Detection And Control Of Stiction In Control Loops

The control systems in the process industry are essentially nonlinear, and the liner object is few, the nonlinear process is researched as the linear process. In the control loop, the valve is the most commonly used actuator, its performance will affect the control loop of the control effect. According to statistics, there are about 30% of the control loops in the process industry are in state of oscillation, in which 1/3 due to the valve stiction. Therefore, stiction is the most common problems in the industrial control process, and its existence seriously impacts on the performance of the control system.The control of stiction is very important to improve the accuracy and stability of the control loops. But there are a few studies in the related fields so far, in recent years, some foreign researcheres present different kinds of theories and methods of the stiction detection.But until now, few research the control of stiction.Based on the analysis of the stiction models which established by foreign researchers and analysis of the advantages and disadvantages of many stiction models, a new stiction model is presented which can accurately reflect the true characteristics of the valve and is suitable for the detection of stiction, and at the same time the methods about detection of stiction presented by foreign researchers are researched in this thesis. In these theories and methods, however, one or more limitations in the areas, such as speed, accuracy, simplicity and so on, constrain the applications of them. Supplied with data of the controller output and process value, the existence of the valve stiction in control loops can be identified using the least-squares method. To resolve the oscillation problem of control loops in the process industry, this thesis focuses on studying the impact of valve stiction, and analyzed Hagglund's method about compensation control of stiction. Also some new approaches are put forward on control of valve stiction. These approaches can effectively minimize the oscillation in control loops.Numerous simulations are conducted according to existing stiction model and control method by using Matlab. The result of simulation shows the reasonableness of the stiction model and the effectiveness and accuracy of the control methods. The results in this thesis can not only be applied to the detection and control of stiction in control loops, but also improve the product quality, reduce energy consumption and maintain the stability of the process, as well as reduce the intensity of the operation, and so on. The research of stiction control can not only fill in the gaps between the field of our country and other countries, but also significantly increase the economic efficiency of the process industry with its broad and huge prospect of application.