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  • Journal article
    Palladino M, Vinter RB, 2014,

    Minimizers That Are Not Also Relaxed Minimizers

    , SIAM Journal on Control and Optimization, Vol: 52, Pages: 2164-2179, ISSN: 1095-7138

    Relaxation is a widely used regularization procedure in optimal control, involving the replacement of velocity sets by their convex hulls, to ensure the existence of a minimizer. It can be an important step in the construction of suboptimal controls for the original, unrelaxed, optimal control problem (which may not have a minimizer), based on obtaining a minimizer for the relaxed problem and approximating it. In some cases the infimum cost of the unrelaxed problem is strictly greater than the infimum cost over relaxed state trajectories; we need to identify such situations because then the above procedure fails. The noncoincidence of these two infima leads also to a breakdown of the dynamic programming method because, typically, solving the Hamilton--Jacobi equation yields the minimum cost of the relaxed, not the original, optimal control problem. Following on from earlier work by Warga, we explore the relation between, on the one hand, noncoincidence of the minimum cost of the optimal control and its relaxation and, on the other, abnormality of necessary conditions (in the sense that they take a degenerate form in which the cost multiplier is set to zero). Two kinds of theorems are proved, depending on whether we focus attention on minimizers of the unrelaxed or the relaxed formulation of the optimal control problem. One kind asserts that a local minimizer which is not also a relaxed local minimizer satisfies an abnormal form of the Hamiltonian inclusion. The other asserts that a relaxed local minimizer that is not also a local minimizer also satisfies an abnormal form of Hamiltonian inclusion.
  • Journal article
    Bettiol P, Frankowska H, Vinter RB, 2014,

    Improved Sensitivity Relations in State Constrained Optimal Control

    , Applied Mathematics and Optimization, Vol: 71, Pages: 353-377, ISSN: 1432-0606

    Sensitivity relations in optimal control provide an interpretation of the costate trajectory and the Hamiltonian, evaluated along an optimal trajectory, in terms of gradients of the value function. While sensitivity relations are a straightforward consequence of standard transversality conditions for state constraint free optimal control problems formulated in terms of control-dependent differential equations with smooth data, their verification for problems with either pathwise state constraints, nonsmooth data, or for problems where the dynamic constraint takes the form of a differential inclusion, requires careful analysis. In this paper we establish validity of both ‘full’ and ‘partial’ sensitivity relations for an adjoint state of the maximum principle, for optimal control problems with pathwise state constraints, where the underlying control system is described by a differential inclusion. The partial sensitivity relation interprets the costate in terms of partial Clarke subgradients of the value function with respect to the state variable, while the full sensitivity relation interprets the couple, comprising the costate and Hamiltonian, as the Clarke subgradient of the value function with respect to both time and state variables. These relations are distinct because, for nonsmooth data, the partial Clarke subdifferential does not coincide with the projection of the (full) Clarke subdifferential on the relevant coordinate space. We show for the first time (even for problems without state constraints) that a costate trajectory can be chosen to satisfy the partial and full sensitivity relations simultaneously. The partial sensitivity relation in this paper is new for state constraint problems, while the full sensitivity relation improves on earlier results in the literature (for optimal control problems formulated in terms of Lipschitz continuous multifunctions), because a less restrictive inward pointing hypothesis is invoked in the proo
  • Journal article
    Bottrell N, Green TC, 2014,

    Comparison of current-limiting strategies during fault ride-through of inverters to prevent latch-up and wind-up

    , IEEE Transactions on Power Electronics, Vol: 29, Pages: 3786-3797, ISSN: 0885-8993

    Transient stability of a power network requires that generators remain synchronized and return to normal power export once a fault is cleared. For inverter-interfaced generators, one must ensure that current and voltage limiters do not latch-up and that controller integrators do not wind-up. A comparison of current-limiting strategies during fault ride-through of inverters to prevent latch-up and wind-up is presented. A voltage-controlled inverter with an inner current controller is used in this paper. Instantaneous limiting (saturation) and latched limiting with a variety of reset strategies are tested to check for correct operation when a fault is applied and cleared. All the cases were tested on an experimental system using 10-kVA inverters and low-impedance three-phase faults. The experimental results showing the current and voltage waveforms of the inverter are presented to test whether each strategy correctly transitioned from current limiting to normal operation once the fault was cleared and to examine the extent to which controller wind-up was a problem. Conclusions are drawn as to which current-limiting strategies provide good performance in ride-through and recovery from faults.
  • Journal article
    Howey DA, Mitcheson PD, Yufit V, Offer GJ, Brandon NPet al., 2014,

