Symbiosis: Cooperative Algorithms for Mobile Robots and a Sensor Network

1. T1 Symbiosis: Cooperative Algorithms for Mobile Robots and a SensorNetwork ? by Maxim Batalin ? T2 Supervisor (USC): Gaurav Sukhatme

2. “Sym·bi·o·sis - the intimate living together of two dissimilar organisms in a mutually beneficial relationship” -Merriam-Webster Online Dictionary - http://www.webster.com

3. Contents • Introduction • Problems and solutions with Mobile Robots (MR) and Sensor Networks (SN) • Coverage and Exploration through Sensor Network Deployment; • Sensor Network Repair and Maintenance – Implicit; • Probabilistic Navigation; • Task Allocation Problem (scheduling); • DINTA; • DINTA-MF; • Sensor Network Repair and Maintenance - Explicit; • Current Work: Task Allocation for NIMS; • Summary CENS Research Seminar

4. Introduction

5. Why use both Mobile Robots and Sensor Networks? • Sensor Network provides distributed: • Sensing; • Computation; • Communication; • Ubiquitous computing is an active area of research and investment, hence can utilize intelligence which will be present in an environment in the future • At the same time Mobile robots deploy, repair and maintain the network, while accomplishing tasks that require mobility; Robots can be simpler, cheaper and don’t need as many; Can solve wider variety of problems CENS Research Seminar

6. Overall objectives • We are motivated by the idea that MRs and SN each could leverage strengths from the other • Goal: Build a collaborative ‘symbiotic’ system in which mobile robots and a sensor network solve tasks cooperatively and coexist benefiting each other • Assumptions: • Global information is not accessible (no GPS, no map, etc.) • Neither robot localization nor mapping is performed – hence robots can be simple • Environment can be dynamically changing • Assume that a collection of nodes “large enough” • Do not consider power constraints CENS Research Seminar

7. Validation Platform • Pioneer 2DX • Laser (for obstacle avoidance) • Compass • Wireless ethernet • PC-104 stack (Pentium 1Ghz) • Motes • Atmel ATmega 128L • Program Flash Memory 128K bytes • Measurement (Serial) Flash 512K bytes • Configuration EEPROM 4 K bytes • Serial Communications UART • 916 MHz Multi-Channel Radio Transceiver • Player/Stage engine CENS Research Seminar

8. Problems and solutions with Mobile Robots and Sensor Networks

9. Coverage and Exploration • Deploy a group of robots maximizing sensor coverage; • Cover/Explore every point of environment with an onboard sensor; • Can’t tell if coverage is complete; • Can’t recover robots; • Require a LOT of ‘expensive’ robots; • Use Sensor Network; CENS Research Seminar

10. Approach Robot Loop If no beacon within radio range deploy beacon Else move in direction suggested by nearest beacon Beacon Loop Emit least recently visited direction M. Batalin, G. S. Sukhatme, Coverage, Exploration and Deployment by a Mobile Robot and Communication Network, Telecommunications Systems, April 2004 (accepted, to appear) M. Batalin, G. S. Sukhatme,Efficient Exploration Without LocalizationProceedings of the 2003 IEEE International Conference on Robotics and Automation (ICRA'03), Taipei, Taiwan, May 12 - 17, 2003. CENS Research Seminar

11. Simulation Experiment CENS Research Seminar

12. Sensor Network Repair and Maintenance - Implicit CENS Research Seminar

13. Graph Model • Consider G = (V, E) - regular square lattice; • Coverage - the time it takes the robot to visit every node of the graph; • No information– Random Walk (RW) RW(G)= ω(VlogV ) and RW(G)= o(V2) • Full information – DFS DFS(G) = Θ(V + E) < 4V • Proposed algorithms data is taken on graphs of 25, 49 and 100 nodes • Every node was tried as starting point; • 50 experiments per starting point; CENS Research Seminar

14. Experimental Results analysis • Conjecture 1: The asymptotic cover time of our algorithm is less than o (V log V); • While proposed algorithm designed for coverage also can be applied for mapping; • The graph mapping time (MT) is: MT < d * maxi(CTi) (i.e. o (V log V)) • d is the maximum degree of G (4 in case of square lattice); • CTi is the i-th cover time (o(VlogV)); • Conjecture 2: Our algorithm produces a map of the environment in asymptotic time faster than o (V log V); CENS Research Seminar

15. Simulation Experiments • Direct correspondence between the graph model and simulations; • Support our conjecture that the cover time is less than o(nlogn); CENS Research Seminar

