What are the characteristics of the design process of PLC control system and relay control system?

Comparison between PLC and relay control system

1 control mode:

The control of relay is realized by hardware wiring, and the combination of the series connection of relay mechanical contacts or the hysteresis action of parallel pole time delay relay forms control logic, which can only complete the established logic control.

PLC adopts storage logic, and its control logic is stored in memory in the form of program. To change the control logic, just change the program, which is called soft wiring.

2 control speed

The relay control logic relies on the mechanical action of the contact to realize control, and the working frequency is low, millisecond, and the mechanical contact has jitter.

PLC is controlled by a semiconductor circuit controlled by program instructions, with high speed, microsecond level, strict synchronization and no jitter.

3 delay control

The relay control system relies on the lag effect of the time relay to realize the delay control, but the timing accuracy of the time relay is not high, which is greatly influenced by the environment and is difficult to adjust the time.

PLC uses semiconductor integrated circuit as timer, and clock pulse is generated by crystal oscillator, which has high precision, convenient adjustment time and is not affected by environment.

What are the advantages of PLC compared with trunk lines?

1, powerful and cost-effective.

A small PLC has hundreds of programming elements for users to use, which is powerful and can realize very complex control functions. Compared with the relay with the same function, it has higher cost performance. Programmable controller can realize decentralized control and centralized management through communication network.

2. Complete hardware, humanization and strong adaptability.

Programmable controller products have been standardized, serialized and modularized, and are equipped with a full range of hardware devices for users to choose from. Users can configure the system flexibly and conveniently to form systems with different functions and scales. The installation and wiring of the programmable controller are also very convenient. Usually, terminals are used to connect external wiring. PLC has strong load capacity and can directly drive general solenoid valves and AC contactors.

3, high reliability and strong anti-interference ability

A large number of intermediate relays and time relays are used in the traditional relay control system. Failure is easy to occur due to poor contact. PLC replaced a large number of intermediate relays and time relays with software, leaving only a small amount of hardware related to input and output. Wiring can reduce110-1100 of relay control system, and the faults caused by poor contact are greatly reduced.

PLC adopts a series of software and hardware anti-interference measures with strong anti-interference ability, and the average time between failures reaches tens of thousands of hours. It can be directly used in industrial production sites with strong interference. PLC has been recognized as one of the most reliable industrial control devices by the majority of users.

4. The workload of system design, installation and debugging is less.

PLC replaces a large number of intermediate relays, time relays, counters and other devices in relay control system with software functions, which greatly reduces the workload of design, installation and wiring of control cabinet.

The ladder diagram program of PLC generally adopts sequential control design method. This programming method is very regular and easy to master. For complex control systems, the design time of ladder diagram is much less than that of relay system circuit diagram.

The user program of PLC can be simulated and debugged in the laboratory. The input signal is simulated by a small switch, and the state of the output signal can be observed by the LED on the PLC. After the installation and wiring of the system is completed, the problems found in the field debugging can generally be solved by modifying the program, and the debugging time of the system is much less than that of the relay system.

5, the programming method is simple

Ladder diagram is the most widely used programming language of programmable controller, and its circuit symbols and expressions are similar to relay circuit schematic diagram. Ladder diagram language is intuitive, easy to learn and understand. Electrical technicians who are familiar with relay circuit diagram can be familiar with ladder diagram language in just a few days and use it to write user programs.

Ladder diagram language is actually a user-oriented high-level language. When the programmable controller executes the ladder diagram program, it is "translated" into assembly language by the interpreter and then executed.

6. Less maintenance workload and convenient maintenance.

PLC has low failure rate and perfect self-diagnosis and display functions. When the PLC or external input devices and actuators fail, the cause of the failure can be quickly found out according to the resident situation provided by the LED or programmer on the PLC, and the failure can be quickly eliminated by replacing the module.

7. Small size and low energy consumption

For complex control systems, after using PLC, a large number of intermediate relays and time relays can be reduced. The size of small PLC is equivalent to several relays, so the size of switch cabinet can be reduced to the original1/2-110.

The wiring of PLC is much less than that of relay control system, so it can save a lot of wiring and accessories, reduce a lot of installation and wiring man-hours and reduce a lot of expenses. Study hard and do it comfortably.