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Publications (10 of 14) Show all publications
Ramesh, C. (2014). State-based Channel Access for a Network of Control Systems. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>State-based Channel Access for a Network of Control Systems
2014 (English)Doctoral 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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. p. x, 230
Series
TRITA-EE, ISSN 1653-5146 ; 2014:016
Keywords
networked control, multiple access, event-based
National Category
Control Engineering
Research subject
Electrical Engineering
Identifiers
urn:nbn:se:kth:diva-144057 (URN)978-91-7595-092-1 (ISBN)
Public defence
2014-04-25, E3, Osquars Backe 14, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140408

Available from: 2014-04-08 Created: 2014-04-07 Last updated: 2019-11-14Bibliographically approved
Ramesh, C., Jenkins, D., Araujo, J., Sandberg, H. & Johansson, K. H. (2014). State-based Priorities for Tournaments in Wireless Networked Control Systems.
Open this publication in new window or tab >>State-based Priorities for Tournaments in Wireless Networked Control Systems
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2014 (English)Manuscript (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.

Keywords
adaptive priorities, CAN bus, networked control
National Category
Control Engineering Communication Systems
Identifiers
urn:nbn:se:kth:diva-152361 (URN)
Note

Qc 20140930

Available from: 2014-09-25 Created: 2014-09-25 Last updated: 2014-09-30Bibliographically approved
Ramesh, C., Sandberg, H. & Johansson, K. H. (2013). Design of State-Based Schedulers for a Network of Control Loops. IEEE Transactions on Automatic Control, 58(8), 1962-1975
Open this publication in new window or tab >>Design of State-Based Schedulers for a Network of Control Loops
2013 (English)In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 58, no 8, p. 1962-1975Article in journal (Refereed) Published
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.

Keywords
Event-based systems, networked control systems, state-based schedulers
National Category
Electrical Engineering, Electronic Engineering, Information Engineering Control Engineering
Identifiers
urn:nbn:se:kth:diva-127482 (URN)10.1109/TAC.2013.2251791 (DOI)000322364300006 ()2-s2.0-84880913768 (Scopus ID)
Funder
Swedish Research CouncilVinnovaSwedish Foundation for Strategic Research Knut and Alice Wallenberg FoundationEU, European Research Council
Note

QC 20130905

Available from: 2013-09-05 Created: 2013-08-30 Last updated: 2017-12-06Bibliographically approved
Ramesh, C., Sandberg, H. & Johansson, K. H. (2012). Stability analysis of multiple state-based schedulers with CSMA. In: 2012 IEEE 51st Annual Conference on Decision and Control (CDC): . Paper presented at 51st IEEE Conference on Decision and Control, CDC 2012; Maui, HI; United States; 10 December 2012 through 13 December 2012 (pp. 7205-7211). IEEE conference proceedings
Open this publication in new window or tab >>Stability analysis of multiple state-based schedulers with CSMA
2012 (English)In: 2012 IEEE 51st Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2012, p. 7205-7211Conference paper, Published 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.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2012
Series
IEEE Conference on Decision and Control. Proceedings, ISSN 0191-2216
Keywords
Closed loop systems, Customer relationship management, Delay, Estimation error, Markov processes, Stability analysis, Steady-state
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-111461 (URN)10.1109/CDC.2012.6426068 (DOI)000327200407072 ()2-s2.0-84874234336 (Scopus ID)978-1-4673-2064-1 (ISBN)
Conference
51st IEEE Conference on Decision and Control, CDC 2012; Maui, HI; United States; 10 December 2012 through 13 December 2012
Funder
ICT - The Next Generation
Note

QC 20130116

Available from: 2013-02-15 Created: 2013-01-11 Last updated: 2013-12-19Bibliographically approved
Ramesh, C. (2011). Contention-based Multiple Access Architectures for Networked Control Systems. (Licentiate dissertation). Stockholm: KTH Royal Institute of Technology
Open this publication in new window or tab >>Contention-based Multiple Access Architectures for Networked Control Systems
2011 (English)Licentiate 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.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. p. vi, 120
Series
Trita-EE, ISSN 1653-5146 ; 2011:006
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-29781 (URN)978-91-7415-866-3 (ISBN)
Presentation
2011-02-18, D3, KTH, Lindtedtsv. 5, Stockholm, 15:41 (English)
Opponent
Supervisors
Note
QC 20110216Available from: 2011-02-16 Created: 2011-02-15 Last updated: 2019-11-14Bibliographically approved
Chen, P., Ramesh, C. & Johansson, K. H. (2011). Network Estimation and Packet Delivery Prediction for Control over Wireless Mesh Networks. In: IFAC Proceedings Volumes (IFAC-PapersOnline): . Paper presented at 18th IFAC World Congress; Milano; Italy; 28 August 2011 through 2 September 2011 (pp. 6573-6579).
Open this publication in new window or tab >>Network Estimation and Packet Delivery Prediction for Control over Wireless Mesh Networks
2011 (English)In: IFAC Proceedings Volumes (IFAC-PapersOnline), 2011, p. 6573-6579Conference paper, Published 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.

