Publications

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. 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.
2013
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. 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.
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, 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.
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.
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, 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, 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.