- freely available
Sensors 2016, 16(12), 2020; doi:10.3390/s16122020
2.1. Principle of Out-of-Plane Measurement
2.2. Principle of In-Plane Measurement
2.3. Principle of 3D Measurement
2.4. Principle of the Wavelet Transform Method and Signal Processing
3. Experiment and Results
3.1. Verification Experiment of One Color CCD Camera Light Separation Capability
3.2. The Experiment of the 3D Displacement Measurement
3.2.1. The Experiment
3.2.2. Multi-Detection Points Processing
- The error of the displacement in the x-direction is the minimum in the three directions. As shown in Figure 11b, the peak to peak values and shapes of the waves after the first one are almost the same. Moreover, the intervals between the waves are also identical. These led to the most uniform change of the phase processed by the wavelet transform. Hence, a better phase unwrapping results are acquired.
- The accuracy of the displacement in the z-direction is more than 1%. Unlike the common-path layout in-plane measurement, the object and the reference is located in a different path, which is easier to be disturbed by the environment. To improve the accuracy, heterodyne speckle pattern interferometry (HSPI) is expected to be employed in future work.
- The experiment adopts several points of the metal plate to measure the accuracy of the system which are regarded as measurements of the full-field deformation and displacement. The whole object deformation and displacement will be obtained by processing each point of the object with the same algorithm.
- The wavelet transform is used to analyze the speckle interferograms. Compared with the Fourier transform, it can show more detail to obtain more accurate displacement information.
- The measurement resolution of the system is less than half a wavelength, which is limited by many factors, such as the stability of the laser source, sampling rate of the CCD camera used, the disturbance of the environment, etc. In our case, the main factor is the phase unwrapping algorithm, particularly the selection of the mother wavelet. The monitored distance of the system from the object can be from a few meters to 10 meters away in the laboratory, mainly depending on the coherence length of the laser source. In theory, as long as the speckle interferograms can be recorded, the measurement can be completed successfully. However, it is difficult to do this outdoors due to the rapid change of background light intensity, vibrations, and air disturbance, which are common problems in interfering instruments. On the other hand, the camera vibrations caused by ground motion affect the measuring accuracy as well.
Conflicts of Interest
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