Advanced Control Research at NCSU
January 1, 2006
By Dr. N. Ma, NCSU

Nonlinear control, as a mainstream research field has found wide applications in various industrial disciplines, such as control of aircraft and robotics. The importance of nonlinear system theory is manifest because all physical systems are nonlinear in nature. To address modeling uncertainties and seek optimal performance, nonlinear robust control theory has been developed. The solution of the nonlinear optimal control problem is related to a difficult Hamilton-Jacobi-Isaacs equation. However, no systematic numerical algorithm is currently available to solve this equation efficiently.

Dr. Ashok Gopalarathnam		Dr. Fen Wu

Sponsored by the National Science Foundation and the National Aeronautics and Space Administration, Professor Fen Wu's group is developing advanced linear parameter-varying (LPV) control algorithms for nonlinear systems. This research is motivated by a popular nonlinear control design technique called gain-scheduling control methodology. The primary advantage of the gain-scheduling approach is that it is possible to meet performance objectives over a wide range of operating conditions while taking advantage of the wealth of tools and experience from the much simpler linear theory. As an alternative to conventional nonlinear optimal control theory, the LPV control techniques have been applied to robotic manipulators, turbo-fan engines, aircraft/missile autopilots and nuclear reactor problems.

Using this powerful control design technique, Professor Wu collaborated with Professor Gopalarathnam on the analysis and control of post-stall flight of highly maneuverable aircraft in a recent NASA project. The potential benefits from controlled flight of aircraft at near-stall and post-stall conditions include reduction of landing distances, capability of evasive maneuvers and emergency landings in unexpected situations, avoiding loss of control when encountering severe atmospheric turbulence and downbursts, etc. The goal of this work is to understand post-stall aerodynamics, and design flight control laws with enhanced control capability and robustness to large aerodynamic uncertainties in the post-stall region.

Due to lack of sufficient aerodynamic controls at post-stall conditions, Professor Wu's graduate students have investigated the use of thrust vectoring control to augment the conventional controls. Concurrently, they have developed a switching LPV control technique to design a family of controllers, each suitable in different angle of attack regions, and the technique to switch among them according to the evolution of angle of attack. Two different switching logics are used to overcome possible transient instability caused by the switching process. The control synthesis problem has been formulated and solved using efficient convex optimization techniques. The proposed switching LPV control technique has been applied to an F-16 aircraft model switching between low and high angle of attack regions with different control objectives and actuator sets (aerodynamic surface and thrust vectoring nozzle). The results thus far look promising.