Way to improve the reliability of PLC automatic control system - Database & Sql Blog Articles

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The method to improve the reliability of the plc automatic control system is as follows:

First, the main reason for the reliability of the control system

Although the industrial control unit and the programmable technology ">the 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 is not pressed. Requiring actions, these may cause errors in the control process, resulting in irreparable economic losses. The main reasons that affect the input of the PLC signal to the field are: 1. Short-circuit or open circuit of the transmission signal line (due to mechanical pull, the line itself is aging, the connection is Loose off, etc., when the transmission signal line fails, the field signal can not 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 it is closed many times, although the hardware adds The filter circuit, the software increases the differential instruction, but because the PLC scan cycle is too short, it may still be in the error of counting, accumulating, shifting, etc., and the error control result occurs. 3. The field transmitter, the mechanical switch itself fails, such as Poor contact of the contacts, the transmitter reflects that the site has a large deviation of the power or does not work properly, etc., these faults will also

1. The contact of the control load cannot be reliably operated. Although the PLC issued an action command, the actuator did not act as required.

2. Control the start of the inverter. Due to the fault of the inverter itself, the motor with the inverter does not work as required.

3, the various electric valve, solenoid valve can not be opened, the off can not be closed, because the actuator failed to follow the PLC control requirements, so that the system can not work properly, reducing system reliability. 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.

Second, the design of a complete 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 indicator light bulb from being damaged, it can not correctly reflect the working condition of the equipment. The fault reset/light test button is specially set. When the system is running for any time, press the button for 3s, all the indicators should be lit. If there is an indication, etc. If the indicator 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. The level 2 fault display is set on the large-screen monitor in the central control room. When the device fails, there is a text display fault type, the corresponding device on the process flow diagram flashes, and the fault is recorded in the historical event table. The level 3 fault display is set in the signal box of the central control room. When the equipment fails, the signal box will prompt the staff with sound and light alarm to deal with the fault in time. When dealing with faults, the faults are classified. Some faults require the system to stop running, but some faults have little effect on the system work. The system can run with faults, and the faults can be eliminated during operation, thus greatly reducing the overall system stop. Run time to improve system reliability operation level.

Third, the input signal reliability research

To improve the reliability of the on-site input to the PLC signal, first select a transmitter with high reliability and various switches to prevent short-circuit, open circuit or poor contact of the transmission signal line for various reasons. Secondly, digital filtering program is added during programming to increase the credibility of the input signal. The digital signal filtering can adopt the following program design method. After the field input contact, a timer is added. The timing time is determined according to the contact jitter condition and the response speed of the system, generally in the order of several tens of ms, so that the contact is surely closed. After that, there are other responses.

The analog signal filtering can adopt the following programming method, and the on-site analog signal is continuously sampled three times, and the sampling interval is determined by the A/D conversion speed and the rate of change of the analog signal. The three sampling data are stored in the data registers DT10, DT11, DT12 respectively. When the last sampling is finished, the data comparison, data exchange instruction, and data segment comparison instruction are used to remove the maximum and minimum values, and the intermediate value is retained as the current sampling result. In the data register DT0.

In practical applications, the tool situation also increases the number of sampling times to achieve better results. Improving the reliability of the signal read into the PLC can also take advantage of the characteristics of the control system and use the relationship between the signals to determine the credibility of the signal. For liquid level control, since the size of the tank is known, the opening and pressure of the inlet or outlet valve are known, and the height of the liquid in the tank is known to be within a certain range of time. At this time, the data sent to the PLC by the liquid level gauge differs greatly from the estimated liquid level height. The judgment may be that the liquid level gauge is faulty, and the operator is notified by the fault alarm system to check the liquid level gauge.

Another example is that each tank has upper and lower liquid level limit protection. When the switch is activated, it sends a signal to the PLC. Whether the signal is authentic or not, we compare this signal with the tank level gauge signal during programming, if the level gauge reads In the extreme position, the signal is true; if the level gauge reading is not at the limit position, it may be judged that the liquid level limit switch is faulty or the transmission signal line is faulty, and the operator is also notified by the alarm system to handle the fault. Since the above method is adopted in programming, the reliability of the input signal is greatly improved.

Fourth, the reliability study of the executive agency

When the signal from the site is accurately input to the PLC, the PLC executes the program and adjusts and controls the field device through the actuator. How to ensure that the executing agency works according to the control requirements. When the executing agency does not work as required, how to find the fault? We take the following measures: When the load is controlled by the contactor, starting or stopping such load turns to control the contactor coil, whether the contactor is reliably sucked at startup, and whether the contactor is reliably released when stopped, which is of concern to us.

We have designed the following procedure to determine if the contactor is acting reliably. X0 is the contactor operating condition, Y0 is the control coil output, and X1 is the contactor auxiliary normally open contact that is led back to the PLC input. The timer timing is greater than the contactor action time. R0 is the set fault bit. When R0 is ON, it indicates that there is a fault, and the alarm is processed. When R0 is OFF, it indicates that there is no fault. The fault has a memory function and is cleared by the fault reset button.

When the electric valve is opened or closed, the delay time is set according to the opening and closing time of the valve. After the delay detection is turned on or off, if these signals cannot be accurately returned to the PLC on time, the valve may be faulty and the valve may be used. Fault alarm processing. The programming is as follows. X2 is the valve opening condition, Y1 is the control valve action output, the timer timing time is greater than the valve opening time, X3 is the valve in-position return signal, and R1 is the valve fault position. In addition, the general switch output has an intermediate relay, more than the more important control can use the other auxiliary contacts of the intermediate relay to feedback the action information to the PLC.