Quantificação e compensação de atrito em válvulas de controle pneumáticas

• Autor(a) principal: Cuadros, Marco Antonio de Souza Leite
• Data de Publicação: 2011
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da Universidade Federal do Espírito Santo (riUfes)
• Texto Completo: http://repositorio.ufes.br/handle/10/4123
• topic: Medidas Elétricas, Magnéticas e Eletrônicas; Instrumentação; Válvulas de controle pneumático; Atrito; 621.3

The control valves are actuators very common in the industry and are important elements in the control loops. The stiction (static + friction) is a non-linearity present in the control valves which increases the variability of the process, compromising the performance, producing losses in quality, cost of raw materials and drop in profitability. The stiction represents the friction present in the valve that increases with the wear. The first contribution of this paper is to propose a new methodology for the quantification of stiction in situations where the graph of the process variable and controller output has the form of a parallelogram. For this pattern the proposed method produces less conservative results compared to other methods of quantifying with similar approach. Moreover, a proposal for estimating the slipjump from the knowledge of the process model is discussed. The proposed method requires only the operating data, without the need of invasive tests. Application to simulations, data from industrial plants and a pilot plant demonstrate the applicability and superiority of the contribution. The second contribution is to propose three methods for compensation of stiction that improve the performance of the process variable without the negative effect of increasing the variability in the movement of the valve stem, something common in other methods of this nature. The first proposed method evaluates the process through a step in order to finding the control signal for bringing the error to zero, through a procedure performed in five steps The second method approximates the error to zero by applying a gentle signal to the valve, followed by a brake signal at the instant when the process variable approaches the reference. The third method applies pulse signals to the valve to minimize the error, similarly to the method known as knocker. However, it is proposed a strategy to detect the instant when the error signal is minimal and the pulses can cease while keeping the minimum error. This makes the method suitable for treating disturbances and setpoint changes. The methods are evaluated through their application in examples of simulation and in a flow control loop of a pilot plant.