16. Benefits • A static sensor network is deployed: • can be used for numerous network applications; • guarantees that every point of environment would be eventually covered by the mobile robots; • etc. • Robots can: • be used for exploration, patrolling and coverage tasks; • restore the static sensor network in case of damage; • retrieve robots; • system knows when full coverage is achieved; CENS Research Seminar

17. Summary • Theoretical analysis shows that trade offs in the assumptions affect the outcome significantly: • RW – robot does not have or assume anything: RW(G)= ω(VlogV ); • Proposed Approach – the number of nodes is ‘enough’: ω(V ) and o(VlogV) • DFS – nodes of three colors, perfect localization and navigation: DFS(G) = Θ(V + E) < 4V • Proposed algorithm outperforms other algorithms if localization and perfect navigation cannot be assumed; CENS Research Seminar

18. Probabilistic Navigation

19. Introduction • How to navigate from point A to point B; • A fundamental problem in robotics; • No a priori information about the environment; • Use Sensor Network; CENS Research Seminar

20. Transition Probabilities • While robot deploys and traverses sensor network from node to node it can determine transition probabilities: • Robot stores determined probabilities on appropriate nodes; CENS Research Seminar

21. Navigation Field computation • Suppose transition probabilities are determined for all nodes; • If a goal node is specified, the information is propagated through the network and the ‘navigation field’ is computed using: • Where - V is the utility value, C(s, a) is the cost associated with an action, P(s’|s, a) is the transition probability of arriving to node s’ if an action a was taken at node s, π(s) is the policy, or direction that the node s is going to suggest for the robot in the vicinity. CENS Research Seminar

22. Distributed Computation • Suppose the SN is flooded with goal node data; • Every node updates own utilities according to utility equation; • After the utilities are computed, every node computes an optimal policy for itself according to policy equation; • Note that the action policy computation is done only once, and does not need to be recomputed, unless the goal changes; • Note that if neighbors of all nodes are known exactly the system converges after a single iteration. CENS Research Seminar

23. Basic algorithm • Assume that SN deployed and Navigation Field is computed; • Closest Node suggests direction of motion; • Robot moves straight in a suggested direction; • Determine if close to the next node based on signal strength: • If no, repeat 2; • If yes, start 1; M. Batalin, G. S. Sukhatme, Coverage, Exploration and Deployment by a Mobile Robot and Communication Network, Telecommunications Systems, April 2004 (accepted, to appear) CENS Research Seminar

24. Simulation Experiment Node-to-node navigation: Move in suggested direction, switch to closest node, repeat; CENS Research Seminar

25. Probabilistic Navigation in Real Environment My Cube CENS Research Seminar

26. Real-World Navigation Challenges • Cubicle environment is ‘narrow’, hence precision is required; • Compass or IMU proved to be useless inside; • Implementation should be simple enough for simple robot; • Algorithm should be based purely on signal strength: • Different antennas, not truly omnidirectional; • Ambient noise in the environment not constant with time; • Hence raw signal strength thresholding or an observation model would not work reliably; CENS Research Seminar

27. Basic algorithm • Assume that SN deployed and Navigation Field is computed; • Robot knows its initial heading and closest node; • Closest Node suggests direction of motion; • Robot moves in a suggested direction: • Vector Field Histogram (VFH)algorithm is used for local navigation and obstacle avoidance • Drives the robot from A to B avoiding obstacles on the path on the local scale (<= 2-3 meters); • Author: J. Borenstein, available in Player/Stage; • Determine if close to the next node (next slide): • If no, repeat 2; • If yes, start 1; M. Batalin, G. S. Sukhatme,M. Hattig, "Mobile Robot Navigation using a Sensor Network,“ To appear IEEE International Conference on Robotics and Automation, 2004 CENS Research Seminar

28. When to switch between the nodes? • Similar to general state estimation problem; • Difficult problem in general; • Proposed algorithm which estimates when the robot is close to a node: • Compute initial maximum average of the first i samples • Compute a running average which is an average of j consecutive samples where j << i • Threshold on ratio of averages CENS Research Seminar

29. Experiments – The Environment My Cube 3 2 1 5 6 4 8 9 7 CENS Research Seminar

30. Experiments – Run 1 1 9 CENS Research Seminar

31. Summary • Algorithm allows the robot to navigate precisely and reliably using a deployed sensor network. • Approach differs from systems described in the literature by assuming that the map, localization, GPS, IMU and compass are not available; • The navigation occurs through node-wise motion from node to node on the path from starting node to the goal node; • We conducted 50 experiments for 5 different goals, totaling in over 1 km of traveled distance; • In each of the 50 cases the robot successfully navigated to the goal node; • Note that we considered an experiment to be successful if the robot approached the goal node to within 3m; CENS Research Seminar

