Sonar (Ultrasonic Sensing)
Active vs Passive
Active Ultrasonic Sensing
Polaroid's Role in Ultrasonic Sensing Technology
Technological Issues: Signal Gain
Other Issues: Sound Reflection
The Devantech SRF04
Pros and Cons

Sonar (Ultrasonic Sensing)

Sound Navigation and Ranging

developed during WWII to find submarines. (works well underwater, where Radar - Radio Detection and Ranging - doesn't work.)
uses sound energy to
- locate objects
- measure their distance
- produce pictures of them

Active vs Passive

Active: uses a transmitter to convert electrical enery to sound energy. Sound waves travel until they strike an object, which reflects them in various directions. Some reflected waves return to sonar, where they strike a receiver. Distance is determined by measuring the time it takes for the sound wave to travel from the transmitter, bounce, and travel back to the receiver.

Passive: only receives sound waves. Can be used to detect the direction of an object, for example.

Active Ultrasonic Sensing

transmitter circuit causes ultrasonic burst ("chirp") to be emitted from a speaker element.
sound travels and is reflected back
receiver circuit tuned to detect specific frequency of sound emitted by the transmitter.

In normal room temperature, sound travels approximately 1.1 ft/millisecond. (i.e., it takes .89 milliseconds to travel 1 foot)

Polaroid's Role in Ultrasonic Sensing Technology

Technology first popularized by Polaroid: used to measure distance from camera to subject for control of autofocus system. (Currently, infrared typically used.)

One popular sonar device, the Polaroid 6500, has a single ultrasonic transducer that acts both as a transmitter and receiver.

Technological Issues: Signal Gain

As sound disperses through air, volume falls at square of distance. Yet more dispersion as sound travels back. So an echo from a faraway object may be one millionth the strength of one nearby.

Solution: Variable gain amplifier that is automatically controlled through a sequence of steps. The gain is increased as time elapses.

Other Issues: Sound Reflection

When sound strikes an irregular object, produces collection of reflected sounds.

When sound strikes a long flat surface, reflects like a beam of light off a mirror. In this case, little or no sound is aimed back at the transducer.

The Devantech SRF04

Has both a transmitter and receiver.

Range: 3cm to 3m. (Approximately 3 inches to 10 feet)

Sensitivity: 3cm stick at > 2m.

Beam pattern:

For more info, go to: http://www.acroname.com/robotics/parts/R93-SRF04.html

Pros and Cons

Pros:

low cost
can effectively provide distance to objects
produces tractable amount of data for interpretation

Cons:

poorer discriminatory ability than vision
susceptible to noise/distortion
can produce erroneous data (reflections)

Best for obstacle detection and avoidance at short range.

Note: can't be used in outer space - needs a medium for transmission of sound wave.