Fault Modeling And Analysis For Liquid Level Control System

As important parts of the liquid level control system, once valves and transmitter go wrong, they will cause serious influence on the control loops, and even affect the safety of device. Thereby, researching on the fault model of valves and transmitter plays an important role in improving reliability and stability of control system. In the liquid level control system, stiction in pneumatic control valves and the failure of pressure measurement in capacitance differential pressure transmitter are studied as the following parts:1. The stiction model of valves and fault model of pressure measurement in transmitter are introduced respectively. In addition, with valves stiction and failure of pressure measurement, the impact on the performance of the liquid level control system is analyzed in this thesis.2. We know that valve stiction gives rise to triangular waves in the controller output, while a sinusoidal external disturbance results in sinusoidal waves. Then, an improved method of calculating area ratio for the average waveform of controller output is proposed according to the above characteristics. The ratios for the area of average waveform with triangular area reference and sinusoidal area reference are calculated respectively. And then the shape of controller output will be distinguished via the two ratios. Thereby, the stiction of self-regulating system or nonself-regulating system can be detected. At last, simulation results verify that stiction can be accurately detected with the proposed detecting strategy.3. In view of the approach of stiction compensation in pneumatic control valves, an optimal stiction compensator under the steady-state position of valves is presented in this paper. Aiming at steady-state position of valves, a novel compensator is designed combined with the information of controller transfer function, process transfer function and stiction parameters. And then in order to eliminate the oscillation in the control loop effectively, the compensator is added to the input of valves at the moment that the stem reverse. Finally, the simulation results verify that the stiction compensator is effective.