  As one of the core parts of flexible manufacturing systems, material handling
involves storage and transportation of materials between workstations with
automated vehicles. The improvement in material handling can impulse the
overall efficiency of the manufacturing system. However, the occurrence of
dynamic events during the optimisation of task arrangements poses a challenge
that requires adaptability and effectiveness. In this paper, we aim at the
scheduling of automated guided vehicles for dynamic material handling.
Motivated by some real-world scenarios, unknown new tasks and unexpected
vehicle breakdowns are regarded as dynamic events in our problem. We formulate
the problem as a constrained Markov decision process which takes into account
tardiness and available vehicles as cumulative and instantaneous constraints,
respectively. An adaptive constrained reinforcement learning algorithm that
combines Lagrangian relaxation and invalid action masking, named RCPOM, is
proposed to address the problem with two hybrid constraints. Moreover, a
gym-like dynamic material handling simulator, named DMH-GYM, is developed and
equipped with diverse problem instances, which can be used as benchmarks for
dynamic material handling. Experimental results on the problem instances
demonstrate the outstanding performance of our proposed approach compared with
eight state-of-the-art constrained and non-constrained reinforcement learning
algorithms, and widely used dispatching rules for material handling.
