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The aim of the project is to design an autonomous robot platform, capable of reaching a desired destination starting from a known position in an environment filled with unknown obstacles. We are using the Virtual Force Field (VFF) method by Dr. J. Borenstein for the navigation purpose. The VFF method is the integration of two concepts: Certainty grids for obstacle representation and potential fields for navigation.
The system can be summarized as follows. In the first phase, distance measuring ultrasonic sensors mounted strategically on the robot chassis scans the environment and represents the obstacles in a grid-type world model. This grid-type world model has been developed at the Carnegie-Mellon University (CMU).
In the certainty grid, the robot’s work area is represented by a two-dimensional array of square elements denoted as cells. Each cell contains a certainty value (CV) that indicates the measure of confidence that an obstacle exists within the cell area. The ultrasonic sensor has a conical field of view and returns a radial measure of the distance to the nearest obstacle within the cone. In the second phase, the idea of imaginary forces acting on the robot has been considered. Location of the obstacles and the target with respect to the robot are fed into the microcontroller. The obstacles exert repellant forces, while the target applies an attractive force to the robot. A resultant force R, comprising the sum of a target-directed attractive force and repellant forces from obstacles, is calculated for a given robot position. This resultant force has a specific magnitude and direction and the robot follows the same.
Traditionally, such systems use 12 to 24 distance measuring ultrasonic sensors placed circumferentially around the robot chassis. However, due to cost considerations, we are using only four sensors (SRF-08 by devantech) mounted at 90 degrees to each other on a base, which is being rotated using a stepper motor. We are using a unique approach to overcome the use of lesser number of sensors. The base is rotated by 30 degrees after every 300 milliseconds and the range readings of the sonars are taken and stored. After 3 rotations the sonars cover the entire circumference and together act as 12 sensors.
The hardware of the system consists of a differentially driven wheeled platform, driven by geared stepper motors and an ultrasonic rangefinder SRF-08 mounted on a stepper motor. SRF-08 gives the distance of the measured obstacle in IIC protocol. The sensors communicate with the AVR microcontroller (ATMEGA32), and the stepper motors are driven as per the algorithm programmed in the microcontroller.
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