The reliability problem of the automatic control system has been studied in depth, and a method to improve the reliability of the system is proposed. Practice has proved that the adoption of these methods is effective in improving the reliability of the system.
1 Introduction
The programmable controller is widely used in the industrial control field due to its strong anti-interference ability, high reliability, simple programming and high performance-price ratio.
As the central control unit, the industrial year machine is equipped with configuration software. It uses a large-screen real-time monitoring interface to realize dynamic display, data modification, fault diagnosis and automatic alarm of each control point. It can also display the historical event record of the query. Accumulated running time of components, process flow chart of each device, structure diagram of each device, etc. The serial communication mode is used for data exchange between the central control unit and the lower-position PLC. Usually, the 485 twisted pair communication mode is selected within 1000m, the optical fiber communication can be selected at a longer distance, and the wireless communication mode can be selected for a longer distance. The lower computer selects PLC control. According to the number of control objects, the range of control objects can be controlled by one or more PLCs. The data exchange between PLCs uses internal link registers to realize data exchange and sharing. Because PLC has high reliability for on-site real-time monitoring, and programming is simple and flexible, it has received more and more attention.
2. The main reason for the decrease in control system reliability
Although the industrial control unit and the programmable controller itself have high reliability, if the switching signal input to the PLC is wrong, the analog signal has a large deviation, and the actuator controlled by the PLC output port does not act as required. These can make the control process go wrong, resulting in irreparable economic losses.
The main reasons that affect the input of the PLC signal to the field are:
1) causing short-circuit or open circuit of the transmission signal line (due to mechanical pull, the line itself is aging, especially the rat), when the transmission signal line fails, the field signal cannot be transmitted to the PLC, causing control error;
2) The mechanical contact is shaken. Although the field contact is only closed once, the PLC thinks that it has been closed many times. Although the hardware adds a filter circuit, the software adds a differential instruction. However, since the PLC scan cycle is too short, it may still count, accumulate, An error occurred in the instruction such as shifting, and an error control result occurred;
3) On-site transmitter, the mechanical switch itself is faulty. If the contact is not good, the transmitter will reflect that the non-electricity deviation of the site is large or cannot work normally. These faults will also make the control system not work properly.
The main reasons that affect the execution of the agency are:
1) The contact of the control load cannot be reliably operated, and the PLC issues an action command, but the actuator does not act as required;
2) Control the inverter to start, due to the fault of the inverter itself, the motor with the inverter does not work as required;
3) All kinds of electric valve and solenoid valve can't be opened, the switch can't be closed, because the actuator can't operate according to PLC's control requirements, the system can't work normally, and the system reliability is reduced. In order to improve the reliability of the entire control system, it is necessary to improve the reliability of the input signal and the accuracy of the actuator action. Otherwise, the PLC should be able to find the problem in time, and prompt the operator with an alarm method such as sound and light to eliminate the fault as soon as possible and make the system safe. Work reliably and correctly.
3. Well-designed fault alarm system
In the design of the automatic control system, we designed a three-level fault display alarm system. The first level is set on the control panel of each control panel. The indicator light indicates the normal operation and fault condition of the equipment. When the equipment is in normal operation, the corresponding indicator light is on. The indicator flashes at 1 Hz when the device is faulty.
In order to prevent the lamp bulb from being damaged, the device's working condition cannot be correctly reflected. The fault reset/light test button is specially set. When the system is running for any time, press the button continuously. All the indicators should be lit. If there is an indication, etc. If the indicator light is broken, it should be replaced immediately. After the button is reset, the indicator still displays the working status of the device according to the original working status.
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