    Online measurement of battery impedance using motor controller excitation

    , IEEE Transactions on Vehicular Technology, Vol: 63, Pages: 2557-2566, ISSN: 0018-9545

    This paper presents a fast cost-effective technique for the measurement of battery impedance online in an application such as an electric or hybrid vehicle. Impedance measurements on lithium-ion batteries between 1 Hz and 2 kHz give information about the electrochemical reactions within a cell, which relates to the state of charge (SOC), internal temperature, and state of health (SOH). We concentrate on the development of a measurement system for impedance that, for the first time, uses an excitation current generated by a motor controller. Using simple electronics to amplify and filter the voltage and current, we demonstrate accurate impedance measurements obtained with both multisine and noise excitation signals, achieving RMS magnitude measurement uncertainties between 1.9% and 5.8%, in comparison to a high-accuracy laboratory impedance analyzer. Achieving this requires calibration of the measurement circuits, including measurement of the inductance of the current sense resistor. A statistical correlation approach is used to extract the impedance information from the measured voltage and current signals in the presence of noise, allowing a wide range of excitation signals to be used. Finally, we also discuss the implementation challenges of an SOC estimation system based on impedance.
  • Conference paper
    Mylvaganam T, Astolfi A, 2014,

    Approximate solutions to a class of nonlinear Stackelberg differential games

    , Conference on Decision and Control
  • Report
    Strbac G, Moreno Vieyra R, Konstantelos I, Aunedi M, Pudjianto Det al., 2014,

    Strategic Development of North Sea Grid Infrastructure to Facilitate Least-Cost Decarbonisation

    , Strategic Development of North Sea Grid Infrastructure to Facilitate Least-Cost Decarbonisation, Publisher: E3G

    Offshore wind power is expected to make a significant contribution towards de-carbonisingthe European energy system. It is envisaged that today’s installed capacity levels of about 5GW of offshore wind generation may reach 150GW by 2030, with approximately half of thiscapacity located in the North Seas. Given Europe’s goal of increased integration of the powermarkets by expanding cross-border interconnectors, there is a significant opportunity tointegrate offshore wind generation and interconnector projects in the North Seas in order totake advantage of potentially significant economies of scale and thus reduce network costs.
  • Journal article
    Carnevale D, Astolfi A, 2014,

    Semi-Global Multi-Frequency Estimation in the Presence of Deadzone and Saturation

    , IEEE TRANSACTIONS ON AUTOMATIC CONTROL, Vol: 59, Pages: 1913-1918, ISSN: 0018-9286
  • Journal article
    Lin D, Hui SYR, Chua LO, 2014,

    Gas Discharge Lamps Are Volatile Memristors

    , IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS I-REGULAR PAPERS, Vol: 61, Pages: 2066-2073, ISSN: 1549-8328
  • Journal article
    Chen H-T, Tan S-C, Hui SYR, 2014,

    Color Variation Reduction of GaN-Based White Light-Emitting Diodes Via Peak-Wavelength Stabilization

    , IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol: 29, Pages: 3709-3719, ISSN: 0885-8993
  • Report
    Schofield J, Carmichael R, Tindemans S, Woolf M, Bilton M, Strbac Get al., 2014,

    Residential consumer responsiveness to time-varying pricing

    , Report A3 for the “Low Carbon London” LCNF project
  • Conference paper
    Kerrigan EC, 2014,

    Co-design of Hardware and Algorithms for Real-time Optimization

    , 2014 European Control Conference (ECC), Publisher: IEEE, Pages: 2484-2489

    It is difficult or impossible to separate the performance of an optimization solver from the architecture of the computing system on which the algorithm is implemented. This is particularly true if measurements from a physical system are used to update and solve a sequence of mathematical optimization problems in real-time, such as in control, automation, signal processing and machine learning. In these real-time optimization applications the designer has to trade off computing time, space and energy against each other, while satisfying constraints on the performance and robustness of the resulting cyber-physical system. This paper is an informal introduction to the issues involved when designing the computing hardware and a real-time optimization algorithm at the same time, which can result in systems with efficiencies and performances that are unachievable when designing the sub-systems independently. The co-design process can, in principle, be formulated as a sequence of uncertain and non-smooth optimization problems. In other words, optimizers might be used to design optimizers. Before this can become a reality, new systems theory and numerical methods will have to be developed to solve these co-design problems effectively and reliably.
  • Journal article
    Mueller MA, Angeli D, Allgoewer F, 2014,

