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Control over Wireless Networks: Research Challenges and Case Studies

Control over Wireless Networks: Research Challenges and Case Studies. Karl Henrik Johansson School of Electrical Engineering Royal Institute of Technology Stockholm, Sweden. GRASP Seminar Series University of Pennsylvania, Philadelphia, USA, March 24, 2006. Royal Institute of Technology.

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Control over Wireless Networks: Research Challenges and Case Studies

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  1. Control over Wireless Networks: Research Challenges and Case Studies Karl Henrik Johansson School of Electrical Engineering Royal Institute of Technology Stockholm, Sweden GRASP Seminar Series University of Pennsylvania, Philadelphia, USA, March 24, 2006

  2. Royal Institute of Technology • Founded 1827 • 1/3 of Sweden’s engineering research and education • EE strengths include • autonomous systems • wireless communication • electrical power engineering Karl H. Johansson, Control over wireless networks, 2006

  3. Wireless control systems Sensors Controllers Actuators Karl H. Johansson, Control over wireless networks, 2006

  4. Wireless control systems Sensors Controllers Actuators Karl H. Johansson, Control over wireless networks, 2006

  5. Machine-to-machine device population forecast 2010 RFID/Sensors: Location Humidity Temperature Vibration Liquid Weight etc. 1+ Trillion Microprocessors: 4-64+ bits CPU’s etc. 500 Billion Smart Devices: Appliances Machinery Vehicles Bldg. Eqpt. etc. 2 Billion 1 Billion Info Devices: 300 Million PC’s: Mobile Phones PDA’s Web Tablets etc. Wireless control systems everywhere Forecast of installed base, 2010 The FocalPoint Group, LLC 2003 http://www.thefpgroup.com Karl H. Johansson, Control over wireless networks, 2006

  6. A Plant S C A Plant S C A A Plant Plant S S A Plant S C C C Feedback control representations A networked control system is a spatially distributed control system with sensor, actuator and controller communication supported by a network Network Karl H. Johansson, Control over wireless networks, 2006

  7. Outline: Control over wireless networks • Motivating examples • Benefits and barriers of networked control • Control-aware networking • Network-aware control • Conclusions Karl H. Johansson, Control over wireless networks, 2006

  8. Motivation for control over wireless networks • Wireless industrial automation • Information networks in vehicles • Multi-robot systems Karl H. Johansson, Control over wireless networks, 2006

  9. Wireless industrial automation • Communication cabling is subject to heavy wear and tear in industry robots and therefore requires frequent maintenance • Replace wires between robot controller and gripper by wireless communication Removing cables undoubtedly saves cost, but often the real cost gains lie in the radically different design approach that wireless solutions permit. […] In order to fully benefit from wireless technologies, a rethink of existing automation concepts and the complete design and functionality of an application is required. Jan-Erik Frey, ABB Wireless Actuator Sensor Interface Karl H. Johansson, Control over wireless networks, 2006

  10. Network architectureof a heavy vehicle • Architecture based on three controller area networks (CANs) • Link up to 40 control nodes • Coloured by criticality Karl H. Johansson, Control over wireless networks, 2006

  11. Road profile Coolant temp Fan & pump speed Wireless information systems for improved fuel efficiency in vehicles • Predict road and driving conditions based on emerging wireless information systems • E.g., congestion info through RDS, digital maps, GPS • Control vehicle subsystems to improve fuel efficiency • E.g., cruise control, automated gear shifting, auxiliary systems Example Optimal control of cooling system based on road profile ahead and other predicted driver conditions [Pettersson & J, 2006] Karl H. Johansson, Control over wireless networks, 2006

  12. Networked control architecture of SMART-1 spacecraft • First European lunar mission, launched Sep 2003 • Two CAN busses for control system and for scientific experiments (“payload”) • Node and communication redundancies [Bodin, 2005]

