Publications

2017
P. Irofti and Stoican, F., “Dictionary learning strategies for sensor placement and leakage isolation in water networks”, in IFAC World Congress Conference, Toulouse, France, 2017.Abstract
This paper deals with the problem of fault detection and isolation in water networks.We consider classification strategies for sensor placement and subsequent dictionary learning and classification for accurate fault detection and isolation. Various sensor placement strategies are proposed and it is shown that faults with varying magnitudes are correctly identified in a detailed emulation benchmark.
T. Nguyen, Prodan, I., Stoican, F., and Lefevre, L., “Reliable nonlinear control for quadcopter trajectory tracking through differential flatness”, in IFAC World Congress Conference, 2017.Abstract
his paper addresses the trajectory tracking problem for a quadcopter system under nominal and fault-affected scenarios, the latter case considers stuck actuator(s)). Differential flatness is employed for trajectory generation and control design. The particularity resides in that a full parametrization of the states and inputs is given without any assumptions or simplifications on the quadcopter dynamics. Furthermore, using the properties of flatness and a combination between computed torque control and feedback linearization, a two layer control design is proposed. The tracking performances and stability gurantees are analyzed for nominal and faulty functioning under extensive simulations.
2016
D. Popescu, Ichim, L., Gornea, D., and Stoican, F., “Complex Image Processing Using Correlated Color Information”, in Advanced Concepts for Intelligent Vision Systems (ACIVS 2016), Lecce, Italy, 2016, p. 723-734.Abstract
The paper presents a method for patch classification and remoteimage segmentation based on correlated color information. During the trainingphase, a supervised learning algorithm is considered. In the testing phase, weused the classifier built a priori to predict which class an input image samplebelongs to. The tests showed that the most relevant features are contrast, energyand homogeneity extracted from the co-occurrence matrix between H and Scomponents. Compared to gray-level, the chromatic matrices improve the processof texture classification. For experimental results, the images were acquiredby the aid of an unmanned aerial vehicle and represent various types of terrain.Two case studies have shown that the proposed method is more effective thanconsidering separate color channels: flooded area and road segmentation. Also it is shown that the new algorithm provides a faster execution time than the similar one proposed.
D. Popescu, Stoican, F., and Ichim, L., “Control and Optimization of UAV Trajectory for Aerial Coverage in Photogrammetry Applications”, Advances in Electrical and Computer Engineering, vol. 16, no. 3, p. 99-106, 2016.Abstract
Photogrammetry is a well-studied and much-used analysis tool. Typical use cases include area surveillance, flood monitoring and related tasks. Usually, an Unmanned Aerial System (UAS) is used as support for image acquisition from an a priori delimited region in a semi-automated manner (via a mix of ground control and autonomous trajectory tracking). This in turn has led to various algorithms which handle path trajectory generation under realistic constraints but still many avenues remain open. In this paper, we consider typical costs and constraints (UAS dynamics, total-path length, line inter-distance, turn points, etc.) in order to obtain, via optimization procedures, an optimal trajectory. To this end we make use of polyhedral set operations, flat trajectory generation and other similar tools. Additional work includes the study of non-convex regions and estimation of the number of photographs taken via Ehrhart polynomial computations.
I. Prodan, Stoican, F., and Grotli, E., “Some remarks on potential field constructions in a multi-obstacle environment”, in 10th IFAC Conference on Control Applications in Marine Systems, Trondheim, Norway, 2016.Abstract
This paper addresses a novel combination between mixed-integer representations and potential field constructions for typical multi-agent marine control problems. First, we prove that for any kind of repulsive functions applied over a function which we denote as sum function, the feasible domain is piece-wise affine (PWA). Next, concepts like hyperplane arrangements together with potential field approaches are used for providing an efficient description of the feasible non-convex domain. This combination offers an original and beneficent computation of control laws under non-convex constraints. Simulation results over a common application of obstacle avoidance, which can be extended for unmanned surface vehicles, prove the effectiveness of the proposed approach.
F. Stoican, Ivanusca, V. - M., Prodan, I., and Popescu, D., “Obstacle avoidance via B-spline parametrizations of flat trajectories”, in 24th Mediterranean Conference on Control and Automation (MED), Athens, Greece, 2016.Abstract
This paper considers the collision avoidance problem in a multi-agent multi-obstacle framework. The originality in solving this intensively studied problem resides in the proposed geometrical view combined with differential flatness for trajectory generation and B-splines for the flat output parametrization. Using some important properties of these theoretical tools we show that the constraints can be validated at all times. Exact and sub-optimal constructions of the collision avoidance optimization problem are provided. The results are validated through extensive simulations over standard autonomous aerial vehicle dynamics.
