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 slot in output modules, which in turn control various output field devices such as solenoids, motor starters, etc.
Figure 1.3: Input modules
1.3.3Output Modules
Output modules provide the physical connection between the processor unit and output field devices to facilitate the PLC processor sending the control signals to output devices (Figure 1.4). Output modules perform the following functions:
•Provide connection terminals for output field devices
•Receive control data from the PLC processor and pass on to control output field devices
•Give isolation between the PLC system and field voltages
Figure 1.4: Output modules
Figure 1.5: Hand-held programming device
Figure 1.6: Programming terminal
1.3.4Programming Devices
The main purpose of a programming device is to enter the control programs to PLCs. There are several ways you can perform this task. The available programming devices you can use for a particular PLC vary from manufacturer to manufacturer. There are three basic types of programming devices: hand-held programmers, programming terminals, and personal computers.
Hand-held programmers are an inexpensive and portable way to program small PLCs (Figure 1.5). They have a display and keyboard with numeric keys and programming instruction keys for input. Programming terminals are used in larger PLCs. They are proprietary products from PLC manufacturers and are more costly. They look like a combination of computer monitor and keyboard (Figure 1.6). One programming terminal can serve multiple PLCs.
Personal computers can work together with PLC programming software to create PLC programs for any PLC types. Each brand name PLC has more than one type of programming software for you to prepare programs; the software allows you to download programs to PLCs and monitor the status of the systems.
1.4 PLC Operation Principle
The successful implementation of a PLC project depends on integrating field devices, the PLC program, and the PLC system. The keys to integrate and coordinate these three elements are interface wiring and address assignment (Figure 1.7).
Figure 1.7: PLC operation principle
1.4.1Input Wiring
There are two types of interface wiring: input wiring and output wiring. Input wiring connects input devices to the input modules. There are a fixed number of connection terminals in each input module. Each connection terminal has its assigned address in the PLC memory (Figure 1.8). The actuation of an input device generates a digital ON (1) or OFF (0) signal to appear at its connected terminal of the input module (Figure 1.9). The PLC processor picks up the signal while it reads the input module and stores the signal in the assigned memory address. In this way, any status change in input devices can be immediately reflected on the PLC memory.
Figure 1.8: PLC memory address for input module
Figure 1.9: Signals produced from input module
1.4.2Output Wiring
Similarly, output wiring connects output devices to the output modules. Each connection terminal has its assigned address in the PLC memory. The PLC processor sends the digital signal to connection terminals