Programmable controllers

Programmable controllers

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. See Digital computer, Distributed systems (control systems)

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 interchangability, 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. See Local-area networks

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. See Microcomputer, Operating system, Real-time systems, Software

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.

McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
JVL (Birkerod, Denmark) has added Profinet Industrial Ethernet capabilities to their NEMA 34 Series of integrated stepper with programmable controllers. The motors are available in four torque ratings of 3, 6, 9, and 12 Nm, and they include necessary electronics within the IP67 rated motor housing Features include:
Herman describes solid state devices common in industrial environments to help students of electricity and engineering understand how many of the control components operate, such as solid-state relays, rectifiers, silicon-controlled rectifier (SCR) drives for direct current motors, variable frequency drives for alternating current motors, and the inputs and outputs of programmable controllers. Most electricians do not troubleshoot circuits on a component level, he agrees, but a basic knowledge of how these electronic devices operate is necessary for understanding how various control components preform their functions.
It explains motor controllers and their theory, operation, installation, and maintenance of various electrical systems, covering tools and equipment, safety, symbols, control circuits and diagrams, switches, magnetism and solenoids, relays, electric motors, timers, sensors and sensing, valves, motor starting methods, solid-state reduced-voltage starters, speed control and monitoring, motor control and protection, three-phase controllers, drives, transformers, power generation, power distribution systems, programmable controllers, troubleshooting and maintenance, and careers in electricity.
All are offered with a choice of digital or programmable controllers and feature control heads that swivel, adjusting to a position that best suits the operator.
Programmable Controllers: theory and Implementation (2nd Edition) Industrial Text Company, U.S.A.
Every aspect of industry - from power generation to automobile painting to food packaging-uses programmable controllers to expand and enhance production.
The EtherNet/IP products include a full range of programmable controllers, managed switches with Internet Group Management Protocol (IGMP) snooping capability, and a variety of I/O devices.
Advanced programming facilities for control of complex processes include storage for up to 50 setpoint programs and control function blocks for logical and mathematical operations, which allow for the elimination of a number of other process control components such as times, counters and programmable controllers.
The use of Profibus-DP at field level reduced the amount of cable required for the installations, and ensured fast data exchange between the programmable controllers and the distributed devices.
According to the company, the L75 adds a new dimension of operational dependability and flexibility to modern processes controlled by computers, programmable controllers, and other electric control equipment.
Some key components of real-time systems are already in place--bar codes, programmable controllers, sensors, and execution and planning systems.

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