N. Stankovic, Stoican, F., Olaru, S., and Niculescu, S. I., “Fault tolerant control design for a class of multi-sensor networked control systems”, International Journal of Adaptive Control and Signal Processing, vol. 30, no. 2, p. 412-426, 2016. WebsiteAbstract
Present article provides a set-based fault tolerant control strategy for multi-sensor systems, where sensors are communicating with a controller via a shared network. Possible faults, such as abrupt sensor outages and network-induced delays, are identified as degradation modes which might affect the information provided by each sensor. Measurements that are transmitted from a sensor to the controller are characterized by a residual signal which is sensitive to the sensor's abrupt faults and network-induced delays. In order to avoid control based on information which is provided by a faulty sensor, we designed a fault detection and isolation mechanism that is implemented through a set membership evaluation. This evaluation differentiates between ``healthy'', ``faulty'' and ``delayed'' data transmission. Unequivocal fault detection and isolation are assured if the corresponding sets are disjoint. Since in general this is not the case, sets separation is enforced by a reference governor. Fault detection and isolation mechanism is design in order to transmit only measurements from sensors which are fully operational, even if potentially affected by delays. If there is a delayed information that reaches the controller, then control action is reconfigured in order to govern the plant as close as possible to the reference signal. Such control action is provided by a model-based controller with compensation block. Sufficient condition that guarantees the existence of the compensation signal is presented as well.
F. Stoican, Prodan, I., Ocampo-Martinez, C., and Puig, V., “Interval model identification in the frequency domain by using mixed-integer representations”, 4th European Conference on Computational Optimization. 2016.
F. Stoican and Prodan, I., “Mixed integer representations in control design: theoretical results and applications”, 4th European Conference on Computational Optimization. 2016.
F. Stoican, Prodan, I., and Grotli, E. I., “A mixed-integer implementation of the corner cutting problem in a multi-obstacle environment”, 4th European Conference on Computational Optimization. 2016.
Mixed-Integer Representations in Control Design
I. Prodan, Stoican, F., Olaru, S., and Niculescu, S. I., Mixed-Integer Representations in Control Design. Springer, 2016. Website
2015
F. Stoican, Dan, P., Vlasceanu, E., and Mateescu, C., “Geometrical considerations for photogrammetry missions in an UAV context”, in 19th International Conference on System Theory, Control and Computing, Cheile Gradistei, Romania, 2015.Abstract
This paper discusses a practical implementation for photogrammetry missions in an UAV setting. Further advances, which exploit geometrical properties of the problem are considered. In particular, a bi-level optimization procedure which minimizes the total path-length is discussed. Various constraints and limitations are taken into account.
F. Stoican, Grotli, E., Prodan, I., and Oara, C., “On corner cutting in multi-obstacle avoidance problems”, in 5th IFAC Conference on Nonlinear Model Predictive Control, Seville, Spain, 2015.Abstract
One challenging and not extensively studied issue in obstacle avoidance is the corner cutting problem. Avoidance constraints are usually imposed at the sampling time without regards to the intra-sample behavior of the dynamics. This paper improves upon state of the art by describing a multi-obstacle environment via a hyperplane arrangement, provides a piecewise description of the forbidden regions and represents them into a combined mixed integer and predictive control formulation. Furthermore, over-approximation constraints which reduce to strictly binary conditions are discussed in detail. Illustrative proofs of concept, comparisons with the state of the art and simulation results over a classical multi-obstacle avoidance problem validate the bene ts of the proposed approach.
I. Prodan, Stoican, F., and Zio, E., “On a fault tolerant strategy for efficient energy management in microgrid systems”, in 5th IFAC Conference on Nonlinear Model Predictive Control, Seville, Spain, 2015.Abstract
This paper addresses the microgrid energy management problem within a coherent framework of control tools based on Mixed-Integer Linear Programming (MILP) and constrained Model Predictive Control (MPC). These help characterize the microgrid components’ dynamics and the overall system control architecture. A fault tolerant strategy is considered in order to ensure the proper amount of energy in the storage devices such that (together with the utility grid) the essential consumer demand is reliably covered. Simulation results on a particular microgrid architecture validate the proposed approach.
F. Stoican, Prodan, I., and Dan, P., “Flat trajectory generation for way-points relaxations and obstacle avoidance”, in 23rd Mediterranean Conference on Control and Automation (MED), Torremolinos, Spain, 2015.Abstract
This paper addresses some alternatives to classical trajectory generation for an autonomous vehicle which needs to pass through a priori given way-points. Using differential flatness for trajectory generation and B-splines for the flat output parametrization, the current study concentrates on constraint relaxations and on obstacle avoidance conditions. The results are validated through simulations over standard UAV dynamics.
I. Prodan, Zio, E., and Stoican, F., “Fault tolerant predictive control design for reliable microgrid energy management under uncertainties”, Energy, vol. 91, p. 20-34, 2015.Abstract
This paper presents an extension of a MPC (Model Predictive Control) approach for microgrid energy management which takes into account electricity costs, power consumption, generation profiles, power and energy constraints as well as uncertainty due to variations in the environment. The approach is based on a coherent framework of control tools, like mixed-integer programming and soft constrained MPC, for describing the microgrid components dynamics and the overall system control architecture Fault tolerant strategies are inserted in order to ensure the proper amount of energy in the storage devices such that (together with the utility grid) the essential consumer demand is always covered. Simulation results on a particular microgrid architecture validate the proposed approach.
F. Xu, Puig, V., Ocampo-Martinez, C., Olaru, S., and Stoican, F., “Set-theoretic Methods in Robust Detection and Isolation of Sensor Faults”, International Journal of Systems Science, vol. 46, no. 13, p. 18, 2015.Abstract
In this paper, we consider a multi-sensor networked control configuration with linear plant which is affected by a bounded additive disturbance. Shared network is used for the communication between sensors and controller. It is assumed that the sensors are prone to abrupt faults, while the controller’s input may be updated with a varying time-delay. In order to identify and isolate the sensor(s) providing faulty information, we equip the controller with a set-based detection and isolation routine. Furthermore, in the case when the network induces time-delays, control is performed based on the knowledge we have on the mathematical model of the plant. In the presence of model inaccuracies or disturbance, such a control action may not guarantee satisfying performance of the system. Therefore, a stabilising controller with delay compensation has been designed. The functioning of the proposed control algorithm has been illustrated through an example.
