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  • 1.
    Chen, Phoebus
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Ramesh, Chithrupa
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Reducing Packet Loss Bursts in a Wireless Mesh Network for Stochastic Guarantees on Estimator Performance2011In: 2011 50th IEEE Conference on Decision and Control andEuropean Control Conference (CDC-ECC), 2011Conference paper (Refereed)
  • 2.
    Chen, Phoebus
    et al.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ramesh, Chithrupa
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl H.
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Electrical Engineering (EES), Automatic Control.
    Reducing Packet Loss Bursts in a Wireless Mesh Network for Stochastic Bounds on Estimation Error2011In: Proceedings of the IEEE Conference on Decision and Control, IEEE conference proceedings, 2011, p. 3130-3135Conference paper (Other academic)
    Abstract [en]

    A big challenge for wireless networked control systems is how to design the underlying networking algorithms and protocols to provide high reliability, defined as the end-to-end probability of packet delivery, despite the high packet loss rates of individual wireless links. This paper formulates the problem of jointly designing a set of packet forwarding policies on a multipath mesh network to meet control application requirements. We derive several results to help understand the problem space. First, we demonstrate that some common approaches, like applying a single forwarding policy to all packets or always routing packets on disjoint paths, are not optimal for the application when the links are bursty. Second, we introduce the notion of dominance to give a partial ordering to sets of forwarding policies, used to prove that an optimal policy schedules all outgoing links at each node and that an upper bound on the performance attained by unicast forwarding policies on the network graph can be computed assuming a flooding policy. Third, we demonstrate how to convert application performance metrics to packet forwarding policy objectives, using the probability that the error covariance of a Kalman filter stays within a bound as our application metric. Fourth, we provide an algorithm to compute the joint probability mass function that a sequence of packets are delivered, given a set of policies and a network graph. Finally, we describe how to obtain optimal policies via an exhaustive search, motivating future research for more computationally efficient solutions.

  • 3.
    Chen, Phoebus
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Ramesh, Chithrupa
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Network Estimation and Packet Delivery Prediction for Control over Wireless Mesh Networks2011In: IFAC Proceedings Volumes (IFAC-PapersOnline), 2011, p. 6573-6579Conference paper (Refereed)
    Abstract [en]

    Much of the current theory of networked control systems uses simple point-to-point communication models as an abstraction of the underlying network. As a result, the controller has very limited information on the network conditions and performs suboptimally. This work models the underlying wireless multihop mesh network as a graph of links with transmission success probabilities, and uses a recursive Bayesian estimator to provide packet delivery predictions to the controller. The predictions are a joint probability distribution on future packet delivery sequences, and thus capture correlations between successive packet deliveries. We look at finite horizon LQG control over a lossy actuation channel and a perfect sensing channel, both without delay, to study how the controller can compensate for predicted network outages.

  • 4.
    Ramesh, Chithrupa
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Contention-based Multiple Access Architectures for Networked Control Systems2011Licentiate thesis, monograph (Other academic)
    Abstract [en]

    Networked Control Systems (NCSs) use a wireless network for communication between sensors and controllers, and require a Medium Access Controller (MAC) to arbitrate access to the shared medium. Traditionally, a MAC for control systems is chosen primarily based on the delay it introduces in the closed loop. This thesis focuses on the design of a contention-based MAC, in a time-varying, resource-constrained network for closed loop systems.

    In this thesis, we advocate the use of a state-aware MAC, as opposed to an agnostic MAC, for NCSs. A state-aware MAC uses the state of the plant to influence access to the network. The state-aware policy is realized using two different approaches in the MAC: a regulatory formulation and an adaptive prioritization.

    Our first approach is a regulatory MAC, which serves to reduce the traffic in the network. We use a local state-based scheduler to select a few critical data packets to send to the MAC. We analyze the impact of such a scheduler on the closed loop system, and show that there is a dual effect for the control signal, which makes determining the optimal controller difficult. We also identify restrictions on the scheduling criterion that result in a separation of the scheduler, observer and controller designs.

