Ultrasonic sensors provide an excellent way to measure the distance of the robot from obstacles (providing a measurement of between 2 cm and 20 cm) and are available at most electrical hobby stores (see the Appendix, Hardware and Software List). The ultrasonic module functions by sending a short burst of ultrasonic pulses and then measures the time it takes for the receiver to detect the echo. The module then produces a pulse on the echo output that is equal to the time measured. This time is equal to the distance traveled divided by the speed of sound (340.29 m/sec or 34,029 cm/s), which is the distance from the sensor to the object and back again. An ultrasonic module is shown in the following photo:
The HC-SR04 ...