  13. Networked control architecture of SMART-1 spacecraft • First European lunar mission, launched Sep 2003 • Two CAN busses for control system and for scientific experiments (“payload”) • Node and communication redundancies Power Earth communication Control system Payload [Bodin, 2005]

  14. Outline: Control over wireless networks • Motivating examples • Benefits and barriers of networked control • Control-aware networking • Network-aware control • Conclusions Karl H. Johansson, Control over wireless networks, 2006

  15. Benefits of wireless networking in control systems Several advantages with wireless networking control systems: • Added flexibility • Sensor and actuator nodes can be placed more appropriately • Less restrictive maneuvers and control actions • More powerful control through distributed computations • Reduced installation and maintenance costs • Less cabling • More efficient monitoring and diagnosis Karl H. Johansson, Control over wireless networks, 2006

  16. Barriers of wireless technology in automation and control • Complexity • Systems designers and programmers need suitable abstractions to hide the complexity from wireless devices and communication • Reliability • Systems should have robust and predictable behaviour despite characteristics of wireless networks • Security • Wireless technology is vulnerable • Security mechanisms for control loops Control application Network Radio Karl H. Johansson, Control over wireless networks, 2006

  17. Wireless network Influence of wireless communication on closed-loop control performance • Communication of sensor and actuator data impose uncertainty, disturbances and constraints on control system • Communication imperfections in control loops include • Delay and jitter • Bandwidth limitations • Data loss and bit errors • Outages and disconnection • Topology variations Karl H. Johansson, Control over wireless networks, 2006

  18. Wireless network Influence of wireless communication on closed-loop control performance • Communication of sensor and actuator data impose uncertainty, disturbances and constraints on control system • Communication imperfections in control loops include • Delay and jitter • Bandwidth limitations • Data loss and bit errors • Outages and disconnection • Topology variations Complementary approaches: • Control-aware networking • Design more suitable network protocols and radio links • Network-aware control • Develop control algorithms to cope with communication imperfections Karl H. Johansson, Control over wireless networks, 2006

  19. Outline: Control over wireless networks • Motivating examples • Benefits and barriers of networked control • Control-aware networking • Network-aware control • Conclusions Karl H. Johansson, Control over wireless networks, 2006

  20. Layered communication networks • Communication networks are layered • Allows independent refinements • Several feedback control mechanisms separate communication layers Example • Reliable data transfer over wireless link through suitable feedback control of • power • modulation scheme • channel coding Karl H. Johansson, Control over wireless networks, 2006

  21. Cross-layer interaction and adaptation • Control mechanisms are not always able to separate communication layers • Undesirable cross-layer interaction may lead to performance deterioration • E.g., wireless link influence end-to-end network performance • Cross-layer adaptation is desirable signaling between communication layers to improve end-to-end performance • Lets the upper layers adapt to communication variations • E.g., measurable wireless bandwidth variations can be used to improve end-to-end transport Cross-layer Karl H. Johansson, Control over wireless networks, 2006

  22. Cross-layer adaptation for improved behavior of interacting cellular and wired networks • Bandwidth variations in radio link give performance degradations due to large end-to-end delays and improper transport protocols • Proxy between cellular and wired networks to adapt sending rate to bandwidth variations obtained from radio network controller (RNC) App Server 3G-SGSN RNC 3G-GGSN Internet BW variations PROXY BTS 3G Cellular Network BTS Terminal Karl H. Johansson, Control over wireless networks, 2006

  23. Hybrid control law • Hybrid controller in proxy regulates sending rate based on • Events generated by bandwidth changes obtained from RNC • Sampled measurements of queue length in RNC [Möller et al., 2005] Karl H. Johansson, Control over wireless networks, 2006

  24. Bandwidth utilization New protocol End-to-end protocol Experimental evaluation • Improved time-to-serve-user and link utilization compared to traditional end-to-end protocol [Möller et al., 2005] Karl H. Johansson, Control over wireless networks, 2006

  25. Outline: Control over wireless networks • Motivating examples • Benefits and barriers of networked control • Control-aware networking • Network-aware control • Conclusions Karl H. Johansson, Control over wireless networks, 2006

