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This paper presents a novel approach to modelling disc temperature distributions for use in health monitoring applications. The method combines singular value decomposition techniques with a series of low order transfer functions to predict the thermal response over a typical flight cycle. The model inputs reflect typical online measurements; engine speed, altitude, and compressor exit temperature. The model’s complexity has been reduced by projection and truncation of the reference data and its decompositions. The model predicts transient temperatures anywhere within the disc’s profile to an accuracy of 20K, and similarly the thermal gradient can be calculated in any direction within the 2D cross-section. The result is a simple model that is suitable for application in a FADEC system, with the accuracy necessary to safely extend a disc’s life through online calculation of fatigue damage incurred for individual flights. The paper concludes by comparing the model with an earlier method using a grey box physics approach.Copyright © 2015 by Rolls-Royce plc

Unlike earlier work, this paper identifies parts of the state space where it is possible to invoke feedback linearization and uses the closed-loop paradigm [8] to accommodate input constraints while utilizing an extremely efficient online receding horizon strategy. The key to this development is a partial invariance property that is derived through the use of linear difference inclusion made possible due to the bilinear nature of the models considered here. For all other parts of the state space the proposed algorithm switches to a bilinear controller which is designed to give invariance and feasibility over low complexity polytopes. Without increasing computational complexity, the use of bilinear controllers affords extra freedom with which to maximize the region of attraction. The improvements in terms of this and closed loop output performance are shown to be very significant.

Non-uniform temperature distributions in aircraft turbine and compressor discs result in high thermal stresses, particularly during engine transients. This has an important role in limiting disc life and subsequently engine time on wing between overhauls. Typically, and in the absence of specific engine real-time information, disc life is computed and declared at the design stage, for the whole fleet assuming worst case scenarios and flight profiles. This approach potentially leads to conservativeness in disc life prediction for the majority of the engines in the fleet. The purpose of this paper is to present a novel methodology to estimate the transient temperature distribution of such components in real-time, with the ultimate aim of using such information to improve knowledge about disc life consumed on an engine by engine basis. The paper describes a reduced order dynamic model that estimates the disc temperature distribution in real-time using only measured parameters such as spool speed, atmospheric pressure, and compressor exit temperatures. The model presented is a grey box model consisting of dynamic Linear Parameter Varying transfer functions that relate measured air temperatures to the relevant air temperatures in the disc cavities. Heat transfer from air to disc is modelled using rotor-stator aerothermal theory matched to validated computational fluid dynamic analyses. Lumped capacitance models and simplified explicit conduction models are used to simulate the disc temperature distribution. It is shown that this approach gives a scientifically accurate estimate of the temperature distribution both radially and axially, whilst being computationally efficient enough for real-time Engine Health Management implementation.Copyright © 2014 by Rolls-Royce plc

Practical observations of the nonreturn valve wear in aero-engine cabin-bleed systems suggest that such valves are subject to unstable behavior. A theoretical model for the prediction of nonreturn valve instabilities in air systems is proposed and a nonlinear state-space model of the nonreturn valve and air volume interaction is derived from first principles. Experimental work is used to identify both the dynamic characteristics and the flow properties of the valve, which are used to identify the coefficients within the model. Through frequency analysis of valve oscillatory behavior, the levels of damping within the system are identified. Finally, using a local linearization of the state-space model an explicit mathematical prediction of valve stability is derived based on system parameters. These predictions are used to generate a map of the transition from stable to unstable system behavior for low-speed air flow, which is in excellent agreement with experimental data.

Future manned and unmanned aircraft are likely to exploit operation at high-angle of attack regions of the flight envelope where there are large unsteady aerodynamic effects. Understanding these effects is of paramount importance for design and certification of flight control systems that will operate correctly in and harness the benefits of such extreme areas of the flight envelope. A significant amount of research effort has gone into developing nonlinear mathematical models that would predict accurately aerodynamic forces and moments in unsteady regimes. The purpose of this paper is to demonstrate an alternative strategy for studying aircraft behaviour in unsteady regimes through the application of hardware-in-the-loop concept to dynamic wind tunnel testing. The paper presents preliminary experimental results for the free flight prediction of a pitching airfoil at high angles of attack and evaluates its accuracy.

