Robust Control Design for Active Flutter Suppression

Abstract

Flutter is an unstable oscillation caused by the interaction of aerodynamics and structural dynamics. It can lead to catastrophic failure and therefore must be strictly avoided. Weight reduction and aerodynamically efficient high aspect ratio wing design reduce structural stiffness and thus reduce flutter speed. Consequently, the use of active control systems to counter these adverse aeroservoelastic effects becomes an increasingly important aspect for future flight control systems. The paper describes the process of designing a controller for active flutter suppression on a small, flexible unmanned aircraft. It starts from a greybox model and highlights the importance of individual components such as actuators and computation devices. A systematic design procedure for an H∞-norm optimal controller that increases structural damping and suppresses flutter is then developed. A second key contribution is the development of thorough robustness tests for clearance in the absence of a high-fidelity nonlinear model.

H∞ Control Design for Active Flutter Suppression of Flexible-Wing Unmanned Aerial Vehicle Demonstrator

“…The selection of margin requirements for a flutter suppression controller is not straightforward, as it does not benefit from the extensive industrial experience that supports the design of controllers for rigid-body aircraft. For instance, the literature reports specifications of minimum phase margins ranging from 60° [32] down to 37° [ 10] and 30° [33]. Thus, in the present case, the requirements used in the design of the flutter controller are a minimum gain margin of 6 dB and a minimum phase margin of 35°.…”

“…This is done to avoid exciting highfrequency aeroelastic modes as well as limiting the coupling with the low-frequency dynamics that are handled by the baseline rigidmotion controller. The same weighting function for each control channel is then chosen and is given byW u s 40 s 2 101s 2200 s 2 127020s 2200 (5) This transfer function is inspired by [10] and configures a bandstop filter centered around the eigenfrequency of the flutter modes, which imposes a wash-out and roll-off effect on the controller's frequency response, thus achieving the desired behavior.…”

“…This technique has been used before, in the frame of flutter suppression studies, in, for example, [9,21], including wind-tunnel validation, but they focus on AFS for wings and airfoils only. More recently, the H ∞ techniques have been applied to the design of AFS controllers for the X-56A model [11] as well as to the related Mini MUTT UAV [10]. The X-56A and the Mini MUTT are flying-wing UAVs and, as such, are subjected to a strong coupling between flexible and rigid modes, a phenomenon known as body freedom flutter (BFF) [5,22].…”

Ground Vibration Tests on a Flexible Flying Wing Aircraft - Invited

“…1, 2 Therefore, designing advanced aeroservoelastic control laws are an active area of reasearch as they are required to suppress these aeroelastic instabilities. 1,3,4,5,6 Control-oriented, low order models are required to design and analyze such controllers. These simple models can be obtained either by model reduction of complex fluid/structure models 3,7 or using a flight dynamics approach.…”

“…Theoretical research is conducted on topics varying from modeling, 17, 18 model reduction 19,20 and control design. 21,6 Experimental work 13,14,22 and flight tests are also conducted to support and validate the theoretical research. Researchers at the UAV lab are working as part of a large industry/academic team for a NASA grant entitled 'Performance Adaptive Aeroelastic Wing' (PAAW).…”