32. Task Allocation Problem

33. Introduction • Task Allocation (TA) is the problem of assigning resources (robots for example) to tasks; • Offline TA is the problem of assigning resources to different tasks (processes) if the tasks‘ arrival distribution is known a priori; • Task assignments are computed offline; • Resembles Traveling Salesperson Problem, it is NP-Complete; • Online TA is the problem of assigning resources to tasks if the distribution of the tasks’ arrival is NOT known a priori; • The task assignment occurs in decision epochs. • A decision epoch is a period of time during which only arrived tasks are considered for assignment. • It is shown in literature that in case of Online TA the optimal solution assigns the tasks in a greedy fashion • We consider an Online TA; • Note that many real life TA problems are Online; CENS Research Seminar

34. Experimental Scenario • We study a particular experimental scenario - emergency handling; • Alarms are detected by nodes in the static network; • The problem is to assign and navigate robots to different alarms; • The goal is to minimize the cumulative alarm OnTime across all alarms, over the duration of the entire experiment; • Alarm’s OnTime is a difference between the time the alarm was turned off by a robot and the time the alarm was detected by one of the nodes of the network; CENS Research Seminar

35. Distributed In-Network Task Allocation (implicit) • Suppose sensor network monitors the environment; • If an event is detected by node n it sends out a packet (n_id, weight, hop_count) • Compute Navigation Field; • This computation results in a direction which maximizes the net utility of the robot; • If there are several events detected at the same time, a node computes direction towards the goal node with largest: CENS Research Seminar

36. Multi Field Distributed In-Network Task Allocation (explicit) • If an event is detected by node n it sends out a packet (n_id, time_of_event, weight, hop_count) • Every node considers task assignment in decision epochs; • At the end of current decision epoch, network synchronizes current positions of available robots; • Since every node receives the same data – the node states are ‘in synch’ – an optimal greedy assignment is possible. • Hence, every node maintains a suggested direction per task; CENS Research Seminar

37. Experimental results • Player/Stage simulations; • Sensor Network of 25 motes; • Groups of 1-4 robots, 10 trials/group • 10 Alarms are drawn from the Poisson distribution with λ=1/60; • Empty environment, A = 576 m2 CENS Research Seminar

38. Sensor Network Repair and Maintenance - Explicit CENS Research Seminar

39. Benefits of DINTA & DINTA-MF compared to other techniques • Sensing, computation and communication are distributed; • Provides sensor that is everywhere at the same time; • Can estimate utilities directly; • Does not rely on global information (no map, GPS, localization, etc.); • Can be combined with other techniques to increase range of applications; M. Batalin, G. S. Sukhatme, “Using a Sensor Network for Distributed Multi-Robot Task Allocation,“ To appearIEEE International Conference on Robotics and Automation, 2004 M. Batalin, G. S. Sukhatme, "Sensor Network-based Multi-Robot Task Allocation," In IEEE/RSJ International Conference on Intelligent Robots and Systems, pp. 1939-1944, 2003 CENS Research Seminar

40. Current Work: Task Allocation for NIMS

41. 47 m Solar Cell NIMS Node 91 84 Power Distribution Cable Imager With Pan/Tilt Actuator 50 m Vertical Node (Ta, RH, PAR) Battery Pack CENS Research Seminar

42. CENS Research Seminar

43. Problem Definition • Assume a NIMS system (s1, s2, s3, …) and a Sensor Network deployed in the same area; • Suppose nodes of the sensor network detect phenomena (p1, p2, p3, …) that require further study by a NIMS system; • Compute an optimal assignment of : (s1, s2, s3, …) (p1, p2, p3, …) • The problem is an instance of an Online Task Allocation problem CENS Research Seminar

44. Current Status • Implemented a version of the algorithm in simulation; • Porting to the lab NIMS system – a model of an actual node; • In march plan experiments in James Reserve; CENS Research Seminar

45. Summary • Coverage and Exploration through Sensor Network Deployment; • Sensor Network Repair and Maintenance – Implicit and Explicit; • Probabilistic Navigation; • Task Allocation Problem (scheduling); • DINTA; • DINTA-MF; • Current Work: Task Allocation for NIMS; Symbiotic systems are beneficial and important to study CENS Research Seminar

46. Contacts Maxim A. Batalin Robotic Embedded Systems Laboratory Center for Robotics and Embedded Systems Computer Science Department University of Southern California Los Angeles, CA 90089, USA maxim@robotics.usc.edu http://www-robotics.usc.edu/~maxim CENS Research Seminar