1. Sonar (Ultrasonic Sensing)
  2. Active vs Passive
  3. Active Ultrasonic Sensing
  4. Polaroid's Role in Ultrasonic Sensing Technology
  5. Technological Issues: Signal Gain
  6. Other Issues: Sound Reflection
  7. The Devantech SRF04
  8. Pros and Cons

Sonar (Ultrasonic Sensing)

Sound Navigation and Ranging

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

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: Cons: 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.