Closed-loop control of shock location to prevent hypersonic inlet unstart
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Hypersonic inlet unstart remains a major technical obstacle in the successful implementation of hypersonic air-breathing propulsion systems such as ramjets and scramjets. Unstart occurs when combustor-induced pressure fluctuations lead to rapid expulsion of the shock system from the isolator, and is associated with loss of thrust. The research presented here attempts to mitigate this behavior through the design and implementation of a closed-loop control scheme that regulates shock location within a Mach 1.8 wind tunnel isolator test section. To localize the position of the shock within the isolator, a set of high frequency Kulite pressure transducers are used to measure the static pressure at various points along the wind tunnel test section. A novel Kalman filter based approach is utilized, which fuses the estimates from two distinct shock localization algorithms running at 250 Hz to determine the location of the shock in real time. The primary shock localization algorithm is a geometrical shock detection scheme that can estimate the position of the shock system even when it is located between pressure transducers. The second algorithm utilizes a sum-of-pressures technique that can be calibrated by the geometrical algorithm in real time. The closed-loop controller generates commands every 100 ms to actuate a motorized flap downstream of the test section in an effort to regulate the shock to the desired location. The closed-loop control implementation utilized a simple logic-based controller as well as a Proportional-Integral (PI) and a Proportional-Derivative (PD) Controller. In addition to the implementation of control algorithms, the importance of various design criteria necessary to achieve satisfactory control performance is explored including parameters such as pressure transducer spacing, shock localization speed, flap-motor actuation speed and actuator resolution. Experimental results are presented for various test scenarios such as regulation of the shock location in the presence of stagnation pressure disturbances as well as tracking of time-varying step inputs. Performance and robustness properties of the tested control implementations are discussed. Further areas of improvement for the closed-loop control system in both hardware and software are discussed, and the need for reduced-order dynamics-based controllers is presented.