The Ultrasonic Sensor measures distances within a range of 1-250 cm (EV3 Robot) and 1-1000 cm (Ruby Robot) , with an accuracy of +/- 1 cm. The sensor is useful for detecting and avoiding obstacles. In some cases the robot might have several ultrasonic sensors: one facing front, one on the side, perhaps one on either side!
It's important to note that even if the Ultrasonic sensor is located at the front of the robot, it does not stick out as much as the other sensors. This means that when the robot hits an object, the ultrasonic sensor is still a few centimeters away from it and its reading will show a value greater than zero:
Want to see the ultrasonic sensor's beam?
Use the keyboard shortcut!
Hit 'u' on your keyboard to see exactly where the robot's ultrasonic sensor(s) are facing.