Elimination method of loop vibration caused by sticking fault of pneumatic control valve

Elimination method of loop vibration caused by sticking fault of pneumatic control valve

The pneumatic control valve takes the compressed gas as the main power source, uses the cylinder as the actuator, and combines valve positioner, gas tank, gas filter and other accessories to switch or proportionally control and adjust the system operation. After receiving the control signal of information automation, adjust various media of the pipeline, mainly including: flow, pressure, temperature, quality and other parameters. In recent years, with the constant updating and development of the control valve, the pneumatic control valve is favored by the majority of operating systems for its advantages of simple control, safety and stability, quick response, and no need to add additional explosion-proof measures. It has not only made certain achievements in the industrial system, but also involved in the development trend of the oil industry, chemical industry, electrical industry and other important national fields, so it plays a key role in the development of our society. In an industrial system, there are hundreds of loops. Only by perfect and scientific control of these loops can we ensure the quality and safety of production. The circuit with poor performance will have an impact on the overall system, such as vibration, which will damage the normal operation of the entire system circuit, and finally cause huge losses and safety impact.

Loop vibration caused by sticking of pneumatic control valve

The pneumatic control valve is the most frequently used executive component in the operating system, so its normal circuit is the key to ensure the scientific operation. However, according to the data of the national industrial survey, only 30% of the industrial control loops in China are within the standard range, and 30% of the unqualified control loops are caused by the viscosity of the control valve. In general, the valve viscosity leads to a limit cycle in the circuit, so the control amount is also in the process of up and down oscillation transformation of the set value. Therefore, eliminating the influence of viscosity on the circuit can not only improve the quality of production, but also reduce the waste of work, improve the economic value, and also promote the daily operation of the pneumatic control valve.

Valve sticking refers to the non-linear fault caused by the greater static friction between the valve stem and the packing after the valve operates for a period of time. In today's system operation, most of the control loops are calculated using the traditional PID control calculation method. In the PID control loop, the controller needs to spend more time to adjust repeatedly to make it reach a certain set state. In this process, the controller will constantly change direction. When there is a sticking fault on the circuit with the pneumatic control valve, the controller will change direction, and the valve will further stick. This situation will continue until the output of the controller is greater than a certain value (S), and then the valve rod will have a runout of a certain value (J) (refer to the following figure). In this unpredictable process, it is inevitable that the adjustment will produce deviation. In order to eliminate this deviation, the controller will change again, and the viscous fault will occur again, eventually leading to the dynamic oscillation of the control variable at the set value. Therefore, PID is the main cause of numerical oscillation.

图1　粘滞阀门的输入输出关系图

Eliminating Method of Circuit Oscillation Caused by Stickiness of Pneumatic Control Valve

Experiments show that in the traditional PI controller, r (k) is the set value in the operating system, y (k) is the actual output value generated by the system during operation, P is set as the proportional coefficient, T is the practical sampling period, and I is the final integral coefficient. Therefore, the expression of discrete PI can be summarized as

Therefore, in the traditional PI controller, when the controller changes, the reason for the valve sticking is that the output of the controller is too small. This shows that the effect of integral can be further changed to control and plan the input when the valve is sticky, so as to eliminate the influence of viscosity on the control loop. The value of the controller's input rate has a great impact on the operation of the pneumatic control valve stem. When the valve stem is sticky, the controller's output rate can further help the valve stem to run quickly, so as to get rid of the impact of viscosity. Conversely, it can reduce a certain amount of output rate to ensure the normal operation of the pneumatic control valve. At the same time, the values of E (k) and Ec (k), which affect the output rate, will also have a different impact on the viscosity phenomenon. According to a number of experimental data, when the two values are not equal to 0, it means that the control variable has not reached the specific set value and is still changing. Therefore, at this time, the valve stem does not have viscosity. When E (k) is not equal to 0 and Ec (k) is equal to 0, its control quantity has not changed even though it has not reached the set value, so the sticking phenomenon has occurred. When E (k) is equal to 0 and Ec (k) is not equal to 0, the control quantity is still changing, so there is no viscosity. Finally, when the two are equal to 0 at the same time, the control amount reaches the set value, and no other changes occur, so there is no viscous phenomenon.

To sum up, the new model is used to carry out fuzzy control on the controller to eliminate loop oscillation. First, the specific parameters should be obtained through the traditional PI controller, and then the same parameters should be given to the fuzzy controller. Finally, the circuit with viscosity should be recuperated through reasonable parameters to finally achieve the goal of improving the working quality of the pneumatic control valve.