Programmable Logic Controllers (PLCs)

 PLCs 

(Programmable Logic Controllers )

PLCs are programmable logic controllers.

A programmable logic controller (PLC) is a computer that may be used to control processes in an industrial environment. The programmable controller has eliminated much of the hardwiring associated with classic relay control circuits. Among the other benefits are quick response, simple programming and installation, fast control speed, network interoperability, simplicity of troubleshooting and testing, and dependability.

The PLC has many input and output configurations, greater temperature ranges, electrical noise immunity, and vibration and impact tolerance. Nonvolatile memory, or battery-backed memory, is often used to store program for controlling and managing manufacturing process equipment and machinery. The system controlled by the PLC is an example of a real-time system since the output is reliant on the input circumstances.

Originally designed to replace relay logic, the PLC is today used in a broader range of applications as its capabilities have evolved. Because the construction of a PLC is based on computer architectural ideas, it can perform operations such as timing, counting, calculating, comparing, and processing analogue data in addition to relay switching.

The programmable controller has eliminated much of the hardwiring associated with classic relay control circuits. It is small and inexpensive when compared to equivalent relay-based process control systems. Although relays are still used in modern control systems, they are rarely used for logic.

PLCs have a plethora of additional benefits, including:

• Increased Reliability

After it has been built and tested, a program may be easily downloaded to various PLCs. Because all of the logic is stored in the PLC's memory, there is no risk of making a logic wiring error (Figure 1-3). The program eliminates most of the external wiring that would otherwise be required for process control. Although hardwiring is still required to connect field equipment, it takes less time. PLCs, like solid-state components, offer the same level of reliability.

Figure 1-3 The memory of the PLC contains all of the logic.

• More adaptability. 

Using a PLC to create and change a program is significantly easier than wiring and rewiring a circuit. The relationships between inputs and outputs in a PLC are determined by the user program, not by how they are linked (Figure 1-4). To provide system upgrades, original equipment manufacturers (OEMs) may simply send out new software. End users can change the program on the fly, and if necessary, security can be provided by hardware features like as key locks and software passwords.

Figure 1-4 The user program determines the relationships between the inputs and outputs.

• Less expensive.

PLCs were designed to replace relay control logic, and the cost savings have been so significant that, with the exception of power applications, relay control is becoming obsolete. In general, installing a PLC is less costly if an application has more than a half-dozen control relays.

• Ability to communicate.

To perform activities like as supervisory control, data gathering, process parameter monitoring, and software download and upload, a PLC may connect with other controllers or computer equipment (Figure 1-5).

Figure 1-5 PLC communication module.

• Faster response time.

PLCs are designed to perform high-speed, real-time tasks (Figure 1-6). The programmable controller operates in real time, which implies that an event in the field will trigger an action or output. Machines that process thousands of items per second and objects that spend only a fraction of a second in front of a sensor demand the PLC's quick-response capacity.

Figure 1-6 High-speed counting.

• Easier to troubleshoot.

PLCs include built-in diagnostics and override functions that make it easy to identify and resolve software and hardware errors. Users may display the control software on a monitor and see it discover and rectify faults in real time (Figure 1-7).

Figure 1-7 Control program can be displayed on a monitor

in real time.

• Field Devices are easier to test.

From a central location, a PLC control panel can analyze field equipment. A control system with hundreds of input and output field devices, for example, may be housed within a large manufacturing area. As a result, examining each device at its location would be time-consuming. Each device's operation could be immediately tested by connecting it to a common point on a PLC module.

Parts of PLC

A traditional PLC can be divided into pieces, as shown in Figure 1-8. These components include the central processing unit (CPU), the input/output (I/O) section, the power supply, and the programming device.

The term architecture include both PLC hardware and PLC software, as well as a combination of the two. Because of the system's open architecture, it can be easily coupled to devices and software from many suppliers.

Off-the-shelf standard-compliant components are used in open designs. A closed architectural system has a proprietary design that makes connecting to other systems more difficult.

Because most PLC systems are proprietary, any generic hardware or software you use must be compatible with your unique PLC.

Figure 1-8: A programmable logic controller's typical components.

Furthermore, while the essential concepts of all programming techniques are the same, subtle differences in addressing, memory allocation, retrieval, and data processing may exist for different models. As a result, PLC program from different manufacturers cannot be utilized interchangeably.

Fixed and modular I/Os (Inputs/Outputs) are two approaches for incorporating I/Os (Inputs/Outputs) into the PLC.

