Trajectory Control for Very Flexible Aircraft
Abstract
This paper focuses on trajectory control of the 6-DOF body fixed reference frame located on a very flexible aircraft. The 6-DOF equations of motion of a reference point on the aircraft are coupled with the aeroelastic equations that govern the geometrically nonlinear structural response of the vehicle. A low-order strain-based nonlinear structural analysis coupled with unsteady finitestate potential flow aerodynamics form the basis for the aeroelastic model. The nonlinear beam finite element structural model assumes constant strain over an element in extension, twist, and in/out of plane bending. The geometrically nonlinear structural formulation, the finite state aerodynamic model, and the nonlinear rigid body equations together provide a low-order complete nonlinear aircraft analysis tool. Due to the inherent flexibility of the aircraft modeling, the low order structural fre quencies are of the same order as the rigid body modes. This creates a coupling which cannot be separated by previous control schemes. The flexibility must be accounted for directly in the controller development. To accomplish this a heuristic approach based upon pilot behavior is developed. This approach separates the problem into two parts: a fast inner-loop and a slower outer-loop. Dominant kinematic nonlinearities are handled in the outer-loop while the inner-loop is further separated into a lateral and longitudinal motion. Control of the inner-loop lateral motion is accomplished using a standard Linear Quadratic Regulator. For the longitudinal motion Dynamic Inversion is utilized. Differences between the desired and actual trajectories are handled in the nonlinear outer-loop using traditional proportional-integral-derivative design guidelines. The closed loop time integration is accomplished using an implicit modified Newmark method. Numerical simulations are presented highlighting the strengths and weaknesses of the method.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 30, 2006
- Accession Number
- ADA462761
Entities
People
- Cesnik E. Carlos
- Christopher M. Shearer
Organizations
- University of Michigan