RPI approximations of the mRPI set characterizing linear dynamics with zonotopic disturbances
F. Stoican, Oara, C., and Hovd, M., “RPI approximations of the mRPI set characterizing linear dynamics with zonotopic disturbances”, in Developments in Model-Based Optimization and Control, vol. 464, Springer, 2015, p. 361-377.
Trajectory Generation with Way-Point Constraints for UAV Systems
F. Stoican and Popescu, D., “Trajectory Generation with Way-Point Constraints for UAV Systems”, in Advances in Robot Design and Intelligent Control, vol. 371, Springer, 2015, p. 379-386.Abstract
This paper addresses some alternatives to classical trajectory generation for an unmanned aerial vehicle (UAV) which needs to pass through (or near) a priori given way-points. Using differential flatness for trajectory generation and B-splines for the flat output parameterization, the current study concentrates on flat descriptions which respect to UAV dynamics and verify way-point constraints.
2014
M. Hovd and Stoican, F., “On the design of exact penalty functions for MPC using mixed integer programming”, International Journal of Computers & Chemical Engineering, vol. 70, p. 104-113, 2014.Abstract
Soft constraints and penalty functions are commonly used in MPC to ensure that the optimization problem has a feasible solution, and thereby avoid MPC controller failure. On the other hand, soft constraints may allow for unnecessary violations of the original constraints, i.e., the constraints may be violated even if a valid solution that does not violate any constraints exists. The paper develops procedures for the minimizing (according to some norm) of the Lagrange multipliers associated with a given mp-QP problem, assumed to originate from an MPC problem formulation. To this end the LICQ condition is exploited in order to efficiently formulate the optimization problem, and thereby improve upon existing mixed integer formulations and enhance the tractability of the problem. The results are used to design penalty functions such that corresponding soft constraints are made exact, that is, the original (hard) constraints are violated only if there exists no solution where all constraints are satisfied.
F. Xu, Puig, V., Ocampo-Martinez, C., Stoican, F., and Olaru, S., “Closed-loop Actuator-fault Detection and Isolation using Invariant Sets and Tubes”, in The 19th World Congress of the International Federation of Automatic Control, Cape Town, South Africa, 2014.
F. Stoican, Olaru, S., Seron, M. M., and De Doná, J. A., “A discussion of sensor recovery techniques for fault tolerant multisensor schemes”, International Journal of Systems Science, vol. 45, no. 8, p. 15, 2014.Abstract
The present paper deals with the interplay between healthy and faulty sensor functioning in a multisensor scheme based on a switching control strategy. Fault tolerance guarantees have been recently obtained in this framework based upon the characterisation of invariant sets for state estimations in healthy and faulty functioning. A source of conservativeness of this approach is related to the issue of sensor recovery. A common working hypothesis has been to assume that once a sensor switches to faulty functioning it can no longer be used by the control mechanism even if at an ulterior moment it switches back to healthy functioning. In the current paper, we present necessary and sufficient conditions for the acknowledgement of sensor recovery and we propose and compare different techniques for the reintegration of sensors in the closed-loop decision-making mechanism.
F. Xu, Puig, V., Ocampo-Martinez, C., Stoican, F., and Olaru, S., “Improved Fault Detection and Isolation Strategy using a Bank of Interval Observers”, in he 19th World Congress of the International Federation of Automatic Control, Cape Town, South Africa, 2014.Abstract
In this paper, an improved algorithm for actuator-fault detection and isolation (FDI) using a bank of interval observers is presented, where each interval observer matches one considered system mode. In this approach, interval observers and invariant sets are simultaneously used for FDI. Under a collection of improved FDI conditions, this new algorithm can detect and isolate the considered actuator faults. At the end of this paper, a circuit example is used to illustrate the e ectiveness of the proposed strategy.
F. Stoican, Olaru, S., Seron, M. M., and De Doná, J. A., “A fault tolerant control scheme based on sensor-actuation channel switching and dwell time”, International Journal of Robust and Nonlinear Control, vol. 24, no. 4, p. 18, 2014.Abstract
The present paper proposes a switching control scheme for a plant with multiple sensor–estimator/control–actuator pairs. The scheme is shown to handle the specific stability problems originated by the switching between the different feedback loops and accommodate to faults in the measurement (sensors) channels. The main contribution is a fault tolerant switching scheme with stability guarantees assured by a pre-imposed dwell time. The detection and the fault tolerance capabilities are achieved through the separation of sets associated with suitable residual signals corresponding to healthy and faulty functioning. Another contribution of the paper resides in a recovery technique for the post-fault reintegration of the biased estimations. This technique makes use of a virtual sensor whose associated estimation, based on an optimization procedure, minimizes the recovery time.
