1 / 75

Networked Control Loops - An Overview

Networked Control Loops - An Overview. Karl-Erik Årzén. Outline. Overview Analysis and Design constant network delays varying network delays JitterBug analysis of networked control loops TrueTime simulation of networked control loops. Control Builder. Controllers. I/O Fieldbuses. IO.

baxter
Download Presentation

Networked Control Loops - An Overview

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Networked Control Loops -An Overview Karl-Erik Årzén HRTC Wien 11-13 Sep 2002

  2. Outline • Overview • Analysis and Design • constant network delays • varying network delays • JitterBug • analysis of networked control loops • TrueTime • simulation of networked control loops HRTC Wien 11-13 Sep 2002

  3. Control Builder Controllers I/O Fieldbuses IO Intra-net OPC Server Data access Control Network (TCP/IP) HRTC Wien 11-13 Sep 2002

  4. Networked Operations • Data presentation & recording • Event notification • Code distribution • Task downloading • Commands • Control loops • …. HRTC Wien 11-13 Sep 2002

  5. Motivation • Reduced cabling costs • Network hardware cheaper • Manufacturer independent nodes • Modularity and flexibility in system design • …. HRTC Wien 11-13 Sep 2002

  6. Temporal Determinism • The key issue in real-time systems • However, temporal determinism is not a yes or no thing. • Levels of determinism rather than hard or soft HRTC Wien 11-13 Sep 2002

  7. Control - Hard or Soft? • Both • However, most feedback control loops can manage deadline misses without any problems. • Probably easier to find hard r-t in discrete (logic) control • “Hard real-time” is a model • often works well • in many cases overly restrictive HRTC Wien 11-13 Sep 2002

  8. Control System Characteristics • For many controllers a worst-case design approach works well • e.g., PI, PID, … • However, a lot of exceptions: • hybrid controllers that switch between different modes with different characteristics • model-predictive controllers (MPC) • convex optimization problem solved every sample • execution time can easily vary an order of magnitude HRTC Wien 11-13 Sep 2002

  9. Delays • Networks induce delays: • limited bandwidth • overhead in network interface • overhead in network • Time delays in control loops: • give rise to phase lag • degenerate system stability and performance HRTC Wien 11-13 Sep 2002

  10. HRTC Wien 11-13 Sep 2002

  11. Control messages • Small in size • Frequent, often periodic • “Best consumed before 2002-09-15” • Loosing occasional messages is often acceptable HRTC Wien 11-13 Sep 2002

  12. Network Types • Different networks give different levels of determinism: • constant delay (no jitter) • stochastically varying delays HRTC Wien 11-13 Sep 2002

  13. CAN: Experimental Data HRTC Wien 11-13 Sep 2002

  14. Ethernet: Experimental data HRTC Wien 11-13 Sep 2002

  15. Two Points of View • Computer Science • Scheduling principles • Resource allocation • What can be guaranteed? • Control • Model the delays • Design of controllers • Robustness, stability, performance HRTC Wien 11-13 Sep 2002

  16. Two design approaches • Maximize temporal determinism • use protocols and scheduling techniques that maximize determinism • e.g. TTA/TTP • well suited for formal analysis, safety critical systems • matches sampled control theory well • non-COTS, requires complete knowledge • Compensate for temporal non-determinism • inherent robustness of feedback • temporally robust off-line design methods • on-line compensation • need to measure delays, e.g. “time-stamping” • gain-scheduling, feedforward, ... • Complementary HRTC Wien 11-13 Sep 2002

  17. Feedback Scheduling in Control • Scheduling of computing resources (CPU time, bandwidth, memory, power) with guaranteed control performance • Co-design of control and scheduling • Control performance as a QoS parameter (QoC) • Negotiation and contracts • Examples: • adjust sampling frequencies dynamically to control CPU utilization • adjust execution time quota for any-time controllers to control CPU utilization HRTC Wien 11-13 Sep 2002

  18. Outline • Overview • Analysis and Design • constant network delays • varying network delays • JitterBug • analysis of networked control loops • TrueTime • simulation of networked control loops HRTC Wien 11-13 Sep 2002

  19. Delay Models • Constant delay • Random delay • independent from transfer to transfer • Random delay • dependent • e.g. probability distribution governed by Markov chain • “Low load”, “Medium load”, “High load” HRTC Wien 11-13 Sep 2002

  20. Constant Delays • Straightforward • Continuous time • e.g. Otto-Smith Controller • e.g. Predictive PI • Discrete Time • sampling of a system with time delay • time-invariant finite-dimensional system HRTC Wien 11-13 Sep 2002

  21. Robust vs Worst-Case • Left: Robust design taking the delay into account • Top Right: Design for zero delay • Bottom Right: Design for worst-case delay HRTC Wien 11-13 Sep 2002

  22. Random Delays • More tricky • Time-varying system • Examples can be found of systems that are stable for all constant delays, but become unstable when the delay varies • It is important to be clear of what the available results cover: • constant delays within a certain range • varying delays within a certain range HRTC Wien 11-13 Sep 2002

  23. Sampling of systems with varying delays • Closed Loop System (plant + controller) • Similar for sampling jitter HRTC Wien 11-13 Sep 2002

