12 Preliminary Characterization of a Plastic Piezoelectric Motor Stator Using High-Speed Digital Holographic Interferometry 91 Fig. 12.2 Plastic stator and high-speed digital holographic (HDH) system on an optical table setup for quantitative imaging with nanometer resolution in full field to observe motor motion from startup to stability Fig. 12.3 High-speed digital holographic imaging of a plastic stator operating at 6.788 kHz and 240 V under normal operation mode. From (a)∼(f), rotatory motion along the stator surface can be observed. The time interval between consecutive images is one frame, which is approximately 14.9μs 12.3 Experimental Results We tested two equivalent manufactured custom plastic stators using the high-speed digital holographic (HDH) system. The two stators were operated in the second mode resonant frequency, respectively, at 6.788 kHz and 6.980 kHz, which yields approximately 10 frames per stator excitation cycle. Figure 12.3a–f shows 6 sequential frames from one of the plastic stators during a single excitation cycle operating at 6.788 kHz frequency with its surface waves moving circumferentially along the stator surface. The time interval between each of the two images is one frame, which is equal to 14.9μs (1/67,000 s). The in-phase operation mode, which indicates the same sine waves-driven crystal setup, was tested and served as an experimental control. Figure 12.4a–f shows another equivalent manufactured plastic stator operating in an in-phase mode, since no rotating motion is expected or observed. The time interval is the same as the normal operation mode, which is 14.9μs aswell. Raw results from holographic images are shown in Fig. 12.5. The stator showed rotatory motion during the experiment. From Fig. 12.5a and b (peak out-of-plane displacement), the wave peak rotated approximately 15◦ solid line position in estimate versus baseline. The peak moved approximately another 15◦ reaching a total rotation of 30◦ as shown by solid line in Fig. 12.5c. Notice that the stator operating frequency was 6.788 kHz, which is not perfectly matched to the high-speed camera operating frequency but is approximately 10 camera frames per stator excitation cycle. Under these circumstances, there will be a few frames mismatch and that will cause the resulting images to appear discontinuous. The time-evolving out-of-plane displacement data was calculated from holographic images using the signal phase unwrapping methodology, and the result is shown in Fig. 12.6. The x-axis represents the frame number in the sequence, and one frame in the figure equals to 14.9μs. The y-axis represents the Z component out-of-plane displacement in nanometers, with baseline at 50 nm (no relative motion) and 100 nm peak-to-peak amplitude. The Z field displacement versus time (frame)
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