PLC is an industrial grade computer used in automation industry to make system reliable through logic. It was invented by General motor, in America and Dick Morley was considered father of PLC. Dick Morley has identified the problem in industry that before PLC there is hardware wiring and this hardware nature made it difficult for design engineer to find troubleshooting and system is also not reliable. Now a days PLC is used in every automation industry and there are so many Industry providing automation digital solution like Schneider electric, Allen Bradley, Siemens industries and so on.
Working of PLC
PLC works in program of scan cycle where it read executes it program repeatedly and there is ladder logic and scan cycle consist of 3 steps.
- Read input
- Execute the program
- Write output
It follows the sequence of instruction and works in real time that is millisecond and in milliseconds for the processor to evaluate all the instruction and update all the output according to instruction.
Component of PLC
They have three components, and these are: processor, power supply input/output section
It is brain of system of plc system, is a solid state device designed to perform a wide variety of production, machine tool and processor control function. Conventionally electromechanical device, relays and their associated wiring formerly performed these function. It operates in 5volt supply and supplied by power supply. Once the ladder diagram program is entered into processor, it remains until changed by the user with one of programming devices and program unaltered through power failure.
- Power supply
It work is to convert line voltage into 24 dc voltage to provide internal circuitry. Basically, it is combination of transformer, rectifier and capacitor. In some cases, it also provides an isolated VDC supply to power dc input circuits, switches and indicators. There are so type power supply found in PLC like DC/DC converter power supply, Frequency converter plc power supply, Linear plc power supply and Switching plc power supply.
- PLC Input/Output
Electrical noise like spike in power lines or load kick back would have series impact on PLC internal circuits this is where input/output portion play a very important role. The I/O both protect CPU from electrical noise. The I/O section is where status signal is filtered to remove noise level and CPU decision are made and put into operation. The PLC inputs provide their status to a storage area within the CPU AND outputs are driven from similar stored status in the CPU. Real world devices like push buttons, limit switches and sensor are connected through input modules in the PLC. These modules detect a change in state of input signal and provide a stored image to input element in ladder logic. The input element simulates action of relay contact within PLC. In turn, output element are energized which produces desired output signal to drive load such as motor controller, contactors, via output modules in the I/O.
- PLC Programming
PLC are simply to program. They use a relay ladder language that is similar to magnetic relay circuitry. Engineers, and electrician can learn to program the PLC without extensive training or experience. There are numerous advantages in using PLC versus a relay or solid electronics. In a PLC, changes can be accomplished quickly and inmost cases, without hardware modification to the controller.
The advantages one gets with PLC are-
- Less wiring.
- Easier and make faster response in real time.
- It make trouble shooting easier and reducing downtime.
- High reliable and flexible.
- Low power consumption.
- Capable of handling complex logic.
PLC applications are typically customized system. It is low compared to cost specific custom- built control design. It requires less maintenance due to absence of moving parts, thus making things they work better. Overall, PLC appear to be an excellent solution for many different problems.