Programmable logic controller - PLC

The manufacturing process requires a series of operations to manufacture a product. And in this case, the sequence can be performed manually or via some kind of controller.

In the late 1960s, sequencing operations were performed using a bank with connected relays to perform a particular job. But there were disadvantages, logical systems where it was difficult to reprogram production changes, and also problem solving is very difficult.

To avoid the above drawbacks, PLCs are used.

The PLCs use the logic function for summing relay and the sequence is not wired but can be programmed and stored in memory.

The memory content can be easily changed.

PLCs are used first by the automotive industry.

PLC working principle:

A programmable logic controller handles binary inputs and binary outputs related to logic statements. The inputs are derived from input elements, such as limit switches, buttons, etc., based on set values ​​for temperature, pressure and the like. The plc programmer output is also a binary mode that activates the output devices, such as solenoid valves, valves, servomotors, etc. Controllers check the status of each input and perform logical functions in the central processing unit according to the program stored in the memory and generate it required binary output. Input scanning is performed cyclically at a fast speed.

The CPU receives instructions from memory and executes them in sequence. An internal clock controls sequential retrieval of instructions.

The heart of PLCs is a computer, but not all computers become programmable controllers. To become the heart of a PLC, a computer must be designed to operate in the industrial environment, which can often be hostile to conventional computers. A PLC must use a computer that can operate over a wide temperature range (0 to 60 Celsius) and a wide humidity range (0 to 90 percent). It must be packed in a sturdy cabinet and be well protected from electromagnetic interference, wire, dirt and moisture. Your memory circuits must be battery-backed. It must be programmable with logical commands, symbols or mnemonics corresponding to the ladder diagrams. Finally, it must be designed for scanning operations. A scan computer solves the logic from the beginning of the memory to a specific specified stop input. When the end is reached, the operation is repeated again.

PLC functions:

A typical programmable logic controller offers many of the following features.

Compact size

Financial costs

Easily mountable.

It uses advanced technology.

The multi-digit alphanumeric display is used for timer values, counter values, process parameters, error messages, etc.

Selection of operating voltages.

High Speed ​​Counter Input (Some Kilo Hertz)

Powerful set of instructions.

Relay outputs.

Programmed using ladder charts or PC-based software.

Advantages or benefits of PLC

Compared to conventional relay controllers, a programmable logic controller has the following advantages or advantages.

The wires to a PLC system are generally reduced by 80 percent compared to the conventional relay control system. Therefore, the size of the cabinet is small, which requires less floor space.

Power consumption is significantly reduced as PLC uses much less energy.

Self-diagnostic functions enable quick and easy problem solving of the system.

A single panel can be used in multiple applications simply by modifying the program.

Spare parts for relays and timers are greatly reduced.

The PLC operating speed is in the interval of milliseconds, and therefore the cycle time of the machine is greatly improved. This increases productivity.

It costs much less, especially when there are many I / O devices and the control functions are complex.

Very reliable operation.

A printout of the PLC program can be obtained in a short time. Therefore, printed documentation can be easily maintained.

It can be easily installed.