Big data is considered to play an important role in the fourth industrial revolution, which requires engineers and computers to fully utilize data to make smart decisions and improve the performance of industrial processes and of their control and safety systems. Traditionally, industrial process control systems rely on a (usually linear) data-driven model with parameters that are identified from industrial/simulation data, and in certain cases, for example, in profit-critical control loops, on first-principles models (with data-determined model parameters) that describe the underlying physico-chemical phenomena. However, modeling large-scale, complex nonlinear processes continues to be a major challenge in process systems engineering. Modeling is particularly important now and into the future, as process models are key elements of advanced model-based control systems, e.g., model predictive control (MPC) and economic MPC (EMPC).
Due to the wide variety of applications, machine learning models have great potential, yet, the development of rigorous and systematic methods for incorporating machine learning techniques in nonlinear process control and operational safety is in its infancy. Traditionally, operational safety of chemical processes has been addressed through process design considerations and through a hierarchical, independent design of control and safety systems. However, the consistent accidents throughout chemical process plant history (including several high profile disasters in the last decade) have motivated researchers to design control systems that explicitly account for process operational safety considerations. In particular, a new design of control systems such as model predictive controllers (MPC) that incorporate safety considerations and can be coordinated with safety systems has the potential to significantly improve process operational safety and avoid unnecessary triggering of alarms systems, where machine learning techniques can be utilized to derive dynamic process models. However, the rigorous design of safety-based control systems poses new challenges that cannot be addressed with traditional process control methods, including, for example, proving simultaneous closed-loop stability and safety. On the other hand, cybersecurity has become increasingly important in chemical process industries in recent years as cyber-attacks that have grown in sophistication and frequency have become another leading cause of process safety incidents. While the traditional methods of handling cyber-attacks in control systems still rely partly on human analysis and mainly fall into the area of fault diagnosis, the intelligence of cyber-attacks and their accessibility to control system information has recently motivated researchers to develop cyber-attack detection and resilient operation control strategies to address directly cybersecurity concerns.
Motivated by the above considerations, this dissertation presents the use of machine learning techniques in model predictive control, operational safety and cybersecurity for chemical processes described by nonlinear dynamic models. The motivation and organization of this dissertation are first presented. Then, the use of machine learning techniques to develop data-driven nonlinear dynamic process models to be used in model predictive controllers is presented, followed by the discussion of real-time implementation with online learning of machine leaning models and of physics-based machine learning modeling methods. Subsequently, the MPC and economic MPC schemes that use control Lyapunov-barrier functions (CLBF) are presented in detail with rigorous analysis provided on their closed-loop stability, operational safety and recursive feasibility properties. Next, the development of machine-learning-based CLBF-MPC schemes is presented with process stability and safety analysis. Finally, the development of an integrated detection and control system for process cybersecurity is developed, in which several types of intelligent cyber-attacks, machine learning detection methods and resilient control strategies are presented. Throughout the dissertation, the control methods are applied to numerical simulations of nonlinear chemical process examples to demonstrate their effectiveness and performance.
