An Ultrasonic ranging module, also called a range finder, is a small device that sends out an ultrasonic sound via a transducer, and since the sound has a very high frequency, a frequency that we can not hear, it bounces off objects pretty easily. Its main job is just to record the time it takes for the sound to bounce back and into the transducer. Then a controller will do the mathematics to determine the distance between the module and an object. there is a down side to some range finders, in robotics, it only can measure up to a few inches....