Networked Control Systems: Control Structures with Bandwidth Limitations

TítuloNetworked Control Systems: Control Structures with Bandwidth Limitations
Publication TypeJournal Article
Year of Publication2009
AuthorsCasanova V, Salt J, Cuenca Á, Pizá R

The aim of this paper is to propose a control structure suitable to be used in Networked Control Systems in which the shared communication medium imposes a limitation in the amount of information that can be transmitted. In this kind of systems, the link to perform the communication between controller and plant is used to close several control loops. The bandwidth must be shared between all the sender devices and the available bandwidth for each loop is reduced as much as the number of devices using the link is increased. With conventional control techniques the control frequency must be decreased to fit the available bandwidth. If the number of devices sharing the link is large, the control frequency may not be enough to reach the desired specifications. When frequency decreasing, imposed by the shared link, is present in the loop it can be modeled as a periodic system. The paper proposes a periodic model of the plant dynamics, as it is seen from the point of view of the controller through the shared medium. This periodic model can be converted into a multivariable one, suitable to be used to design a multivariable controller. A second control structure, easier to be implemented and designed to make the most advantage the available bandwidth, is proposed. This is a periodic control that uses the model of the plant to supply the information that could not be sent due to the bandwidth limitations and uses a conventional control, designed without considering the limitations imposed by the shared medium. One of the main advantages of the proposed control is that the model of the plant in which is based the information recovery does not need to be a LTI one. Non linear behavior, periodicity and any kind of irregularities can be included for a better recovery of the information loss. The proposed periodic control is tested on the control of X and Y axis from a model crane, using Profibus-DP as the shared medium to close the loop.