F. Xu, Puig, V., Ocampo-Martinez, C., Stoican, F., and Olaru, S., “Actuator-fault detection and isolation based on set-theoretic approaches”, Journal of Process Control, vol. 24, no. 6, p. 10, 2014. WebsiteAbstract
In this paper, an actuator-fault detection and isolation (FDI) approach is proposed. The FDI approach is based on a bank of interval observers, each of which is designed to match a healthy or faulty system mode. To ensure reliable FDI for all considered actuator faults, a collection of invariant set-based FDI conditions are established for the proposed technique. Under these guaranteed FDI conditions, all the considered faults can be detected and isolated during the transition induced by fault occurrences. Comparing with the existing set-based FDI approaches, the advantage of the proposed technique consists in that it combines the advantages of interval observers in the transient-state functioning and the advantages of invariant sets in the steady-state functioning. This paper is completed with the study of a continuous stirred-tank reactor (CSTR), which illustrates the effectiveness of the proposed method.
F. Stoican, Prodan, I., Strutu, M. - I., and Popescu, D., “Geometrical interpretation on the coverage problems for a mobile agent”, in 18th International Conference on System Theory, Control and Computing, Sinaia, Romania, 2014.
D. Clipici, Necoara, I., Stoican, F., Patrascu, A., and Hovd, M., “A linear MPC algorithm for embedded systems with computational complexity guarantees”, in 18th International Conference on System Theory, Control and Computing, Sinaia, Romania, 2014.
2013
M. - I. Strutu, Stoican, F., Prodan, I., Popescu, D., and Olaru, S., “A characterization of the relative positioning of mobile agents for full sensorial coverage in an augmented space with obstacles”, in Proceedings of the 21st Mediterranean Conference on Control and Automation, Platania-Chania, Crete, Grecee, 2013, p. 936-941.Abstract
This paper addresses the coverage problem for a collection of agents and fixed obstacles (e.g., the “gallery” and the “patrolling” problems). A collection of sufficient conditions over the positions of the agents are provided such that whenever these are verified there is no “blind” region in the feasible space. These conditions are expressed by making use of hyperplane arrangements which lead to a mixed-integer formulation. Practical applications regarding the coverage problem inside an augmented space with obstacles validate these concepts and provide an efficient implementation (in terms of computing power).
F. Xu, Stoican, F., Puig, V., Ocampo-Martinez, C., and Olaru, S., “Fault Detection and Isolation based on the combination of a Bank of Interval Observers and Invariant Sets”, in Proceedings of the 21st Mediterranean Conference on Control and Automation, Platania-Chania, Crete, Grecee, 2013, p. 807-813.Abstract
In this paper, a fault detection and isolation (FDI) approach using a bank of interval observers is developed. From the methodological point of view, a bank of interval observers is designed according to different dynamical models of the system under different modes (healthy or faulty). Each interval observer matches one system mode while all the interval observers monitor the system simultaneously. In order to guarantee FDI, a set of FDI conditions based on invariant set notions are established. These conditions ensure that the considered faults can be accurately isolated after a period of monitoring time. Finally, simulation results are used to present the effectiveness of the approach.
F. Stoican, Prodan, I., and Olaru, S., “Hyperplane Arrangements in Mixed-Integer Programming Techniques. Collision avoidance application with Zonotopic Sets”, in Proceedings of the 2013 European Control Conference, Zurich, Switzerland, 2013, p. 3155-3160.Abstract
The current paper addresses the problem of minimizing the computational complexity of optimization problems with non-convex and possibly non-connected feasible region of polyhedral type. Using hyperplane arrangements and Mixed-Integer Programming we provide an efficient description of the feasible region in the solution space. Moreover, we exploit the geometric properties of the hyperplane arrangements and adapt this description in order to provide an efficient solution of the mixed-integer optimization problem. Furthermore, a zonotopic representation of the sets appearing in the problem is considered. The advantages of this representation are highlighted and exploited through proof of concepts illustrations as well as simulation results.
F. Stoican, Hovd, M., and Olaru, S., “Explicit invariant approximation of the mRPI set for LTI dynamics with zonotopic disturbances”, in 52nd IEEE Conference on Decision and Control, Florence, Italy, 2013, p. 3237-3242.Abstract
In this paper we provide a RPI over-approximation of the mRPI set associated for linear dynamics with zonotopic disturbances.We prove that the RPI construction converges toward the mRPI set and its conservatism diminishes monotonically with respect to the complexity of the representation (a “tightness” coefficient is calculated a priori). The results are tested in illustrative examples.
F. Xu, Puig, V., Ocampo-Martinez, C., Stoican, F., and Olaru, S., “Actuator-Fault Detection and Isolation based on Interval Observers and Invariant Sets”, in 52nd IEEE Conference on Decision and Control, Florence, Italy, 2013, p. 4385-4390.Abstract
In this paper, an actuator-fault detection and isolation (FDI) approach using interval observers is proposed. An interval observer designed according to the healthy model of the supervised system is used to monitor the system. When the system is under different modes, state or output interval vectors predicted by the interval observer manifest different dynamical behaviors, which is the basis for FDI. To guarantee FDI, a group of set-based sufficient conditions based on invariant sets are established. Under these conditions, actuator faults can be accurately detected and isolated during the transition between different system modes. Finally, a numerical example is used to present the effectiveness of the proposed approach.
