雷射都卜勒振動儀之自動化全場聲振量測系統開發

Abstract

本研究基於單點式的雷射都卜勒振動儀(Laser Doppler Vibrometry, LDV)，搭配兩種不同的全域式掃瞄儀器，分別為龍門式二維位移平台(Gantry Platform System)以及微動振鏡系統(Galvano Scanning System)，整合為掃描型雷射都卜勒振動儀(Scanning Laser Doppler Vibrometry, SLDV)。SLDV系統中整合了訊號產生器為激振儀器，示波器或資料擷取系統(NI-DAQ)為量測資料接收儀器。分別透過MATLAB及LabVIEW為控制語言，撰寫出其對應的圖形用戶介面(GUI)，方便量測者進行SLDV系統的使用。 接著對SLDV的量測訊號進行後處理，透過比較LDV的速度量測訊號及訊號產生器的輸入訊號，兩者之間的相位差，決定量測點的振動方向，並以頻率及相位判斷的方式優化量測資料，搭配所有離散點的座標內插後，即可繪出2D及3D的模態振形圖。隨後與不同的全域式面外振動量測法:電子斑點干涉術(Electronic Speckle Pattern Interferometry, ESPI) 量測的面外振動結果做比較，本研究開發的SLDV方法除了有更廣的速度量測範圍，同時可以數值化的顯示振動振幅與正確判斷全場振動相位。 成功量測出聲學元件振膜的振動模態後，透過聲學理論解析找出振動速度和聲壓關係，將振動場推估到聲學場上，並將多個量測的離散點用雷利積分(Rayleigh Integral)的方式，計算出聲學元件整體的聲壓貢獻，是為分布參數法(Distributed Parameter Method, DPM)。與無響室(Anechoic Room)中使用麥克風對自由音場的聲壓量測結果進行比較，本研究開發的DPM方法，得以在任何環境中利用光學方法量測出試片的聲壓情形，大幅節省建造無響室所需要的成本；與KLIPPEL公司利用雷射位移計的聲壓量測方法相比，DPM方法在高頻(3000Hz以上)的區域有更佳的量測能力；與Polytec 公司利用LDV結合振鏡系統的全場速度量測系統相比，本研究花費大約四分之一的建造成本，便可達成相同的功能，並可搭配聲學理論解析成功的將振動訊號直接推估到聲學領域上。 最後就本研究開發的SLDV進行改良。首先，開發出SLDV搭配掃頻(Chirp)訊號量測，此方法利用短時間內連續變化的頻率訊號為激振源，一次性完成所有頻率的量測，如此得以顯著提升量測效率，並快速判斷出試片全域的共振頻率；接著開發連續掃描型雷射都卜勒振動儀(Continuous Scanning Laser Doppler Vibrometry, CSLDV)，將原先逐點振動量測的系統改為逐線段量測，一次性量測整條線段的振動資料，顯著地提升系統的量測效率，同時維持不錯的量測精度。

This research is based on a single-point Laser Doppler Vibrometry (LDV) combined with two different full-field scanning instruments, a gantry platform system and a Galvano scanning system, integrated into a Scanning Laser Doppler Vibrometry (SLDV) system. The system incorporates a signal generator as an excitation instrument and an oscilloscope or NI-DAQ data acquisition system as a measurement data receiver. MATLAB or LabVIEW was used to develop corresponding graphical user interfaces (GUIs) to control and operate the entire system efficiently. Following that, we need to post-process the measurement data. We can determine the phase of vibration at each measurement point by utilizing the input signal from the signal generator and the measured signal from the LDV. The measurement data is then optimized using frequency and phase-based analysis methods. 2D and 3D mode shapes can successfully plot based on the optimized data. Subsequently, a comparison is made between the results obtained from the SLDV and Electronic Speckle Pattern Interferometry (ESPI). The aim is to validate SLDV's feasibility and evaluate the strengths and weaknesses of the two different measurement methods. The SLDV method developed in this research not only has a wider range of velocity measurement but also enables the numerical display of vibration amplitudes and accurate determination of the phase of full-field vibrations. After successfully measuring the global out-of-plane vibration of the acoustic component, acoustic theory analyzes and extrapolates the vibration field to the acoustic field. The relationship between the vibration velocity and the sound pressure is determined. Rayleigh Integral calculates the contribution of multiple discrete measurement points to a specific location. This method is known as the Distributed Parameter Method (DPM). Compared with the measurement results of free sound field using a microphone in an anechoic room, the DPM method allows for optical measurements of the sound pressure of the test sample in any environment, significantly reducing the cost of constructing an anechoic room. Compared to KLIPPEL's sound pressure measurement method using a laser displacement sensor, the DPM method exhibits superior measurement capabilities, particularly in the high-frequency range (above 3000 Hz). Compared to Polytec's full-field velocity measurement system using LDV combined with a Galvano scanning system, this research achieves the same functionality at approximately one-fourth of the construction cost. Additionally, it successfully applies acoustic theory to directly estimate vibration signals in the field of acoustics. Finally, improvements were made to the developed Scanning Laser Doppler Vibrometry (SLDV) system. Firstly, the SLDV system was integrated with the Chirp signal measurement method. This method utilizes a rapidly changing frequency excitation signal within a short time frame to measure all frequencies simultaneously. By measuring all frequencies at one time, the measurement efficiency is significantly improved, allowing for the rapid determination of the global resonance frequencies of the test sample. Next, a Continuous Scanning Laser Doppler Vibrometry (CSLDV) system was developed, replacing the previous point-by-point vibration measurement approach with a line-by-line measurement technique. It enables the simultaneous measurement of vibration data along the entire line segment, resulting in a significant improvement in measurement efficiency while maintaining a good level of accuracy.