    On the performance of economic model predictive control with self-tuning terminal cost

    , Journal of Process Control, Vol: 24, Pages: 1179-1186, ISSN: 1873-2771

    In this paper, we analyze the closed-loop performance of a recently introduced economic model predictive control (MPC) scheme with self-tuning terminal cost. To this end, we propose to use a generalized terminal region constraint instead of a generalized terminal equality constraint within the repeatedly solved optimization problem, which allows us to obtain improved closed-loop asymptotic average performance bounds. In particular, these bounds can be obtained a priori. We discuss how the necessary parameters for the generalized terminal region setting can be calculated, and we illustrate our findings with two numerical examples.
  • Conference paper
    Padoan A, Astolfi A, 2014,

    Model reduction by moment matching for ZIP systems

    , 53rd IEEE Conference on Decision and Control, Pages: 3631-3636, ISSN: 0191-2216
  • Journal article
    Angeli D, Enciso GA, Sontag ED, 2014,

    A small-gain result for orthant-monotone systems under mixed feedback

    , Systems and Control Letters, Vol: 68, Pages: 9-19, ISSN: 0167-6911

    This paper introduces a small-gain result for interconnected orthant-monotone systems for which no matching condition is required between the partial orders in input and output spaces. Previous results assumed that the partial orders adopted would be induced by positivity cones in input and output spaces and that such positivity cones should fulfill a compatibility rule: namely either be coincident or be opposite. Those two configurations correspond to positive feedback or negative feedback cases. We relax those results by allowing arbitrary orthant orders. A linear example is provided showing that the small-gain iteration used for the negative feedback case is not sufficient for global attractivity under mixed feedback. An algebraic characterization is given of the new small-gain condition, generalizing a result known in the negative feedback case. An application is given to nonlinear protein networks with one positive and one negative feedback loop. © 2014 Elsevier B.V. All rights reserved.
  • Journal article
    Pin G, Chen B, Parisini T, Bodson Met al., 2014,

    Robust Sinusoid Identification With Structured and Unstructured Measurement Uncertainties

    , IEEE TRANSACTIONS ON AUTOMATIC CONTROL, Vol: 59, Pages: 1588-1593, ISSN: 0018-9286
  • Journal article
    Zhang Q, Zhang X, Polycarpou MM, Parisini Tet al., 2014,

    Distributed sensor fault detection and isolation for multimachine power systems

    , INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Vol: 24, Pages: 1403-1430, ISSN: 1049-8923
  • Journal article
    Pudjianto D, Castro M, Strbac G, Liu Z, van der Sluis L, Papaefthymiou Get al., 2014,

    Asymmetric impacts of European transmission network development towards 2050: Stakeholder assessment based on IRENE-40 scenarios

    , Energy Economics, Vol: 53, Pages: 261-269, ISSN: 1873-6181

    This paper presents the assessment of stakeholder impacts of European electricity transmission network investments for a set of future system development scenarios. A techno-economic analysis is adopted, which quantifies the economic impacts on different stakeholders including electricity producers, consumers, and network investors, under the future de-carbonisation pathways described in the IRENE-40 scenarios, ranging from 2010 to 2050. To quantify the impact of transmission infrastructure development, for each pathway, two distinct scenarios of future European transmission development are assessed: “low” (no new transmission between 2010 and 2050) and “high” (optimal transmission development between 2010 and 2050 to accommodate the generation pathway).The geographical scope of the analysis covers the EU 27+2 nations. A detailed case study of Spain and France is presented to illustrate the so-called ‘asymmetric’ impacts towards different stakeholders in different importing and exporting zones. The resulting arbitrage trades shift the market equilibriums, which eventually affect asymmetrically the welfare of stakeholders.
  • Conference paper
    Ge M, Kerrigan E, 2017,

    Noise Covariance Estimation for Time-varying and Nonlinear Systems

    , The 19th World Congress of the International Federation of Automatic Control
  • Journal article
    Singh A, Pal BC, Singh R, 2014,