F. Xu, Stoican, F., Puig, V., Ocampo-Martinez, C., and Olaru, S., “On the Relationship between Interval Observers and Invariant Sets in Fault Detection”, in 2nd International Conference on Control and Fault-Tolerant Systems, Nice, France, 2013, p. 49-54.Abstract
In this paper, the relationship between two set-based fault detection (FD) approaches, the interval observer-based and the invariant set-based approaches, is investigated. In FD, an interval observer has been shown to be suitable to generate adaptive thresholds for residuals, which can monitor the system behavior in real time. Invariant sets focus more on the steady state behavior of the system rather than on the transient behavior. This paper discusses these two approaches, presents a relationship between them and compares them in the FD task. At the end, simulation examples are used to compare and discuss these two approaches.
F. Xu, Puig, V., Ocampo-Martinez, C., Stoican, F., and Olaru, S., “Sensor-fault Detection and Isolation using Interval Observers”, in 2nd International Conference on Control and Fault-Tolerant Systems, Nice, France, 2013, p. 55-60.Abstract
This paper proposes an interval observer-based sensor fault detection and isolation (FDI) approach for closed-loop systems. In the proposed approach, residuals are defined in such a way that their components are independent of each other at the time instant after fault occurrence, namely kf +1, where kf denotes the fault occurrence time instant. In this way, it is guaranteed that at kf +1 the changes in each component of the residuals are only related to the fault in the corresponding sensor. By detecting the threshold violation of the corresponding residual interval components, the proposed approach can detect and isolate sensor faults at the same time instant. At the end of this paper, a numerical example is used to present the effectiveness of the proposed approach.
Mixed-Integer Programming Techniques in Distributed MPC Problems
I. Prodan, Stoican, F., Olaru, S., Stoica, C. N., and Niculescu, S. I., “Mixed-Integer Programming Techniques in Distributed MPC Problems”, in Distributed Model Predictive Control Made Easy, J. M. Maestre and Negenborn, R. R. New York: Springer, 2013, p. 273-288. Online infoAbstract
This chapter proposes a distributed approach for the resolution of a multiagent problem under collision and obstacle avoidance conditions. Using hyperplane arrangements and mixed integer programming, we provide an efficient description of the feasible region verifying the avoidance constraints. We exploit geometric properties of hyperplane arrangements and adapt this description to the distributed scheme in order to provide an efficient Model Predictive Control (MPC) solution. Furthermore,we prove constraint validation for a hierarchical ordering of the agents.
Set-Theoretic Fault Tolerant Control in Multisensor Systems
F. Stoican and Olaru, S., Set-Theoretic Fault Tolerant Control in Multisensor Systems, Engineering & Materials Science. London: Wiley - ISTE, 2013, p. 176. Buy onlineAbstract
Fault-tolerant control theory is a well-studied topic but the use of the sets in detection, isolation and/or reconfiguration is rather tangential. The authors of this book propose a systematic analysis of the set-theoretic elements and devise approaches which exploit advanced elements within the field. The main idea is to translate fault detection and isolation conditions into those conditions involving sets. Furthermore, these are to be computed efficiently using positive invariance and reachability notions. Constraints imposed by exact fault control are used to define feasible references (which impose persistent excitation and, thus, non-convex feasible sets). Particular attention is given to the reciprocal influences between fault detection and isolation on the one hand, and control reconfiguration on the other.
2012
F. Stoican and Hovd, M., “Efficient solution of a qp optimization problem with zonotopic constraints”, in Proceedings of the 21th IEEE International Conference on Control Applications, part of 2012 IEEE Multi-Conference on Systems and Control, Dubrovnik, Croatia, 2012, p. 457–462.Abstract
In this paper we analyze the advantages of describing the constraint set of a constrained optimization problem by an (inner-approximating) zonotope. We compare this with the usual polytopic description and note that by using the generator description characterizing zonotopes we can exploit their special structure in order to obtain a simpler formulation of the optimization problem. We test the results on a typical MPC setting and observe the improvements.
F. Stoican, Olaru, S., and Hovd, M., “MPC techniques in fault tolerant control design”, in Proceedings of the 17th Nordic Process Control Workshop, Lyngby, Denmark, 2012, p. ?–?Abstract
In the present paper we provide a robust approach for fault tolerant control (FTC) schemes using the methodology detailed in Seron et al. [2008], Olaru et al. [2010]. We guarantee the detection and isolation of a fault through a set-separation condition (FDI mechanism) and use this condition further in the reconfiguration control (RC) mechanism in order to stabilize the closed-loop system and respect performance criteria.
G. Marafioti, Stoican, F., Hovd, M., and Bitmead, R. R., “Persistently Exciting Model Predictive Control for SISO systems”, in Proceedings of the 4th IFAC Nonlinear Model Predictive Control Conference, Noordwijkerhout, Netherlands, 2012, p. 448–453.Abstract
A formulation of Persistently Exciting Model Predictive Control (PE-MPC) for Single-Input Single-Output (SISO) systems is presented. PE-MPC is an extension of a conventional model predictive control where a Persistence of Excitation Condition (PEC) is included as inequality constraint, to allow for adaptive implementation and on-line tuning of the model. The PEC makes the PE-MPC feasible region non-convex. For SISO systems the non-convex region can be represented as the union of two convex regions. Therefore an ad-hoc solution of the PE-MPC optimization problem can be eciently computed. This is done by exploiting the particular structure of the PEC constraint. Finally a numerical example of SISO sytem is given and several scenarios are simulated to analyze the PE-MPC properties.
