PROGRAMMING LOGIC CONTROL

PROGRAMMING LOGIC CONTROL 

HISTORY

PLC development began in 1968 in response to a request from an US car manufacturer (GE). The first PLCs were installed in industry in 1969.

Communications abilities began to appear in approximately 1973. They could also be used in the 70′s to send and receive varying voltages to allow them to enter the analog world.

The 80′s saw an attempt to:
standardize communications with manufacturing automation protocol (MAP), reduce the size of the PLC, and making them software programmable through symbolic programming on personal computers instead of dedicated programming terminals or handheld programmers.

The 90′s have seen a gradual reduction in the introduction of new protocols, and the modernization of the physical layers of some of the more popular protocols that survived the 1980′s.

The latest standard “IEC 1131-3″ has tried to merge plc programming languages under one international standard. We now have PLCs that are programmable in function block diagrams, instruction lists, C and structured text all at the same time.

What does ‘PLC’ mean?
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 LOGIC CONTROLLER Defined as:
“A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions such as logic sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes”.

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.

Advantages of PLC control
* Rugged and designed to withstand vibrations, temperature, humidity, and noise.
* Have interfacing for inputs and outputs already inside the controller.
* Easily programmed and have an easily understood programming language.

PLC LAN APPLICATIONS

Centralized data acquisition and distributed control are the most common applications of LANs. Data collection and processing, when performed by an individual controller, can burden the processor’s scan time, consume large amounts of memory, and complicate the control logic program.

A data highway configuration, in which all data is passed to a host computer that performs all data processing, eliminates these problems.

Also, distributed control applications allocate control functions, once performed by a
single controller, among several controllers, this eliminates dependence on a single controller and improves performance and reliability.

To use the distributed processing approach, a LAN and the PLCs attached to it must provide the functions:
- communication between PLCs
- upload capability to a host computer from any PLC
- download capability from a host computer to any PLC
- reading/writing of I/O values and registers to any PLC
- monitoring of PLC status and control of PLC operation