Programmable Logic Controllers (PLCs) are essential components in many industrial and manufacturing processes. They are used to control and monitor a wide range of equipment and machinery including conveyor belts, pumps, motors and other automated systems. PLCs are made up of several different components each with a specific function. In this article, we will explore the different components of a PLC and their functions including the central processing unit (CPU), input/output (I/O) modules and power supply. Understanding these components and their functions is crucial for anyone working with PLCs or involved in the maintenance and troubleshooting of industrial equipment.
Main PLC Components
The major sections of a programmable logic controller (PLC) system and their basic functions are:
1. Power supply
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 the 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 of 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.
2. Central processing unit (CPU)
The central processing unit (CPU) is also called the processor and associated memory from 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.
3. Input module
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.
4. Output module
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.
5. Programming device
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.