Emanuele Garone

False data injection attacks against state estimation in wireless sensor networks

In this paper we study the effect of false data injection attacks on state estimation carried over a sensor network monitoring a discrete-time linear time-invariant Gaussian system. The steady state Kalman filter is used to perform state estimation while a failure detector is employed to detect anomalies in the system. An attacker wishes to compromise the integrity of the state estimator by hijacking a subset of sensors and sending altered readings. In order to inject fake sensor measurements without being detected the attacker will need to carefully design his actions to fool the estimator as abnormal sensor measurements would result in an alarm. It is important for a designer to determine the set of all the estimation biases that an attacker can inject into the system without being detected, providing a quantitative measure of the resilience of the system to such attacks. To this end, we will provide an ellipsoidal algorithm to compute its inner and outer approximations of such set. A numerical example is presented to further illustrate the effect of false data injection attack on state estimation.

LQG Control for MIMO Systems Over Multiple Erasure Channels With Perfect Acknowledgment

This technical note concerns control applications over lossy data networks. Sensor data is transmitted to an estimation-control unit over a network and control commands are issued to subsystems over the same network. Sensor and control packets may be randomly lost according to a Bernoulli process. In this context, the discrete-time linear quadratic gaussian (LQG) optimal control problem is considered. In Schenato , a complete analysis was carried out for the case that sensor measurements and control inputs are delivered into a single packet to the estimator and to the actuators respectively. Here, a nontrivial generalization for MIMO systems is presented under the assumption that each sensor and each actuator exchange data with the control unit in an independent way by using their own data packet (no aggregation). In such a framework, it is shown that the separation principle still holds in the case where packet arrivals are acknowledged by the receiver. Moreover, the optimal LQG control is a linear function of the state that explicitly depends on the loss probabilities of the actuator channels. Such a dependence is not present in the single channel case considered in mean-square. In the infinite horizon case, stability conditions on the packet arrival probabilities are provided in terms of linear matrix inequalities (LMIs).

Stochastic Sensor Scheduling for Energy Constrained Estimation in Multi-Hop Wireless Sensor Networks

Wireless Sensor Networks (WSNs) enable a wealth of new applications where remote estimation is essential. Individual sensors simultaneously sense a dynamic process and transmit measured information over a shared channel to a central fusion center. The fusion center computes an estimate of the process state by means of a Kalman filter. In this technical note we assume that the WSN admits a tree topology with one fusion center at the root. At each time step only a subset of sensors can be selected to transmit observations to the fusion center due to a limited energy budget. We propose a stochastic sensor selection algorithm that randomly selects a subset of sensors according to a certain probability distribution, which is opportunely designed to minimize the asymptotic expected covariance matrix of the estimation error. We show that the optimal stochastic sensor selection problem can be relaxed into a convex optimization problem and thus efficiently solved. We also provide a possible implementation of our algorithm which does not introduce any communication overhead. The technical note ends with some numerical examples that show the effectiveness of the proposed approach.

Reference and command governors: A tutorial on their theory and automotive applications

This paper provides a tutorial overview of reference governors and command governors, which are add-on control schemes for reference supervision and constraint enforcement in closed-loop feedback control systems. The main approaches to the development of such schemes for linear and nonlinear systems are described. The treatment of unmeasured disturbances and parametric uncertainties is addressed. Generalizations to extended command governors, feedforward reference governors, reduced order reference governors, parameter governors, networked reference governors, decentralized reference governors, and virtual state governors are summarized. Examples of applications of these techniques to automotive systems are given. A comprehensive list of references is included. Comments comparing reference and command governor approaches with Model Predictive Control and input shaping, and on future directions in reference and command governor research are included.