F. Stoican and Hovd, M., “Some remarks upon the characteristics of the explicit representation of the MPC problem”, in Proceedings of the 4th IFAC Nonlinear Model Predictive Control Conference, Noordwijkerhout, Netherlands, 2012, p. 126–131.Abstract
In this paper we revisit the explicit MPC representation and related notions. We point to the special structure of the constraint matrices and exploit it in order to provide novel results. We give an upper bound for the collection of admissible active sets with use in the mixed integer representation of the KKT problem and a partial recursive description of the explicit partitioning of the MPC problem. The results are tested over illustrative examples.
N. Stankovic, Stoican, F., Olaru, S., and Niculescu, S. I., “Reference governor design with guarantees of detection for delay variation”, in Proceedings of the 10th IFAC Workshop on Time Delay Systems, Boston, Massachusetts, USA, 2012, p. 67–72.Abstract
The present paper deals with fault tolerant control for linear dynamics with additive disturbances. The control action is generated based on information collected from a redundant, multi-sensors network. Delays that may appear during plant measurements transmission through real communication channels are considered as faults. Depending on presence of delay in feedback loop, di erent invariant sets can be computed. We show that fault tolerant control can be achieved through invariant sets separation with respect to di erent delay values. Sets separation is accomplished for speci c values of the reference signal. Therefore, we introduce in the loop a reference governor which is designed by a receding horizon technique. Thus, we provide reference signals which practically guarantee fault detection and identi cation in real time.
F. Stoican, Olaru, S., and Bitsoris, G., “Controlled invariance-based fault detection for multisensory control systems”, IET Control Theory & Applications Journal, vol. 7, no. 4, p. 606 – 611, 2012.Abstract
In this study, set theoretic methods are used to design a fault-tolerant scheme for a multisensor control application. The basic principle is the separation of the invariant sets for the estimations of the state and tracking error under healthy and faulty functioning. The fault scenario assumes abrupt changes of the observation equations. The main contribution of this paper is the introduction of controlled invariant sets in the fault detection mechanism. The control action is chosen in order to guarantee the closed-loop positive invariance of a candidate region when the exogenous signals (additive disturbances, noise and reference/set-points) are bounded.
I. Prodan, Stoican, F., Olaru, S., and Niculescu, S. I., “Enhancements on the hyperplanes arrangements in mixed integer techniques”, Journal of Optimization Theory and Applications, vol. 154, no. 2, p. 549-572, 2012.Abstract
This paper is concerned with improvements in constraints handling for mixed-integer optimization problems. The novel element is the reduction of the number of binary variables used for expressing the complement of a convex (polytopic) region. As a generalization, the problem of representing the complement of a possibly not connected union of such convex sets is detailed. In order to illustrate the benefits of the proposed improvements, a typical control application, the control of multiagent systems using receding horizon optimization techniques, is considered.
F. Stoican, Olaru, S., Seron, M. M., and De Doná, J. A., “Reference governor design for tracking problems with fault detection guarantees”, Journal of Process Control, vol. 22, no. 5, p. 829–836, 2012.Abstract
The present paper deals with the reference tracking problem for processes with linear dynamics and multisensor information subject to abrupt sensor faults. A key point for fault tolerance will be the separation between healthy and faulty closed-loop behavior upon a set-characterization approach. This is achieved through set theoretic operations involving the healthy/faulty behavior of residual signals related to the system dynamics. As a main contribution, a reference governor scheme is designed using a receding horizon technique. It is shown that fault detection guarantees can be achieved by appropriate adjusting of the governor's delay/prediction window under mild assumptions on the fault scenario.
2011
F. Stoican, Prodan, I., and Olaru, S., “On the hyperplanes arrangements in mixed-integer techniques”, in Proceedings of the 30th American Control Conference, San Francisco, California, USA, 2011, p. 1898–1903.Abstract
This paper is concerned with the improved constraints handling in mixed-integer optimization problems. The novel element is the reduction of the number of binary variables used for expressing the complement of a convex (polytopic) region. As a generalization, the problem of representing the complement of a possibly non-connected union of such convex sets is detailed. In order to illustrate the benefits of the proposed improvements, a practical implementation, the problem of obstacle avoidance using receding horizon optimization techniques is considered.
H. N. Nguyen, Olaru, S., and Stoican, F., “On maximal robustly positively invariant sets”, in Proceedings of the 8th International Conference on Informatics in Control, Automation and Robotics, Noordwijkerhout, Netherlands, 2011, p. 300-305.Abstract
This paper addresses the problem of computing maximal robustly positively invariant sets for discrete-time linear time-invariant systems with disturbance inputs. It is assumed that the disturbance is unknown, additive, but bounded. The main contribution is the determination of bound of the number of steps in the iterative construction of the maximal invariant sets.
F. Stoican, Raduinea, C. - F., and Olaru, S., “Adaptation of set theoretic methods to the fault detection of a wind turbine benchmark”, in Proceedings of the 18th IFAC World Congress, Milano, Italy, 2011, p. 8322–8327.Abstract
The last decade has seen the emergence of set-theoretic methods in fault detection and identification mechanisms. These techniques are seen as restrictive and mathematically challenging due to the strict conditions (e.g. signal boundedness) imposed for reactivity to faults by means of set separation. The present paper aims at implementing such methods to a practical application proposed by a wind turbine benchmark setup. It is shown that strict boundedness conditions can be adjusted in order to obtain robust fault detection.
