Ultrasonic or sonar sensors are widely used for range finding for indoor and outdoor applications in robotics. However, for outdoors applications, they pose different environmental challenges. Ultrasonic sensor can be used both in air and underwater. It emits acoustic pulses in a cone shaped form in its surroundings and waits for the echoes from the objects nearby that lie within its working range. Ultrasonic sensors have convincing advantages over other sensors. However, sonar sensors have different practical limitations as well which need to be carefully dealt with while working with these sensors. Ultrasonic sensors have several applications in electronics and robotics including obstacle detection and avoidance, mapping and navigation, object recognition and identification. Ultrasonic sensors are widely used in automatic car parking systems in modern vehicles, where two to four sensors are mounted in rear bumper for detecting obstacles up to 2.5 meter and assisting the driver about the parallel parking. The thesis is mainly divided into two parts. In the first part, background studies and literature review is presented which describes sonar sensing principle, applications, advantages, limitations and outdoor sensing challenges. In the second part, a sonar system for obstacle detection for a mobile machine is implemented and its tests and results are discussed. The study indicates the testing of ultrasonic sensors for obstacles detection for an autonomous mobile vehicle outdoor. The sensors were tested both on static frame and on real machine detecting different obstacles from 60 cm up to five meters. The results are better when the object is in front or moving along the axis of the sensor. The sensors are connected in series and are in ranging mode all the time. The experimental results show that the environmental factors like, air turbulence and temperature change affect the speed of sound in air and measuring range. The ranging value is better indoors than the outdoors for same obstacles. However, the results are better on less windy day and also when the surface is strong reflector. It is noted that the results get improved when a cone made of paper or plastic is wrapped around the transducer. The sensor is protected with a water proof casing made of PVC plastic material and it is noted that the casing made of aluminum does not yield good results as compared with the plastic casing. The two or more sensors attached in line increase the covering area of the system.
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