Ultrasonic transducers can be used for under water sonar and proximity measurement in air. It uses the theory of time of flight (ToF), which means the time difference between emitted ultrasound waves and reflected waves from the object, to calculate the distance between the ultrasonic transducer and the object. Furthermore, the difference in the penetration energy of ultrasound waves between a single sheet and multiple sheets can be used for double-sheet detection to determine whether objects overlap. In general, for ultrasonic detection, the object can be detected by ultrasonic transducers regardless of solids, liquids, or powders. The type and nature of the object to be detected are not affected by its shape, material, color, transparency, hardness, etc. Therefore, ultrasonic transducers nowadays are widely used in under water sonar, parking sensor, level sensor, double feed detection and flow meter, etc.
Structure of Ultrasonic Transducers
As shown in Figure 1, the ultrasonic transducer is composed of piezoelectric ceramics, acoustic matching layers and damping layers. The main component of the piezoceramics is Lead Zirconate Titanate (PZT) with conductive layers coated on both sides. By applying high frequency alternating voltage, the piezoelectric ceramic can produce high frequency vibration by the inverse piezoelectric effect (conversion of electrical energy to mechanical energy).The high frequency vibration is a kind of sound wave. If the frequency of this sound wave is greater than 20 kHz that is ultrasonic vibrations. In contrast, ultrasound waves can be received by using the positive piezoelectric effect (mechanical energy to electrical energy) of piezoelectric ceramics.
Ultrasonic Transducer Design Principle
Ultrasonic transducers can be divided into transmitters, receivers and transceivers by functions, as shown in Figure 2. For the example of a transducer operating at 40 kHz, the resonant frequency (fr) of the transmitters are designed at a frequency close to the operating frequency of the applied electrical signal, as shown in Figure 3, to optimize the efficiency of emission. On the contrary, designing the anti-resonant frequency (fa) of the receivers close to the received ultrasonic frequency, as shown in Figure 4, to optimize the efficiency of reception. The operating frequency of the transceiver is designed to be between the resonant frequency (fr) and the anti-resonant frequency (fa) of the transceiver, as shown in Figure 5. The higher the operating frequency of the transducers, the better resolution but shorter detecting range.
However, in order to make the generated ultrasonic waves be transmitted efficiently from the piezoelectric ceramic to the object or fluid (e.g., in air or water), the acoustic impedance between the piezoelectric ceramic and the object or fluid must be matched through acoustic matching layers. The sound velocity and acoustic impedance characteristics of common substances are as follows:
|substance||density(ρ) Kg/M3||Speed of sound(C) m/sec||acoustic impedance (Z) 106 Kg/M2∙sec|
For the example of an ultrasonic air transducer, the acoustic impedance of piezoelectric ceramics is about 35 MRayl (106 kg/m2∙s), while the acoustic impedance of air is as low as around 414 Rayl (kg/m2∙s). Therefore, the acoustic matching layer becomes a necessary part of the ultrasonic transducer, which is placed between the piezoelectric ceramic and air, so that the acoustic impedance of the two can be matched and the ultrasonic energy can be effectively transmitted into air.
The ideal value of acoustic impedance for the matching layer of an ultrasonic air transducer is Rayl, which is about 0.122 MRayl, but it is difficult to find materials with acoustic impedance lower than 1 MRayl and durable in nature. At present, the commonly used material for acoustic matching layers is a kind of composite material made of polymer matrix and hollow powder to achieve lower acoustic impedance with reasonable reliability. Depending on applications, ultrasonic transducers can be used in either a pitch-catch mode or a pitch-echo mode. It should be noted that ultrasonic transducers have ringing characteristics, inherently. When people design ultrasonic transducers for proximity measurement, the ringing limits the minimum detection distance. Generally, a damping layer is used to let the ultrasonic transducer quickly return to its static state to reduce its ringing.
How to Choose The Appropriate Ultrasonic Transducer?
Major characteristics of an ultrasonic transducer include sensitivity, directivity and blind zone, explained below:
Before understanding the sensitivity, it is necessary to introduce the Sound Pressure Level (SPL).
SPL is the effective sound pressure measured by a logarithmic scale relative to a reference value, defined as:
SPL = 20*log(P/Pref); the unit of measurement is dB
Where P is the sound pressure, Pref is the standard reference sound pressure. Generally, Pref is defined as 0.0002 ubar (derived from the human hearing threshold). However, for the convenience of expressing the performance of transducers, the sensitivity is used to express the reception intensity.
The unit of sensitivity is usually V/Pa or mV/Pa (V: Receiving voltage, Pa: Pascal). The transmitting intensity is defined as the sound pressure obtained by a standard condenser microphone (S.C.M.) at a certain distance after inputting a specific frequency and power signal to the ultrasonic transducer, as shown in Figure 6. Receiving sensitivity is defined as a fixed sound pressure emitted by an ultrasonic transducer and received by a standard condenser microphone and the ultrasonic transducer in parallel to be tested at certain distance, as shown in Figure 7. Next to the standard condenser microphone, the transducer receive this sound pressure and then convert it to voltage signal, by comparing the two, the sensitivity of the transducer can be determined. Therefore, the output voltage of the receiver can be used to determine the level of its sensitivity. The higher the output voltage of the receiver, the higher sensitivity, and vice versa.
The directivity of an ultrasonic transducer is defined as the angle which ultrasound intensity decayed -3 dB at specific distance from the transducer with respect to the maximum intensity defined as zero dB in the main emitting direction, as shown in Figure 8. The directivity angle of an ultrasonic transducer is affected by the wavelength of the ultrasound wave and the dimension of the emitting surface. The higher the frequency, the smaller the wavelength and the narrower the directivity angle; the larger the dimension of the emitting surface, the smaller directivity angle.
The blind zone means the minimum detection distance of the ultrasonic transducer, and the main factor affecting the size of the blind zone is the ringing time. When the transducer receives a specific frequency of electronic signal, the piezoelectric ceramic correspondingly generates vibration and emits ultrasonic waves, but this vibration does not stop immediately as a circuit, it gradually tends to calm down to static after the main vibration with the assistance of the damping layer. The period of time between the end of the main vibration and the static state is called the ringing time. Ringing time will affect the range of blind zone, for the example of transceivers, we use the time difference between input signal and reflected wave to measure the distance of the object to be detected. When the ringing time is too long, causing the reflected signal to overlap with the ringing time, the signal interpretation will be affected, as shown in Figure 9.