Department of Materials Science and Engineering, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA.
Sensors (Basel). 2018 Nov 9;18(11):3861. doi: 10.3390/s18113861.
Receiving displacement sensitivities (Rx) of ultrasonic transducers and acoustic emission (AE) sensors are evaluated using sinewave packet excitation method and compared to the corresponding data from pulse excitation method with a particular emphasis on low frequency behavior below 20 kHz, down to 10 Hz. Both methods rely on the determination of transmitter displacement characteristics using a laser interferometric method. Results obtained by two calibration methods are in good agreement, with average spectral differences below 1 dB, indicating that the two calibration methods yield identical receiving sensitivities. At low test frequencies, effects of attenuation increase substantially due to increasing sensor impedance and Rx requires correction in order to evaluate the inherent sensitivity of a sensor, or open-circuit sensitivity. This can differ by more than 20 dB from results that used common preamplifiers with 10 kΩ input impedance, leading to apparent velocity response below 100 kHz for typical AE sensors. Damped broadband sensors and ultrasonic transducers exhibit inherent velocity response (Type 1) below their main resonance frequency. In sensors with under-damped resonance, a steep sensitivity decrease occurs showing frequency dependence of f²f⁵ (Type 2), while mass-loaded sensors exhibit flat displacement response (Type 0). Such behaviors originate from sensor characteristics that can best be described by the damped harmonic oscillator model. This model accounts for the three typical behaviors. At low frequencies, typically below 1 kHz, receiving sensitivity exhibits another Type 0 behavior of frequency independent Rx. Seven of 12 sensors showed this flat region, while three more appear to approach the Type 0 region. This appears to originate from the quasi-static piezoelectric response of a sensing element. In using impulse method, a minimum pulse duration is necessary to obtain spectral fidelity at low frequencies and an approximate rule is given. Various factors for sensitivity improvement are also discussed.
接收超声换能器和声发射（AE）传感器的位移灵敏度（Rx）采用正弦波包激励法进行评估，并与脉冲激励法的相应数据进行比较，特别强调低于 20 kHz、低至 10 Hz 的低频行为。两种方法都依赖于使用激光干涉法确定发射器的位移特性。两种校准方法得到的结果非常吻合，平均频谱差异低于 1 dB，表明两种校准方法产生的接收灵敏度相同。在低测试频率下，由于传感器阻抗增加，衰减效应会大大增加，因此需要进行 Rx 校正，以评估传感器的固有灵敏度或开路灵敏度。与使用具有约 10 kΩ输入阻抗的常见前置放大器的结果相比，这可能相差超过 20 dB，从而导致典型 AE 传感器的表观速度响应低于 100 kHz。带阻尼宽带传感器和超声换能器在其主共振频率以下表现出固有速度响应（类型 1）。在具有欠阻尼共振的传感器中，灵敏度会急剧下降，表现出频率依赖性 f²~f⁵（类型 2），而质量加载传感器则表现出平坦的位移响应（类型 0）。这种行为源于传感器特性，最好用阻尼谐波振荡器模型来描述。该模型解释了三种典型行为。在低频下，通常低于 1 kHz，接收灵敏度表现出另一种与频率无关的 Rx 类型 0 行为。12 个传感器中有 7 个表现出这种平坦区域，而另外 3 个似乎接近类型 0 区域。这似乎源于传感元件的准静态压电响应。在使用脉冲法时，为了在低频获得频谱保真度，需要最小脉冲持续时间，并给出了一个近似规则。还讨论了各种提高灵敏度的因素。
