st 1 HYCON PhD School on Hybrid Systems www.ist-hycon.org www.unisi.it Hybrid Control and Communication Karl Henrich Johansson KTH Stockholm, Sweden kallej@s3.kth.se scimanyd suounitnoc enibmoc smetsys dirbyH lacipyt (snoitauqe ecnereffid ro laitnereffid) scimanyd etercsid dna stnalp lacisyhp fo fo lacipyt (snoitidnoc lacigol dna atamotua) fo senilpicsid gninibmoc yB .cigol lortnoc ,yroeht lortnoc dna smetsys dna ecneics retupmoc dilos a edivorp smetsys dirbyh no hcraeser ,sisylana eht rof sloot lanoitatupmoc dna yroeht fo ngised lortnoc dna ,noitacifirev ,noitalumis egral a ni desu era dna ,''smetsys deddebme`` ria ,smetsys evitomotua) snoitacilppa fo yteirav ssecorp ,smetsys lacigoloib ,tnemeganam ciffart .(srehto ynam dna ,seirtsudni HYSCOM IEEE CSS Technical Committee on Hybrid Systems Siena, July 1 9-22, 2005 - Rectorate of the University of Siena 14
Hybrid Control and Hybrid Control and Communication Communication Karl H. Johansson Department of Signals, Sensors and Systems Royal Institute of Technology Stockholm, Sweden www.s3.kth.se/~ kallej K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Outline Outline • Lecture I: Control under communication constraints – Motivating applications – Communication constraints – Compensation for delay and loss – Integrated design of control and communication – References • Lecture II: Hybrid control of communication systems – Packet-switched networks – Hybrid model of congestion control – References • [Lecture III (by L. Palopoli): Stabilization of quantized systems] K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Lecture I: Lecture I: Control under communication constraints Control under communication constraints • Classical control theory are A Plant S based on perfect exchange of information • Modern control systems are C often networked + added flexibility + cheaper implementation A Plant S • Sensor and actuator data are then transmitted over a shared network resource C – added uncertainty – higher complexity K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Motivating applications Motivating applications • Scania truck • Volvo XC90 • SMART-1 spacecraft • Power control in wireless system • Congestion control in communication network K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Networked control architecture Networked control architecture of a Scania Scania truck truck of a • Control units connected through 3 controller area networks (CANs) coloured by criticality • CAN is a standard introduced by Bosch 1986 Diagnostic bus Red bus COO 1 AUS Coordinator system Audio system ACS 2 EMS 1 GMS BMS SMS SMD CSS Gearbox management Articulation control Engine management Brake management Suspension Suspension Crash safety system system system system system management management dolly t t SMS SMS Suspension Suspension ACC management system management system Automatic climate LAS control EEC Locking and alarm Exhaust Emission system Control ISO11992/2 WTA Auxiliary heater AWD system water-to-air All wheel drive ISO11992/3 system ATA ICL 1 Auxiliary heater Instrument cluster system air-to-air system 15-pole 7-pole CTS TCO Tachograph system Clock and timer system VIS 1 RTG Visibility system Road transport informatics gateway Trailer APS Air prosessing system RTI Road transport informatics system BWS Body Builder Body work system Truck Green bus Yellow bus BCS 2 Body Builder Body chassis system Bus
Networked control architecture of a Volvo XC90 Networked control architecture of a Volvo XC90 • 3 CAN networks connect up to 40 control units • Example of control system using CAN is vehicle dynamics control (electronic stability program)
Networked control architecture Networked control architecture of the SMART- -1 spacecraft 1 spacecraft of the SMART • First European lunar mission, launched Sep 2003 • CAN networks for control system (“system”) and for scientific experiments (“payload”) • Node and communication redundancies Sun sensors Sun sensors (3 in total) (3 in total) Star tra Star tra Hydr Hydr Reaction wheels Reaction wheels (4 in (4 in (4 in total) (4 in total) EP thruster and orien EP thruster and orien mechanism mechanism
Distributed power control in cellular systems Distributed power control in cellular systems • Power control in each mobile station tries to keep signal-to-interference ratio (SIR) at a threshold value • Disturbances from channel fluctuations and interfering traffic • Control in mobile station based on quantized estimate of SIR communicated from base station
Congestion control in packet- -switched switched Congestion control in packet data communication network data communication network • Each sender regulates sending Receiver rate based on congestion information from receiver • Variations in available Receiver bandwidth and traffic load • Congestion indicated implicitly through missing acknowledgement packets • Quantized control command Sender [Discussed in detail next lecture] Sender
Examples of Examples of networked control architectures networked control architectures C A Plant S A Plant S A Plant S C C A Plant S A Plant S C C K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
What is hybrid in networked control? What is hybrid in networked control? • Networked control systems are inherently hybrid, not only because interaction of physical plant and computer control, but also because they have – mixture of event- and time-triggered communication protocols – asynchronous network nodes (no global clock) – quantized sensor data to limit network traffic – symbolic control commands to simplify design and operation • Now on we mainly discuss modelling and compensating some communication constraints, cf., Mitter’s lecture K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Control with constrained communication Control with constrained communication • Limitations in the communication of sensor and actuator data impose constraints on the control system • Communication imperfections include – Delay and jitter – Quantization [Lecture by Palopoli] – Packet loss – Bit error – Outage (lost connection) K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Time delays Time delays • Delays in communication due to buffering and propagation delays • Delays are bad for control loops (avoid if possible) • Delays can be fixed or varying, known (measurable) or unknown • Data loss can be interpreted as infinity delay K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Unknown and fixed time delay Unknown and fixed time delay K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Unknown and varying time delay Unknown and varying time delay Remark: Closed-loop gain needs to be sufficiently small. Easy to check through Bode plot. K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Proof Proof K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Relation to Nyquist Nyquist Criterion Criterion Relation to K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Unknown and varying delay jitter Unknown and varying delay jitter K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Known time delay Known time delay K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Time delay estimation Time delay estimation • Major improvement in control performance if delays are known/measurable (or can be accurately estimated) Example • CAN protocol (discussed earlier) is event-triggered and does not give timing guarantees in general • TTCAN (Time-triggered communication on CAN) is an extension to the CAN standard targeting the need from sampled-data control K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Compensating known delays: Compensating known delays: State feedback controller feedback controller State K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Compensating known delays: Compensating known delays: Output feedback controller feedback controller Output K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Large delays and out- -of of- -order delivery order delivery Large delays and out 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 also increase delays • Don’t wait for late data, but when they arrive use them to adjust old estimates K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
Packet loss Packet loss Modify traditional observer; simplest case: • Can be hard to handle packet loss: when decide that ? – E.g., TCP uses TimeOut variable to decide when a packet is lost • It can be better to drop data, than use old information for control K. H. Johansson, 1 st HYCON PhD School on Hybrid Systems, Siena, 2005
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