Feedback control of nanostructured coatings processing accounting for powder size distribution

This work presents a novel formulation of the control problem and a feedback control system for the high velocity oxygen-fuel (HVOF) thermal spray process, which explicitly accounts for the effect of powder size distribution. Initially, based on available model predictions and experimental data, the control problem is formulated as the one of regulating appropriate averages of the temperature and velocity of the particles at the point of impact on substrate by manipulating the oxygen/fuel ratio and the combustion chamber pressure, respectively. Then, a feedback control system is developed and applied to a detailed mathematical model of the process. Closed-loop simulations show that the average particle velocity and temperature at the point of impact on substrate reach the desired values in a short time, which validates the feasibility of real-time implementation of feedback control on the HVOF thermal spray system. It is also shown that the proposed formulation of the control problem leads to a solution of the control problem that is superior to a solution that assumes a monodisperse powder size distribution. Finally, the proposed control problem formulation and feedback control system are shown to be robust with respect to disturbances in initial particle velocity and distance between gun exit and substrate.