De-Centralized Multi Robot Co-Ordination and Communication

The aim of this research is to develop a decentralized communication system of multiple robots which have capability to communicate and coordinate with each other, by which all robots work individually while keeping connectivity among them. Decentralized communication between robots is the main part of this project. This type of communication shows great potential in many aspects. A decentralized system is the combination of both intelligent and decision making inspired from nature. To meet this object two identical robots are developed which successfully communicate with each other to complete an assigned task. A single task is given to both robots; to find specific object/target location. Once the target is found by any of the robot that robot will then communicate with second robot to guide the path of object/target location. Guiding path to the second robot is the challenging task as discussed in this paper. The Robots use Bluetooth devices to calculate and extract the RSSI (Received Signal Strength Indication)values for communication purpose.Both robots work together as a team. By working in communicated way their time is reduced to achieve the task and create synergy. Any task could be easily attained in team work than by the work of the single individual robot.

D ecentralized robotic system is an effective area that has gained attention by many researchers over past few years [1][2]. This type of system gives authority to every single member to make decision on its own. There is no single centre robot that takes control for all the system. It makes the system more flexible.
Due to low cost and wide availability of robotic hardware, there is growing interest in robotics which consists of

INTRODUCTION
multiple simple robots instead of one highly-capable robot.
In this work two swarm type robots are developed in such a way that both robots have same configuration and are designed in similar manner. Each robot has multiple sensors to detect and avoid obstacles and to reach the target location. Both robots have capability to communicate with each other by using Bluetooth. When any of the robot reaches the target first, it stops there and communicates other robot (which is still moving in the area for the search of target) to reach the same target location.
Using single robot to find the target in any area may take more time. Single robot has to move in diverse directions of area. For working together as a team every robot may take its own specific direction, which reduced lot of time. The control of the robot is decentralized so there is no "master" robot that controls the rest of the system; all the robots work independently to achieve the given task.
The paper is organized into five sections. Following the introduction, Section II presents the related work, Section III describes working methodology, Section IV illustrates experimental results and finally, conclusion is drawn in Section V.

LITERATURE
In literature, two obvious approaches available for motion planning of multi-robot systems are the centralized and decentralized (distributed) approaches. In centralized systems, the central or the main robot decides the path for every robot and the motion takes place after each robot is assigned its path. In the decentralized approach, there is no single coordinator in the system. Each agent coordinates its own movement and ensures that it does not collide with any obstacle or any other robot during goal accomplishment [3].

METHODOLOGY
Two robots have been developed in such a way that both robots have same configuration and are designed in similar manner to achieve the specific task. The Robots are composed of a Controller [15], three-wheeled Robot chassis [16], Ultrasonic Sensor [17], Color Sensor [18] and Bluetooth [19]. Fig. 1 shows the block diagram of basic architecture of robot. Fig. 2 shows the final design of the robot.
The task has been accomplished in two steps. According to the algorithm, if any obstacle appears in the path while moving towards master robot. The slave robot repeats the above mentioned methodology to differentiate obstacle and target. If it is obstacle it will take left or right direction and rotate its Bluetooth to get largest RSSI values and move towards master robot. It repeats the same for every obstacle before reaching the master robot. Fig. 3 shows the complete algorithm developed for achieving task.

RESULTS
The complete robotic system was tested in specified area to materialize the decentralized communication system of multi robots. Fig. 4(a-f) show the snapshots of experimental results to reach the target by two robots. The sequence of operations is as follows: In Fig. 4(a) two identical robots start moving to search the target object. In Fig. 4(b) Robot1 moving randomly faces the obstacle (white colored box) in the path, while Robot2 continues to move in different directions. Fig.   4(c) shows that Robot1 pauses there pivot left and measures distance and Fig. 4(d) shows that Robot1 pivots right, measures distance. By comparing both distances in Fig. 4(e) the robot1 advances towards right.

FIG. 2. FINAL DESIGN OF ROBOT EQUIPPED WITH MULTIPLE SENSORS AND BLUETOOTH AT THE TOP
In Fig. 4(f) Robot2 reaches the target object, while Robot1 keeps on moving.
After reaching the target object, Robot2 sends signal via Bluetooth to guide robot1 about the targeted location.  To measure the effectiveness of team work the experiment was performed with two robots and single robot individually. Different number of obstacles were placed in the specified area. Four trials were taken with each number of obstacles by single and by two Robots. Table  1 shows the average time taken by single and two robots to reach the target.
With the completion of this project it is experienced that the robots performed the assigned tasks by communicating and making decisions between them without any human guidance. In addition, it is also observed that two Robots working together as a team take less time as compared to single Robot doing the same task alone.

CONCLUSION
Two robots were assigned a task to find a target with certain parameters. Color sensors were used for this purpose. To make system efficient enough ultrasonic sensors were installed to avoid the collision. When one robot reached the target object it then successfully communicated second robot by using Bluetooth device and guided its location by transmitting signals. The second robot calculated the RSSI values and successfully traveled in a direction where there is greatest signal strength value.

FUTURE WORK
Although this project successfully completed the assigned task and worked properly, but still there are some limitations and areas that can be looked and reviewed for further improvements.