F. Stoican, Olaru, S., De Doná, J. A., and Seron, M. M., “Zonotopic ultimate bounds for linear systems with bounded disturbances”, in Proceedings of the 18th IFAC World Congress, Milano, Italy, 2011, p. 9224–9229.Abstract
This paper deals with robust invariant sets construction for discrete-time linear timeinvariant dynamics. The case of a zonotopic disturbance set is analysed in detail by exploiting the properties of these geometrical structures. A constructive method is provided for diminishing the conservatism of ultimate bound invariant sets. It is shown that the resulting zonotopic set is related to the minimal robust positively invariant set in the sense that their boundaries have common points.
F. Stoican, Prodan, I., and Olaru, S., “Enhancements on the hyperplane arrangements in mixed integer techniques”, in Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference, Orlando, Florida, USA, 2011, p. 3986–3991.Abstract
The current paper addresses the problem of optimizing a cost function over a non-convex and possibly non-connected feasible region. A classical approach for solving this type of optimization problem is based on Mixed integer technique. The exponential complexity as a function of the number of binary variables used in the problem formulation highlights the importance of reducing them. Previous work which minimize the number of binary variables is revisited and enhanced. Practical limitations of the procedure are discussed and a typical control application, the control of Multi-Agent Systems is exemplified.
F. Stoican, Minoiu Enache, N., and Olaru, S., “A lane control mechanism with fault tolerant control capabilities”, in Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference, Orlando, Florida, USA, 2011, p. 2245–2250.Abstract
The paper discusses the problem of lane departure avoidance for a vehicle. A corrective mechanism imposes its control action whenever the vehicle is no longer inside a nominal region centered along the middle of the lane. Set theoretic methods are used in order to design this control action and to guarantee global stability. Additionally, for the same lane departure avoidance system, a fault tolerant control mechanism is proposed in order to discard faulty sensors in a redundant measurement setting, thus guaranteeing stability even in the presence of faults.
Fault tolerant control based on set-theoretic methods
F. Stoican, “Fault tolerant control based on set-theoretic methods”, École Superieure d'Électricite (SUPELEC) , 2011. Online archiveAbstract
The scope of the thesis is the analysis and design of fault tolerant control (FTC) schemes through the use of set-theoretic methods. In the framework of multisensor schemes, the faults appearance and the modalities to accurately detect them are investigated as well as the design of control laws which assure the closed-loop stability. By using invariant/contractive sets to describe the residual signals, a fault detection and isolation (FDI) mechanism with reduced computational demands is implemented based on set-separation. A dual mechanism, implemented by a recovery block, which certificates previously fault-affected sensors is also studied. From a broader theoretical perspective, we point to the conditions which allow the inclusion of FDI objectives in the control law design. This leads to static feedback gains synthesis by means of numerically attractive optimization problems. Depending on the parameters selected for tuning, is shown that the FTC design can be completed by a reference governor or a predictive control scheme which adapts the state trajectory and the feedback control action in order to assure FDI. When necessary, the specific issues originated by the use of set-theoretic methods are detailed and various improvements are proposed towards: invariant set construction, mixed integer programming (MIP), stability for switched systems (dwell-time notions).
2010
F. Stoican and Olaru, S., “Fault Tolerant Positioning System for a Multisensor Control Scheme”, in Proceedings of the 19th IEEE International Conference on Control Applications, part of 2010 IEEE Multi-Conference on Systems and Control, Yokohama, Japan, 2010, p. 1051–1056.Abstract
The present paper deals with a fault tolerant control scheme for a multisensor plant based on set theoretic methods under the assumption of bounded exogenous signals. Robust guarantees for the global stability of the system and the separability and identification of abrupt faults occurring in the sensor outputs are provided. The methodology is exemplified on a positioning system showing improved detection and isolation capabilities even for reference signals passing with oscillations around the position corresponding to faulty functioning of the sensors.
F. Stoican, Olaru, S., and Bitsoris, G., “A fault detection scheme based on controlled invariant sets for multisensor systems”, in Proceedings of the 2010 Conference on Control and Fault Tolerant Systems, Nice, France, 2010, p. 468–473.Abstract
The present paper uses set theoretic methods for the design of a fault tolerant control scheme in the case of a multisensor application. The basic principle is the separation of invariant sets for the estimations of the state and tracking error under healthy and faulty functioning. The fault scenario is based on abrupt changes of the observation equations. The main contribution is the introduction of controlled invariant sets in the fault detection mechanism. The control action is chosen so that the closed loop invariance is assured for a candidate region which accounts for the bounds on the exogenous signals (additive disturbances, noise and reference/set-points).
F. Stoican, Olaru, S., Seron, M. M., and De Doná, J. A., “Reference governor for tracking with fault detection capabilities”, in Proceedings of the 2010 Conference on Control and Fault Tolerant Systems, Nice, France, 2010, p. 546–551.Abstract
This paper presents a fault tolerant multisensor strategy for feedback control of a class of nonlinear systems upon a geometrical approach. A key point to ensure fault tolerance is the separation between healthy and faulty closed-loop behavior. Here we achieve this through set theoretic operations upon sets describing the healthy/faulty behavior of the system. The results rely both on an appropriate choice for the exogenous signals and on fixed point conditions for a nonlinear mapping which describes the gap between the nonlinear system and a linearized model in the functioning interval. A reference governor is employed such that, under a receding horizon technique, only feasible exogenous signals are provided to the system.
