PLCs are digital electronic apparatus with a programmable memory for storing instructions to implement logic, sequencing, timing, counting, and arithmetic functions. Since its inception in 1969, PLC technology has staged through six generations of development. Memory size and added functions have been increased from one generation to the next.
There are four major components in a PLC: processor unit, input modules, output modules, and programming device. A processor unit is the brain of the PLC that consists of three elements: central process unit, memory, and power supply. Input modules provide the physical connection between the processor unit and input field devices such as limit switches. Output modules provide the physical connection between the processor unit and output field devices such as motor starters. A programming device allows creating PLC programs and entering the programs to PLCs.
The basic operation principle of a PLC is program scanning. A PLC processor scans a program in a cyclic manner, starting from left to right at the top rung, then proceeding to the next rung. During scanning, the processor simultaneously updates the status of input and output instructions in both the program and the memory, as well as interacting with input and output modules.
The benefits of using PLCs are very obvious. They are flexible and programmable, reliable, and cost saving. PLCs provide ease of installation and implementation, as well as ease of maintenance and troubleshooting. There are five categories of PLCs according to their size and configuration, including micro PLCs, small PLCs, medium PLCs, large PLCs, and very large PLCs.
1.1 Definition of PLC
The term PLC is the abbreviation for Programmable Logic Controller. PLCs were initially called Programmable Controllers (PCs). The use of this term, PCs, caused some confusion when personal computers (PCs) became popular. To avoid this confusion, PCs are referred to as personal computers and PLCs programmable logic controllers. 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.”
More specifically, a PLC can be considered as an industrial computer that is especially designed for use in industrial, rugged environments. It performs the following functions:
•Receives and interprets the signals from various input switches and sensors
•Implements the control logics designed in the form of programs
•Outputs the control signals to activate the power devices such as motor starters, solenoids, contactors, etc.
1.2 History of PLCs
The inception of PLCs resulted from the necessity and the emergence of computer technology. Electromechanical relays that provide logic controls for industrial systems had been successfully implemented for many generations. The main problems with those hard-wired, relay-based systems include:
•Lack of flexibility of reprogramming
•Limit to relay types of applications
•Susceptive to mechanical failure due to physical contact and wear
•Prone to wiring errors
•Difficulty of trouble shooting
•Limited to small-to-medium size of control systems
•Costly to implement when the size of control systems increase
GM’s Hydromatic Division realized the need for using a solid-state system with computer flexibility to replace hard-wired relay control panels, which were huge, costly, and inflexible. In 1968, Hydromatic defined the design specifications for the first programmable logic controller. Some of the initial specifications are outlined below:
•The new control system had to be a solid-state device with the flexibility of a computer.
•The system had to sustain an industrial environment (vibration, heat, dirt, etc.).
•The system had to be re-programmable and reusable for other tasks.
•The input and output interfaces had to be easily replaceable.
•The system had to be easily programmed and maintained by plant electricians and technicians.
•The system had to be cost competitive with the use of hard-wired relay systems.
GM solicited interested companies to develop a system that met the above design specifications. Richard E. Morley, founder of the Modicon Corporation, built the first practical PLC in 1969. Since then, PLC technology has steadily advanced, in both hardware and software, with the computer technology. We can grossly classify the PLC development into six generations. The year, added functions, advancement in hardware and software, and the applications of each generation are summarized in Table 1.1.
1.3 PLC Components
A typical PLC system consists of four major components: processor unit, input modules, output modules, and programming device (Figure 1.1).
1.3.1Processor Unit
The processor unit is the brain of the PLC. It consists of three parts: central process unit (CPU), memory, and power supply (Figure 1.2). They are briefly described below:
Table 1.1: PLC Development
Figure 1.1: PLC components
Figure 1.2: Processor unit
CPU: executes operating system, manages memory, receives inputs, implements ladder logic instructions, outputs control signals to field devices, and handles communications with other devices.
Memory: has two types — system memory and user memory. The operating system is stored in a Read-Only Memory (ROM) type of system memory for converting the ladder diagram to instructions that the CPU can recognize. User memory is normally of Random Access Memory (RAM) type. It is used to (1) store status of inputs and outputs, (2) store contents of variables for timers and counters, and (3) reserve spaces for the processor work area.
Power Supply: produces low voltage DC power used by the logic circuits of the processor and input and output modules.
1.3.2Input Modules
Input modules provide the physical connection between the processor unit and the input field devices. They detect and transmit the input signals from input field devices to the processor (Figure 1.3). Using input modules, the PLC processor can sense status conditions such as Open or Closed, and actuated or not-actuated switches, as well as measure process quantities such as pressure, temperature, flow, and position. Using the input data gathered from input modules, the PLC processor processes the data according to the instructions in the PLC program, then issues control commands to the proper
