The basics of applying ultrasonic transducers for sensing objects or fluid flow
In operation, a brief pulse of acoustic energy is generated by a transducer, which is usually a piezoelectric device. After the pulse ends, the system switches to receive mode and awaits the reflection (echo) of that pulse. When the transmitted acoustic energy encounters an impedance transition or discontinuity, such as between air and a solid object, some of that energy is reflected and can be detected, usually by a piezoelectric device. Acoustic impedance is based on the density and acoustic velocity of a given material, and it is important to determine the amount of reflection that occurs at the boundary of two materials having different acoustic impedances. The proportion of energy that is reflected is a function of the material type and its absorption coefficient, as well as the impedance differential at the boundary between materials. Hard materials such as stone, brick, or metal reflect more than soft materials such as fabric or cushions. The acoustic impedance of air is four orders of magnitude less than that of most liquids or solids. As a result, the majority of ultrasonic energy is reflected to the transducer based on the large difference in reflection coefficients. The acoustic cross section is a metric analogous to radar cross
section and is determined by the target object’s material and size. This detection and distance sensing is similar to what happens when radar RF energy or lidar optical energy encounters an impedance discontinuity, and some of that energy is reflected back to the source. However, while the overall concept is the same, there is a big difference: ultrasound energy is not electromagnetic energy. Its use of the frequency spectrum is not regulated, and it has very few restrictions. One pertinent restriction is excessive sound pressure level (SPL), a consideration that is generally not relevant to sensing/detection applications, as most of these operate at fairly low power levels. Propagation and media matter There’s one other big difference: ultrasound sensing/detection can only be used in a propagating medium such as air, other gases, or liquids. The attenuation and propagation characteristics of acoustic energy through various media are the opposite of RF and optical energy. Acoustic energy propagates well through liquids, while RF energy generally does not. Optical energy also has high attenuation in most liquids.
Further, unlike acoustic energy, both RF and optical have low attenuation in a vacuum. In its simplest implementation, the ultrasonic system is used solely to detect the presence or absence of an object or person within an overall zone of interest by detecting a return signal of sufficient strength. By adding a timing measurement, the distance to the target can also be determined. In more sophisticated systems where the distance to the object must also be calculated, a simple equation can be used: distance = ½ (velocity × time), using the round-trip time between the emitted pulse and received reflection, and the established speed of sound in air which is about 343 meters per second (m/s) at +20°C (+68°F). If the medium is a fluid or gas other than air, the appropriate propagation speed must be used. Note that the speed of sound in air varies slightly with temperature and humidity. Therefore, ultraprecise distance sensing applications require that one or both of those factors must be known, and a correction factor added to the basic equation. Interestingly, as an example of engineers turning a negative factor into a positive one, there
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