Browsing by Author "Pajic, M"

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    Attack-resilient sensor fusion for safety-critical cyber-physical systems
    (ACM Transactions on Embedded Computing Systems, 2016-02-01) Ivanov, R.; Pajic, M; Lee, I.
    © 2016 ACM.This article focuses on the design of safe and attack-resilient Cyber-Physical Systems (CPS) equipped with multiple sensors measuring the same physical variable. A malicious attacker may be able to disrupt system performance through compromising a subset of these sensors. Consequently, we develop a precise and resilient sensor fusion algorithm that combines the data received from all sensors by taking into account their specified precisions. In particular, we note that in the presence of a shared bus, in which messages are broadcast to all nodes in the network, the attacker's impact depends on what sensors he has seen before sending the corrupted measurements. Therefore, we explore the effects of communication schedules on the performance of sensor fusion and provide theoretical and experimental results advocating for the use of the Ascending schedule, which orders sensor transmissions according to their precision starting from the most precise. In addition, to improve the accuracy of the sensor fusion algorithm, we consider the dynamics of the system in order to incorporate past measurements at the current time. Possible ways of mapping sensor measurement history are investigated in the article and are compared in terms of the confidence in the final output of the sensor fusion. We show that the precision of the algorithm using history is never worse than the no-history one, while the benefits may be significant. Furthermore, we utilize the complementary properties of the two methods and show that their combination results in a more precise and resilient algorithm. Finally, we validate our approach in simulation and experiments on a real unmanned ground robot.
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    Attack-Resilient State Estimation in the Presence of Noise
    Pajic, M; Tabuada, P.; Lee, I.; Pappas, G.J.
    We consider the problem of attack-resilient state estimation in the presence of noise. We focus on the most general model for sensor attacks where {any} signal can be injected via the compromised sensors. An $l_0$-based state estimator that can be formulated as a mixed-integer linear program and its convex relaxation based on the $l_1$ norm are presented. For both $l_0$ and $l_1$-based state estimators, we derive rigorous analytic bounds on the state-estimation errors. We show that the worst-case error is linear with the size of the noise, meaning that the attacker cannot exploit noise and modeling errors to introduce unbounded state-estimation errors. Finally, we show how the presented attack-resilient state estimators can be used for sound attack detection and identification, and provide conditions on the size of attack vectors that will ensure correct identification of compromised sensors.
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    Automatic verification of linear controller software
    (2015 Proceedings of the International Conference on Embedded Software, EMSOFT 2015, 2015-11-04) Pajic, M; Park, J; Lee, I; Pappas, GJ; Sokolsky, O
    © 2015 IEEE.We consider the problem of verification of software implementations of linear time-invariant controllers. Commonly, different implementations use different representations of the controller's state, for example due to optimizations in a third-party code generator. To accommodate this variation, we exploit input-output controller specification captured by the controller's transfer function and show how to automatically verify correctness of C code controller implementations using a Frama-C/Why3/Z3 toolchain. Scalability of the approach is evaluated using randomly generated controller specifications of realistic size.
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    Design methodologies for securing cyber-physical systems
    (2015 International Conference on Hardware/Software Codesign and System Synthesis, CODES+ISSS 2015, 2015-11-17) Faruque, MAA; Regazzoni, F; Pajic, M
    © 2015 IEEE.Cyber-Physical Systems (CPS) are in most cases safety- and mission-critical. Standard design techniques used for securing embedded systems are not suitable for CPS due to the restricted computation and communication budget available in the latter. In addition, the sensitivity of sensed data and the presence of actuation components further increase the security requirements of CPS. To address these issues, it is necessary to provide new design methods in which security is considered from the beginning of the whole design flow and addressed in a holistic way. In this paper, we focus on the design of secure CPS as part of the complete CPS design process, and provide insights into new requirements on platform-aware design of control components, design methodologies and architectures posed by CPS design. We start by discussing methods for the multi-disciplinary modeling, simulation, tools, and software synthesis challenges for CPS. We also present a framework for design of secure control systems for CPS, while taking into account properties of the underlying computation and communication platforms. Finally, we describe the security challenges in the computing hardware that is used in CPS.
