On Maintaining Connectivity of a Colony of Autonomous Explorer Mobile Robots ıas Palma Olate 1 and Cristian Duran-Faundez 2 Jonathan Mat´ Departamento de Ingenier´ ıa El´ ectrica y Electr´ onica Universidad del B´ ıo-B´ ıo, Concepci´ on, Chile. October 21, 2014 JMP & CDF (UBB) October 21, 2014 1 / 37
Indice 1 Introduction to Problem of Maintaining Connectivity. Methods of solution 2 A multi-robot exploring application Tethering Optimal Point Orientation Method 3 Algorithm Evaluation Method. Simulation. 4 Conclusion Conclusion and Comments Acknowledgment JMP & CDF (UBB) October 21, 2014 2 / 37
Introduction to Problem of Maintaining Connectivity. Robotics. JMP & CDF (UBB) October 21, 2014 3 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methods to maintain a communication link of explorer robot. JMP & CDF (UBB) October 21, 2014 4 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methods to maintain a communication link of explorer robot. A) Direct Communication. JMP & CDF (UBB) October 21, 2014 5 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methods to maintain a communication link of explorer robot. A) Direct Communication. -Limited coverage Transmit. -Unique link, not robust. -High power transmission over long distances. JMP & CDF (UBB) October 21, 2014 6 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methods to maintain a communication link of explorer robot. B)Communication with Static Router. JMP & CDF (UBB) October 21, 2014 7 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methodsto maintain a communication link of explorer robot. B)Communication with Static Router. -Previous infrastructure. -High cost of implementation and maintenance. -Robustness to the failure of a unit. -A subset of the total units the network participateing actively in the link communication. JMP & CDF (UBB) October 21, 2014 8 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methods to maintain a communication link of explorer robot (Proposal) . C)Communication using Mobile Router. JMP & CDF (UBB) October 21, 2014 9 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methods to maintain a communication link of explorer robot (Proposal) . C)Communication using Mobile Router. -Dynamic deployment. -Minimize energy and cost. -Design flexibility. -High level of technical complexity in implementation. today. JMP & CDF (UBB) October 21, 2014 10 / 37
Introduction to Problem of Maintaining Connectivity. Methods of solution Methods to maintain a communication link of explorer robot (Proposal) . C)Communication using Mobile Router. -Dynamic deployment. -Minimize energy and cost. -Design flexibility. -High level of technical complexity in implementation. today. JMP & CDF (UBB) October 21, 2014 11 / 37
A multi-robot exploring application Tethering Tethering Tethering is the robot task of The reference we propose includes following a mobile agent (human, the following kind of nodes: robot, etc.), with all the different -A set Base Station : Gateways required capabilities to it, in order ( GW ). to provide network connectivity -A set Explorer Robot : Targets (Zickler and Veloso 2010). ( TG ). -A set Router Robots : Gangway of data ( GG ). JMP & CDF (UBB) October 21, 2014 12 / 37
A multi-robot exploring application Tethering Network Topology Options Link type to use? Model network of interest simple-link. Figure Network Topology link green. Figure: Network Topology. JMP & CDF (UBB) October 21, 2014 13 / 37
A multi-robot exploring application Tethering This scenario entails a set of sub-problems, including: - Definition of a link quality metric. - Drive router robots to optimal positions to forward data packets. - Minimizing the amount of robot routers maintaining global performance. - Addressing robustness. Which involves methods to deal with communication errors and incidentals such as robot failures. - Sharing tasks. To provide ways to make robots share some tasks, e.g., to make a router robot take other router’s job allowing the second one to go to the base station for recharge or maintenance. JMP & CDF (UBB) October 21, 2014 14 / 37
A multi-robot exploring application Optimal Point Definition of Link Quality Metric. Metric quality standard of wireless communication. GW − 20 - Packet Loss Rate PLR. − 30 ) − 40 B - link quality indicator LQI. d − 50 ( I RSSI (dB) S − 60 S - Received Signal Strength − 70 R − 80 Indicator RSSI. − 90 − 100 3 2.5 2 1.5 0 1 1 2 0.5 3 4 0 5 6 Metros (M) Metros (m) Metros (m) Figure: Example RSSI vs Distance JMP & CDF (UBB) October 21, 2014 15 / 37
