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http://en.wikipedia.org/wiki/Acoustic_camera

Ultrasonic sensor

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Figure 1

Ultrasonic sensors emit a high frequency sound waves and evaluet the echo which is received back by the sensor. After The principle in ultrasonic sensors is pretty much the same as on radars and sonars, after sending the sound wave, the sensor calculates Image Removedthe time it takes between sending the signal and receiving the echo to determine how far the object is from the source. Capacitive sensors are also called as transceivers/transducers, and this is because the sensor sends and receives at the same time.

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Figure 2

The formula for calculating how far the object is, is really simple, basically you just measure the time it takes for the pulse to travel, multiply it with the speed of sound which is 341m/s in air, and divide the whole thing with 2, since the pulse travels from the sensor to the object and back.

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One of the drawbacks is that the ultrasonic sensor is that there is no way of telling the difference between small and large objects because the pulse that's emitted is cone shaped, because of the shape, an echo will be returned by all objects the pulse comes into contact with. One way to work around this is to use rotating sensor or multiple sensors to find out the shape and objects size.

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Figure 3 (Ultrasonic pulse from the transducer sensor)

In industry, ultrasonic sensors are used  to detect movement of targets and to measure the distance to targets in many automated factories. As for example, they can be used to sense the edge of material as part of a web guiding system.

Sources:

1. http://en.wikipedia.org/wiki/Ultrasonic_sensor
2. http://www.ehow.com/how-does_4947693_ultrasonic-sensors-work.html?ref=Track2&utm_source=ask
3. http://www.education.rec.ri.cmu.edu/products/nxt_video_trainer2/resources/helpers/nxt_sensors/ultrasonic.html

Position sensors characteristics comparison

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