Keywords
Control and estimation with data loss; Networked embedded control systems; Stochastic control
National Category
Control Engineering
Identifiers
urn:nbn:se:kth:diva-47686 (URN)10.3182/20110828-6-IT-1002.00828 (DOI)2-s2.0-84866744930 (Scopus ID)
Conference
18th IFAC World Congress; Milano; Italy; 28 August 2011 through 2 September 2011
Funder
TrenOp, Transport Research Environment with Novel PerspectivesICT - The Next Generation
Note

QC 20140902

Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2014-10-03Bibliographically approved
Ramesh, C., Sandberg, H., Bao, L. & Johansson, K. H. (2011). On the Dual Effect in State-based Scheduling of Networked Control Systems. In: Proceedings of the 2011 American Control Conference. Paper presented at 2011 American Control Conference (ACC) on O'Farrell Street, San Francisco, CA (pp. 2216-2221). IEEE
Open this publication in new window or tab >>On the Dual Effect in State-based Scheduling of Networked Control Systems
2011 (English)In: Proceedings of the 2011 American Control Conference, IEEE , 2011, p. 2216-2221Conference paper, Published 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.

Place, publisher, year, edition, pages
IEEE, 2011
Series
Proceedings of the American Control Conference, ISSN 0743-1619
Keywords
Estimation error, Optimal control, Optimal scheduling, Processor scheduling, Scheduling, Tin
National Category
Control Engineering
Identifiers
urn:nbn:se:kth:diva-47682 (URN)000295376002139 ()2-s2.0-80053155550 (Scopus ID)978-1-4577-0080-4 (ISBN)
Conference
2011 American Control Conference (ACC) on O'Farrell Street, San Francisco, CA
Funder
TrenOp, Transport Research Environment with Novel PerspectivesICT - The Next Generation
Note
© 2011 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. QC 20111117Available from: 2011-11-17 Created: 2011-11-11 Last updated: 2012-02-07Bibliographically approved
Chen, P., Ramesh, C. & Johansson, K. H. (2011). Reducing Packet Loss Bursts in a Wireless Mesh Network for Stochastic Bounds on Estimation Error. In: Proceedings of the IEEE Conference on Decision and Control: . Paper presented at 50th IEEE Conference of Decision and Control (CDC)/European Control Conference (ECC) (pp. 3130-3135). IEEE conference proceedings
Open this publication in new window or tab >>Reducing Packet Loss Bursts in a Wireless Mesh Network for Stochastic Bounds on Estimation Error
2011 (English)In: Proceedings of the IEEE Conference on Decision and Control, IEEE conference proceedings, 2011, p. 3130-3135Conference paper, Published 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.

Place, publisher, year, edition, pages
IEEE conference proceedings, 2011
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-121401 (URN)10.1109/CDC.2011.6161196 (DOI)000303506203119 ()2-s2.0-84860662466 (Scopus ID)978-1-61284-801-3 (ISBN)
Conference
50th IEEE Conference of Decision and Control (CDC)/European Control Conference (ECC)
Note

QC 20130429

Available from: 2013-04-29 Created: 2013-04-29 Last updated: 2020-03-05Bibliographically approved
Chen, P. & Ramesh, C. (2011). Reducing Packet Loss Bursts in a Wireless Mesh Network for Stochastic Guarantees on Estimator Performance. In: 2011 50th IEEE Conference on Decision and Control andEuropean Control Conference (CDC-ECC). Paper presented at 2011 50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC) Orlando, FL, USA, December 12-15, 2011.
Open this publication in new window or tab >>Reducing Packet Loss Bursts in a Wireless Mesh Network for Stochastic Guarantees on Estimator Performance
2011 (English)In: 2011 50th IEEE Conference on Decision and Control andEuropean Control Conference (CDC-ECC), 2011Conference paper, Published paper (Refereed)
National Category
Control Engineering
Identifiers
urn:nbn:se:kth:diva-89713 (URN)
Conference
2011 50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC) Orlando, FL, USA, December 12-15, 2011
Note
QC 20120330Available from: 2012-02-15 Created: 2012-02-15 Last updated: 2012-03-30Bibliographically approved
Ramesh, C., Sandberg, H. & Johansson, K. H. (2011). Steady State Performance Analysis of Multiple State-based Schedulers with CSMA. In: Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference. Paper presented at 50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC), Orlando, FL, USA, December 12-15, 2011 (pp. 4729-4734).
Open this publication in new window or tab >>Steady State Performance Analysis of Multiple State-based Schedulers with CSMA
2011 (English)In: Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference, 2011, p. 4729-4734Conference paper, Published 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.

National Category
Control Engineering
Identifiers
urn:nbn:se:kth:diva-75307 (URN)10.1109/CDC.2011.6161118 (DOI)000303506205056 ()2-s2.0-84860663427 (Scopus ID)
Conference
50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC), Orlando, FL, USA, December 12-15, 2011
Note

QC 20120413

Available from: 2012-02-05 Created: 2012-02-05 Last updated: 2020-03-05Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-7228-0249

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