  26. Network observer Wireless network Control law Plant observer Network-aware control architecture • Estimate network state • Delay • Data loss probability • Bandwidth • Adjust controller accordingly Karl H. Johansson, Control over wireless networks, 2006

  27. Network observer Wireless network Control law Plant observer Network-aware controllers Control algorithms to cope with communication imperfections • Control under network delay • Control under data loss • Control under bandwidth limitation • Control under topology constraints Characteristics depend on network technology Karl H. Johansson, Control over wireless networks, 2006

  28. Control under network delay • Delays in communication due to buffering and propagation • Delays are bad for control loops (avoid if possible) • Delays can be fixed or varying, known (measurable) or unknown Delays and losses INRIAUMd [Bolot, 1993] • Vast literature, e.g., IFAC Workshop on Time-Delay Systems Karl H. Johansson, Control over wireless networks, 2006

  29. Unknown and fixed time delay Karl H. Johansson, Control over wireless networks, 2006

  30. Unknown and varying time delay Remark: Closed-loop gain should be small. Easy to check through Bode plot. [Kao and Lincoln, 2004] Karl H. Johansson, Control over wireless networks, 2006

  31. Proof Karl H. Johansson, Control over wireless networks, 2006

  32. Relation to Nyquist Criterion Karl H. Johansson, Control over wireless networks, 2006

  33. Known time delay Karl H. Johansson, Control over wireless networks, 2006

  34. Time stamps and synchronization • Major improvement in control performance if delays are measurable (or can be estimated) Example • CAN is event-triggered and does not give timing guarantees • TTCAN (Time-triggered communication on CAN) is an extension to the CAN standard targeting the need for time-synchronized nodes Karl H. Johansson, Control over wireless networks, 2006

  35. Compensating known delays in output feedback controller [Zhang et al., 2001] Karl H. Johansson, Control over wireless networks, 2006

  36. Large delays and out-of-order delivery Large known delays can be treated as before by extending the estimator state (one dim per extra sampling period delay) • Buffers can handle out-of-order delivery, but may increase delays • “Don’t wait for late data, but use them to adjust old estimates” Karl H. Johansson, Control over wireless networks, 2006

  37. Delay estimation • Internet round-trip time (RTT) data are noisy with piecewise constant average • Complex network dynamics hard to model • RTT estimation in TCP: • Improved estimation thru Kalman filter with hypothesis test (CUSUM filter) [Jacobsson et al., 2004] Karl H. Johansson, Control over wireless networks, 2006

  38. Control under data loss • Estimation under data loss • Control under data loss • Controlled data loss Karl H. Johansson, Control over wireless networks, 2006

  39. Estimation under data loss [Sinopoli et al., 2004] Karl H. Johansson, Control over wireless networks, 2006

  40. Estimation using smart sensor S [Hu & Hespanha, 2005] Karl H. Johansson, Control over wireless networks, 2006

  41. Controlled data loss • Compensate data loss through added redundancy • Base amount of redundancy on feedback information from decoder • Adaptive forward error correction Enc Dec Karl H. Johansson, Control over wireless networks, 2006

  42. Controlled data loss • Compensate data loss through added redundancy • Base amount of redundancy on feedback information Track network variations through variable redundancy [Flärdh et al., 2005] Karl H. Johansson, Control over wireless networks, 2006

  43. Control under either delay or data loss • Communication channel with two modes: • Error-free transmission, but delay • Transmission errors, but no delay • Receding horizon control optimized under communication constraints [Quevedo & Goodwin, 2005] Karl H. Johansson, Control over wireless networks, 2006

  44. Control under bandwidth limitation • Maximize sensor sleeping to maximize battery lifetime • Periodically sampled sensors waste resources Idea: Event-based sensing with optimized use of communication bandwidth Karl H. Johansson, Control over wireless networks, 2006

