An Overview of Integral Quadratic Constraints for Delayed Nonlinear and Parameter-Varying Systems

A general framework is presented for analyzing the stability and performance of nonlinear and linear parameter varying (LPV) time delayed systems. First, the input/output behavior of the time delay operator is bounded in the frequency domain by integral quadratic constraints (IQCs). A constant delay is a linear, time-invariant system and this leads to a simple, intuitive interpretation for these frequency domain constraints. This simple interpretation is used to derive new IQCs for both constant and varying delays. Second, the performance of nonlinear and LPV delayed systems is bounded using dissipation inequalities that incorporate IQCs. This step makes use of recent results that show, under mild technical conditions, that an IQC has an equivalent representation as a finite-horizon time-domain constraint. Numerical examples are provided to demonstrate the effectiveness of the method for both class of systems

Robust Modal Damping Control for Active Flutter Suppression

Flutter is an unstable oscillation caused by the interaction of aerodynamics and structural dynamics. It is current practice to operate aircraft well below their open-loop flutter speed in a stable flight regime. For future aircraft, weight reduction and aerodynamically efficient high aspect ratio wing design reduce structural stiffness and thus reduce flutter speed. Active control of the flutter phenomena can counter adverse aeroservoelastic effects and allow operation of an aircraft beyond its open-loop flutter speed. This paper presents a systematic robust control design method for active flutter suppression. It extends the standard four block mixed sensitivity formulation by a means to target specific dynamic modes and add damping. This enables a control design to augment damping of critical flutter modes with minimal impact on the rigid-body autopilots. Finally, the design scheme uses a manageably low number of tunable parameters with a clear physical interpretation. Tuning the controller is hence considerably easier than with standard approaches. The method is demonstrated by designing an active flutter suppression controller for a small, flexible unmanned aircraft and verified in simulation

Polynomial Chaos Approximation of the Quadratic Performance of Uncertain Time-Varying Linear Systems

This paper presents a novel approach to robustness analysis based on quadratic performance metrics of uncertain time-varying systems. The considered time-varying systems are assumed to be linear and defined over a finite time horizon. The uncertainties are described in the form of real-valued random variables with a known probability distribution. The quadratic performance problem for this class of systems can be posed as a parametric Riccati differential equation (RDE). A new approach based on polynomial chaos expansion is proposed that can approximately solve the resulting parametric RDE and, thus, provide an approximation of the quadratic performance. Moreover, it is shown that for a zeroth order expansion this approximation is in fact a lower bound to the actual quadratic performance. The effectiveness of the approach is demonstrated on the example of a worst-case performance analysis of a space launcher during its atmospheric ascent

A Comparison of Centralised and Decentralised Scheduling Methods Using a Simple Benchmark System

This paper is intended to provide a comparison of a centralised scheduling system, a simple Multi-Agent System (MAS), and a Mixed Integer Linear Programming (MILP) formulation. The systems are tested on a simulation of a small scale flexible job shop that has machines in series and in parallel. The performance of the systems is assessed by running a batch of randomized jobs and comparing the number of late jobs and the length of time by which they are delayed. Additionally, simulations with random product failure are included to assess how well the systems perform with disruptions

Robust Path-following Control with Anti-Windup for HALE Aircraft

In this paper, a robust path-tracking controller for a High Altitude Long Endurance (HALE) aircraft is presented. The main control paradigm for operating a HALE aircraft consists of a basic path following control, i.e. tracking a reference flight path and airspeed while dealing with very limited thrust. The priority lies in keeping airspeed inside the small flight envelope of HALE aircraft even during saturated thrust. For the basic path following objective, a mixed sensitivity approach is proposed which can easily deal with decoupled tracking and robustness requirements. To deal with saturated control inputs, an anti-windup scheme is incorporated in the control design. A novel observer-based mixed sensitivity design is used which allows directly using classical anti-windup methods based on back-calculation. The control design is verified in nonlinear simulation and compared to a classical total energy control based controller

Actuator and sensor selection for robust control of aeroservoelastic systems

Abstract — This paper proposes an approach for actuator and sensor selection for a small flexible aircraft. The approach is based on the synthesis of robust controllers accounting for model uncertainty. The objective is to find, out of a finite set of actuator/sensor configurations available in the aircraft, the best configuration that provides sufficient robustness and desired performance. The results show that the ability to stabilize and achieve performance objectives of aeroservoelastic systems is highly dependent on the selection of actuators and sensors for feedback control. I

Robust autopilot design for landing a large civil aircraft in crosswind

A comprehensive autolanding design for a representative model of a twin-engined commercial aircraft is presented in this paper. To facilitate the design task and minimize control law switching, a cascaded control structure is selected which resembles integrator chains. Classical loopshaping and robust control techniques are used to design the individual control loops. The emphasis is on providing a complete and comprehensive qualitative design strategy. The control system’s ability to safely land the aircraft despite strong crosswind in a variety of possible scenarios is demonstrated in an industry-grade verification campaign. Nonlinear Monte Carlo simulations of the airliner model are used to assess the risk of unsafe landing conditions and provide insight into the performance characteristics and limitations of the proposed control system

Air drag coefficient of textile-covered elastic cylinders – preliminary aerodynamic studies

This paper presents preliminary experimental results on the influence on the aerodynamic drag of a cylinder from the cylinder type (i.e., rigid or soft) and its textile surface. Both a rigid cylinder and a soft-body cylinder, with a gelatin layer, each with five different textile surfaces were measured in the wind tunnel using force measurement technology. The drag coefficient was determined for several Reynolds numbers. The study shows that the elasticity of a cylinder has a significant influence on the drag force and the airflow type. However, the influence of the soft-body cylinder depends on the respective fabric. With the given measurements, no exact statements can yet be made to quantify the influence. This influence must be studied independently and in conjunction with the textile surface in order to gain understanding of the overall system of airflow, textile and elastic body

Robust Performance Analysis: a Review of Techniques for Dealing with Infinite Dimensional LMIs

This paper compares three techniques for dealing with infinite dimensional linear matrix inequalities (LMIs) for robust performance analysis: the gridding based approximation, the polytopic relaxation and the linear fractional representation based relaxation. The latter draws on the Full Block S-Procedure with different types of multipliers. All three techniques are applied in two benchmark studies at the example of an aeroelastic system. The studies are backed up by results from the Robust Control Toolbox for Matlab