    Stability Analysis of Networked Control in Smart Grids

    , IEEE Transactions on Smart Grid, Vol: 6, Pages: 381-390, ISSN: 1949-3053

    A suitable networked control scheme and its stability analysis framework have been developed for controlling inherent electromechanical oscillatory dynamics observed in power systems. It is assumed that the feedback signals are obtained at locations away from the controller/actuator and transmitted over a communication network with the help of phasor measurement units (PMUs). Within the generic framework of networked control system (NCS), the evolution of power system dynamics and associated control actions through a communication network have been modeled as a hybrid system. The data delivery rate has been modeled as a stochastic process. The closed-loop stability analysis framework has considered the limiting probability of data dropout in computing the stability margin. The contribution is in quantifying allowable data-dropout limit for a specified closed loop performance. The research findings are useful in specifying the requirement of communication infrastructure and protocol for operating future smart grids.
  • Journal article
    Hartley EN, Jerez JL, Suardi A, Maciejowski JM, Kerrigan EC, Constantinides GAet al., 2014,

    Predictive control using an FPGA with application to aircraft control

    , IEEE Transactions on Control Systems Technology, Vol: 22, Pages: 1006-1017, ISSN: 1558-0865

    Alternative and more efficient computational methodscan extend the applicability of MPC to systems with tightreal-time requirements. This paper presents a “system-on-a-chip”MPC system, implemented on a field programmable gate array(FPGA), consisting of a sparse structure-exploiting primal dualinterior point (PDIP) QP solver for MPC reference tracking anda fast gradient QP solver for steady-state target calculation. Aparallel reduced precision iterative solver is used to accelerate thesolution of the set of linear equations forming the computationalbottleneck of the PDIP algorithm. A numerical study of the effectof reducing the number of iterations highlights the effectivenessof the approach. The system is demonstrated with an FPGA-inthe-looptestbench controlling a nonlinear simulation of a largeairliner. This study considers many more manipulated inputsthan any previous FPGA-based MPC implementation to date,yet the implementation comfortably fits into a mid-range FPGA,and the controller compares well in terms of solution quality andlatency to state-of-the-art QP solvers running on a standard PC.
  • Journal article
    Lee J, Angeli D, 2014,

    Cooperative economic model predictive control for linear systems with convex objectives

    , European Journal of Control, Vol: 20, Pages: 141-151, ISSN: 0947-3580

    In this paper we propose a cooperative distributed economic model predictive control strategy for linear systems which consist of a finite number of coupled subsystems. The suggested feedback strategy is generating control input which converges to a set of Nash equilibria of the corresponding game provided infinite iterations are allowed at each sampling time. Moreover, the control for each subsystem is computed in itself without coordination layer except for a synchronization requirement between subsystems.We first introduce distributed linear systems with two subsystems and economic model predictive control, then show the convergence and stability properties of a suboptimal model predictive control strategy for the system. The optimization problem for the implementation of MPC is stated with a terminal equality constraint and a terminal cost.
  • Journal article
    Sassano M, Astolfi A, 2014,

    Dynamic generalized controllability and observability functions with applications to model reduction and sensor deployment

    , AUTOMATICA, Vol: 50, Pages: 1349-1359, ISSN: 0005-1098
  • Journal article
    Gan CK, Pudjianto D, Djapic P, Strbac Get al., 2014,

    Strategic Assessment of Alternative Design Options for Multivoltage-Level Distribution Networks

    , IEEE TRANSACTIONS ON POWER SYSTEMS, Vol: 29, Pages: 1261-1269, ISSN: 0885-8950
  • Journal article
    Falugi P, Mayne DQ, 2014,

    Getting Robustness Against Unstructured Uncertainty: A Tube-Based MPC Approach

    , IEEE TRANSACTIONS ON AUTOMATIC CONTROL, Vol: 59, Pages: 1290-1295, ISSN: 0018-9286
  • Journal article
    Parisini T, 2014,

    Untitled

    , IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, Vol: 22, Pages: 825-826, ISSN: 1063-6536
  • Journal article
    Ng BF, Hesse H, Palacios R, Graham JMR, Kerrigan ECet al., 2014,

    Aeroservoelastic state-space vortex lattice modeling and load alleviation of wind turbine blades