  45. Joint encoder-decoder and control design • Optimize closed-loop performance based on plant model and disturbance statistics • Model of communication system [Bao et al., 2006] Karl H. Johansson, Control over wireless networks, 2006

  46. Plant output Control signal Event-triggered feedback control for bandlimited channels • Train encoder and decoder through sequential Monte Carlo estimation [Bao et al., 2006] Karl H. Johansson, Control over wireless networks, 2006

  47. Outline: Control over wireless networks • Motivating examples • Benefits and barriers of networked control • Control-aware networking • Network-aware control • Conclusions Karl H. Johansson, Control over wireless networks, 2006

  48. Conclusions • Wireless control systems have a growing application domain • E.g., industrial automation, vehicles, multi-robot systems • Several open research problems related to complexity, reliability and security • Compensate for and explore communication characteristics: • Delays, bandwidth limitations, data loss, outages, topology variations • A need for an integrated approach: • Control-aware networking • Network-aware control • Challenge traditional approaches in control and communication theory http://www.ee.kth.se/~kallej Karl H. Johansson, Control over wireless networks, 2006

  49. Acknowledgements Presented work was done together with colleagues and students at KTH: • Lei Bao, Carlo Fischione, Oscar Flärdh, Håkan Hjalmarsson, Krister Jacobsson, Mikael Johansson, Niels Möller, Niklas Pettersson, Mikael Skoglund Financially supported by • Swedish Research Council • Swedish Foundation for Strategic Research • Swedish Programme Council for Vehicle Research • European Commission IST 6th Framework Programme • RUNES Integrated Project • HYCON Network of Excellence Karl H. Johansson, Control over wireless networks, 2006

  50. Literature • Handbook of Networked and Embedded Control Systems, D. Hristu-Varsakelis and W. S. Levine, Editors, Birkhäuser, 2005 • High-Performance Communication Networks, J. Walrand and P. Varaiya, 2nd Edition, Morgan Kaufmann, 2000 • Fundamentals of Wireless Communication, D. Tse and P. Viswanath, Cambridge University Press, 2005 • Special Issue on Networks and Control, L. G. Bushnell, Ed., IEEE Control Systems Magazine, 21(1), 2001 • Special Issue on Networked Control Systems, P. Antsaklis and J. Baillieul, Eds., IEEE Transactions on Automatic Control, 49(9), 2004 • “Networked control systems: analysis and design”, J. P. Hespanha, P. Naghshtabrizi, and Y. Xu. Manuscript submitted for journal publication. 2005. • “Simple stability criteria for systems with time-varying delays”, C.-Y. Kao and B. Lincoln, Automatica, Volume 40, Issue 8, August 2004 • "Kalman filtering with intermittent observations", B. Sinopoli, L. Schenato, M. Franceschetti, K. Poolla, M. Jordan, S. Sastry, IEEE Transactions on Automatic Control, 2004, • “Encoder–decoder design for event-triggered feedback control over bandlimited channels”, L. Bao, M. Skoglund, and K. H. Johansson, IEEE ACC, Minneapolis, Minnesota, USA, 2006. • “Influence of power control and link-level retransmissions on wireless TCP”, N. Möller and K. H. Johansson. Quality of Future Internet Services, Vol. 2811 of Lecture Notes in Computer Science. Springer-Verlag. 2003. • “Using radio network feedback to improve TCP performance over cellular networks”, N. Möller, I. Cabrera Molero, K. H. Johansson, J. Petersson, R. Skog, and Å. Arvidsson. IEEE CDC-ECC, Seville, Spain, 2005. • “A new feedback control mechanism for error correction in packet-switched networks”, O. Flärdh, K. H. Johansson, and M. Johansson. IEEE CDC-ECC, Seville, Spain, 2005. • “Modeling and control of auxilary loads in heavy vehiles”, N. Pettersson and K. H. Johansson. International Journal of Control, 2006. Special issue on advanced design methodologies in automotive control. Karl H. Johansson, Control over wireless networks, 2006

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