Figure 1-8: B programmable logic controller's typical components.

Fixed I/O

(Figure 1-9) is a typical example of a tiny PLC that is packed as a single unit with no changeable components. The processor and I/O are combined, and the I/O terminals will have a predetermined number of input and output connections. The main advantage of this packing method is that it is less costly. The available I/O points vary and are normally enhanced by acquiring more fixed I/O devices. Fixed I/O has the disadvantage of being rigid; you are limited to what you can get in the amounts and types specified by the package. Furthermore, on some models, if any part of the unit fails, the complete unit must be replaced.

.Figure 1-9 Fixed I/O configuration

Modular I/O (Figure 1-10) is divided into compartments into which various modules can be inserted. This feature broadens your options as well as the unit's versatility. You may choose from the manufacturer's modules and combine them whatever you like. A simple modular controller consists of a rack, a power supply, a processing module (CPU), input/output (I/O) modules, and an operator interface for programming.

The modules are secured to a rack. When a module is inserted into the rack, it establishes an electrical connection with the backplane, which is a series of contacts at the rack's rear. The PLC processor is connected to the backplane by the backplane, allowing it to communicate with all of the modules in the rack.

Figure 1-10 Modular I/O configuration.

The power source provides DC power to other modules that plug into the rack (Figure 1-11). This power source is not often used to power field devices in large PLC systems. Field devices in larger systems are powered by an external alternating current (AC) or direct current (DC) supply. Some small micro PLC systems may utilize the power supply to power field devices.

Figure 1-11 Power Supply unit.

The processor (CPU) is the "brain" of the PLC. A typical processor (Figure 1-12) consists of a microprocessor that implements logic and controls communication between modules. The CPU needs memory to store user program instructions, numerical values, and the status of I/O devices.

Figure 1-12 Typical PLC processor modules.

The CPU controls all PLC activities and is designed so that the user may enter the desired program using relay ladder logic. The PLC program is executed as part of a scan, which is a time-consuming activity (Figure 1-13).

A conventional PLC scan begins with the CPU reading the condition of the inputs. After that, the application program is launched.

Figure 1-13 Typical PLC scan cycle.

Following the completion of the program execution, the status of all outputs is updated. The CPU then performs a number of internal diagnostic and communication tasks. This action is repeated endlessly as long as the PLC is in run mode.

The I/O

The I/O system serves as the link between field devices and the controller (Figure 1-14). The function of this interface is to condition signals that come in from or go out to external field devices. Input devices such as pushbuttons, limit switches, and sensors are linked to the input terminals. Output devices such as small motors, motor starters, solenoid valves, and indicator lights are linked to the output terminals.

To electrically separate the internal components from the input and output terminals, PLCs commonly utilize an optical isolator, which connects the circuits together by light.

External inputs and outputs are frequently referred to as "real-world" or "field" inputs and outputs. The terms field and real world are used to distinguish between genuine external devices that must be physically wired and an inside user program that simulates the working of relays, timers, and counters.

Figure 1-14 Typical PLC input/output (I/O) system connections.

A programming device is used to load the appropriate software into the processor's memory. One of the most frequent programming languages, relay ladder logic, may be used to input the program. To represent the intended output, the ladder logic programming language use graphical symbols rather than words.

A relay control circuit diagram is analogous to a ladder logic program.

It is a bespoke language developed to make PLC programming simple for people who are familiar with relay logic control. Hand-held programming devices are occasionally used to program micro PLCs since they are inexpensive and simple to use.

Once linked to the PLC, they may be used to enter and monitor programs. Small hand-held devices and laptop computers are frequently used on the manufacturing floor to repair equipment, alter programs, and transfer programs to many machines. A personal computer is the most common piece of programming equipment (PC). Most PLCs have software that enables a PC to be used as a programming device. With this software, users may create, alter, document, save, and troubleshoot ladder logic programs (Figure 1-15). The computer monitor may show more logic than handheld counterparts, making software understanding easier.

Figure 1-15 Typical PC software used to create a ladder logic program.

Relay ladder logic is a common programming language for PLCs (RLL). Its roots can be found in the control of electromechanical relays. The relay ladder logic program graphically illustrates contact rungs, coils, and special instruction blocks. RLL was designed with the purpose of being easy for users to use and understand, and it has subsequently been modified to suit the ever-increasing demands of industrial control needs.