Sensing Distance

Have you ever shouted in a big empty gym or a tunnel and heard your voice come back to you? That echo is sound bouncing off a wall and returning to your ears. Now here is a clever question: if you knew exactly how fast sound travels, could you figure out how far away the wall was just from how long it took the echo to come back? The answer is yes! And this trick — sending out a signal and timing the echo — is how many robots measure distance.

Ultrasonic Sensors: Echoes You Cannot Hear

An ultrasonic sensor is a distance-sensing tool that sends out a burst of sound — so high-pitched that you cannot hear it — and then listens for the echo. Ultrasonic means sound waves that are too high for human ears to detect. Bats use ultrasonic sound to navigate in the dark. They squeak, wait for the echo, and figure out exactly where the walls and flying insects are. It is called echolocation. Robots can do the same thing. The sensor sends out an ultrasonic beep, the beep bounces off a wall or an object, and the echo comes back. The sensor measures the time between the beep and the echo. Since sound travels at a known speed, a tiny calculation gives the distance: about this many centimeters away. This all happens in a fraction of a second, over and over, dozens of times per second.

Some robots use light instead of sound to measure distance. This is called a LIDAR sensor or a laser rangefinder. LIDAR stands for Light Detection And Ranging. Instead of a sound beep, it shoots out a beam of laser light in every direction, thousands of times per second. The light bounces back from everything around the robot, and the sensor times each return. The result is a detailed three-dimensional map of everything around the robot — walls, furniture, trees, people — all from bouncing light. Self-driving cars use LIDAR to understand the road around them. The spinning tower on top of some self-driving cars is a LIDAR sensor. Whether using sound or light, the idea is the same: send a signal out, wait for the echo, calculate the distance.

Distance sensing is incredibly important for robots that move around. Think about a robot that delivers food in a hotel. It needs to travel down a hallway without bumping into walls, other robots, or guests. By constantly firing its distance sensors in all directions, it always knows how much space it has on each side. Or think about a robot lawn mower. It needs to know when it is getting close to the edge of the grass, to a garden bed, or to a fence. Distance sensors let it glide around the yard without going too close to anything. Without distance sensing, a moving robot is basically moving blind.