SUMMARY Here we consider how new experimental approaches in biomechanics can be used to attain a systems-level understanding of the dynamics of animal flight control. Our aim in this paper is not to provide detailed results and analysis, but rather to tackle several conceptual and methodological issues that have stood in the way of experimentalists in achieving this goal, and to offer tools for overcoming these. We begin by discussing the interplay between analytical and empirical methods, emphasizing that the structure of the models we use to analyse flight control dictates the empirical measurements we must make in order to parameterize them. We then provide a conceptual overview of tethered-flight paradigms, comparing classical `open-loop' and `closed-loop'setups, and describe a flight simulator that we have recently developed for making flight dynamics measurements on tethered insects. Next, we provide a conceptual overview of free-flight paradigms, focusing on the need to use system identification techniques in order to analyse the data they provide,and describe two new techniques that we have developed for making flight dynamics measurements on freely flying birds. First, we describe a technique for obtaining inertial measurements of the orientation, angular velocity and acceleration of a steppe eagle Aquila nipalensis in wide-ranging free flight, together with synchronized measurements of wing and tail kinematics using onboard instrumentation and video cameras. Second, we describe a photogrammetric method to measure the 3D wing kinematics of the eagle during take-off and landing. In each case, we provide demonstration data to illustrate the kinds of information available from each method. We conclude by discussing the prospects for systems-level analyses of flight control using these techniques and others like them.

This paper builds on previous work on optimal methodologies for the design of numerical subsystems for non-linear, uncertain hardware-in-the-loop (HWIL) simulators. Firstly, three important extensions to the existing methods are presented: the limitation to SISO systems is removed, allowing full MIMO design; a method for the design of an important and previously neglected component of the numerical system is described; and finally measurement noise and other unstructured uncertainty is tackled more rigorously and explicitly than previously. Secondly, as a case study the extended method is used to design a numerical system for a real HWIL simulator, and the results are shown to outperform those produced using classical design methods. The system used in the case study is a high-performance simulator for small aerodynamic objects that is in the final stages of development.

In this paper we present a methodology for quantifying the accuracy of hardware-in-the-loop (HWIL) simulators. HWIL simulation refers to the process of interfacing a subset of system hardware to a controller containing a numerical model of the rest of the system. Quantification of prediction accuracy is fundamentally important in all simulations, and presents unique challenges in the HWIL context. Hardware included in HWIL is often uncertain, non-linear, time varying, or generally unpredictable. Therefore, it is impractical to expect rich sources of information about this hardware's behaviour (e.g. probability distributions) as a basis for accuracy quantification. The methodology in this paper is able to produce a priori hard guaranteed bounds on simulation error based on a minimal set of assumptions regarding the behaviour of the hardware under test. The method is applicable to simulators with uncertain, non-linear, time varying hardware, provided the software subsystem, sensors and actuators are linear. A convergence proof is given. The method has been successfully demonstrated on a real experimental HWIL simulator.

Hardware-in-the-loop simulation is often used to evaluate the effectiveness of missile seeker sensors and guidance control systems in engagement scenarios. This paper describes a novel application of hardware-inthe-loop concept to wind-tunnel testing of aircraft. As unsteady aerodynamics are difficult to model numerically we propose a framework within which we can predict experimentally trajectory of aircraft in free flight. To demonstrate the concept, we consider an example for predictions of an airfoil trajectory in pitch in a falling free flight.

Here we present the first measurements of 3D acceleration and turn rate obtained from a bird in wide-ranging free flight. These data on body kinematics are accompanied by simultaneous onboard video sequences of the head and tail kinematics, and by video of the bird taken from the ground. The bird, a Steppe eagle Aquila nipalensis, carried a miniature inertial measurement unit outputting data on body orientation, acceleration and rate of turn, together with a data logger and two wireless video cameras pointing forward over the head and aft over the tail. The complete instrumentation package and harness weighed

This paper describes a new methodology for the design of control systems for hardware-in-the-loop (HWIL) simulators. A HWIL simulator consists of a subset of hardware from a real object interfaced to a control system, with the goal of matching the closed loop behaviour of the simulator to that of the complete object. The simulator design problem is qualitatively different from the standard control design problem; while classical control design aims to reduce uncertainty in plant behaviour, controllers in HWIL simulators must instead match simulator behaviour to the behaviour of the real object, which are both sensitive to the underlying uncertainty in the hardware. This represents a new design challenge that has not yet been addressed in the literature. The design method presented here is general and non-heuristic, and is applicable to a large class of uncertain, non-linear systems. The method is based on the computation of a sub-optimal solution to an optimisation over the space of possible controllers. Experimental results are presented that verify the practicality of the design method.

