What types of distance sensors are there?

The two most common types of distance sensors used in robotics are the ultrasonic and infrared, but there are also laser rangefinders, sonar, and radar depending on the complexity and budget of the robot in question.

Ultrasonic - Ultrasonic rangefinders send out a directed sound wave and measure the amount of time to get a response to calculate distance. Since sound travels in waves, ultrasonic sensors are able to sense objects slightly to either side of straight ahead. This can cause some irregularities in distance measurements if something is within the path of the sound waves, but not the target of the distance reading. They usually have a small deadzone close to the sensor where readings will fail.

Infrared - There are two types of infrared range finders. The first type sends out a beam of infrared light and measures the amount of light bouced back to calculate distance. The closer and object is to the sensor, the more light that will be reflected. The second type of IR sensor sends out a beam of infrared light and measures the angle at which the beam is bounced back to the receiving sensor. If an object is close to the sensor, the angle will be greater than if it is far away. IR sensors work best at short distances and only sense objects directly in front of them.

Laser Rangefinders send out a pulse of light. For small distance measurements, the angle that the light is returned at can be used for measuring the distance. For longer distances, the amount of time that it takes the light to be bounced back to the device is used to calculate the distance. Like the IR sensor, Laser rangefinders only work in a straight line.

Sonar - There are two types of sonar. Active sonar works much like an ultrasonic - sonar is in fact a more general type of sensing using sound that includes ultrasonic frequencies. Sound waves are sent out and the time it takes them to return is measured to calculate distance. Passive sonar listens for sound without emitting, allowing the user to detect presence, but not as accurately detect distance. Sonar is typically used for underwater activities, where sound carries further than through the air.

Radar - Radio Detection and Ranging uses radio waves (or microwaves) to measure distance. Radio waves are emitted from a source at a known frequency. When they hit objects, they are scattered and a small amount is reflected back to a collector at the origin. The signal can then be examined to show the distance, as shown by the time that the radio waves take to return to the point of origin and the speed relative to the origin. The speed can be deduced by examining the frequency of the reflected waves. An object moving towards the origin will cause waves to increase in frequency, moving away from the origin will decrease the frequency - the Doppler effect. Radar is effective because the reflected signals can be amplified (as is done in a common stereo receiver) and analyzed, allowing for a large range in addition to covering a large area.

The most common type of distance sensor is the ultrasonic. An ultrasonic works like echolocation, the way bats find food. It emits a high frequency noise that bounces off a target, and the time that the sound takes to return to the sensor tells the user how far away the object is. The major downside is that the sound is not in a straight beam, it is an arc, and objects that are not directly in front of the robot can throw off the readings.