    Our second approach is a prioritized MAC that uses state-based priorities called Attentions, to determine access to the network. We use a dominance protocol called tournaments, to evaluate priorities in a contention-based setting, and analyze the resulting performance of the MAC.

    We also consider a NCS that uses a wireless multihop mesh network for communication between the controller and actuator. We design an optimal controller, which uses packet delivery predictions from a recursive Bayesian network estimator.

    Download full text (pdf)
    CR_Lic2011
  • 5.
    Ramesh, Chithrupa
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    State-based Channel Access for a Network of Control Systems2014Doctoral thesis, monograph (Other academic)
    Abstract [en]

    Wireless networked control systems use shared wireless links to communicate between sensors and controllers, and require a channel access policy to arbitrate access to the links. Existing multiple access protocols perform this role in an agnostic manner, by remaining insular to the applications that run over the network. This approach does not give satisfactory control performance guarantees. To enable the use of wireless networks in emerging industrial applications, we must be able to systematically design wireless networked control systems that provide guaranteed performances in resource-constrained networks.

    In this thesis, we advocate the use of state-based channel access policies. A state-based policy uses the state of the controlled plant to influence access to the network. The state contains information about not only the plant, but also the network, due to the feedback in the system. Thus, by using the state to decide when and how frequently to transmit, a control system can adapt its contribution to the network traffic, and enable the network to adapt access to the plant state. We show that such an approach can provide better performance than existing methods. We examine two different state-based approaches that are distributed and easy to implement on wireless devices: event-based scheduling and adaptive prioritization.

    Our first approach uses events to reduce the traffic in the network. We use a state-based scheduler in every plant sensor to generate non-coordinated channel access requests by selecting a few critical data packets, or events, for transmission. The network uses a contention resolution mechanism to deal with simultaneous channel access requests. We present three main contributions for this formulation. The first contribution is a structural analysis of stochastic event-based systems, where we identify a dual predictor architecture that results in separation in design of the state-based scheduler, observer and controller. The second contribution is a Markov model that describes the interactions in a network of event-based systems. The third contribution is an analysis of the stability of event-based systems, leading to a stabilizing design of event-based policies.

    Our second approach uses state-based priorities to determine access to the network. We use a dominance protocol to evaluate priorities in a contention-based setting, and characterize the resulting control performance. An implementation and evaluation of this channel access mechanism on sensor nodes is also presented.

    The thesis finally examines the general networked control problem of jointly optimizing measurement and control policies, when a nonlinear measurement policy is used to perform quantization, event-triggering or companding. This contribution focuses on some of the fundamental aspects of analyzing and synthesizing control systems with state-based measurement policies in a more generalized setting. We comment on the dual effect, certainty equivalence and separation properties for this problem. In particular, we show that it is optimal to apply separation and certainty equivalence to a design problem that permits a dynamic choice of the measurement and control policies.

    Download full text (pdf)
    cramesh_PhDthesis
  • 6.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Jenkins, Dick
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Araujo, José
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    State-based Priorities for Tournaments in Wireless Networked Control Systems2014Manuscript (preprint) (Other academic)
    Abstract [en]

    We introduce a state-based distributed prioritization mechanism for a sensor associated with a dynamical system to access a wireless network, when multiple such systems share the same network link. The priorities, designated Attention Factors, are assigned by each sensor to its data packets, based on measurements of the system state. The Attention Factor represents a quantized value of the minimum risk in not transmitting a given measurement. The Attention Factors from different sensors are evaluated and allotted slots, in a distributed manner, using a dominance-based protocol called tournaments. Packets with the same Attention Factor in a tournament collide, and are lost. We analytically evaluate the probability of a successful transmission using this access mechanism. We also find an upper bound for the estimation and control performance of a system using tournament access, which shows the benefits of using state-based priorities. The proposed tournament mechanism is implemented on the IEEE 802.15.4 standard protocol stack, and evaluated in a hardware-in-the-loop experimental setup.

