Introduction to Programmable Logic Controllers (PLCs)
What is a Programmable Logic Controller (PLC)?
A programmable logic controller, PLC, or programmable controller is a user-friendly, microprocessor based specialized computer that carries out control functions of many types and levels of complexity. Its purpose is to monitor crucial process parameters and adjust process operations accordingly.
A PLC is a digital computer used for automation of typically industrial electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many machines, in many industries.
- Is easy to set up and program.
- Behaves predictably, ruggedized.
- It can be programmed (to a degree), controlled, and operated by a person unskilled in operating (programming) computers.
Essentially, a PLC’s operator draws the lines and devices of ladder diagrams with a keyboard/mouse onto a display screen. The resulting ladder diagram is converted into computer machine language and run as a program.
The PLC has input lines where sensors are connected to notify upon events (e.g. temperature above/below a certain level, liquid level reached, etc.), and output lines to signal any reaction to the incoming events (e.g. start an engine, open/close a valve, etc.).
Function of PLC
Modular PLCs have extended I/O
Components in a PLC system
- Network interfaces: to allow PLCs to function in a networked environment
- Communication adapters for remote I/O devices: so I/O devices do not have to be physically close to the CPU module
- Operator interface devices: allow monitoring and/or data entry by operators
Definition And History Of The PLC
The PLC can be operated on the input side by on-off devices (discrete, or digital) or by variable (analog) input devices. The first PLC systems evolved from conventional computers in the late 1960s and early 1970s. These first PLCs were installed primarily in automotive plants.
Traditionally, the auto plants had to be shut down for up to a month at model changeover time. The early PLCs were used along with other new automation techniques to shorten the changeover time. One of the major time-consuming changeover procedures had been the wiring of new or revised relay and control panels.
The PLC keyboard reprogramming procedure replaced the rewiring of a panel full of wires, relays, timers, and other components. The new PLCs helped reduce changeover time to a matter of a few days.
Implementing Changes and Correcting Errors
Large Quantities of Contacts
Ladder or Boolean Programming Method
Reliability and Maintainability
Often, the file prints for relay panels are not properly kept up to date. A PLC printout is the circuit at the present time; no wire tracing is needed for verification.