Cascade nonlinear feedforward-feedback control of stagnation pressure in a supersonic blowdown wind tunnel

Current trends in the development of control systems involve disproportionately high costs for embedded software integration and testing techniques that rely almost exclusively on exhaustive testing of more or less complete versions of complex systems. Wind tunnel control systems are not an exception. A formal methodology for supersonic flow control does not exist, which is not acceptable from a perspective of cost, reliability and safety of wind tunnel operations. The cascade nonlinear feedforward-feedback stagnation pressure controller proposed here is intended to address this deficiency in the operation of a supersonic blowdown wind tunnel. By focusing on a model-based approach using physical principles and hierarchical design methodologies, a systematic design method is offered for stagnation pressure control in particular, and control of flow parameters in general. The suggested mathematical model of supersonic flow in a blowdown wind tunnel is analyzed and main challenges of using a model-based approach are identified, with an emphasis on high process nonlinearity and an infinite number of possible operating conditions. The model is applied to the VTI Belgrade T-38 blowdown wind tunnel to identify the feedforward component that accurately predicts the nonlinear response of the facility. The Simulink® models of the facility and the proposed controller are developed to tune the feedback component in numerical simulations and verify the controller. The wind tunnel control system is implemented as an embedded distributed hierarchical system and experiments to verify the suggested control method are realized at Mach numbers 1.0-4.0. Both simulations and experiments demonstrate that feedback calculation successfully captures nonlinearities in the facility response, enabling a simple linear feedback controller with a single set of control terms to be used only to trim out additional deviations for an entire operating range of the facility. The feedforward-feedback architecture thus improves setpoint reference tracking, while the cascade architecture improves disturbance rejection performance compared to common single loop solutions. Combined within a single system, they eliminate large transient pressure overshoots typical for blowdown facilities, decrease the setpoint settling time and improve overall stagnation pressure control accuracy. In addition, system integration and testing time and costs are significantly reduced by analyzing physical properties of the process and taking them into consideration during early stages of the system development.