Retrofit Control: Localization of Controller Design and Implementation

TL;DR

This paper introduces a retrofit control method for stable network systems that allows controller design based on subsystem models, improving stability and performance analysis without requiring full system models.

Contribution

The paper presents a novel hierarchical state-space expansion approach for designing retrofit controllers using only subsystem models, enabling systematic stability and performance analysis.

Findings

Effective in power systems control scenarios

Applicable to vehicle platoon control

Demonstrates improved stability and performance

Abstract

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.