    , Wind Energy, Vol: 18, Pages: 1317-1331, ISSN: 1095-4244

    An aeroservoelastic model, capturing the structural response and the unsteady aerodynamics of turbine rotors, will be used to demonstrate the potential of active load alleviation using aerodynamic control surfaces. The structural model is a geometrically non-linear composite beam, which is linearized around equilibrium rotating conditions and coupled with time-domain aerodynamics given by a linearized 3D unsteady vortex lattice method. With much of the existing work relying on blade element momentum theory with various corrections, the use of the unsteady vortex lattice method in this paper seeks to complement and provide a direct higher fidelity solution for the unsteady rotor dynamics in attached flow conditions. The resulting aeroelastic model is in a state-space formulation suitable for control synthesis. Flaps are modeled directly in the vortex lattice description and using a reduced-order model of the coupled aeroelastic formulation, a linear-quadratic-Gaussian controller is synthesized and shown to reduce root mean square values of the root-bending moment and tip deflection in the presence of continuous turbulence. Similar trend is obtained when the controller is applied to the original non-linear model of the turbine. Trade-offs between reducing root-bending moment and suppressing the negative impacts on torsion due to flap deployment will also be investigated.
  • Journal article
    Li S, Chen H, Tan SC, Hui SYR, Waffenschmidt Eet al., 2014,

    Critical design issues of retrofit light-emitting diode (LED) light bulb

    , Conference Proceedings - IEEE Applied Power Electronics Conference and Exposition - APEC, Pages: 531-536

    For retrofit applications, some high-brightness (HB) light-emitting diode (LED) products have the same form factor restrictions as existing light bulbs. Such form factor constraints may restrict the design and optimal performance of the LED technology. In this paper, some critical design issues for a commercial LED bulb designed for replacing an E27 incandescent lamp are quantitatively analyzed. The analysis involves a power audit on such densely packed LED system so that the amounts of power consumption in (1) the LED driver, (2) the LED wafer, (3) the phosphor coating, and (4) the bulb translucent cover are quantified. The outcomes of such an audit enable R&D engineers to identify the critical areas that need further improvements in a compact LED bulb design. © 2014 IEEE.
  • Journal article
    Yaclin Y, Goren-Sumer L, Astolfi A, 2014,

    Some results on disturbance attenuation for Hamiltonian systems via direct discrete-time design

    , International Journal of Robust and Nonlinear Control, Vol: 25, Pages: 1927-1940, ISSN: 1099-1239

    The disturbance attenuation and robust disturbance attenuation problems for Hamiltonian systems in the discrete-time setting are considered and some new results are presented. The new results are derived utilizing the recently presented dissipativity equality obtained by adding the dissipation rate function to the classical dissipativity inequality. A selection of the dissipation rate function yields new results. These results include a condition on the dissipation structure of the system to achieve the desired disturbance attenuation level and gives direct construction of optimal control laws for any desired disturbance attenuation level. The results remove the need to solve Hamilton–Jacobi–Isaacs inequalities.
  • Patent
    Chaudhuri B, Hui SYR, Chaudhuri NR, Lee CKet al., 2014,

    A power compensator

    , WO2013153075 A3

    A power compensator for compensating voltage at a location along a power transmission line, the compensator having a controller for controlling a voltage generated across the compensator, wherein the voltage is controlled to maintain a power transmission line voltage at a value dependent on the power transmission line location.
  • Journal article
    Dzafic I, Jabr RA, Halilovic E, Pal BCet al., 2014,

    A Sensitivity Approach to Model Local Voltage Controllers in Distribution Networks

    , IEEE Transactions on Power Systems, Vol: 29, Pages: 1419-1428, ISSN: 0885-8950

    Local controllers are essential in distribution networks;they are employed in classical devices such as loadtap-changing (LTC) transformers and switchable shunt capacitors,and more recently in distributed generation (DG). Theeffective use of distribution management system (DMS) applicationsrequires an accurate model of the interaction betweenthe local controllers through the distribution system. This paperpresents a new sensitivity matrix approach for modeling suchinteractions, and demonstrates its application in the implicitGaussmethod for power flow computation. The sensitivity methodmodels both PV buses (for the connection of DG) and tap positionadjustments through current source injections, and consequentlyavoids re-factorization of the network bus admittance matrix.Numerical results on distribution networks with up to 3145 busesshow that the sensitivity-based power flow method for simulatingthe operation of local controllers is superior to a sequentialcontrol action adjustment approach previously proposed in theliterature, and that its computing time is commensurate with theperformance requirements in real-time DMS applications.

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