Showing papers by "N.H. McClamroch published in 1998"

Output tracking for nonlinear non-minimum phase VTOL aircraft

Abstract: Vertical takeoff and landing (VTOL) aircraft provide interesting examples of nonlinear non-minimum phase systems. We introduce a new approach to output tracking for VTOL aircraft. First the nonminimum phase aircraft model is decomposed into a minimum phase part and a non-minimum phase part. For the minimum phase part dynamic inversion is used. The nonminimum phase part is simpler than the original and linear control can be used to achieve output tracking with stability. A robust control design methodology is proposed that incorporates perturbations that arise due to the tracking commands; this leads to restrictions on the class of tracking commands. The resulting control laws are nonlinear. Computer simulations indicate that the controllers are efficient and robust with respect to aircraft parameter uncertainties.

A decomposition approach to output tracking for multivariable nonlinear non-minimum phase systems

Abstract: It is common wisdom that if a multivariable nonlinear system, affine in the controls, is non-minimum phase then dynamic inversion is not applicable for output tracking. We introduce a new approach in which a multivariable nonlinear control system is decomposed into two parts: a minimum phase part and a non-minimum phase part. For the minimum phase part dynamic inversion can be used to achieve output tracking. The non-minimum phase part can be suitably controlled by linear feedback, but the decomposition often leads to a simpler non-minimum phase problem than the original. It is necessary to restrict the class of output commands to be tracked according to the properties of the non-minimum phase part. These restrictions are identified by using results from robust control theory for commands that are perturbations of a command corresponding to a closed loop equilibrium.

Control problems for planar motion of a rigid body with an unactuated internal degree of freedom

Abstract: We study control problems for a specific mechanical system consisting of a rigid base body with an unactuated internal degree of freedom. The key assumptions are that the translational and rotational motion of the base body can be completely controlled by external forces and moments, while the internal degree of freedom is unactuated. This specific example is representative of a class of underactuated mechanical control systems that are not linearly controllable. This class of control problems presents major theoretical and practical difficulties, and such models do arise in important terrestrial and spacecraft applications. Our focus in this paper is on the simplest mechanical example in this class; it illustrates important features and difficulties associated with the class of such problems.

Toward less conservative designs of multimode controllers for systems with state and control constraints

Abstract: Multimode controllers are an effective stabilization tool for linear and nonlinear systems under state and control constraints. The multimode controller switches between a family of controllers or a family of set-points in a way that guarantees satisfaction of state and control constraints without sacrificing the ultimate performance of the closed loop system. When unmeasured disturbances affect the operation of the system, the worst-case scenario approach to their treatment may result in unnecessary degree of conservatism in the multimode controller operation. This paper focuses on ways to reduce the conservatism by incorporating additional information about the disturbance such as rate bounds or stochastic description.

Three dimensional attitude and shape control of spacecraft with articulated appendages and reaction wheels

Abstract: Three dimensional attitude and shape control problems are studied for a class of spacecraft with articulated appendages and reaction wheels. We provide a new unified formulation which includes many of the attitude control problems that have been studied previously. The development is based on a nonlinear, drift-free, control model that characterizes the attitude and shape change dynamics, assuming zero angular momentum of the system. Controllability results are presented for the general case, and specialized results are identified for interesting multibody spacecraft configurations. These results are made explicit by providing computable formulas for the Lie brackets in terms of the spacecraft geometry, inertial properties, and shape. Emphasis is given to the interplay between the shape change dynamics and the attitude change dynamics.

Symbolic computations in nonlinear control of multibody space systems

Abstract: Symbolic computations can facilitate the development of models and control systems for multibody spacecraft systems. This paper describes a program written for Maple that performs symbolic model derivation and Lie bracket computations that are required for control development.