  24. Stability • Delays that change according to a finite, repeating cycle • Delays that change randomly • Lyapunov stability theory • Stable if we can find a common quadratic Lyapunov function for all delays HRTC Wien 11-13 Sep 2002

  25. Stability • New stability criterion for systems with varying time delays (Lincoln, 2002) • simple & graphical • Small gain theorem • so far only for open-loop stable processes HRTC Wien 11-13 Sep 2002

  26. Frequency Domain Criterium HRTC Wien 11-13 Sep 2002

  27. left right • Bo Lincoln: “A simple stability criterion for digital control systems with varying delays”, IFAC World Congress, 2002 HRTC Wien 11-13 Sep 2002

  28. SISO Control - Basic Setup • Computational Model: • different possibilities • our approach (Nilsson): time-driven sensor & event-driven controller and actuator HRTC Wien 11-13 Sep 2002

  29. Alternative Approach • Luck and Ray • Make invariant through max-delay buffers • Longer delays than necessary • Almost always worse than having shorter, varying delays HRTC Wien 11-13 Sep 2002

  30. LQG Control - Independent delays • Johan Nilsson - PhD • “Real-Time Control Systems with Delays” • Time stamping • Old delays known when calculating uk • sensor-controller delay up to k • controller-actuator delay up to k-1 • Independent random delays with known distributions • State feedback • full state information • all states from the same node in the same frame HRTC Wien 11-13 Sep 2002

  31. LQG Control • Stochastic Riccati equation • Updating of S not always possible in real-time HRTC Wien 11-13 Sep 2002

  32. HRTC Wien 11-13 Sep 2002

  33. Simplifications • Off-line calculation of stationary Riccati • tabular for L • interpolation • linear approximation • Suboptimal scheme • delay-free feedbackvector • predict from time k overaverage delay HRTC Wien 11-13 Sep 2002

  34. HRTC Wien 11-13 Sep 2002

  35. Extensions • Optimal state estimator • Optimal output feedback controller • separation principle holds HRTC Wien 11-13 Sep 2002

  36. LQG Control - Dependent delays • Delay distributions governed by Markov chain • Optimal state feedback • requires knowledge of the delay mode (Markov chain state) • Optimal state estimate & output feedback HRTC Wien 11-13 Sep 2002

  37. LQG Control: Extensions • Markov chain with two transitions every sample • different delay distributions for sender-controller message and controller-actuator message • Sampling interval jitter in sensor node • optimal state feedback, estimator & output feedback • requires the solution of the Riccati in every sample • Estimation of the Markov state HRTC Wien 11-13 Sep 2002

  38. LQG Control: Extensions • MIMO Control • multiple sensor and actuator nodes • time-driven sampling (synchronized clocks) • optimal state feedback and estimator results has been derived • based on the delay of the latest received sensor measurement HRTC Wien 11-13 Sep 2002

  39. Timeout • Can control performance be improved by having a timeout on sensor values? HRTC Wien 11-13 Sep 2002

  40. Timeout Control • Prediction-Based Controller • Two versions: • Lost samples: The delayed samples will eventually arrive and can be used for updating the filters • Vacant samples: The delayed samples are lost HRTC Wien 11-13 Sep 2002

  41. Timeout Control • 2nd order process, uniform delay on [0,h] • LQG optimal control Why wait for a noisy measurement? HRTC Wien 11-13 Sep 2002

  42. Multi-rate Periodic Control • Asynchronous periodic loops HRTC Wien 11-13 Sep 2002

  43. Interesting Delay Patterns Small change in timing patterns Björn Wittenmark, Lund HRTC Wien 11-13 Sep 2002

  44. MIMO Controllers - Other approaches • Strobe connection • time-driven controller multicasts a message telling the sensors to sample • Poll connection • time-driven controller sends individual messages to each sensor node in turn, requesting them to sample HRTC Wien 11-13 Sep 2002

  45. Dynamic Delay-Jitter Compensation • New approach by Bo Lincoln • Assumptions: • time-stamping • full process model not required (only at high frequencies) • delay statistics not needed • Approach: • linear compensator as an add-on to an existing controller • frequency domain conditions for stability and performance • loop shaping design • stability compensation and performance compensation HRTC Wien 11-13 Sep 2002

  46. Dynamic Jitter Compensation • Bo Lincoln: “Jitter Compensation in Digital Control Systems”, ACC 02 HRTC Wien 11-13 Sep 2002

  47. Collision Detection • Opens up interesting possibilities • Sensor nodes that detect collisions: • re-send old sample • discard sample • resample and send new sample … • increase sampling interval to reduce communication load - tradeoff • Layered model inadequate HRTC Wien 11-13 Sep 2002

  48. Outline • Overview • Analysis and Design • constant network delays • varying network delays • JitterBug • analysis of networked control loops • TrueTime • simulation of networked control loops HRTC Wien 11-13 Sep 2002

  49. JITTERBUG • Matlab-based toolbox for analysis of real-time control performance • Developed by Bo Lincoln and Anton Cervin • Calculation of a quadratic performance criterion function • Linear process, linear controller • Stochastic timing description • Theory for jump-linear systems HRTC Wien 11-13 Sep 2002

  50. JITTERBUG Analysis HRTC Wien 11-13 Sep 2002

More Related