Hardware-in-the-loop simulation is a well established technique used in design and evaluation of control systems. The purpose of this paper is twofold. First it aims to identify research questions related to the design of hardware-in-the-loop simulation. Second it suggests possible measures of assessing the simulation fidelity of hardware-the-loop simulation.

This paper presents some novel ideas on building a low cost hardware-in-the-loop simulator for free flight simulation of unmanned air vehicles. We postulate that to achieve maximum transparency of hardware-in-the-loop simulation it is necessary to match the boundary conditions around the aircraft such that in terms of interaction with the environment the aircraft does not feel a difference between being flown in the lab and being in free flight. We introduce a pefromance index based on some recent research and we present a possible hardware and software architecture for a 2-DOF free flight simulator that is currently under development.

This paper describes a method for calculating invariant/feasible sets with respect to feedback linearising control law for input-affine nonlinear systems. Polytope simplification and partial invariance/feasibility are used to reduce the conservativeness of the affine difference inclusion. The output optimality of a feedback linearising control law provides a good basis for using such invariant sets in dual mode prediction MPC settings. Finally, a feasibility study of the proposed method is carried out on a benchmark continous-stirred tank reactor example. © IEE, 2005.

This paper describes a method for calculating invariant/feasible sets with respect to feedback linearizing control law for bilinear systems. The output optimality of a feedback linearizing control law provides a good basis for using such invariant sets in a dual mode MPC settings. © 2003 IEEE.

Nonminimum phase difficulties prevent the use of input-output feedback linearization. To avoid this problem, earlier work proposed the use of synthetic outputs with the view of placing zeros in prescribed locations under a condition of static equivalence. The objective here is also to introduce a synthetic output. However this is defined through the use of dynamic perturbations aimed at yielding maximum relative degree. Resetting of the initial conditions of the additional dynamics allows design freedom for closing the gap between the behaviour of the synthetic and actual output. The efficacy of the strategy in meeting pole placement is demonstrated by a simple numerical example. Copyright © 2005 IFAC.

The use of graph theory for NMPC approach which is used for fast sampling applications was discussed. The efficacy of the approach was analyzed by a design study. It was found that the main benefit of this approach is the significant reduction of on-line computation which makes it ideal for fast sampling applications.

This paper investigates interpolation based predictive control and presents a study of the properties and therefore limitations of the approach. This understanding is used to develop an efficient algorithm with guarantees of recursive feasibility and stability.

Relative degree and nonminimum phase difficulties limit the applicability of input-output feedback linearization; hence the need for approximations. Earlier work on predictive control of bilinear systems overcame these problems by means of interpolation between feedback linearization and state feedback, the former providing optimality and the latter guaranteeing feasibility and stability through the use of invariant/feasible polytopes. The current work also makes use of polytopes in preference to ellipsoids but achieves distinctly different objectives. First, it is shown that feedback linearization can be used over particular polytopes without needing to resort to either approximation or interpolation. Then, it is shown that invariant polytopes based on bilinear controllers can be much larger. These two approaches are combined in an algorithm that guarantees stability over much larger initial condition sets and gives much improved closed-loop performance.

In this paper we present a novel approach to NMPC based on state-space partitioning in conjunction with one-step ahead prediction and judicious use of graph theory to deliver an optimized control law which is robust, and is suitable for fast sampling applications. The only on-line computation required is a one-step ahead QP minimization which is linear in the number of inputs. The paper describes the general idea of the method and presents results, whose efficiency is demonstrated by means of a design study.

Non-minimum phase difficulties prevent the use of input-output feedback linearization. To avoid this problem earlier work proposed the use of synthetic outputs in order to place zeros in prescribed locations under a condition of static equivalence. The objective here is also to introduce a synthetic output but this is defined through the use of perturbations aimed at yielding maximum relative degree. Resetting of the initial conditions allows design freedom for closing the gap between the behaviour of the synthetic and actual output. © 2006 Elsevier Ltd. All rights reserved.

This paper considers interpolation between nonlinear control laws and their pre-computed invariant sets. It is shown that the resulting control law ensures the invariance and feasibility of the convex hull of the individual invariant sets. The method presented here is the extension of M. Bacic, et al. (2003) for the case of input-affine nonlinear systems.