  In this paper, we propose a retrofit control method for stable network
systems. The proposed approach is a control method that, rather than an entire
system model, requires a model of the subsystem of interest for controller
design. To design the retrofit controller, we use a novel approach based on
hierarchical state-space expansion that generates a higher-dimensional cascade
realization of a given network system. The upstream dynamics of the cascade
realization corresponds to an isolated model of the subsystem of interest,
which is stabilized by a local controller. The downstream dynamics can be seen
as a dynamical model representing the propagation of interference signals among
subsystems, the stability of which is equivalent to that of the original
system. This cascade structure enables a systematic analysis of both the
stability and control performance of the resultant closed-loop system. The
resultant retrofit controller is formed as a cascade interconnection of the
local controller and an output rectifier that rectifies an output signal of the
subsystem of interest so as to conform to an output signal of the isolated
subsystem model while acquiring complementary signals neglected in the local
controller design, such as interconnection signals from neighboring subsystems.
Finally, the efficiency of the retrofit control method is demonstrated through
numerical examples of power systems control and vehicle platoon control.
