Programmable logic controllers are solid state system designated to perform logic functions previously accomplished by components such as electromechanical relays, drum switches, Mechanical timers/counters etc. for the control and operation of manufacturing process equipment and machinery.
Even though the electromechanical relay has served well for many generations, often under adverse conditions, the ever increasing sophistication and complexity of modern processing equipment requires faster acting more reliable control functions than electromechanical relays and /or timing devices can offer.
The requirements of highly specialized, high speed manufacturing processes has created a demand for smaller, faster acting, more reliable control devices.
The National Electrical Manufacturing Association (NEMA) defines a programmable controller as follows:
A programmable controller is a digital electronic apparatus with a programmable memory for storing instructions to implement specific functions, such as logic, sequencing, timing, counting, and arithmetic to control machines and processes.
A PLC consists of the following
- Processor Unit
- Program Memory
- Power Supply
- Input/output Section
- Display Device
Block Diagram of PLC
The processor unit houses the processor, which is the decision maker or the brain of the system. The brain is a microprocessor based system that is designed so that the user can enter the desired program in relay ladder logic.
The processor then makes all the decisions necessary to carry out the user program, based on the status of the inputs and outputs for control of a machine or process. It can also perform arithmetic functions, data manipulation, and communications between the local input/output section, remotely located I/O sections, and /or other networked PLC systems.
The processor memory consists of hundreds or thousands of locations that are referred to as words. Each word is capable of storing binary data in the form of binary digits or bits. A binary digit, like a binary signal, can only be a 1 or 0. The number of bits that a word can store will depend on the system or PLC. Words can be made up of 32 bits, 16 bits or 8 bits.
The 16 bit word is most common, in order to locate the information stored in each word, each word is numbered or given an address. Addressing words in the memory serves the same function as the addresses used for homes or apartments.
The power supply is necessary to convert 120 or 240 volts AC voltages to the low voltage DC required for the logic circuits of the processor, and for the internal power required for the I/O modules. The power supply can be a separate unit or an integral part of the processor depending on the manufacturer.
Input/ Output Section:
The input/ output section consists of input and output modules. The number of input and output modules necessary is dictated by the requirements of equipment that is to be controlled by a PLC.
The I/O section contains the circuitry necessary to convert input voltages of 120-240V AC or 0-24V DC, etc. from discrete input devices to low-level DC voltages (typically 5V) that the processor uses internally to represent the status or condition.
The I/O section can also convert 4-20 mill ampere (mA) input signals to low-level DC voltages for the processor. Similarly, the output module changes low-level DC signals from the processor to 120-240V AC or DC voltages required to operate the discrete output devices.