Programmable Logic Controllers (PLC)

SLC 500 programmable logic controller from Allen-Bradley.
ABOVE: SLC 500 programmable logic controller from Allen-Bradley.

Programmable Logic Controllers (PLC) implement specific functions such as:
  • I/O control
  • logic
  • timing
  • communication
  • report generation
  • data file manipulation
  • arithmetic
  • counting
Programmable controllers are electronic computers that are used for the control of machines and manufacturing processes through the implementation of specific functions such as logic, sequencing, timing, counting, and arithmetic. They are also known as programmable logic controllers (PLCs). Historically, process control of a single or a few related devices has been implemented through the use of banks of relays and relay logic for both the control of actuators and their sequencing. The advent of small, inexpensive microprocessors and single-chip computers, or microcontroller units, brought process control from the age of simple relay control to one of electronic digital control while neither losing traditional design methods such as relay ladder diagrams nor restricting their programming to that single paradigm. The computational power of programmable controllers and their integration into networks has led to capabilities approaching those of distributed control systems, and plantwide control is now a mixture of distributed control systems and programmable controllers. Applications for programmable controllers range from small-scale, local process applications in which as few as 10 simple feedback control loops are implemented, up to large-scale, remote supervisory process applications in which 50 or more process control loops spread across the facility are implemented. Typical applications include batch process control and materials handling in the chemical industry, machining and test-stand control and data acquisition in the manufacturing industry, wood cutting and chip handling in the lumber industry, filling and packaging in food industries, and furnace and rolling-mill controls in the metal industry.

SIMATIC Controller from Siemens
ABOVE: SIMATIC Controller from Siemens

Although programmable controllers have been available since the mid-1970s, developments-such as the ready availability of local area networks (LANs) in the industrial environment, standardized hardware interfaces for manufacturer interchangeability, and computer software to allow specification of the control process in both traditional (ladder logic) and more modern notations such as that of finite-state machines-have made them even more desirable for industrial process control.

Programmable logic controllers are typically implemented by using commonly available microprocessors combined with standard and custom interface boards which provide level conversion, isolation, and signal conditioning and amplification. Microprocessors used in programmable controllers are similar or the same as those used in personal computers. The software of a programmable controller must respond to interrupts and be a real-time operating system, characteristics which the typical operating system of a personal computer does not possess.

Perhaps the biggest benefit of programmable controllers is their small size, which allows computational power to be placed immediately adjacent to the machinery to be controlled, as well as their durability, which allows them to operate in harsh environments. This proximity of programmable controllers to the equipment that they control allows them to effect the sensing of the process and control of the machinery through a reduced number of wires, which reduces installation and maintenance costs. The proximity of programmable controllers to processes also improves the quality of the sensor data since it reduces line lengths, which can introduce noise and affect sensor calibration.

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