Programmable Logic Controller (PLC)

A Programmable Logic Controller (PLC) is a device that is used to program for performing control functions. Automotive industry was innovated the first PLC in the late 1960s to replace relay logic controls .It has been become most important introduction for automation industry. Because of that reason, lot of industrial processes have been made using automation technology.

Relay logic controls was used in past time but it had lot of difficulties when taking desired performances .Because of that reason PLC was introduce to minimize the difficulties .There are lot of advantages when using PLC ,those advantages can be listed as following 

• Easy programming and installation,
• High control speed
• Hardware and software security
• Network compatibility
• Troubleshooting and testing convenience
• High reliability
• Can be used in a harsh environment conditions
• There are not any moving parts
• Easy wiring system 

Industrial and commercial environments are being widely used PLC in present time. PLC can be found in almost any manufacturing facility. There are several manufacturers of PLCs. Each brands have unique programing method, but basically all PLC‘s hardware structures and programming concepts are very similar .There are lot of different PLC brands, famous brands can be mentioned as following.

• Allen Bradley PLC
• GE Fanuc PLC
• Horner electric PLC
• Siemens PLC
• Array PLC
• Xinje PLC

Figure No 01 : PLC brands 

When considering PLC, every PLC consist of following component

• Central processing unit (CPU)
• Memory
• Inputs Output modules
• Power supply
• Programming Terminal

Connection between above mentioned components can be demonstrated using following figure

       Figure No 02 : Connection among PLC components

Central processing unit

PLC has a central processing unit like other computerized devices. It is the brain of the PLC It is controlled by operating system software The operating system program is a supervisory programs that are loaded and stored permanently in the PLC’s memory by PLC programmer .CPU does following operations

• Updating inputs outputs ,using inputs status it energizes or de-energizes it’s output
• Performing logic and arithmetic operations
• Communicating with memory ,Programmed data are stored in memory  .So CPU can read or change the content of memory locations
• Scanning application program
• Communicating with a programming terminal

Figure No 03 : Central processing unit 

Memory

Memory is the most important component that stores information, programs, and data in a PLC. There are two process that are being done using memory .The process of putting new information into a memory location is called writing. The process of retrieving information from a memory location is called reading.

There two common types of memory used in PLC, they are Read Only Memory (ROM) and Random Access Memory (RAM). A ROM location can be read, but it cannot be written. ROM is used to store programs and data that should not be altered. Because of that reason the PLC’s operating programs are stored in ROM.

A RAM location can be read or written. Therefore the information stored in a RAM location can changed. Ladder logic programs are stored in RAM, programing langue may be different with the PLC brands. When a new ladder logic program is loaded into a PLC’s memory, the old program that was stored in the same locations is over-written and essentially erased. The memory capacities of PLCs can vary. Memory capacities are often expressed in terms of kilo-bytes (K). One byte is a group of 8 bits. One bit is a memory location that may store one binary number that has the value of either 1 or 0. (Binary numbers are addressed in Module 2). 1K memory means that there are 1024 bytes of RAM. 16K memory means there are 16384 bytes of RAM.  

Input modules and output modules

A PLC is a control device. It takes information from inputs and makes decisions to energize or de-energize outputs. The decisions are made based on the statuses of inputs and outputs and the ladder logic program that is being executed. The input signal can be given to the PLC by using push buttons, limit switches, relay contacts, photo sensors, proximity switches, temperature sensors, and the like. These input devices can be AC (alternating current) or DC (direct current). The input voltages can be high or low. The input signals can be digital or analog. Differing inputs require different input modules. An input module provides an interface between input devices and a PLC’s CPU, which uses only a low DC voltage. The input module’s function is to convert the input signals to DC voltages that are acceptable to the CPU. Standard discrete input modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and transistor-transistor logic (TTL) level.

The devices controlled by a PLC include relays, alarms, solenoids, fans, lights, and motor starters. These devices may require different levels of AC or DC voltages. Since the signals processed in a PLC are low DC voltages, it is the function of the output module to convert PLC control signals to the voltages required by the controlled circuits or devices. Standard discrete output modules include 24 V AC, 48 V AC, 120 V AC, 220 V AC, 24 V DC, 48 V DC, 120 V DC, 220 V DC, and TTL level.    

         Figure No 04 : Input modules and output modules

 Power supply

Standard commercial AC power lines are being used to power the PLC. But many PLC components are utilizing 5V or another low voltage of DC power. Those components are CPU and memory. The PLC power supply converts AC power into DC power to support those components of the PLC.

Programming Terminal

A PLC requires a programming terminal and programming software for operation. The programming terminal is used for programming the PLC and monitoring the PLC’s operation. It may also download a ladder logic program (the sending of a program from the programming terminal to the PLC) or upload a ladder logic program (the sending of a program from the PLC to the programming terminal). 

Programming device

Generally personal computers are used to program the PLC. Working software allows users to modify, store, troubleshoot the program. Personal computers communicate with PLC using processor via a serial or parallel data communications link

Hand-held unit are often used for modifying, troubleshooting or transferring programs to multiple machines on the factory floor.