F. Stoican, Olaru, S., De Doná, J. A., and Seron, M. M., “Improvements in the Sensor Recovery Mechanism for a Multisensor Control Scheme”, in Proceedings of the 29th American Control Conference, Baltimore, Maryland, USA, 2010, p. 4052–4057.Abstract
This paper deals with a multisensor scheme based on set theoretic principles, whereby different invariant sets that characterize healthy and faulty functioning of system components are computed offline. Such sets allow to partition the ensemble of sensors into ‘healthy’, ‘faulty’ and ‘under recovery’ subclasses. Fault detection and isolation consists of online setmembership verifications with low computational complexity. Sensors that are deemed healthy are utilized in the computation of the feedback control law, while sensors that are deemed ‘faulty’ or ‘under recovery’ are prevented from participating in the feedback control action. The main focus of this paper is on the reintegration of ‘under recovery’ sensors, that is to say, the transition of sensors from the ‘under recovery’ to the ‘healthy’ sensor subclass. This transition, in contrast to all other possible transitions, is particularly difficult to evaluate since it involves set membership conditions based on unmeasurable quantities. This difficulty is circumvented by resorting to necessary and sufficient conditions for the recognition of recovery, which are based exclusively upon measurable quantities. The interplay between the necessary conditions and the sufficient conditions, together with the particular system structure and fault detection mechanism, allows to obtain further important improvements in the recovery procedure in terms of transient times and sensitivity to the topology of the invariant sets.
F. Stoican, Olaru, S., Seron, M. M., and De Doná, J. A., “A fault tolerant control scheme based on sensor switching and dwell time”, in Proceedings of the 49th IEEE Conference on Decision and Control, Atlanta, Georgia, USA, 2010, p. 756–761.Abstract
The present paper deals with a switching control scheme for a plant with multiple estimator-controller-actuator pairs. The scheme has to deal with specific problems originated by the switching between the different feedback loops and accommodate to faults in the observation channels (sensors outputs). The main contribution is a fault tolerant switching scheme with stability guarantees assured by a pre-imposed dwell-time. The detection and the fault tolerance capabilities are assured through set separation for the residual signals corresponding to healthy and faulty functioning. Another contribution of the paper resides in a recovery technique for faulty sensors which makes use of a virtual sensor whose estimation, based on an optimization procedure, minimizes recovery time.
S. Olaru, De Doná, J. A., Seron, M. M., and Stoican, F., “Positive invariant sets for fault tolerant multisensor control schemes”, International Journal of Control, vol. 83, no. 12, p. 2622–2640, 2010.Abstract
This article deals with fault tolerant multisensor control schemes for systems with linear dynamics. Positive invariance is a common analysis and control design tool for systems affected by bounded constraints and disturbances. This article revisits the construction of \epsilon-approximations of minimal robust positive invariant sets for linear systems upon contractive set-iterations. The cases of switching between different sets of disturbances and the inclusion of a predefined region of the state space are treated in detail. All these results are used in multisensor control schemes which have to deal with specific problems originated by the switching between different estimators and by the presence of faults in some of the sensors. The construction of positive invariant sets for different operating regimes provides, in this context, effective fault detection information. Within the same framework, global stability of the switching strategies can be assured if the invariant sets topology allows the exclusive selection of estimates obtained from healthy sensors.
2009
S. Olaru, Stoican, F., De Doná, J. A., and Seron, M. M., “Necessary and Sufficient Conditions for Sensor Recovery in a Multisensor Control Scheme”, in Proceedings of the 7th IFAC Symposium on Fault Detection, Supervision and Safety of Technical Processes, Barcelona, Spain, 2009, p. 977–982.Abstract
The present paper deals with a multisensor scheme based on a switching control strategy. Fault tolerance guarantees were obtained in this framework lately upon the characterization of invariant sets for state estimations in healthy and faulty functioning. A source of conservativeness of this approach is related to the issue of sensor recovery. Thus, in the previous work, it was supposed that the sensors are functioning under healthy dynamics for a long enough time, in order to enter the respective invariant sets, before being considered for feedback. In the current paper we present necessary and sufficient conditions for the acknowledgement of sensor recovery and the reintegration of sensors in the closed-loop decision making mechanism.
F. Stoican, Olaru, S., Nesic, M., and Marinkovic, S., “Control Design of a Positioning System upon a Fault Tolerant Multisensor Scheme”, in Proceedings of the 17th Telecommunications forum TELFOR, Belgrade, Serbia, 2009, p. 685–688.Abstract
The present paper deals with a fault tolerant control scheme for a multisensor plant under the assumption of bounded noises. A practical example, concerning a positioning system is detailed. Robust guarantees for the global stability of the system and the separability and identification of abrupt faults occurring in the sensor outputs are provided.
2008
S. Olaru, Seron, M. M., De Doná, J. A., and Stoican, F., “Receding horizon optimisation for control and reconfiguration of multisensor schemes”, in International Workshop on Assessment and Future Directions of NMPC, Pavia, Italy, 2008.