基于宽频激励电磁声谐振 7075 铝合金板厚检出概率

电磁超声换能器(EMAT)换能效率低、激发信号弱,而基于长周期激励信号以产生声波共振的电磁声谐振技术(EMAR)虽然可以提高接收信号的信噪比,然而可能导致主脉冲加宽并扩大超声波检测盲区,降低测量精度。本文应用一种宽频激励电磁声谐振技术(BE-EMAR)和一种Halbach阵列纵波EMAT,以单周期宽频激励作为EMAT输入信号,并利用EMAR方法对7075型铝合金试件板厚进行测量,借助检出概率(POD)模型来表征该技术针对微金属板厚的测量精度。实验结果表明,在未降噪时BE-EMAR得到的三种信噪比下的POD曲线中,对应的a_(50)或a_(90/95)最大差异值为0.05 mm,同一信噪比下降噪前后的最大差异值为0.07 mm,而在时差法(ToF)得到的POD曲线中这一最大差异值则为0.21 mm,因此说明BE-EMAR降噪前后得到的结果并无较大差别,该方案对于常规噪声信号不敏感;且降噪前后,在BE-EMAR得到的POD曲线中,a_(50)和a_(90/95)均可稳定在0.55 mm以内,证明了BE-EMAR比ToF测厚具有更优的精度、稳定性和抗噪能力。

Probability of detection of 7075 aluminum alloy plate thickness based on BE-EMAR

The electromagnetic acoustic transducer (EMAT) has low energy conversion efficiency and weak excitation signal, although the electromagnetic acoustic resonance (EMAR) technology based on long-period excitation signal for generating acoustic resonance can improve the signal-to-noise ratio of the received signal, it may cause the main pulse widen, enlarge the blind area of ultrasonic detection and reduce the measurement accuracy. In this paper, a broadband excitation electromagnetic acoustic resonance technology ( BEEMAR) and a Halbach array longitudinal wave EMAT are applied, a single-cycle broadband excitation is used as the EMAT input signal, and the EMAR method is used to measure the plate thickness of 7075 aluminum alloy specimens. The probability of detection (POD) model is used to characterize the measurement accuracy for micro metal plate thickness. The experiment results show that in the POD curves obtained with BE-EMAR under three signal-to-noise ratios when the noise is not reduced, the corresponding maximum difference of a50 or a90/ 95 is 0. 05 mm, while the maximum difference under the same signal-to-noise ratio before and after the noise reduction is 0. 07 mm. However, in the POD curve obtained with the time of flight (ToF) method the maximum difference is 0. 21 mm. Therefore, there is no significant difference between the results obtained with BE-EMAR before and after noise reduction, and the scheme is insensitive to conventional noise signals. Before and after noise reduction, in the POD curve obtained with BE-EMAR, a50 and a90/ 95 both can be stabilized within 0. 55 mm, which proves that the BE-EMAR has better accuracy, stability and anti-noise ability than ToF method in thickness measurement.