Programmable Logic Controllers (PLC)
Description
Programmable logic controllers (PLCs) are usually referred as small industrial computers, containing hardware and software for performing control functions. PLCs contain a CPU and input-output interface system. The interfaces are used to activate relays and switches to control equipment. The CPU “reads” the input information through the interfaces and “performs” the controlling program which has been already stored in the memory. Programs are usually developed in ladder logic which is saved in the memory before the operation. Based on the program, the PLC “writes” the output devices. PLC’s I/O devices are connected to industrial equipment, such as magnetic relays, electric motors, pneumatic cylinders and hydraulic cylinders for controlling purposes. Some of the important PLC system specifications include data and program memory, number of instructions, and scan time.
Specifications
The specifications which are considered for programmable logic controllers are:
Advantages
The following are some of the advantages offered by programmable logic controllers:
I/O devices are easily built into programmable logic controllers
The programmable logic controllers can also be attached to a computer network by external I/O modules
They have input and output for analog and digital sensors, hence it is easy to operate in real time applications. Other than communication, they can also perform diagnosing and reporting functions on their own
Programmable logic controllers can be reprogrammed for various applications
Applications
Programmable logic controllers perform a wide variety of functions, motion control, process control, including basic relay control, complex networking. Programmable logic controllers have number of applications in industries, such as automobile industry, process plant, power plant and other automation industry. Programmable logic controller units are used in machine tools, such as automatic CNC milling, CNC grinding, CNC lathe and sheet metal industry.