Active FTC for dissolved oxygen in WWTP: An integrated method based on an improved projection observer and sliding mode control

Accurate control of dissolved oxygen (DO) concentration is crucial for the efficiency of the biochemical process in urban wastewater treatment. However, sensor faults can severely compromise measurement reliability, which in turn degrades control-loop performance, leading to deteriorated tracking accuracy and operational efficiency. Current approaches to sensor faults typically implement fault diagnosis and fault-tolerant control in isolation, instead of integrating them into a cohesive framework. To bridge this gap, this paper proposes a unified framework that cohesively integrates fault diagnosis with active fault-tolerant control for dissolved oxygen concentration control. First, a DO dynamics model is established, incorporating unmodeled dynamics and additive sensor faults. Subsequently, an improved projection-based adaptive observer is designed for the simultaneous real-time estimation of system states and sensor fault magnitudes. The convergence and uniform ultimate boundedness of the estimation errors are rigorously established via Lyapunov stability analysis, leveraging the inherent physical dissipativity and nonnegative control inputs of the biological process. Building on the state estimates, a sliding mode fault-tolerant controller is synthesized. An active fault-tolerant strategy is introduced, featuring an environment-adaptive fault detection threshold derived analytically from Lyapunov stability conditions. Upon fault detection, the control law is automatically switched to one with enhanced robust gains, ensuring prompt fault compensation and the preservation of system stability and tracking performance. Theoretical analysis guarantees the stability of the overall closed-loop system and provides explicit upper bounds for the tracking error. The proposed method is evaluated quantitatively in terms of control performance and key metrics. Simulation results demonstrate the framework’s precise estimation capability and superior fault-tolerant performance across different environmental scenarios.