Effect of defocusing amount on thickness measurement based on laser ultrasound
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摘要: 为了研究离焦量对激光超声信号幅值的影响,采用相同能量的脉冲激光辐照不同厚度的铝质试块进行了工件测厚的实验。光路会聚采用焦距100mm的平凸球面聚焦透镜,将该透镜固定于5维光学调整架上,调节水平轴旋钮以改变离焦量;利用中心频率5MHz的压电探头接收激光超声波信号,并记录所有试块在每个离焦量下的超声信号数据;依据信号之间的时间间隔,求出每个铝质试块的厚度。结果表明,测厚结果与工件实际厚度之间的相对误差均在3%范围内;在等量激光脉冲激励下,当工件表面处于离焦量-10mm时,获得信号的幅值最大。Abstract: In order to study the influence of defocusing amount on laser ultrasonic signal amplitude, thickness measurement experiment was carried out. Aluminum test blocks in different thickness were irradiated by the same pulse laser while using plano-convex spherical lens whose focal length was 100mm. The lens was fixed at a 5-D optical stage and the horizontal axis knob was adjusted to change the defocusing amount. The ultrasound signals were received by piezoelectric probe with 5MHz center frequency and the ultrasonic signal data of all test blocks under each defocusing amount were recorded. The thickness of each aluminum test block was obtained according to the time interval between the signals. The results indicate that relative errors between the measurement value and the actual value are in the range of 3% and the maximum signal amplitude can be obtained when the defocusing amount is -10mm under the same laser irradiation.
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