Hierarchical Control Systems: An Introduction

The purpose of this paper is to describe the main concepts, ideas and operating principles of hierarchical control systems. The mathematical treatment is rather elementary; the emphasis of the paper is on motivation for using hierarchical control structures as opposed to centralized control. The paper starts with a discussion of multilayer control hierarchies, i.e. hierarchies where either the functions or the time horizons of the subsequent layers of control are different. Some attention has been paid, in this part, to the question of structural choices such as designation of control variables and the selection of the time horizons. Next part of the paper treats decomposition and coordination in steady-state control: direct coordination, penalty function coordination and price coordination are discussed. The focus is on model-reality differences, that is on finding structures and operating principles that would be relatively insensitive to disturbances. The last part of the paper gives a brief presentation of the broad and still developing area of dynamic multilevel control. It was possible, within the restricted space, to show the three main structural principles of this kind of control and to provide for a comparison of their properties. A list of selected references is enclosed with the paper