Session 12 Hierarchical Systems Theory

The design and analysis of large and complex system requires frequently that the system is divided into smaller units, called sub-systems. The structure of overall system resulting from the interconnections of sub-systems can be very complex. One of the most common structures, is the hierarchical structure, i.e. the lay-out of the structure is vertical. Consequently in this report only hierarchical systems are considered, and a special emphasis is put on twolevel hierarchical systems. A diagram of a standard two-level hierarchical system is shown in Fig. 12.1, where, as expected, two levels can be found, namely the lower level and the upper level. The lower level consists of the process level, where the process level has been divided into N sub-systems. The sub-systems are connected to each other because there is either material or information flows between these sub-systems. Each sub-system has its own decision unit, which tries to control the behaviour of the sub-system so that the objectives of this particular sub-system would be met. The decision unit can also use feedback information from the sub-system to improve its ’control policy’. However, quite often the objectives of the sub-systems are conflicting, resulting in a poor overall performance. Hence an upper-level decision unit