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Pharos: A Testbed for MobileCyber-Physical Systems
CS 795 Cyber-Physical SystemsApril 15, 2013
Harshith Reddy Bandi
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Problem/ Motivation• “Mobile cyber-physical systems (MCPS) are gaining
importance as key enablers of emerging applications; this necessitates reliable, robust, and rapid validation and evaluation mechanisms for integrated communication, coordination, and control solutions.”
• Individual pieces of the solutions are commonly evaluated rigorously through mathematical arguments or statistically through software simulation.
• But these can lead to myopic solutions, which when brought together can result in outcomes inconsistent.
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Challenges Undertaken
• “Design steps for the Pharos Testbed to support heterogeneity and extensibility in both hardware and software to enable a wide variety of experiments with mobile cyber-physical systems;
• Creation of a supporting software infrastructure that enables push-button repeatability, including repeatability of mobility patterns and communication capabilities to the extent possible; and
• Understanding of and quantifying the similarities and differences between experimental results and simulated ones with the purpose of replicating experiments” 3
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THE PHAROS TESTBED 4
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The Pharos Testbed
• a networked system of autonomously mobile devices that can coordinate with each other and with networks of embedded sensors and actuators
• an autonomous mobile testbed for extensive validation and evaluation of mobile cyber-physical systems
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Proteus Platform
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Proteus Platform• The Proteus design focuses on componentization and reuse of
commercial-off-the-shelf (COTS) equipment to maximize both robustness and flexibility.
• Physical Mobility:1. iRobot Create2. Segway RMP503. customized Traxxas Stampede• Behavior and Communication:1. A low-power VIA EPIAR x86 Linux-based computer coupled
with a FreescaleTM 9S12 micro-controller provides the platform for Proteus node behaviors. 7
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Proteus Platform continues..• Basic communications are provided by an on-board 802.11 b/g
wireless network interface controller with a 5.5 dBi antenna.• Environmental Interaction:1. Range-finding sensors, digital compass, global positioning
system (GPS), and cameras, as well as ambient sensing devices including MEMSIC R motes are used for sensing and actuating.
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Proteus Software Architecture
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Software Architecture• Pharos Client - responsible for assigning motion scripts to
Proteus nodes and initiating the execution of the motion script.
• Pharos Server - consists of a Motion Script Follower and a Navigation component.
• Player Server - provides a popular middleware abstraction for obtaining GPS and compass data and issuing movement commands to control movement.
• µController - has a Compass; a Tachometer Driver; a Motor Driver, and a Steering Driver.
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CHARACTERIZING REPEATABILITY 11
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Test Path
• “Lollipop” motion script• Different Segment
Lengths• Different Angles
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Motion Divergence• Absolute Divergence - At any moment, the node's ideal
location is the point on the ideal trajectory that is shortest distance to the node's actual location.
• Relative Divergence - the shortest distance between the node's current location and the recalculated ideal path based on the node's previous location.
• Relative-Speed Divergence - the distance between the node's current location and the location it should be at if it had traveled at the same speed along the recalculated ideal path.
• Reflective Divergence - it compares one execution of a motion script with another execution of the same motion script.
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Types of Path Divergence
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Lonestar Test Runs
• Repeatability Testing• Execute motion script 7 times• Constant speed of 1.5m/s
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Absolute Divergence of Lonestar
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Relative Divergence of Lonestar
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Reflexive Divergence of Lonestar
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Relative-Speed Divergence of Lonestar
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Repeatability Across Multiple Nodes
Absolute Divergences• Lonestar 1.34±0.08m• Shiner
2.49±0.18m• Wynkoop1.20±0.06m
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Instant-Simulation Replay of Experiments
• Log files can be fed directly into a simulator to create “instant replays” of a test
• Useful to visualize what occurred for debugging purposes
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DIVERGENCE FROM SIMULATION 22
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Divergence From Simulation
• “Real-world connectivity between wireless nodes often varies, sometimes to a large degree, from simulated connectivity.”
• Characterize the difference between simulated connectivity among mobile nodes and the real-world connectivity of the nodes in the Pharos testbed.
• “This variance in communication characteristics is one of the most compelling reasons to evaluate mobile cyber-physical system solutions using real-world experiments in addition to simulations.”
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Experimental Setup• Measured connectivity by sending wireless beacons between
the nodes in all of our experiments and recorded when any node saw another nodes' beacon.
• OMNeT++ network simulator is used for simulation.• The unit disk radio model which considers a nodes' wireless
range to be a perfect circle is used as radio in the simulator.• Compared one real-world run of this experiment with
simulations using simulated radio ranges of {10m, 25m, 50m, 75m, 100m, 150m, and 200m}.
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Real-World Connections vs. Simulated Connections
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Comparing Effective Radio Ranges
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LIMITATIONS AND LESSONS LEARNED 27
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Limitations in Architecture• High node complexity leads to frequent device failures thus
limiting the scale of experiments• Limited software flexibility─ the current software only
supports one form of motion script based on GPS waypoints
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Limitations in Hardware & Device Drivers
• μC is highly sensitive to interrupt latencies• Excessive current draw during acceleration was
tripping the safety shut-offs of the batteries• Atheros wireless chipset drivers were unreliable• The compass was highly sensitive to voltage
fluctuations• The GPS sometimes had trouble locking on to
satellites
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Limitations of Experiments to Date
• Did not start all node in the same exact position and orientation
• Only tested a single motion script• Did not test speed vs. motion repeatability or wireless
connectivity
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RELATED WORK 31
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Related Work
• Hydra- a wireless emulator that focuses on repeatability at a very fine level of granularity at the physical layer
• MiNT- a miniaturized multi-hop wireless network testbed that connects live emulations with running simulations in real time
• EXC Toolkit- focuses on the software components of a wireless multi-hop network
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Acknowledgements
Information and figures are from “Pharos: A Testbed for Mobile Cyber-Physical Systems” by: Chien-Liang Fok, Agoston Petz, Drew Stovall, Nicholas Paine, Christine Julien, and Sriram Vishwanath at The University of Texas
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Thank you
Questions??34