In this era of automation, PLC (Programmable Logic Controller) is like a hero behind the scenes, quietly supporting the beam of industrial production. Although PLC is the mainstay of industrial automation. But don't be intimidated by its seemingly complicated appearance. In fact, mastering the basics of PLC is like getting a golden key to open the door to industrial automation.
Today, let us explore those who learn PLC must know the basics, so that you in the automation of the road, not only go fast, more stable.
Basic interface components of PLC
The PLC is not just a simple control box, it contains a variety of high-tech components. In addition to the core CPU, memory and communication interfaces, it is equipped with a variety of interfaces that are closely related to the industrial field:
Input Interface: Receives signals from the controlled device and passes these signals to the internal circuitry via opto-coupled devices.
Output Interface: Transmit the execution result of the programme through the photocoupling device to control the action of external devices.
2. Basic unit components of PLC
A PLC system is like a miniature computer that consists of several key components:
CPU: The brain of the PLC, directing all operations, processing data and executing programmes.
Memory: Holds system and user programmes and data.
I/O interface: bridge between PLC and field devices, accepts signals and outputs control commands.
Communication Interface: A window for exchanging information with other devices (e.g., monitors, printers).
Power supply: the heart that provides energy for the entire system.
3. Types of switching output interfaces for PLCs
Depending on the need, PLC output interfaces can be divided into several types, each with its own unique characteristics:
Thyristor output type: Suitable for AC loads, fast response speed, suitable for high frequency operation.
Transistor Output Type: Designed for DC loads with equally fast response time for high frequency operation.
Relay output type: can control both AC and DC loads, but slower and suitable for low frequency applications.
4. Types of PLC structures
The structural design of PLCs can be divided into several types, each with its own advantages and disadvantages:
Integral: All components in one chassis, compact and suitable for small applications.
Modular: each part is divided into independent modules, easy to expand and maintain, suitable for medium and large systems.
Stacked: Combines the advantages of monolithic and modular, flexible configuration and compact size.
5. PLC scan cycle
The PLC operates on a certain time cycle, which is called the scan cycle. The scan cycle consists of the following phases:
Internal processing: system self-tests and internal data processing.
Communication services: Handles communication with other devices.
Input processing: Acquisition of input signals.
Programme execution: Execution of the user programme according to the input signals.
Output processing: Outputs the execution result to the control device.
The length of the scan cycle depends on the speed of the CPU, the hardware configuration and the complexity of the user programme.
6. User programme execution mode for PLCs
The PLC uses cyclic scanning to execute the user programme, which consists of three main phases:
Input Sampling: Reads all input signals.
Programme execution: Execute the user programme in sequence.
Output Refresh: Outputs the results of programme execution to the control device.
7. PLC vs. relay control systems
PLC has the following significant advantages over traditional relay control systems:
Flexibility: Control is achieved through programming, making it easy to change and extend control functions.
Anti-interference ability: adopting serial working mode reduces the influence of electromagnetic interference.
Fast: PLC contacts are equivalent to microsecond triggers, much faster than mechanical contacts.
Timing and Counting Function: PLC has high precision timing and counting capabilities that are unmatched by relay systems.
Reliability: PLC adopts microelectronic technology with high reliability and self-test function, which can detect faults in time.
8. PLC output response hysteresis phenomenon
Since the PLC uses a cyclic scanning method with centralised sampling and outputs, input signals are only read in during the input sampling phase, and the results of program execution are only output during the output refresh phase. This design may result in a lag in output response. To improve the I/O response speed, time-sharing sampling and refreshing can be used, or an intelligent I/O interface can be used.
9. How to choose the right PLC
When selecting a PLC, there are several aspects to consider:
Model selection: Selection is based on factors such as structural form, functional requirements, response speed and reliability.
Capacity Selection: Selection is based on the number of I/O points and user storage capacity.
I/O module selection: including the choice of switching and analogue modules, as well as the need for special function modules.
Power supply modules and programmers: these ancillary devices are also important considerations when selecting a PLC.
10. Features of centralised sampling and centralised outputs
The PLC works by both centralised sampling and centralised output:
Concentrated sampling: During a scanning cycle, the input signal is only captured during the input sampling phase, and the inputs are blocked at other times.
Centralised Output: During a scanning cycle, the output state is updated only during the output refresh phase, and the output state remains unchanged at other times.
This approach improves the system's immunity to interference, but it can also lead to lagging response of the inputs/outputs.
11. How PLCs work
The PLC operates with centralised sampling, centralised output and cyclic scanning. Its features include:
Concentrated sampling: The input signal is captured only in the input sampling stage during a scan cycle.
Focused Output: Updates the output status only during the output refresh phase during a scan cycle.
Cyclic Scanning: The PLC repeatedly performs multiple operations in a certain order, running week after week.
12. Components and operating principles of electromagnetic contactors
Electromagnetic contactor consists of electromagnetic mechanism, contacts, arc extinguishing device, release spring mechanism, bracket and base and other parts. Its working principle is as follows:
When the electromagnetic coil is energised, a magnetic field is generated, causing the static iron core to attract the armature and drive the contacts into action. The normally closed contact is disconnected and the normally open contact is closed.
When the coil is de-energised, the electromagnetic force disappears, the armature returns to its original position under the action of the releasing spring, and the contacts return to their original state.
13. What is a programmable logic controller (PLC)?
A PLC is an electronic device designed specifically for industrial environments with programmable memory for storing and performing operations such as logic operations, sequence control, timing and counting. It controls various types of machinery or production processes through digital or analogue inputs and outputs.PLCs and their peripherals are designed to be easily integrated with industrial control systems and have the flexibility to expand their functionality.
14. Differences in operating principles between PLC systems and relay contactor systems
PLC systems and conventional relay contactor systems differ in several ways:
Component devices are different: PLCs use microelectronic devices, while relay systems use mechanical contacts.
Number of contacts: PLC contacts are programmed and infinitely expandable, whereas relays have a limited number of contacts.
Control method: PLCs are controlled through software programmes, while relay systems rely on hardware connections.
PLCs have become the modern industrial control system of choice due to their flexibility, speed and reliability. With these basics in hand, you've taken the first step towards becoming a PLC expert. If you have more curiosity and questions about PLCs, take a deeper dive and you'll find a whole new world of automation waiting for you.
