Colored Petri Net based Timing Analysis for UAS Application Software

Transcription

1 Colored Petri Net based Timing Analysis for UAS Application Software Charles Hartsell, Vanderbilt University Gabor Karsai, Vanderbilt University Michael Lowry, NASA ARC Acknowledgements: NASA ARC Contract NNX14AN61A

2 Outline 1. Background 2. Core Flight System (CFS) 3. Colored Petri Nets (CPN) 4. CFS Model 5. Model Analysis 6. Example System

3 Background Cyber Physical Systems (CPS) commonly used in mission- and safety-critical applications System verification necessary Timing analysis of real-time deadlines CPS typically component based Built on suitable middleware Common execution structure for components

4 Background Unmanned Aerial Systems (UAS) often must perform wide range of missions Software updates & reconfiguration on per-mission basis Autonomous systems may need new autonomy plans Verification methods must also be reconfigurable and quick NASA GlobalHawk. Photo by Tom Miller. Available: FactSheets/FS-098-DFRC.html

5 Core Flight System (CFS) [1] Core Flight System Common application execution model Pub/Sub messaging between apps through software bus Well defined message inputs/outputs Time/Event triggered applications Applications developed once Verified independently for correctness Integrated system must still be verified [1] J. Wilmot A core flight software system. In 2005 Third IEEE/ACM/IFIP Int. Conf. on Hardware/Software Codesign and System Synthesis (CODES+ISSS 05)

6 Core Flight System (CFS)

7 Colored Petri Nets (CPN) [2] Petri Nets use data-less, indistinguishable tokens CPN extends Petri Nets by adding colors Colored tokens can contain complex data defined by color set Color sets are data structures similar to C-style structs A single colored token can encode several properties A token may also contain a list of data structures List token can encode several complex entities (e.g. multiple application models) [2] Kurt Jensen and Lars M. Kristensen Coloured Petri Nets - Modelling and Validation of Concurrent Systems. Springer.

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12 CFS Model Constructed in parameterized manner Component models shown as rectangles Ovals indicate system parameters - input as initial tokens New system configurations can be analyzed without changing model structure System behavior encoded as parameters Loaded as new initial tokens External tools automate token generation

13 Generic Execution Cycle CFS Scheduler sends wakeup at regular intervals App processes all messages in queue Each message type has associated handler May produce periodic messages Each action has set WCET Not an all-encompassing model CMD Response Wakeup MSG Software Bus Heater Controller WCET = 800 Priority = 1600 Turn_On_Heater CMD Turn_On_Heater Handler WCET = 500 Possible Responses: Success/Failure Queue

14 Heater Controller CPN Representation {name="heater_controller", WCET=800, priority=1600, wakeup_cnt=0, exe_time=0, handlers= [{msg_name="turn_on_heater", msg_id=1, WCET=500, responses= [{sender="heater_controller", destination="", msg_id=2, sys_time=0, msg_type="data", entries=[{name="turn_on_heater", id="echo", value="echo", outcome="command_success", entry_type="command_response"}]}, periodic=[]}, {sender="heater_controller", destination="", msg_id=2, sys_time=0, msg_type="data", entries=[{name="turn_on_heater", id="echo", value="echo", outcome="command_failed", entry_type="command_response"}]}]}],

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16 Model Analysis Generate state space of CPN model Use reduction techniques to prevent state space explosion

17 Model Analysis Generate state space of CPN model Use reduction techniques to prevent state space explosion

18 Model Analysis Search state space for best and worst case response times Exhaustive search between sets of states with stimulus/response arrival Provides detailed execution trace for each case Users can also specify custom searches

19 Example System Simple UAS 8 apps (6 generic) 250 ms frequency 16 node autonomy plan 6 external events 4 seconds of system time State Space 443,685 states 26 minutes to generate

20 Results

21 Results

22 Summary Cyber Physical Systems need rapid verification methods CPS typically built upon a suitable middleware Parameterized CPN-based model of the Core Flight System Allows for analysis of many applications and system configurations Future Work Scalability Non-deterministic Environment Generate system model from architectural model