    Download full text (pdf)
    StateBasedPriorities2014
  • 7.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Rusu, Ana
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Ismail, Mohammed
    KTH, School of Information and Communication Technology (ICT).
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Communication Theory.
    System co-optimization in wireless receiver design with TrACS2008In: Analog Integrated Circuits and Signal Processing, ISSN 0925-1030, E-ISSN 1573-1979, Vol. 57, no 1-2, p. 117-127Article in journal (Refereed)
    Abstract [en]

    System co-optimization of the analog receiver front end circuit and the digital baseband processing could enable receiver designs with lower power budgets, as the signal processing in the digital receiver is asymmetric across circuit topologies. This paper presents a simulation tool that could assist with such co-optimized designs. TrACS (Transceiver Architecture and Channel Simulator) is an RF/DSP co-simulator, capable of providing an application-specific system-level perspective to receiver design. The simulator is especially relevant in the context of energy-constrained wireless sensor node design, where the simulator's system perspective determines the compatibility of circuit topologies, modulation techniques and synchronization methods for various wireless scenarios. A few case studies are presented, which illustrate co-optimization of a ZigBEE receiver using TrACS.

    Download full text (pdf)
    SystemCooptimizationTrACS
  • 8.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Rusu, Ana
    KTH, School of Electrical Engineering (EES), Communication Theory.
    Ismail, Mohammed
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Skoglund, Mikael
    KTH, School of Electrical Engineering (EES), Communication Theory.
    TrACS: Transceiver Architecture and Wireless Channel Simulator2007In: SBCCI2007: 20TH SYMPOSIUM ON INTEGRATED CIRCUITS AND SYSTEMS DESIGN, NEW YORK: ASSOC COMPUTING MACHINERY , 2007, p. 128-132Conference paper (Refereed)
    Abstract [en]

    This paper presents the design of a system-level simulator for radio receivers, including receiver circuits, in Matlab. The system level outlook offers a better characterization of circuit design, as the signal processing in the digital receiver is asymmetric across topologies. Also, circuit models in the simulator make it more precise and realistic compared to baseband models, which assume a single-step error-free down conversion. This interpretation is especially relevant in the design of energy-constrained wireless sensor network solutions. The simulator results for binary FSK in AWGN confirm the importance of system level simulation.

    Download full text (pdf)
    p128-ramesh_TrACS
  • 9.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Bao, Lei
    KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    On the Dual Effect in State-based Scheduling of Networked Control Systems2011In: Proceedings of the 2011 American Control Conference, IEEE , 2011, p. 2216-2221Conference paper (Refereed)
    Abstract [en]

    In this paper, we show that there is a dual effect with state-based scheduling. In general, this makes the optimal scheduler and controller hard to find. However, by removing past controls from the scheduling criterion, we find that certainty equivalence holds. This condition is related to the classical result of Bar-Shalom and Tse, and it leads to the design of a sub-optimal scheduler with a certainty equivalent controller. Furthermore, we show that a mapping of the state-based scheduler into one which fulfills this condition, and consequently has an optimal certainty equivalent controller, does not result in an equivalent class of design in the sense of Witsenhausen. Computing the estimate remains hard, but can be simplified by introducing a symmetry constraint on the scheduler.

    Download full text (pdf)
    DualEffect_ACC11
  • 10.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl H.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Design of State-Based Schedulers for a Network of Control Loops2013In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 58, no 8, p. 1962-1975Article in journal (Refereed)
    Abstract [en]

    For a closed-loop system with a contention-based multiple access network on its sensor link, the medium access controller (MAC) may discard some packets when the traffic on the link is high. We use a local state-based scheduler to select a few critical data packets to send to the MAC. In this paper, we analyze the impact of such a scheduler on the closed-loop system in the presence of traffic, and show that there is a dual effect with state-based scheduling. In general, this makes the optimal scheduler and controller hard to find. However, by removing past controls from the scheduling criterion, we find that certainty equivalence holds. This condition is related to the classical result of Bar-Shalom and Tse, and it leads to the design of an innovations-based scheduler with a certainty equivalent controller. However, this controller is not an equivalent design for the optimal controller, in the sense of Witsenhausen. The computation of the estimate can be simplified by introducing a symmetry constraint on the scheduler. Based on these findings, we propose a dual predictor architecture for the closed-loop system, which ensures separation between scheduler, observer and controller. We present an example of this architecture, which illustrates a network-aware event-triggering mechanism.

