Control Introduction to PLC | Controller Circuit

Friday, August 19, 2011

Control Introduction to PLC

Programmable logic controllers, also called programmable controllers orPLCs, are solid-state members of the computer family, using integratedcircuits instead of electromechanical devices to implement control functions.
They are capable of storing instructions, such as sequencing, timing,counting, arithmetic, data manipulation, and communication, to controlindustrial machines and processes.What Does PLC means?
A PLC (Programmable Logic Controllers) is an industrial computer used to monitor inputs, and depending upon their state make decisions based on its program or logic, to control (turn on/off) its outputs to automate a machine or a process.
Programmable controllers have many definitions. However, PLCs can be
thought of in simple terms as industrial computers with specially designed architecture in both their central units (the PLC itself) and their interfacing circuitry to field devices (input/output connections to the real world).
Advantages of PLC control
  • Faster scan times are being achieved using new, advanced microprocessor and electronic technology.
  • Small, low-cost PLCs (see Figure 1-2), which can replace four to ten
    relays, now have more power than their predecessor, the simple relay
    replacer.
  • High-density input/output (I/O) systems (see Figure 1-3) provide
    space-efficient interfaces at low cost.
  • Intelligent, microprocessor-based I/O interfaces have expanded distributed processing. Typical interfaces include PID (proportional
    integral-derivative), network, CANbus, fieldbus, ASCII communication, positioning, host computer, and language modules (e.g., BASIC, Pascal).
  • Mechanical design improvements have included rugged input/output enclosures and input/output systems that have made the terminal an integral unit.
  • Special interfaces have allowed certain devices to be connected
    directly to the controller. Typical interfaces include thermocouples,
    strain gauges, and fast-response inputs.
  • Peripheral equipment has improved operator interface techniques,
    and system documentation is now a standard part of the system.

Traditional PLC Applications

  • In automated system, PLC controller is usually the central part of a process control system.
  • To run more complex processes it is possible to connect more PLC controllers to a central computer.

Disadvantages of PLC control
- Too much work required in connecting wires.
- Difficulty with changes or replacements.
- Difficulty in finding errors; requiring skillful work force.
- When a problem occurs, hold-up time is indefinite, usually long.

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