Simulation and Function Approximation Based Iterative Approach To Process Control

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Research:The PI plans to investigate a simulation and function approximation-based strategy for bringing an evolutionary improvement to a control policy. The investigation is motivated by some inherent limitations of the current model-based predictive control formulation with respect to handling systems of large-scale complex dynamics, and large amount of uncertainty. The development will be rooted in an approach developed in the field of artificial intelligence - referred to by various names such as Neuro-Dynamic Programming and Reinforced Learning - which has shown great success in handling highly complex multi-stage discrete decision problems like backgammon playing, elevator dispatch problem, and job-shop scheduling. The approach, when extrapolated to the problem of process control, begins by performing closed-loop simulations with a given suboptimal control policy for an extensive set of possible operating conditions. The simulation results are then used to generate data for state versus "cost-to-go" or "reward" function, typically by fitting a neural network to the data. The approximation is improved by additional off-line calculations, either by "value iteration" based on iterating the Bellman Equation or by "policy iteration" based on iterating between policy evaluation and policy improvement. The improved approximation of the "cost-to-go" function can be used to implement optimal control in a computationally efficient way, either by reducing a large-horizon problem into an equivalent short-horizon problem or by allowing an off-line parameterization of the improved control law. To make the approach practicable for process control a number of issues need to be resolved. The success of the approach will depend on the ability to obtain an accurate and robust approximation of the cost-to-go function. An immediate question to ask is what types of function approximators are best suited for the approximation. Also, the level of confidence in the neural network's cost predictions through interpolation and extrapolation need to be taken into account in the control calculations. The PI plans to investigate these and other fundamental issues to arrive at systematic and practically useful answers. The PI will collaborate with industrial partners Weyerhauser, Owens Corning, LG Chemicals, and Aspen Technology, to test the developed tools on real industrial process and to incorporate them into commercial process control software packages. The application portion of the project will be carried out by students and visitors supported by these companies.Broader Impact:The Chemical Process Industries (CPI) are replete with nonlinear control problems involving significant uncertainties, which can benefit from this work. In addition to process control, the strategy fits naturally to planning and scheduling problems under uncertainty as well as supply chain operation problems.