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    Opportunistic Control Over Shared Wireless Channels
    (IEEE Transactions on Automatic Control, 2015-12-01) Gatsis, K; Pajic, M; Ribeiro, A; Pappas, GJ
    © 2015 IEEE.We consider a wireless control architecture with multiple control loops over a shared wireless medium. A scheduler observes the random channel conditions that each control system experiences over the shared medium and opportunistically selects systems to transmit at a set of non-overlapping frequencies. The transmit power of each system also adapts to channel conditions and determines the probability of successfully receiving and closing the loop. We formulate the optimal design of channel-aware scheduling and power allocation that minimize the total power consumption while meeting control performance requirements for all systems. In particular, it is required that for each control system a given Lyapunov function decreases at a specified rate in expectation over the random channel conditions. We develop an offline algorithm to find the optimal communication design, as well as an online protocol which selects scheduling and power variables based on a random observed channel sequence and converges almost surely to the optimal operating point. Simulations illustrate the power savings of our approach compared to other non-channel-aware schemes.
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    Recognition of Planar Segments in Point Cloud Based on Wavelet Transform
    (IEEE Transactions on Industrial Informatics, 2015-04-27) Jakovljevic, Z; Puzovic, R; Pajic, M
    © 2005-2012 IEEE.Within industrial automation systems, three-dimensional (3-D) vision provides very useful feedback information in autonomous operation of various manufacturing equipment (e.g., industrial robots, material handling devices, assembly systems, and machine tools). The hardware performance in contemporary 3-D scanning devices is suitable for online utilization. However, the bottleneck is the lack of real-time algorithms for recognition of geometric primitives (e.g., planes and natural quadrics) from a scanned point cloud. One of the most important and the most frequent geometric primitive in various engineering tasks is plane. In this paper, we propose a new fast one-pass algorithm for recognition (segmentation and fitting) of planar segments from a point cloud. To effectively segment planar regions, we exploit the orthonormality of certain wavelets to polynomial function, as well as their sensitivity to abrupt changes. After segmentation of planar regions, we estimate the parameters of corresponding planes using standard fitting procedures. For point cloud structuring, a z-buffer algorithm with mesh triangles representation in barycentric coordinates is employed. The proposed recognition method is tested and experimentally validated in several real-world case studies.
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    Robust estimation using context-aware filtering
    (2015 53rd Annual Allerton Conference on Communication, Control, and Computing, Allerton 2015, 2016-04-04) Ivanov, R; Atanasov, N; Pajic, M; Pappas, G.J.; Lee, I
    © 2015 IEEE.This paper presents the context-aware filter, an estimation technique that incorporates context measurements, in addition to the regular continuous measurements. Context measurements provide binary information about the system's context which is not directly encoded in the state; examples include a robot detecting a nearby building using image processing or a medical device alarming that a vital sign has exceeded a predefined threshold. These measurements can only be received from certain states and can therefore be modeled as a function of the system's current state. We focus on two classes of functions describing the probability of context detection given the current state; these functions capture a wide variety of detections that may occur in practice. We derive the corresponding context-aware filters, a Gaussian Mixture filter and another closed-form filter with a posterior distribution whose moments are derived in the paper. Finally, we evaluate the performance of both classes of functions through simulation of an unmanned ground vehicle.
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    Robustness of attack-resilient state estimators
    (2014 ACM/IEEE International Conference on Cyber-Physical Systems, ICCPS 2014, 2014-01-01) Pajic, M; Weimer, J; Bezzo, N; Tabuada, P; Sokolsky, O; Lee, Insup; Pappas, GJ
    The interaction between information technology and phys ical world makes Cyber-Physical Systems (CPS) vulnerable to malicious attacks beyond the standard cyber attacks. This has motivated the need for attack-resilient state estimation. Yet, the existing state-estimators are based on the non-realistic assumption that the exact system model is known. Consequently, in this work we present a method for state estimation in presence of attacks, for systems with noise and modeling errors. When the the estimated states are used by a state-based feedback controller, we show that the attacker cannot destabilize the system by exploiting the difference between the model used for the state estimation and the real physical dynamics of the system. Furthermore, we describe how implementation issues such as jitter, latency and synchronization errors can be mapped into parameters of the state estimation procedure that describe modeling errors, and provide a bound on the state-estimation error caused by modeling errors. This enables mapping control performance requirements into real-time (i.e., timing related) specifications imposed on the underlying platform. Finally, we illustrate and experimentally evaluate this approach on an unmanned ground vehicle case-study. © 2014 IEEE.
