An acoustic proximity sensor can be used by a robot to detect the presence of, and determine the  distance to, an object or barrier at close range. It works based on acoustic wave interference.

The principle is similar to that of sonar; but rather than measuring the time delay between the transmission of a pulse and its echo, the system analyzes the phase relationship between the transmitted wave and the reflected wave.

When an acoustic signal having a single, well-defined, constant frequency(and therefore a single,well-defined, constant wavelength) reflects from a nearby object, the reflected wave combines with the incident wave to form alternating zones at which the acoustic energy adds and cancels in phase. If the robot and the object are both stationary, these zones remain fixed. Because of this, the zones are called standing waves. If the robot moves with respect to the object, the standing waves change position. Even a tiny shift in the relative position of the robot and the sensed object can produce a considerable change in the pattern of standing waves. This effect becomes more pronounced as the acoustic wave frequency increases, because the wavelength is inversely proportional to the frequency.

The characteristics and effectiveness of an acoustic proximity sensor depend on how well the object or barrier reflects acoustic waves. A solid concrete wall is more easily detected than a sofa upholstered with cloth. The distance between the robot and the obstacle is a factor; in general, an acoustic proximity sensor works better as the distance decreases, and less well as the distance increases. The amount of acoustic noise in the robot’s work environment is also important. The higher the noise level, the more limited is the range over which the sensor functions, and the more likely are errors or false positives. Ultrasound waves provide exceptional accuracy at close range, in some cases less than 1 cm. Audible sound can allow the system to function at distances on the order of several meters. However, audible signals can annoy people who must work around the machine.



This video shows the simulation of proximity sensors in the Virtual Robot Experimentation Platform (V-REP: ). Proximity sensors will make a geometrically correct detection (i.e. detect the closest part of an objects that is inside its detection volume. Unlike other simulation software, V-REP does not rely on simple collision detection with a sample of rays or segments to simulate proximity sensors.

Moreover, it is possible to specify a maximum angle between the detection ray and the normal vector of a surface in order to realistically simulate ultrasonic sensors (ultrasonic sensors can’t easily detect specular surfaces whose normal vector is not oriented enough towards the sensor).

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