On the Application of Different Event-Based Sampling Strategies to the Control of a Simple Industrial Process

This paper is an experimental study of the utilization of different event-based strategies for the automatic control of a simple but very representative industrial process: the level control of a tank. In an event-based control approach it is the triggering of a specific event, and not the time, that instructs the sensor to send the current state of the process to the controller, and the controller to compute a new control action and send it to the actuator. In the document, five control strategies based on different event-based sampling techniques are described, compared, and contrasted with a classical time-based control approach and a hybrid one. The common denominator in the time, the hybrid, and the event-based control approaches is the controller: a proportional-integral algorithm with adaptations depending on the selected control approach. To compare and contrast each one of the hybrid and the pure event-based control algorithms with the time-based counterpart, the two tasks that a control strategy must achieve (set-point following and disturbance rejection) are independently analyzed. The experimental study provides new proof concerning the ability of event-based control strategies to minimize the data exchange among the control agents (sensors, controllers, actuators) when an error-free control of the process is not a hard requirement

Contribuciones a la sintonía de controladores basados en eventos

Dentro del paradigma del control automático, las estrategias de control basado en eventos se postulan como una posible solución para satisfacer los requisitos de los actuales procesos de control distribuidos. La creciente descentralización, escala y número de dispositivos implicados, está demandando esquemas de muestreo más eficaces conforme a las nuevas necesidades. Los incipientes sistemas basados en red (tales como los sistemas de control en red) son una prueba de tal demanda. En estas aplicaciones el desafío consiste en reducir el intercambio de información entre dispositivos distribuidos con la menor pérdida de rendimiento y, para tal fin, los enfoques basados en eventos podrían aportar un compromiso satisfactorio entre el esfuerzo de muestro y el rendimiento del lazo, a través de un uso oportuno de los recursos y canales de comunicación. A pesar de las muchas ventajas potenciales del control basado en eventos, este campo de investigación se encuentra aún en sus primeros pasos. Aunque la última década ha sido prolífica en resultados analíticos, la carencia de una teoría comprensible es todavía evidente. El diseño de un esquema eficiente sigue siendo un problema difícil de resolver y cuestiones tales como la sintonía del controlador o las condiciones para la estabilidad global y la ausencia de ciclos límite son dificultades que todavía están lejos de ser completamente solventadas. En particular, la cuestión de la sintonía no ha sido tratada debidamente. En la mayoría de los sistemas de control propuestos por los autores, la sintonía o no es tratada o se terminan proponiendo reglas que por las propias condiciones de diseño restringen el espacio de parámetros y comprometen el rendimiento global del sistema. Ante tal escenario, la presente Tesis se centra en el diseño de controladores basados en eventos con ventajas prácticas desde el punto de vista del diseño y desarrollo de la sintonía. Para tal objetivo, la presente Tesis realiza tres principales aportaciones. En primer lugar, se propone un nuevo esquema de control basado en eventos. En su desarrollo se considera el diseño global del generador de eventos y del controlador, de manera que el sistema pueda tratar la problemática del retardo del proceso y el seguimiento de referencias y, a la vez, se obtenga un sistema basado en eventos de fácil diseño y ajuste. El enfoque se centra en sistemas de primer orden con retardo, pero puede ser extendido sistemas de orden mayor. Se ha desarrollado el análisis de estabilidad y robustez del sistema y los resultados experimentales y de simulación prueban la eficacia de la...In the paradigm of automatic control, event-based strategies are proposed as a solution to satisfy the current requirements of distributed process control. The increasing decentralization, large scale and the number of involved devices in current control applications demand more effective sampling schemes for signals. Systems based on networks (such as Networked Control Systems, NCS) are proof of this demand. In this context, the challenge is to reduce the exchange of information between distributed devices without losing performance and, to this aim, event-based approaches could provide a satisfactory trade-off between the sampling effort and the loop performance through an opportune use of sources and information channels. Despite many conceptual advantages of event-based control, this field is still in its infancy. Although last decade has been prolific in analytical results, the lack of a comprehensive theory is still evident. The design of an efficient scheme is a challenging problem and issues such as controller tuning, conditions for global stability and the absence of limit cycles are topics that are far from being fully solved yet. In particular, the question of controller tuning has not been properly addressed. In most control systems proposed for authors, the controller tuning task is omitted or the provided rules become complex and restrictive, compromising the overall system performance. In this context, this thesis is focused on the designing of event-based controllers with advantages from the point of view of a practical design and an intuitive tuning methodology. To this end, this thesis makes three main contributions. Firstly, a novel eventbased control scheme has been proposed. The overall design of the event generator and the controller is considered in order to cope with the problem of the process delay, the set-point tracking and to obtain an easy-to-tune event-based system. The approach is focused on FOPTD systems but can be easily extended to higher order systems. Stability and robustness analyses are conducted, and the experimental and simulation results verify the effectiveness of the approach. Secondly, the tuning framework is presented. The complexity of the design is simplified by using only two parameters with a very intuitive influence. The methodology is illustrated through the definition of the tuning regions and the guidelines for the tuning of parameters are provided. Thirdly, a new interactive software tool developed using Sysquake is described. By using the..