Richard J. Adams

Advanced gain-scheduling techniques for uncertain systems

This paper is concerned with the design of gain-scheduled controllers for uncertain linear parameter-varying systems. Two alternative design techniques for constructing such controllers are discussed. Both techniques are amenable to linear matrix inequality problems via a gridding of the parameter space and a selection of basis functions. These problems are then readily solvable using available tools in convex semidefinite programming. When used together, these techniques provide complementary advantages of reduced computational burden and ease of controller implementation. The problem of synthesis for robust performance is then addressed by a new scaling approach for gain-scheduled control. The validity of the theoretical results are demonstrated through a two-link flexible manipulator design example. This is a challenging problem that requires scheduling of the controller in the manipulator geometry and robustness in face of uncertainty in the high-frequency range.

Stable haptic interaction with virtual environments

This paper addresses fundamental stability and performance issues associated with haptic interaction. It generalizes and extends the concept of a virtual coupling network, an artificial link between the haptic display and a virtual world, to include both the impedance and admittance models of haptic interaction. A benchmark example exposes an important duality between these two cases. Linear circuit theory is used to develop necessary and sufficient conditions for the stability of a haptic simulation, assuming the human operator and virtual environment are passive. These equations lead to an explicit design procedure for virtual coupling networks which give maximum performance while guaranteeing stability. By decoupling the haptic display control problem from the design of virtual environments, the use of a virtual coupling network frees the developer of haptic-enabled virtual reality models from issues of mechanical stability.

Control law design for haptic interfaces to virtual reality

The goal of control law design for haptic displays is to provide a safe and stable user interface while maximizing the operators sense of kinesthetic immersion in a virtual environment. This paper outlines a control design approach which stabilizes a haptic interface when coupled to a broad class of human operators and virtual environments. Two-port absolute stability criteria are used to develop explicit control law design bounds for two different haptic display implementations: the impedance display and admittance display. The strengths and weaknesses of each approach are illustrated through numerical and experimental results for a three degree-of-freedom device. The example highlights the ability of the proposed design procedure to handle some of the more difficult problems in control law synthesis for haptics, including structural flexibility and noncollocation of sensors and actuators.

Robust multivariable flight control

Manual flight control system design for fighter aircraft is one of the most demanding problems in automatic control. Fighter Aircraft dynamics generally have highly coupled uncertain and nonlinear dynamics. Multivariable control design techniques offer a solution to this problem. Robust Multivariable Flight Control provides the background, theory and examples for full envelope manual flight control system design. It gives a versatile framework for the application of advanced multivariable control theory to aircraft control problems. Two design case studies are presented for the manual flight control of lateral/directional axes of the VISTA-F-16 test vehicle and an F-18 trust vectoring system. They demonstrate the interplay between theory and the physical features of the systems.

A two-port framework for the design of unconditionally stable haptic interfaces

A haptic interface is a kinesthetic link between a human operator and a virtual environment. This paper addresses stability and performance issues associated with haptic interaction. It generalizes and extends the concept of a virtual coupling network, an artificial connection between a haptic display and a virtual world, to include both the impedance and admittance models of haptic interaction. A benchmark example exposes an important duality between these two cases. Linear circuit theory is used to develop necessary and sufficient conditions for the stability of a haptic simulation, assuming the human operator and virtual environment are passive. These equations lead to an explicit design procedure for virtual coupling networks which give maximum performance while guaranteeing stability. By decoupling the haptic display control problem from the design of virtual environments, the use of a virtual coupling network frees the developer of haptic-enabled virtual reality models from issues of mechanical stability.

Design of Nonlinear Control Laws for High-Angle-of-Attack Flight

High-angle-of-attack flight control laws are developed for a supermaneuvera ble fighter aircraft. The methods of dynamic inversion and structured singular value synthesis are combined into an approach which addresses both the nonlinearity and robustness problems of flight at extreme operating conditions. The primary purpose of the dynamic inversion control elements is to linearize the vehicle response across the flight envelope. Structured singular value synthesis is used to design a dynamic controller which provides robust tracking to pilot commands. The resulting control system achieves desired flying qualities and guarantees a large margin of robustness to uncertainties for high-angle-of-attack flight conditions. High-fidelity nonlinear simulation results show that the combined dynamic inversion /structured singular value synthesis control law achieves a high level of performance in a realistic environment.

Robust flight control design using dynamic inversion and structured singular value synthesis

A direct methodology for the design of flight control systems is introduced. The design approach uses a control selector and an inner/outer loop structure to achieve robustness and performance across the flight envelope. A control selector normalizes control effectiveness with respect to generalized inputs. An inner loop uses dynamic inversion to equalize plant dynamics across the flight envelope. An outer loop is designed around this equalized plant, using mu -synthesis to achieve performance and robustness goals. The methodology is applied to the design of a manual flight control system for the lateral axis of a fighter aircraft. Analysis shows that the inner/outer loop approach produces designs with excellent performance and robustness for a broad range of operating conditions. The direct incorporation of design goals such as flying qualities requirements and the elimination of gain scheduling make this direct methodology an effective and efficient alternative to traditional flight control design approaches. >

Stable haptic interaction using the Excalibur force display

Creating a compelling haptic sense of immersion in a virtual environment is a challenging task for the control engineer. A haptic display must render both low impedance free-space motion and high impedance rigid constraints while ensuring stable interaction. This paper outlines a control design approach for the most common haptic display implementation, the impedance display. Two-port absolute stability criteria are used to develop explicit design bounds for virtual coupling networks which guarantee system stability for a broad class of human operators and virtual environments. The technique is applied to the Excalibur three-axis force display. The resulting absolutely stable haptic interface is the centerpiece of a virtual building block simulation which emulates the behavior of LEGO/sup TM/ bricks in a virtual environment.

Reduced-Order H^ Compensator Design for an Aircraft Control Problem

A recently introduced method for designing //<» compensators based on minimal-order observers is considered for an aircraft control problem. The purpose of this paper is to bridge the gap between theory and application by presenting a practical utilization of a new design approach. Manual flight control systems for the lateral axis of a fighter aircraft are developed using both full-order and reduced-order compensators, and the results are compared. It is demonstrated that this method can be used to directly design reduced-order compensators that result in a system satisfying a closed-loop //« bound.

Full envelope multivariable control law synthesis for a high-performance test aircraft

A full envelope multivariable flight control system is developed for the Variable Stability In-Flight Simulator Test Aircraft (VISTA). Separate control laws are designed for the longitudinal and lateral directional axes. Output feedback controllers with integral error feedback are created using linear quadratic synthesis and simple linear transformations. Longitudinal stick inputs are used to generate an angle-of-attack command at low-dynamic pressure and a normal acceleration command at high-dynamic pressure. Lateral stick inputs are used to generate a stability axis roll rate command, and rudder pedal inputs are used to generate a sideslip command. Linear point designs are integrated into a gain schedule to create a full envelope nonlinear control system. Flying qualities are evaluated according to military standards and shown to be satisfactory for a wide operating envelope. Classical gain and phase margins and analysis of the structured singular value show robustness to be acceptable for a wide operating envelope.