at a time with reference to the memory copy of the information recorded in the RAM data table for the inputs checked in step 2. This way the PLC processor is said to be solving logic.

D) Output Control: This is the last step of a program scan cycle, in which the CPU updates the status of the output modules to control various output field devices. The PLC processor updates output status based on the execution results of the programmed logic in Step 3 and the state of the input variables (pre-programmed data in Step 2).
After this step, the PLC processor will restart the program scan process by initiating a self-test for internal faults, and the other steps are also repeated. All four steps continually take place in a repeating loop, hence the name operating cycle.

What can a PLC do? 

There are many functions that a PLC can perform, here are just a few: 

A PLC specializes in performing complex, simultaneous, and multi-stage control operations. For example, you can use a PLC to manage multiple assembly lines which funnel parts to palletizing and packaging robots. Such robots are programmed to package the right parts at a precise time in the correct order to be shipped out on a specific schedule. Then, if there are any errors or inefficiencies the PLC will detect them and notify the user, who can reprogram the entire process or specific parts via a user-friendly programming terminal.  

A PLC can act as the physical interface between a SCADA (Supervisory control and data acquisition) or HMI (Human-Machine Interface) system and devices on the plant floor. This could allow users to create a comprehensive SCADA and MES(Manufacturing Execution Systems) platform, Alarming and Reporting solution, HMI system, or an enterprise-wide solution from which they can view and control data with a PLC at any level of the organization.

PLCs can quickly troubleshoot the systems they control in a very simple manner. This makes them a cost-effective solution for controlling complex systems. In addition, a PLC can conduct counting, comparison, and calculation of analog process values. It also offers high flexibility in modifying the control logic within a very short period of time, whenever required.

A PLC is capable of performing a sequence or single set of tasks only, with higher performance and greater reliability, except when it’s operated under real-time constraints. This improves the overall reliability of a PLC control system. Unlike regular PCs and smartphones which are designed to simultaneously execute any number of roles within the Windows framework.

Applications of PLCs 

In the past few years, the role of PLCs in industrial applications is becoming increasingly more important as more industries seek to automate their production processes. Here are some of the key applications of PLC systems:

Automation and Process Control: The fundamental role performed by PLC systems is of automated industrial processes, they accomplish this by sending programmed control functions to the output field devices based on the input signals collected from input field devices. Input devices such as sensors, switches, relays, or thermometers that are connected to a PLC system measure and transmit data from the system being controlled to the PLC processor. On the contrary, output devices connected to the PLC system receive data or commands from the PLC processor to execute a specific function.

For example, a pressure sensor may send a signal to the PLC indicating that the pressure in a certain flow line is too high. The PLC will then automatically send a programmed command signal to a solenoid valve to open, thereby effectively reducing the pressure.

PLC systems are also used to control other automated and process actions such as switching a motor ON or OFF to run a conveyor system; raising or reducing operating temperatures of a machine through a heat exchanger, or displaying an LED status ON or activating an alarm on a Human Machine Interface (HMI) screen whenever abnormal system parameters are detected.

Many industries are employing “safety PLCs” which remain on standby until an abnormal event occurs, such as an emergency stop in an assembly line of a manufacturing plant. When that happens, the safety PLC steps in to control the timing, positioning, and direction of movement of the automated robots to ensure that any products on the assembly line are not damaged.

Data Collection, Machine Monitoring, and SCADA: PLCs monitor and collect data from connected inputs and machinery. This allows other software programs such as Machine Monitoring Systems and SCADA to connect to the PLC, in order to process the collected data and display relevant information such as live-trends, production reports, OEE (Overall Equipment Effectiveness) metrics, batch or cycle status, alarm notifications, and more.

When the operational information is visualized, operators and managers are able to better analyze the production performance and make data-driven decisions to improve output and efficiency. In addition, with immediate alarm notifications, operators are able to respond more quickly to any diagnosis and remedy faults; this ultimately reduces downtime and production costs.

Industrial Machine Learning and IIoT: Industrial Internet of Things (IIoT) and Industrial Machine Learning algorithms are new and evolving data technologies that are making their way into manufacturing facilities. However, PLCs continue to play an essential role as processors for real-time manufacturing data.  

Algorithms for Industrial Machine Learning function by improving themselves through experience. In that regard, PLCs collect the necessary production data through the IIoT connectivity to enable Machine Learning algorithms to make predictions and decisions without the need for pre-programming. This results in a high-end PLC control system that provides predictive maintenance while relaying real-time information to improve product quality control with actionable insights. Also, the information provided by PLCs is useful in proposing responses to system issues that may arise, thereby preventing downtime.  

Types of PLCs 

There are three distinct types of PLCs available for use in industrial automation applications. These include: 

A) Integrated PLCs: These have a fixed type architecture which makes them the simplest PLC types. They are also referred to as compact or unitary PLCs. They utilize a single unit architecture in which all the hardware components of the PLC are embedded into the single unit. The integrated CPU with connection ports, the memory, power supply, communication interfaces, and a number of input/output points are all built into a fixed PLC. Usually, you can connect compact PLCs directly to the machine or application in question. They are suitable for simple processes like small sized and less complex industrial applications.

B) Modular or Rack-Mounted PLCs: They utilize a modular type architecture in which every hardware component is a separate module. The hardware modules are then interconnected through a common mounting system. They provide users with more flexibility as you can have a base module to include CPU, power supply, and I/O, and configure the PLC for a specific application. Or you can customize the modular PLC with additional I/O modules, as many as you would like, or even insert an Analog-to-Digital (ADC) signal converter.

Modular PLCs are commonly used for automating medium sized and more complex industrial applications, where large numbers of I/O devices and higher powered processors are required. Some industries that use modular PLCs include mining, food & beverage, logistics and manufacturing processes associated with higher level of complexity in regards to operation, monitoring and control. 

C) Distributed PLCs: These are high-end PLC systems that make use of modular architecture but they have the capability to interconnect hardware components (processors and distributed I/O) across different locations through high-speed communication links. Each location in a distributed PLC system has multiple PLC hardware modules, housed in a mounting system referred to as a rack, drop, or node.  

Distributed PLCs are suited for large-sized and extensive applications spread across multiple locations since they are not limited to physical locations. They are mainly used in large processing facilities and large factories as they can provide a site-wide process control solution. 

Conclusion 

In conclusion, a Programmable Logic Controller (PLC) is a rugged, specialized computer designed to monitor and control complex industrial processes, like running motors and other machinery. It does so by collecting input data from the system it’s controlling and actuating the necessary outputs. Modern PLCs are quite easy to program and they are fully scalable to an application’s requirements. Also, despite being an upgrade over hard-wired relay control systems previously used in industrial control systems, PLCs are capable of performing much more complex control tasks.

This entry was posted on February 28th, 2022 and is filed under Allen-Bradley, Education, PLC, Uncategorized. Both comments and pings are currently closed.