They are digital computers used to control mechanical automation in industries by monitoring outputs and inputs. They make logic-based commands for electro-mechanical processes and industries. They are made in a special way to survive unfriendly conditions such as moisture, dust cold, and heat.
How a PLC is developed
Inside the PLC, there is a programmed microprocessor which uses computer language. The program is usually developed on another computer then sent to the controller via a cable. There is a specific language known as Ladder Logic that is used in the programming of a PLC.
Components of a Programmable Logic Controller
A PLC has a well-designed hardware system that makes is highly effective in its work areas. These consist of a CPU, microprocessor, input and output sections, power supply unit and memory. Each of this component has its functions that keep the controller at work.
- CPU- This is also known as “the heart of the controller”. It is responsible for keeping the computer in check as well as performing functions such as arithmetic operations, managing computer interfaces, and logic operations.
- Microprocessor- This device is used to feed the computer memory with the program.
- Memory- Data is usually stored in this part. The system ROM stores the fixed data used by the CPU. There is also a Random Access Memory (RAM) that stores output and input data, time values and internal device’s functioning.
- Input and Output Devices- PLSS use either analog or digital devices. These includes visual systems and robots. The output devices are tools such as solenoid valves and motors, lights and pumps.
- Power Supply Unit- Many PLCs work under the normal power of about 220 VAC or 24 VDC. However, there are certain controllers that work under isolated power supply.
- System Bus- There are four system buses that maintain the flow of digital signals within the controller. These buses are
- System Bus
- Data Bus
- Control Bus
- Address Bus
Types of Multiple Controller Systems
There are 3 types of PLCs.
Large Control Systems
They are made to suit your personal automation requirements. These include modular architecture, network options, and other Input/ Output functions. They are the most powerful and deliver top-range services if used effectively. They are made under maximum supervision in order to provide maximum performance and reliability.
Micro and Nano Control Systems
They are designed to provide a solution to basic control needs. They are used on simple machines to perform light duties such as compact packaging, simple logic, and control timing and relay replacements. They are also ideal for applications such as security systems, conveyor automation, and parking and building lot lighting.
Small Control Systems
They are ideal for mid-range uses. Without large systems, they give flexibility and features that maintain maximum performance. They are safety certified and standard for use, with modular, packaged and chassis-based designs. They perform activities such as building automation, batch processing, and machine control.
PLC Selection Criteria
When choosing a PLC, there are various requirements that you should consider.
They affect the performance of a PLC. Normal PLCs should operate between 0 to 55 C. The number of I/O connections are determined by the number of analog and discrete devices.
Analogue devices require little program memory than digital devices. These devices include; sensors, current, voltage and temperature meters.
It is important to check the RAM and ROM specifications. Timers and counters store points in RAM. For long time data storage, there should be an appropriate CPU memory.