Hirobumi Ohta

A Synthesis of Reduced-order H∞ Controllers for Active Flutter Suppression Systems

Abstract The objective of this paper is to present a synthesis method of reduced-order active flutter suppression (AFS) systems. AFS systems of a two-dimensional airfoil (2D-AFS) are first designed by mixed sensitivity reduction problem in H ∞ control theory (H ∞ -mix). Secondary to reduce the order of the H ∞ controller, the coprime factorization (weighted) controller reduction method (CFW) is then extended lo apply the controllers that are designed by using other than the LQG control theory. It is shown in the numerical example that H ∞ -mix controllers give more increase of the flutter velocity than the LQG and H 2 -mix ones, and the extended CFW method is useful to reduce the order of Hie H ∞ -mix controllers

Synthesis of Adaptive Quadratic Control Systems Using Parameter Region.

There still remain many problems to be solved for conventional adaptive control theory to be applied to a real plant. Some of them are restrictive conditions imposed on the plant, low robustness and poor transient properties. To overcome these problems, we present a new design method of adaptive controllers that are robust to plant noise and uncertainties and have a good transient property. We first propose an algorithm to evaluate and reduce the parameter region in which the transfer function of a real plant exists. According to these plant informations, we can judge how closely the identification is made towards real parameters and whether it is better to continue identification or to execute some control strategy. Stabilizability of the plant is determined by one Riccati equation so that the plant can be stabilized by a quadratic stabilization controller. Some numerical examples together with application to a flight controller design are given to illustrate and substantiate the effectiveness of the proposed method.

Effects of Nonlinear Aerodynamics on the Stability of the Longitudinal Motion of a High-Speed Surface Transport.

The objective of this study is to describe effects of nonlinear aerodynamics on the stability of the longitudinal motion of a high-speed surface transport. This paper focuses on the aerodynamic nonlinearity due to ground effects or wall effects that are often observed for high-speed vehicles. First of all, 1 DOF (degree-of-freedom) calculations based on the inherent nonlinearity of the aerodynamic characteristics near the ground are presented. Then, 2 DOF analyses taking into account the interactive nonlinear effects of the aerodynamics are discussed. The trajectories are examined on the phase plane and the center manifold theory is applied to the analyses. It is shown that the stability of motion depends not on the nonlinearity of 1 DOF but on that of 2 DOF.

Sensor Location and Zeros in Active Control of Aeroelastic Airfoils.

Transfer function zeros of active control systems are studied for a two dimensional typical airfoil in an in-compressible flow. Both steady and unsteady aerodynamics are used for the modeling of the dynamics. Sensor location which gives us a minimum phase transfer function is examined over the airfoil area between the leading edge and the hinge line of the tailing edge control surface. Three types of zeros exist in the transfer function with an unsteady aerodynamic modeling. It is shown that the zeros corresponding to the ones derived from the steady aerodynamics determine an upper and lower boundaries of the flight velocity through which the transfer function changes its phase characteristics. The zeros that are originally contained in the unsteady aerodynamic modeling do not contribute to the phase change. However, two other zeros that are generated through the modeling determine a front boundary of the sensor location. It is to be noted in flutter control systems that a zero always exists near the critical pole and the transfer function shows a situation of near pole-zero cancellation around the flutter point.

Analysis of Aircraft Cruise Using Singular Perturbation Methodology.

The problem of determining optimal cruise trajectories is examined for a performance index that is a linear combination of time and fuel, and an application of the singular perturbation method to the optimization problem is discussed. It is shown in this paper that use of the singular perturbation technique combined with a multiple time scaling approach leads us to obtain a control solution that has an algebraic feedback form. The condition of optimality for a steady-state cruise is checked by the II test which consists of the second order derivatives of the criterion. Concerning with the optimality of the steady-state arcs, it is found that there exists a periodic control that can reduce fuel consumption if the weighting coefficient on fuel is large. The optimal trajectories using the periodic control are calculated in a numerical example and show that about 1 percent increase of fuel economy is achieved for the case of the minimum fuel trajectory.

