The Trade-Off between the Controller Effort and Control Quality on Example of an Electro-Pneumatic Final Control Element

Abstract

For many years, the programmable positioners have been widely applied in structures of modern electro-pneumatic final control elements. The positioner consists of an electro-pneumatic transducer, embedded controller, and measuring instrumentation. Electro-pneumatic transducers that are used in positioners are characterized by a relatively short mean time-to-failure. The practical and economical method of a reasonable prolongation of this time is proposed in this paper. It is principally based on assessment and minimizing the effort of the embedded controller. For this purpose, some measures were introduced: The control value variability, mean-time, and the cumulative controller’s effort. The diminishing of controller effort has significant practical repercussions because it reduces the intensity of mechanical wear of the final control element components. On the other hand, the reduction of the cumulative effort is important in the context of process economy due to limitation of the consumption of energy of compressed air supplying the final control element. Therefore, the minimization of control effort indicators has an impact on the increase of the functional safety and economics of the controlled process. The simulations were performed in the Matlab-Simulink environment with the use of the liquid level control system in which a phenomenological model of a final control element was deployed. As a result of the performed simulations, the recommendations regarding the selection of the structure of positioner controller were elaborated.

Keywords

• final control element
• electro-pneumatic transducer
• controller effort
• control quality factors
• wear
• mean-time-between-failures