LQG control for distributed systems over TCP-like erasure channels

This paper is concerned with control applications over lossy data network. Sensor data is transmitted to an estimation-control unit over a network and control commands are issued to subsystems over the same network. Sensor and control packets may be randomly lost according to a Bernoulli process. In this context the discrete-time Linear Quadratic Gaussian (LQG) optimal control problem is considered. In B. Sinopoli, et al., (Aug. 2003) a complete analysis was carried out for the case the network is composed of a single sensor and control channel. Here a nontrivial generalization to the case of sensor and actuator networks with p distinct sensor channels and m control channels is presented. It has been proven that the separation principle still holds for all protocols where packets are acknowledged by the receiver (e.g. TCP-like protocols). Moreover it has been pointed out for the first time that the optimal LQG control is a linear function of the state that explicitly depends on the command channels lost probabilities. Such a dependence is not present in pre-existing literature, since the amplitude of each control input has to be weighted by the loss probability associated to its own channel. This is not observed in the single channel case. In the infinite horizon case stability conditions on the arrival are derived. Their computation requires the use of linear matrix inequalities (LMIs).

Taut Cable Control of a Tethered UAV

This paper focuses on the design of a stabilizing control law for an aerial vehicle which is physically connected to a ground station by means of a tether cable. When the cable is taut, the resulting dynamic model is shown to be characterized by a new set of equilibria which untethered aircraft are unable to maintain in steady state. The control objective is to steer the UAV to a desired set-point while maintaining the cable taut at all times. This leads to a nonlinear control problem subject to constraints. A cascade control scheme is proposed and proven to asymptotically stabilize the overall system by means of ISS arguments. Constraint satisfaction is guaranteed using a modified thrust vector control coupled with a reference governor strategy. The effectiveness of the proposed control strategy is shown via numerical simulations.

Reference and command governors for systems with constraints

Reference and command governors are add-on control schemes which enforce state and control constraints on pre-stabilized systems by modifying, whenever necessary, the reference. This paper surveys the extensive literature concerning the development of such schemes for linear and nonlinear systems. The treatment of unmeasured disturbances and parametric uncertainties is also detailed. Generalizations, including extended command governors, feedforward reference governors, reduced order reference governors, parameter governors, networked reference governors, and decentralized/distributed reference governors, are discussed. Practical applications of these techniques are presented and surveyed as well. A comprehensive list of references is included. Connections with related approaches, including model predictive control and input shaping, are discussed. Opportunities and directions for future research are highlighted.

Sensorless supervision of linear dynamical systems: The Feed-Forward Command Governor approach

This paper proposes a novel class of Command Governor (CG) strategies for input and state-related constrained discrete-time LTI systems subject to bounded disturbances in the absence of explicit state or output measurements. While in traditional CG schemes the set-point manipulation is undertaken on the basis of either the actual measure of the state or its suitable estimation, it is shown here that the CG design problem can be solved, with limited performance degradation and with similar properties, also in the case that such an explicit measure is not available. This approach, which will be referred to as the Feed-Forward CG (FF-CG) approach, may be a convenient alternative CG solution in all situations whereby the cost of measuring the state may be a severe limitation, e.g. in distributed or decentralized applications. In order to evaluate the method proposed here, numerical simulations on a physical example have been undertaken and comparisons with the standard state-based CG solution reported.

Adaptive fault tolerant actuator allocation for overactuated plants

This paper presents an adaptive actuator allocation scheme that is fault-tolerant with respect to actuator faults and loss of effectiveness. The main idea is to use an ad-hoc online parameter estimator coupled with an allocation algorithm to perform on-line control reconfiguration whenever necessary. A preliminary algorithm is proposed for nonlinear discrete-time systems. Its main properties are summarized in the disturbance-free case and its effectiveness shown by means of two numerical examples.

Distributed Command Governor strategies for constrained coordination of multi-agent networked systems

In this paper a novel distributed FeedBack Command Governor (FB-CG) supervision strategy is presented for multi-agent networked systems subject to pointwise-in-time coordination constraints on the overall network evolutions. Specifically, a sequential distributed strategy where only one agent at a time is allowed to manipulate its own reference signal is fully detailed and its stability, feasibility and viability (liveness) properties rigorously proved. It is shown that, unlike the centralized case, liveness (the absence of deadlock situations) can be ensured only if the constraints satisfy a Constraints Qualification (CQ) condition. Several results and open problems in the liveness analysis are discussed and a counterexample is also provided for showing that for multi-input agents case the issue requires further investigations.