Ian J. Fialho

Road adaptive active suspension design using linear parameter-varying gain-scheduling

This paper presents a novel approach to the design of road adaptive active suspensions via a combination of linear parameter-varying control and nonlinear backstepping techniques. Two levels of adaptation are considered: the lower level control design shapes the nonlinear characteristics of the vehicle suspension as a function road conditions, while the higher level design involves adaptive switching between these different nonlinear characteristics, based on the road conditions. A quarter car suspension model with a nonlinear dynamic model of the hydraulic actuator is employed. The suspension deflection, car body acceleration, hydraulic pressure drop, and spool valve displacement are used as feedback signals. Nonlinear simulations show that these adaptive suspension controllers provide superior passenger comfort over the whole range of road conditions.

Design of Nonlinear Controllers for Active Vehicle Suspensions Using Parameter-Varying Control Synthesis

Nonlinear suspension controllers have the potential to achieve superior performance compared to their linear counterparts. A nonlinear controller can focus on maximizing passenger comfort when the suspension deflection is small compared to its structural limit. As the deflection limit is approached, the controller can shift focus to prevent the suspension deflection from exceeding this limit. This results in superior ride quality over the range of road surfaces, as well as reduced wear of suspension components. This paper presents a novel approach to the design of such nonlinear controllers, based on linear parameter-varying control techniques. Parameter-dependent weighting functions are used to design active suspensions that stiffen as the suspension limits are reached. The controllers use only suspension deflection as a feedback signal. The proposed framework easily extends to the more general case where all the three main performance metrics, i.e., passenger comfort, suspension travel and road holding are considered, and to the design of road adaptive suspensions.

On the design of LPV controllers for the F-14 aircraft lateral-directional axis during powered approach

Presents the preliminary design and testing of a linear parameter-varying (LPV) controller for the F-14 aircraft lateral-directional axis. The controller is designed for the powered approach flight envelope, which consists of angle-of-attack/airspeed variations from 2 degrees/182 knots to 14 degrees/126 knots. The design is based on four linearized models at /spl alpha/=2, 6, 10.5, and 14 degrees angle-of-attack. The resulting LPV controller performs well when implemented in a Simulink nonlinear simulation of the F-14 aircraft, in the full order Fortran nonlinear simulation, and in pilot-in-the-loop simulations at the Naval Air Warfare Center at Patuxent River, Maryland.

Worst case analysis of nonlinear systems

The authors work out a framework for evaluating the performance of a continuous-time nonlinear system when this is quantified as the maximal value at an output port under bounded disturbances-the disturbance problem. This is useful in computing gain functions and L/sub /spl infin//-induced norms, which are often used to characterize performance and robustness of feedback systems. The approach is variational and relies on the theory of viscosity solutions of Hamilton-Jacobi equations. Convergence of Euler approximation schemes via discrete dynamic programming is established. The authors also provide an algorithm to compute upper bounds for value functions. Differences between the disturbance problem and the optimal control problem are noted, and a proof of convergence of approximation schemes for the control problem is given. Case studies are presented which assess the robustness of a feedback system and the quality of trajectory tracking in the presence of structured uncertainty.

Gain-Scheduled Lateral Control of the F-14 Aircraft during Powered Approach Landing

The design of a linear fractional transformation gain-scheduled controller, scheduled on angle of attack, for the F-14 aircraftlateral-directionalaxisis presented.Thecontrolleris designed forthe powered approach e ight phase, during which the angleof attack and corresponding airspeed varies from 2 deg and 182 kn to 14 deg and 126 kn. A linearfractional modelof the lateral dynamics is constructed based on four linearized models that correspond to 2, 6, 10.5, and 14 deg angle of attack. Using parameter-dependentfunctions, a controller is designed that dependsin a linear fractional manner on angle of attack and delivers uniform handling quality over angle-of-attack variations that lie between 2 ‐14 deg. The resulting controller performs well when implemented in a nonlinear simulation model of the F-14 aircraft. Nomenclature p = roll rate r = yaw rate v = lateral velocity yac = lateral acceleration a = angle of attack b = sideslip angle d dsp = differential spoiler dee ection d dstab = differential stabilizer dee ection d lstk = lateral stick input d rud = rudder dee ection d rudp = rudder pedal input u = bank angle

l/sub 2/ state-feedback control with a prescribed rate of exponential convergence

In this paper we consider the l/sub 1/-state feedback problem with an internal stability constraint. In particular, we establish the connection between controlled-invariant contractive sets and static control laws that achieve a level of l/sub 1/ performance as well as a desired unforced rate of convergence. We outline two algorithms for computing controlled-invariant contractive sets. The first is a modification of standard recursive techniques used in the literature, whereas the second is based on dynamic games and involves solving an appropriate discrete Isaacs recursion. The latter approach results in a min-max characterization of l/sub 1/-state feedback controllers. We point out that the Isaacs recursion provides a one-shot (as opposed to iterative) computation of the optimal l/sub 1/ performance.

On stability and performance of sampled-data systems subject to wordlength constraint

Studies the effects of finite precision controller implementation on the stability and performance of sampled-data systems. A framework is developed is which these effects can be analyzed from a statistical point of view. The authors analyze the behavior of finite precision controllers at fast sampling rates and consider the problem of /spl Lscr//sub 2/-induced norm minimization, subject to a wordlength constraint.<<ETX>>

Linear fractional transformation control of the F-14 aircraft lateral-directional axis during powered approach landing

Presents the design of a linear fractional transformation (LFT) gain-scheduled controller, scheduled on angle-of-attack, for the F-14 aircraft lateral-directional axis. The controller is designed for the powered approach flight envelope during which angle-of-attack/airspeed variations range from 2 degrees/182 knots to 14 degrees/126 knots. A linear fractional model of the lateral dynamics is constructed based on four linearized models that correspond to 2,6,10.5, and 14 degrees angle-of-attack. The resulting LFT controller performs well when implemented in a Simulink nonlinear simulation of the F-14 aircraft.

A Linear Parameter-varying Framework for Adaptive Active Microgravity Isolation

This paper presents a novel approach to the design of adaptive active vibration isolation systems using linear parameter-varying (LPV) control techniques. The proposed LPV controller is scheduled based on the relative position of the vibrating system, as well as a parameter that characterizes the harshness of the base motion. By scheduling on relative position, the controller is able to shift its focus from a “soft” setting to a “stiff” setting depending on the need for acceleration minimization or relative displacement reduction. An outer loop that is scheduled based on a parameter that quantifies base motion harshness controls the way in which the system transitions between the “soft” and “stiff” settings. Parameter-dependent weighting functions are used to achieve these objectives. A single degree-of-freedom rack-level microgravity vibration isolation model is used to demonstrate the proposed adaptive design framework. The objective is to provide stringent closed-loop isolation characteristics and at the same time restrict the relative motion of the system, so as to prevent it from bumping into its hardstop bumpers. Simulations show that the parameter-varying controller provides excellent isolation with simultaneous position control despite the large variability in the harshness of environmental disturbances.

Adaptive vehicle suspension design using LPV methods

In order to realize the full potential of active suspensions the controller should have the capability of adapting to changing road environments. We present a framework for the design of such adaptive suspension controllers using a combination of linear parameter-varying (LPV) control and nonlinear backstepping techniques. Two levels of adaptation are considered: the lower level control design shapes the nonlinear characteristics of the vehicle suspension based on road conditions, while the higher level design involves adaptive switching between these different nonlinear characteristics. Nonlinear simulations show that these adaptive suspension controllers provide superior passenger comfort over the whole range of road conditions.