A programmable logic controller i.e. PLC is a specialized computer that is used to control machines and processes. A lot of terms that we use for PC’s like central processing unit, memory, software, etc are also used for PLC’s. The PLC’s are designed in such a way so that it can survive in a rugged industrial atmosphere. It is also very flexible in interfacing with inputs and outputs to the real world. If the system requires more controls, a larger PLC Siemens racks will be required. Smaller PLCs have their fixed I/O points.
The essential components that make a PLC work are:
- The power supply and rack
- The central processing unit (CPU)
- The input/output (I/O) section
The Power Supply and Rack
If we remove all the modules then we are left with only naked PLC that is equipped with power supply and rack only. The kind of PLC we need depends on the needs of control system. Just like human spines, rack is well designed with a backplane at the rear. This helps the cards to communicate with the CPU. The power is then supplied to the rack and a regulated DC power is then supplied to other modules that are plugged into the rack. The most commonly used power supply is 120 VAC or 24 VDC.
The CPU
CPU is not only the brain of PC but is also called brain of the PLC. This module exists in the slot that is provided beside the power supply. There are different kinds of CPUs available depending on the need of the system. The basic components of CPU are microprocessor, memory chip and integrated circuits that allow control, monitoring and communication. It also has different modes like programming mode and run mode. Programming mode accepts logics downloaded from a PC whereas in the run mode it executes and operates the process. The CPU stores the program while holding the status of I/O.
I/O System
The I/O system is used as a physical connection that holds the equipment and the PLC together. The Opening doors on an I/O card indicate a terminal strip that shows where the devices connect. There are different kinds of I/O cards that suits different types of input or output and enable CPU to use its logic. It determines the kind of input and output needed, filling racks with the most appropriate cards and then addressing them promptly in the CPUs program.
Inputs
Input devices are mainly composed of digital or analog devices. A digital input card can handle discrete devices that give signal as a pushbutton, limit switch, or selector switches. An analog input card is used to convert a voltage or current into a digitally equivalent number.
Outputs
Output devices also consist of digital or analog types. A digital output card is used to turn on or off a device such as lights, LEDs, etc. An analog output card is used to convert a digital number that it receives from CPU to its real voltage or current. Output signal ranges from 0 – 10 VDC or 4 – 20 mA. It is used to drive mass flow controllers, pressure regulators and position its control.
Programming a PLC
The most commonly used form of programming is the ladder logic. This form makes use of symbols instead of words. The symbols are interconnected with each other with the help of lines. This is used to indicate the flow of current through contacts and coils. The completed program seems to be like a ladder which can be downloaded from the PC and transferred to the PLC using a data cable.
