A.N. Tarau, B. De Schutter, and J. Hellendoorn, "Hierarchical route control in DCV-based baggage handling systems," International Journal of Services Operations and Informatics, vol. 6, no. 1/2, pp. 5-29, Jan. 2011.
State-of-the-art baggage handling systems transport the baggage at high speeds, on a network of tracks, using destination coded vehicles (DCV). In order to ensure the optimal routing of DCVs, in this paper we propose a hierarchical control framework. In this framework switch controllers provide position instructions for each switch in the network. The switch controllers are then supervised by a so-called network controller that mainly takes care of flows of DCVs. The routing control problem for the network controller is a nonlinear, mixed integer optimization problem, with high computational requirements, which makes it intractable in practice. Therefore, we present an alternative approach for reducing the complexity of the computations by approximating the nonlinear optimization problem and rewriting it as a mixed integer linear programming (MILP) problem. The advantage is that for MILP problems solvers are available that allow us to efficiently compute the global optimal solution. The solution of the MILP problem is then used for computing optimal switch control actions. For a benchmark case study we compare the hierarchical route control with switch control approaches that have been developed previously. Results indicate that the proposed hierarchical control offers a balanced trade-off between optimality and computational efficiency.