Programmable Logic Controller (PLC)

A Programmable Logic Controller (PLC) is a specialized computer engineered for industrial purposes, specifically tailored for the control of manufacturing processes. It excels in environments demanding reliability, programmable functionalities, real-time data processing, and fault diagnosis. PLCs exhibit robustness, enabling them to endure challenging conditions such as extreme temperatures, electrical noise, vibration, and physical impact. 

The timeline of the programmable logic controller can be traced back to the late 1960s, where its inception was driven by the need to replace relay logic systems prevalent in the United States automotive industry. Engineered for modularity and scalability, the initial iterations of PLCs embraced solid-state components, enabling the retention of programmed instructions even in the face of power interruptions. Dick Morley, engineer at Modicon Inc., is acknowledged as the inventor of the first PLC in 1969, characterized by a rudimentary relay design featuring four outputs, garnering rapid adoption within the automotive manufacturing sector. 

As technology advanced, PLCs evolved from their large and expensive first-generation counterparts of the 1970s to become smaller, more powerful, and cost-effective. This evolution was marked by significant milestones such as the introduction of the IEC 61131-3 standard in 1982, which brought consistency to PLC software products. By the end of the 1990s, PLCs had begun to incorporate Ethernet connectivity, bringing megabytes of processor memory and user-defined data types to the factory floor.  

Central Processing Unit (CPU)

Serving as the core of the PLC, it executes control instructions present in the PLC’s program. It also performs arithmetic and logic operations while managing data communication between the PLC and connected devices. 

Input/Output (I/O) Modules

Input modules receive signals from input devices, and output modules transmit control signals to actuators. These modules serve as the direct interface between machinery and the PLC. 

Power Supply

This component converts main AC power to low-voltage DC power required by the various components of the PLC. 

Programming Device

Typically, a computer equipped with specialized software, used for writing, testing, and downloading programs into the PLC. 

Communication Ports 

These ports enable PLCs to communicate with other PLCs, computers, distributed I/Os and other interfaces, supporting various communication protocols. 

Memory

A PLC contains different types of volatile or non-volatile memory components. For example, the control program is stored remanently in a flash memory and loaded into the RAM when it is started and executed there. 

The functionality of a PLC follows a cyclical process, encompassing several stages: 

Input: The PLC monitors the status of its input devices, including sensors, switches, or other inputs. 

Program: Based on the received inputs, the PLC executes a user-created control program. 

Output: The PLC makes decisions to control the state of output devices, such as motors, valves, lights, and relays, according to the logic defined in the program. 

Housekeeping: This phase involves communications, diagnostics, and other background tasks that ensure the continuous operation of the PLC. 

Example of application

Take, for instance, an automated bottling plant tasked with filling, capping, and labelling bottles before packaging. The PLC plays a pivotal role in orchestrating this process: 

Input devices: Sensors are deployed to detect the presence of bottles, measure fill levels, or validate proper capping. Each sensor is connected to the PLC's input modules. 

Control logic: The PLC's CPU is loaded with control logic, encompassing conditions such as "initiate fill if the bottle is in place" or "cease filling if the fill sensor is activated." 

Output devices: When the conditions for filling are met, the PLC instructs the actuator to open the fill valve via an output module. Upon completion of filling, another signal may prompt the capper to cap the bottle. 

Communication and monitoring: Throughout the entire process, the PLC can transmit status signals to a central control room where operators can oversee the operation. 

The PLC guarantees the seamless progression of the entire process, intervening in case of errors like misalignment or a defective cap by halting the line and triggering an alarm. 

This illustrates how PLCs interpret sensory data, make informed logical decisions, and effectively manage the production line with minimal human intervention, leading to a highly streamlined and automated process. 

PLCs are indispensable components of industrial communication systems, functioning as robust interfaces that streamline the automation of intricate processes in manufacturing and plant operations. Engineered for high reliability, straightforward programming, and effective fault diagnosis, these industrial computer systems are crucial for overseeing and controlling machinery by encompassing the following virtues: 

Ruggedness and Reliability

Engineered to operate in demanding conditions, PLCs are resilient against factors that typically affect sensitive electronics, including temperature extremes, mechanical vibrations, dust, electrical noise and humidity. Consistent performance in tough environments guarantees that automation processes proceed uninterrupted, which is crucial for continuous production lines. 

Real-Time Control and Monitoring

PLCs operate by continuously cycling through a series of steps: reading inputs, executing the control program, and updating outputs. This scan cycle is executed at high speed and typically in milliseconds, allowing PLCs to facilitate real-time response and adjustments to the system. Furthermore, PLCs offer deterministic execution times for processing the control logic. This means that for every cycle, the processing time should remain consistent, ensuring that real-time responses are predictable which is crucial for synchronization and timing-critical applications. The use of PLCs in predictive maintenance is particularly effective in pre-empting equipment failures and optimizing performance.  

Interoperability and Integration

As one of the key components in industrial automation, PLCs need to communicate and interact with other systems, such as HMIs, sensors, actuators, robots and networks. Because of this, interoperability is a critical feature of PLCs, allowing them to operate in conjunction with equipment, systems, apps or products from different vendors. Therefore, a variety of different industrial communication protocols enables communication between the various automation components, from fieldbus protocols such as Profibus or Modbus, to real-time Ethernet protocols such as EtherCAT or Ethernet/IP, to IIoT standards such as OPC UA or MQTT. A PLC should be as flexible as possible in terms of integration with different protocols. 

Ease of Programmability

PLCs comes pre-equipped with various input/output (I/O) modules, both digital and analogue, which can be duly added or removed to fit the needs of the automation task at hand. This modular approach allows a single PLC program to handle a small or large number of I/O points, making it well-suited for both simple and complex industrial applications. In addition, PLCs offer various integrated development environments (IDEs) such as Ladder Logic (LL), Structured Text (ST), Function Block Diagram (FBD), Sequential Function Charts (SFC), and Instruction List (IL), providing flexibility to engineers and programmers to choose the most suitable language for their specific application needs. Hence, enabling teams to work on parallel development efforts, keeping them productive. 

With its broad and industry-leading product and solution portfolio for industrial communication, Hilscher offers a suitable communication interface for every type of control system. The multiprotocol-capable netX communication controllers for controller applications and embedded modules and PC cards based on them offer a flexible solution for all common fieldbus and Ethernet protocols – and for any desired level of integration. All components are based on the netX chip technology developed in-house and enable communication via protocol technologies such as CANopen, CC-Link, DeviceNet, EtherCAT, EtherNet/IP, Modbus/TCP, Ethernet POWERLINK, PROFIBUS, PROFINET, Sercos, CC-Link IE Field Basic or Varan by simply reloading the firmware.