  • 11.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    LQG and Medium Access Control2009In: Proceedings of the 1st IFAC Workshop on Estimation and Control of Networked Systems (NecSys2009), 2009, p. 328-333Conference paper (Refereed)
    Abstract [en]

    The communication channel is a shared resource in networked control systems, and channel access at every instant cannot be guaranteed. In this paper, we propose a novel architecture for control over wireless networks with integrated medium access control (MAC).We evaluate the impact of constrained channel access on the cost of controlling a single plant over a network and establish that the separation principle holds under certain conditions on the MAC. We arrive at a classification of random access methods for networked control systems and identify a structure for each method. Then, by evaluating the increase in cost compared to a conventional setup, we identify an adaptive random access method which uses a threshold-based decision criteria on the current data to determine channel access. Finally, we give stability criteria for control applications using these medium access methods.

    Download full text (pdf)
    LQGmac_Necsys09
  • 12.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Multiple access with attention-based tournaments for monitoring over wireless networks2009In: Proceedings of the European Control Conference 2009, EUCA , 2009, p. 4302-4307Conference paper (Refereed)
    Abstract [en]

    Wireless sensor networks for control and moni- toring applications introduce critical constraints on the design of multiple access schemes. Controlling dynamic processes requires that priority must be given to critical systems for the use of the wireless medium. Tournaments in the medium access control (MAC) layer are presented as a way to evaluate priorities and assign channel resources in a distributed manner. The priorities are dynamically assigned based on the attention that each data packet requires. A mathematical formulation of attention is presented together with the corresponding performance analysis of the multiple access scheme. Priorities based on the attention emphasize the information content in the data to be transmitted and the related process dynamics. It is shown that under certain conditions, the performance of this distributed scheme converges to a scheduling policy based on minimizing the per-sample variance of the error in the estimates obtained with limited communication resources. Sustainable data rates for a cluster of linear processes are also derived.

    Download full text (pdf)
    fulltext
  • 13.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Stability analysis of multiple state-based schedulers with CSMA2012In: 2012 IEEE 51st Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2012, p. 7205-7211Conference paper (Refereed)
    Abstract [en]

    In this paper, we identify sufficient conditions for Lyapunov Mean Square Stability (LMSS) of a contention-based network of first-order systems, with state-based schedulers. The stability analysis helps us to choose policies for adapting the scheduler threshold to the delay from the network and scheduler. We show that three scheduling laws can result in LMSS: constant-probability laws and additively increasing or decreasing probability laws. Our results counter the notions that increasing probability scheduling laws alone can guarantee stability of the closed-loop system, or that decreasing probability scheduling laws are required to mitigate congestion in the network.

    Download full text (pdf)
    fulltext
  • 14.
    Ramesh, Chithrupa
    et al.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Sandberg, Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Johansson, Karl Henrik
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Steady State Performance Analysis of Multiple State-based Schedulers with CSMA2011In: Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference, 2011, p. 4729-4734Conference paper (Refereed)
    Abstract [en]

    In this paper, we analyze the performance ofmultiple event-based systems that share access to the samenetwork. Transmissions are attempted only when a local statebasedscheduler generates an event, and access to the networkis determined using a Carrier Sensing Multiple Access (CSMA)protocol. In general, the interactions in such a multiple accessnetwork introduce correlations between the system variablesof the various loops, and the respective traffic contributionsas well. Hence, analyzing the performance of this network isdifficult. However, a class of state-based schedulers, introducedin the paper, permits a joint analysis of the scheduler andthe Contention Resolution Mechanism (CRM). The analysisis based on a Markov model, which is validated throughsimulations. The resulting steady-state model makes it possibleto characterize the statistics of packet arrivals in this network.

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