Decentralised controller structure design and retrofit of process systems based on graph theory

A relative degree matrix-based design method is proposed in this article by using graph theoretical methods for optimal control structure design consisting of single input–single output control loops. This approach enables us to use an efficient algorithm for finding a maximum weighted matching to solve the controller structure selection problem. The resulting optimal structures have been refined by analysing the zero dynamics of the input–output pairs. A novel method for decentralised controller structure retrofit is also proposed in this article. A graph theoretic algorithm is developed, which is based on finding the closest weighted maximum matching. Possible changes in efficiency in the controller structure, which may arise after retrofit, are illustrated using a heat exchanger network retrofit case study.