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 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 be not 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 control structure based on the periodic behavior
of the plant, as it is seen from the point of view of the controller through the shared
medium. The control is easy to be implemented and it has been designed to make the
most advantage of the available bandwidth. This periodic control uses the model of the
plant to supply the information that could not be sent due to the bandwidth limitations.
Once the information is recovered, a conventional control designed without considering
the limitations imposed by the shared medium, can be used. 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 linear and time-invariant. Non linear behavior, periodicity
and any kind of irregularities can be included for a better recovery of the information loss.
The proposed 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.