A multi-robot exploring application Optimal Point Optimal point (PO) to ideal model. PO Geometric: midpoint between PO based on the RSSI: coordinate the line formed superior and where the RSSI is equal and also the inferior units. sum of them is maximum GW GW − 10 − 20 RSSI (dB) − 30 − 40 RSSI (dB) − 50 − 60 − 70 0 0.5 − 80 1 Figure: Network: two GW and on 0 0.5 1.5 1 1.5 2 2.5 3 2 Metros (M) Metros (m) GG Metros (m) Figure: RSSI to GW the network. JMP & CDF (UBB) October 21, 2014 16 / 37
A multi-robot exploring application Orientation Method Orientation Method. Indicator MIn Dif . Dif a is relevant because PO Let us define VS and VI as the difference of RSSI to PO it zero. communication links between a Status indicators can be defined. router robot and the neighboring Decrease Dif a , which is calculated as: node closest to the explorer and the base station. Obteniandiendo � 1 if Dif a ( k ) − Dif a ( k − 1) < 0 las relaciones. MIn Dif = 0 otherwise Dif a = | RSSI VS − RSSI VI | Dif d = RSSI VS − RSSI VI Dif a JMP & CDF (UBB) October 21, 2014 17 / 37
A multi-robot exploring application Orientation Method Orientation Method. Indicators Max and Min link Vx. The VX it VL farthest link and VC closer link. Decreases of RSSI values for a communication link VX , can be calculated as: � 1 if RSSI VX ( k ) − RSSI VX ( k − 1) < 0 Min VC = (1) 0 otherwise Increase of RSSI values for a communication link VX , which is defined as: � 1 if RSSI VX ( k ) − RSSI VX ( k − 1) > 0 Max VL = (2) 0 otherwise JMP & CDF (UBB) October 21, 2014 18 / 37
A multi-robot exploring application Orientation Method Actions. Discrete displacements. The Figure presents the eight possible actions [ A 1 , A 2 , ..... A 8 ]. It also implements a ninth action A 9 that is return. Figure: Actions. JMP & CDF (UBB) October 21, 2014 19 / 37
A multi-robot exploring application Orientation Method States based on the indicators Min Dif , Min VC and Max VL . Are defined the state S to function. The Indicators are binaries i ∈ [1 , 2 ..... 2 3 ], having eight possible states S i = ϕ ( Min Dif , Min VC , Max VL ) State compact SS : Are defined the state compact SS 1 and SS 2 . . SS 1 = ϕ ( Min Dif , Min VC , Max VL ); Min Dif = Min VC = Max VL = 1 SS 2 =otherwise JMP & CDF (UBB) October 21, 2014 20 / 37
Algorithm Algorithm Proposal JMP & CDF (UBB) October 21, 2014 21 / 37
Algorithm Objectives. -Design an algorithm that achieves converge to units in the vicinity of the optimum point (PO), maximizing both of RSSI values link. -Design an algorithm that orientation method depends only on current and past RSSI values, does not consider additional information of the environment. -Evaluate the performance of the algorithm in a simulation problem Tethering. JMP & CDF (UBB) October 21, 2014 22 / 37
Algorithm Algorithm, Heuristics based to Q-learning. The action selection is performed by Require: RSSI captures Require: Calculation of goals and using a Q-learning-based vector categorizations for states SS ( k ) and Q ( SS × A i ). We adopted as learning SS ( k − 1). coefficient α = 0 . 5, and we assign a 1: if SS k − 1 = SS 1 and SS k = SS 2 reward +1. then For the proposed model there are return A 9 2: four transitions ( SS k − 1 → SS k ), i.e. 3: end if transitions of passing from an state 4: if SS k − 1 = SS 2 and SS k = SS 2 SS k − 1 in previous iteration k − 1 to then return random A i ; i ∈ [1 , 8] 5: the current state SS k . These 6: end if transitions are: ( SS 1 → SS 2 ), 7: A c = MaxA c InQ ( Q , SS ) ( SS 2 → SS 2 ), ( SS 1 → SS 1 ), and 8: Q ( SS , A c ) ← (1 − α ) Q ( SS , A c ) + ( SS 2 → SS 2 ). ′ , A ′ )] α [ r ( SS , A c ) + λ max A ∈ A s ′ Q ( s 9: return A c JMP & CDF (UBB) October 21, 2014 23 / 37
Algorithm Evaluation Method. Evaluation performance the algorithm. Evaluation of the proposed algorithm was by the mean and standard deviation of a set of simulations to k = 500 iterations. The Network used it the figure. Model to Maintaining Connectivity the Robot Explore. The graphs are the mean and standard deviation for 1000 simulations. JMP & CDF (UBB) October 21, 2014 24 / 37
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