A programmable logic controller (PLC) is an industrial-grade computer that is capable of being programmed to perform control functions. The programmable controller has eliminated much of the hard wiring associated with conventional relay control circuits. Other benefits PLCs provide include easy programming and installation, fast control response, network compatibility, troubleshooting, and testing convenience, and high reliability. Programmable logic controllers are now the most widely used industrial process control technology.
The major sections of a programmable logic controller system and their basic functions are as follows:
The power supply of a PLC system converts either AC line voltage or, in some applications, a DC source voltage, into low-voltage DC required by the processor and I/O modules. In addition to voltages required for the internal operation of these components, the power supply in specific applications may provide low-voltage DC to external loads as well.
Power supplies are available for different input voltages including 120 V AC, 240 V AC, 24 V AC, and 24 V DC. The required output current rating of the power supply load on the power supply is based on the type of processor, the number and types of input/output (I/O) modules, and any external loads that may be required to be connected to the power supply.
The internal logic of a PLC generally operates on 5 to 24 volts DC, depending on the type of controller. This voltage must be free of voltage spikes and other electrical noise and be regulated to within 5% of the required voltage value. Some manufacturers of PLCs build a separate power supply, and others build the power supply into the CPU.
Central processing unit (CPU)
The central processing unit (CPU), also called processor, and associated memory form the intelligence of a PLC system. Unlike other modules that simply direct input and output signals, the CPU evaluates the status of inputs, outputs, and other data as it executes a stored program. The CPU then sends signals to update the status of outputs. Processors are rated as to their available memory and I/O capacity, as well as the different types and number of available programming instructions.
The CPU often has a key located on the front panel. This switch must be turned on before the CPU can be programmed. This is done to prevent the circuit from being changed accidentally. Plug connections on the CPU provide links for the programming terminal and I/O racks.
CPUs are designed so that once a program has been developed and tested, it can be stored on some type of media such as tape, disk, CD, or other storage devices. As a result, if a CPU fails and has to be replaced, the program can be downloaded from the storage medium. This eliminates the time-consuming process of having to reprogram the unit by hand.
Input modules enable the PLC to sense and control the system it is operating. The prime function of an input module is to take the input signals from the field devices switches or sensors and convert them to logic signals that can be used by the CPU. In addition, the input module provides electrical isolation between the input field devices and the PLC. The types of input modules required to depend on the types of input devices used. Some input modules respond to digital inputs, also called discrete inputs, which are either on or off. Other input modules respond to analog signals that represent conditions as a range of voltage or current.
Output modules control the system by operating motor starters, contactors, solenoids, and the like. They convert control signals from the CPU into digital or analog values that can be used to control various output field devices (loads). They also provide electrical isolation between the input field devices and the PLC.
The programming device is used to enter or change the PLC’s program or to monitor or change stored values. Once entered, the program is stored in the CPU. A personal computer (PC) is the most commonly used programming device and communicates with the CPU via a communications port.
Small-size fixed PLCs, such as the Micro PLC are stand-alone, self-contained units.
A fixed controller consists of a power supply, processor (CPU), and fixed number of input/outputs (I/Os) in a single unit. They are constructed in one package with no separate, removable units. The number of available I/O points varies and can usually be increased by adding expansion modules. Fixed controllers are small and less expensive but limited to smaller, less complex applications.
A modular PLC is made up of several different physical components.
It consists of a rack or chassis, power supply, processor (CPU), and I/O modules. The chassis is divided into compartments into which separate modules can be plugged.
The complete assembly provides all of the control functions required for a particular application. This feature greatly increases your options and the system’s flexibility. You can choose from a variety of modules available from the manufacturer and mix them any way you desire.