Robust Flight Control Using a Plant Description of Normalized Coprime Factorization

A New H∞ control theory which uses a plant description of the normalized left coprime factorization (NLCF) is applied to a pitch attitude control of the short period mode of airplanes. In this theory, the selection of pre and/or post compensators called shaping functions is of practical importance to determine the closed-loop characteristics. This paper presents that the closed-loop poles determined by the theory are given as the eigenvalues of two matrices, one for the augmented plant poles and the other for the controller poles. It is shown as well that the closed-loop poles of the augmented plant are the same as those of the linear quadratic regulator (LQR) theory. These properties enable us to design a compensator that takes not only frequency domain properties (e. g. robustness) but also time domain ones (e. g. output step response) into consideration. Some trade-off is necessary between robustness and fast, less overshooted output responses.

Designs of a gust load alleviation system for a cantilevered elastic rectangular wing and wind-tunnel tests.

This paper describes designs of the gust load alleviation (GLA) system and its wind-tunnel tests. The objective is to verify the effectiveness of control system syntheses which we have examined theoretically in the active flutter suppression (AFS). Applying the syntheses to the GLA system of a cantilevered elastic rectangular wing, the 1st or the 2nd order controllers can be constructed. It is shown in the wind-tunnel tests that the bending moment at the wing root is reduced to 66.2% of the control-off value as the best by using a designed reduced-order controller. Furthermore, the robustness for the parameter perturbation of the plant is verified in the tests where the wind velocity is 50% higher than the designed wind velocity.

CONTROLLER DESIGNS OF A GUST LOAD ALLEVIATION SYSTEM FOR AN ELASTIC RECTANGULAR WING

This paper proposes two design methods of reduced-order controllers for gust load alleviation (GLA) systems of an elastic wing, and examines the control performance using both simulation studies and wind-tunnel experiments. One of the methods is based on the use of the generalized Hessenberg representation (GHR) in the time domain, and the other method is the one in the frequency domain termed the Nyquist frequency approximation (NFA). The former yields quasi-optimal controllers in the sense of minimizing quadratic performance indices, whereas the latter can yield controllers with increased stability margin. Applying these methods to the design of GLA systems of a cantilevered elastic rectangular wing, low-order controllers, the 1st- or the 2nd-order, can be constructed, and they showed as good performance as the LQG compensator.

Gust generation in a wind-tunnel and estimation of the model.

In this paper three types of gust generators are tested in a wind-tunnel to examine their characteristics. The generators which were investigated are the flag method, turbulent-flow-glid method and their combined method. The gust velocities generated by these methods are measured using a hot wire anemometer of X type, and the characteristics of the power spectra are examined for several parameters such as wind velocity, the size and thickness of a flag. To estimate a gust model for controller designs, discrete models based on an ARMA process are first identified by the AIC method. A continuous model with a lower dimension is then derived by selecting the coefficients of a rational function. These selections are based on the quasi-Newton method which minimizes the square sum of errors at some discrete frequencies. It is found that the gust generators considered here can furnish one with a convenient mean for wind-tunnel experiments. In particular, the estimated gust model may be expected to provide a simple mathematical model for designing controllers.

Controller Designs of a Gust Load Alleviation System for an Elastic Rectangular Wing

Abstract This paper proposes two design methods of reduced-order controllers for gust load alleviation (GLA) systems of an elastic wing, and examines the control performance using both simulation studies and wind-tunnel experiments. One of the methods is based on the use of the generalized Hessenberg representation (GHR) in the time domain, and the other method is the one in the frequency domain termed the Nyquist frequency approximation (NFA). The former yields quasi-optimal controllers in the sense of minimizing quadratic performance indices, whereas the latter can yield controllers with increased stability margin. Applying these methods to the design of GLA systems of a cantilevered elastic rectangular wing, low-order controllers, the 1st- or the 2nd-order, can be constructed, and they showed as good performance as the LQG compensator.