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    Safety-critical medical device development using the UPP2SF model translation tool
    (Transactions on Embedded Computing Systems, 2014-01-01) Pajic, M; Jiang, Z; Lee, I; Sokolsky, O; Mangharam, R
    Software-based control of life-critical embedded systems has become increasingly complex, and to a large extent has come to determine the safety of the human being. For example, implantable cardiac pacemakers have over 80,000 lines of code which are responsible for maintaining the heart within safe operating limits. As firmware-related recalls accounted for over 41% of the 600,000 devices recalled in the last decade, there is a need for rigorous model-driven design tools to generate verified code from verified software models. To this effect, we have developed the UPP2SF model-translation tool, which facilitates automatic conversion of verified models (in UPPAAL) to models that may be simulated and tested (in Simulink/Stateflow). We describe the translation rules that ensure correct model conversion, applicable to a large class of models. We demonstrate how UPP2SF is used in themodel-driven design of a pacemaker whosemodel is (a) designed and verified in UPPAAL (using timed automata), (b) automatically translated to Stateflow for simulation-based testing, and then (c) automatically generated into modular code for hardware-level integration testing of timing-related errors. In addition, we show how UPP2SF may be used for worst-case execution time estimation early in the design stage. Using UPP2SF, we demonstrate the value of integrated end-to-end modeling, verification, code-generation and testing process for complex software-controlled embedded systems. © 2014 ACM.
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    The wireless control network: A new approach for control over networks
    (IEEE Transactions on Automatic Control, 2011-10-01) Pajic, M; Sundaram, S; Pappas, GJ; Mangharam, R
    We present a method to stabilize a plant with a network of resource constrained wireless nodes. As opposed to traditional networked control schemes where the nodes simply route information to and from a dedicated controller (perhaps performing some encoding along the way), our approach treats the network itself as the controller. Specifically, we formulate a strategy for each node in the network to follow, where at each time-step, each node updates its internal state to be a linear combination of the states of the nodes in its neighborhood. We show that this causes the entire network to behave as a linear dynamical system, with sparsity constraints imposed by the network topology. We provide a numerical design procedure to determine appropriate linear combinations to be applied by each node so that the transmissions of the nodes closest to the actuators will stabilize the plant. We also show how our design procedure can be modified to maintain mean square stability under packet drops in the network, and present a distributed scheme that can handle node failures while preserving stability. We call this architecture a Wireless Control Network, and show that it introduces very low computational and communication overhead to the nodes in the network, allows the use of simple transmission scheduling algorithms, and enables compositional design (where the existing wireless control infrastructure can be easily extended to handle new plants that are brought online in the vicinity of the network). © 2011 IEEE.
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    Topological conditions for in-network stabilization of dynamical systems
    (IEEE Journal on Selected Areas in Communications, 2013-04-04) Pajic, M; Mangharam, R; Pappas, GJ; Sundaram, S
    We study the problem of stabilizing a linear system over a wireless network using a simple in-network computation method. Specifically, we study an architecture called the Wireless Control Network (WCN), where each wireless node maintains a state, and periodically updates it as a linear combination of neighboring plant outputs and node states. This architecture has previously been shown to have low computational overhead and beneficial scheduling and compositionality properties. In this paper we characterize fundamental topological conditions to allow stabilization using such a scheme. To achieve this, we exploit the fact that the WCN scheme causes the network to act as a linear dynamical system, and analyze the coupling between the plant's dynamics and the dynamics of the network. We show that stabilizing control inputs can be computed in-network if the vertex connectivity of the network is larger than the geometric multiplicity of any unstable eigenvalue of the plant. This condition is analogous to the typical min-cut condition required in classical information dissemination problems. Furthermore, we specify equivalent topological conditions for stabilization over a wired (or point-to-point) network that employs network coding in a traditional way-as a communication mechanism between the plant's sensors and decentralized controllers at the actuators. © 1983-2012 IEEE.