15. Appendix G - Sonar TipsHexotica - The Design and Implementation of a Small Walking Robot
The following information was obtained from [12] .
Echolocation is a field of sensing, which as its name indicates, finds the location of an object through the use of propagated wave echoes. Many types of propagation techniques can be utilized in echolocation but ultrasonic sensing systems are relatively easy to use and are very cost efficient for low-scale robotics applications.
Echolocation that uses sound waves for detection is called sonar (SOund Navigation And Ranging) and has three distinct characteristics. These characteristics include the transmission medium, the velocity of propagation and the wavelength of the sound. Unlike electromagnetic radiation, sound requires a medium for transmission. Common transmission mediums for sound include air and water. The velocity at which sound waves travel is much slower (331.6 + 0.6 x ° C m/s in air) than light waves (» 250,000 m/s in vacuum), enabling the controller to use the "time-of-flight" to estimate an object’s distance from the transmitter. The wavelength of a 50 kHz sound wave is 6.872 mm. This is large enough to overcome the roughness of indoor surfaces and yield fairly accurate results.
Specular reflection occurs when sound waves reflect off a surface with near mirror-like precision. Indoor environments tend to have smoother surfaces (i.e. walls) that result in specular reflections. Diffuse reflections, on the other hand, occur when the wave’s reflection is not consistent and may reflect off other objects in the area before returning to the source. The majority of reflections are diffuse in outdoor environments.
The most common ultrasonic transducer available on the market today is the Polaroid 6500 Series Sonar Ranging Module. It acts as both a transmitter and a receiver. This transducer converts sound waves to electrical potential. A small thin gold-plated plastic foil forms the outer surface of the sensor. Below this foil rests a concentric groove aluminum backplate. These two conductive surfaces act as a capacitor when 300 V is applied to the backplate. When the sensor is acting as a transmitter, the voltage across the capacitor is varied, thus varying the electrostatic force applied on the foil. This change causes the foil to act like a speaker propagating sound waves into the air.
The reflected wave applies a varying pressure to the gold foil that alters the capacitance of the conductive surface. This changing capacitance results in variable voltages that can be sampled by a controller. The width of the transmission beam is approximately 15° .
The sonar sensor produces a 1 ms acoustical ping every 200 ms. This ping is made up of 56 pulses. These pulses consist of multiple frequencies to minimize the effects of sound absorption that occurs in some materials. Another benefit of using multiple frequencies is that if a diffuse reflection occurs, the echo may cause a large enough phase shift that it cancels out other echo signals. In most cases, the first echo is the one that is measured.
There are a number of shortcomings of sonar that may limit its effectiveness as a sensing technology. Sonar is very accurate at measuring the perpendicular distance from the sensor to a wall. When an object is placed in front of a wall, the sonar can accurately calculate the distance to the object but its lateral position is only known to be within a certain range. Another limitation arises when the sensor’s main axis is at an angle to a wall. If the outer edge of the beam is perpendicular to the wall, this reflection will set the range for the sensor. This range is shorter than the axial range. Decreasing the beam angle can minimize this problem but if a surface is very smooth, a small beam angle will have fewer echoes.
If the angle between the transmitted beam and the reflected beam is greater than the beam angle, the entire beam will reflect off the wall and the wall will appear invisible. Finally, sonar can also be hampered by multiple reflections occurring in a concave corner. The time-of-flight will be longer and the wall will appear farther away than it actually is.
The following web sites discuss a number of design issues relevant to the implementation of Polaroid 6500 sonar sensors. They may prove helpful in the future for designing the interface to the Polaroid 6500 transducers. These sites are likely to be updated, so they should be revisited when the interface is designed.
http://www.wirz.com - Hardware supplier that sells the Polaroid 6500 modules.
http://www.cs.umd.edu/users/musliner/sonar/notes – contains notes on implementation issues and problems.
http://www-personal.engin.umich.edu/~johannb/ - comprehensive source of academic papers relating to obstacle avoidance using sonar and other technologies.
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