The programmable logic controller or commonly known as PLC is a specialized computer logic program that is used to control processes and machines. It shares common terms with traditional personal computers like memory, a central processing unit, communications, and software.
Not like personal computers though, the PLC is designed to survive the brutal and rugged industrial environment and at the same time be very flexible in how they interact with inputs and outputs in the real world. The parts that make a programmable logic controller work can be divided into different core areas.
The rack and power supply, the CPU or central processing unit, and the I/O or input and output section. PLCs come in many sizes and shapes. There are some PLCs that are too small, and they can fit in your shirt pocket. PLCs that are more involved in controlling systems will require a more massive rack.
Smaller programmable logic controller, also known as bricks, is designed with a fixed input and output points. For our consideration, we will look at the more modular rack-based PLC system. It is called modular because they can accept a lot of different types of input and output modules that can slide into the rack and instantly plugin. If you want a thorough explanation on how PLC works, you can always check PLCs for dummies and other materials found on the internet.
Rack and power supply
Let us start by removing all of the modules which will leave us with a naked programmable controller with only the rack and the power supply. The frame is a part that will hold every component together. Depending on the control system’s needs, it can be ordered in different shapes and sizes to carry more modules effectively.
Like a spinal column in a human body, the rack has a backplane at the back which allows all the cards (spinal cord) to communicate properly with the central processing unit (the brain). The power supply (electrical impulse) is placed in the rack as well and a regulated direct current power all the modules that is plugged into the frame. The usual power supply works well with a 24V DC or a 120V AC power source.
The Central Processing Unit or the CPU
PLCs brain is the CPU or the Central Processing Unit module. The module can be found behind the power supply slot. Manufacturers offer a variety of CPUs, based on the needs of the system. The CPU consist of memory, microprocessor, and integrated circuits to control the communications, logic, and monitoring.
The Central Processing Unit has a different operating system mode. In the programming mode, it will accept the downloaded logic from a personal computer. The CPU is placed in a run mode so that it can proceed with the program and operate the whole process. Since PLC is a dedicated system, it will only process one program over and over.
One cycle through the system is called scan time, and it involves reading other module’s inputs, execute logic based on the inputs and update the outputs according to the data it receives. The scan time happens quickly, at least 1/1000th of a second. The CPU’s memory stores the program while holding the status of the inputs and outputs and provide a way to store the values. To know more about Central Processing Units, you can visit https://en.wikipedia.org/wiki/Central_processing_unit.
Input and output system
The Input/Output system provides the physical connection between the PLC and the equipment. Opening the doors on the I/O slot card reveals a terminal strip where devices will be connected. There are a lot of different kinds of Input/Output cards that serve to condition the I/O so that the CPU can use it to run the logic. It is a matter of determining the inputs and outputs that are needed, fill up the rack with the right cards and address them correctly in the central processing unit’s program.
Input devices consist of analogue or digital devices. Digital input cards handle devices that give provide the signal that is either off or on like the pushbutton, sensors, selector switch, or the limit switch. The analogue input card will convert a current or voltage into a digital equivalent data that can be understood by the system’s CPU (an example of this would be a signal that can be found anywhere from 0 mA to 20 mA). The best example of analog devices is flow meters, thermocouples for reading temperature, and pressure transducers.
Output devices consist of analogue or digital types. An analogue output card converts the digital number sent by the central processing unit to the real world using current or voltage. A digital output either turns the device off or on like the lights, small motors, relays, and the LEDs. A typical output signal can range from 0 VDC to 10 VDC or 4 mA to 20 mA. They are used to drive flow controllers, position controls, and pressure regulators.
In today’s modern world, a personal computer with a dedicated software from programmable logic controller manufacturer is used by most companies to program a PLC. The most widely used types of programming software are called ladder logic.
Want to read more about ladder logic? You can click here to know more.
This ladder logic applies a lot of symbols instead of words to emulate real-world relay logic system, which is derived from PLCs history. The symbols are then interconnected by specific lines to indicate the current flow through the relays like coils and contacts.
Through the years, the number of symbols used has increased to provide quality and high-level functionalities. If the program is completed, it will look like a real ladder, but in reality, it only represents an electrical circuit. The left side and the right-side rails indicate the ground and positive of a power supply