PLC (Programmable Logic Controller) PLC or Programmable Logic Controller Is a small digital computer Embedded on a control system in order to get easy access in automation. It is typically used In the industrial electro mechanical processes such as control of a machinery on a factory or in an amusement park ride Its Advantages Are PLCs are designed for multiple analogue and digital inputs and output arrangements Extended temperature ranges Immunity to electrical noise Resistance to vibration and impact Non volatile memory with the help of an internal battery A PLC is an example of a “hard” real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could number in the hundreds or even thousands Early PLCs were designed to replace relay logic systems. These PLCs were programmed in “ladder logic”, which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver. The basic architecture of a PLC The basic architecture of a PLC consists of main components-the processor module, the power supply, and the I/O modules. The processor module consists of the central processing unit (CPU) and memory. In addition to a microprocessor, the CPU also contains at least an interface to a programming device and may contain interfaces to remote I/O and other communication networks. The power supply is usually a separate module, and the I/O modules are separate from the processor. The types of I/O modules include discrete (on/off), analog (continuous variable), and special modules like motion control or high-speed counters. The field devices are connected to the I/O modules.
With the many I/O modules available, a designer can mix and match the modules to fit application needs. I/O devices can consist of digital or analog signals. Digital I/O is a discrete signal that is simply on or off, such as a limit switch for input or relay on output. Analog I/O converts a voltage or current into an equivalent value by the CPU, such as an input thermocouple, or an output pressure regulator. Most modular systems allow for I/O expandability, only restricted by the CPU capabilities. PLC power supplies typically have an input ranging from 24 VDC to 240 VAC, and will fit most designed power requirements. Depending on the manufacturer, power supplies can be a separate unit, or integrated into the chassis. The power supply should match the load requirements of the PLC system. Since development of the first PLC in 1968, the world has depended on the unmatchable abilities, and nearly nonstop operation offered by this technology. The original unit from 1968 operated for 25 years uninterrupted, it is possible with today’s PLC architecture and technology, to see systems operating for a lifetime