Abstract
This study showed that an image data acquisition system connecting a high-speed camera or webcam to a notebook or personal computer (PC) can precisely capture most dominant modes of vibration signal, but may involve the non-physical modes induced by the insufficient frame rates. Using a simple model, frequencies of these modes are properly predicted and excluded. Two experimental designs, which involve using an LED light source and a vibration exciter, are proposed to demonstrate the performance. First, the original gray-level resolution of a video camera from, for instance, 0 to 256 levels, was enhanced by summing gray-level data of all pixels in a small region around the point of interest. The image signal was further enhanced by attaching a white paper sheet marked with a black line on the surface of the vibration system in operation to increase the gray-level resolution. Experimental results showed that the Prosilica CV640C CMOS high-speed camera has the critical frequency of inducing the false mode at 60 Hz, whereas that of the webcam is 7.8 Hz. Several factors were proven to have the effect of partially suppressing the non-physical modes, but they cannot eliminate them completely. Two examples, the prominent vibration modes of which are less than the associated critical frequencies, are examined to demonstrate the performances of the proposed systems. In general, the experimental data show that the non-contact type image data acquisition systems are potential tools for collecting the low-frequency vibration signal of a system.
Highlights
Because of the rapid development of Web sites and computer software and hardware, peripheral devices such as electret condenser microphones [1] and webcams are becoming increasingly popular.In addition, low-level high-speed digital video cameras with a price and frame speed of less thanUS$3,000 and 300 FPS, respectively, are common products
The experimental data show that the non-contact type image data acquisition systems are potential tools for collecting the low-frequency vibration signal of a system
Any small region of a series of resulting image frames involves the following temporal information: intensity variations of all incident light rays reaching the point of interest, the surface motion of the target, and the motion of the camera [7,8,9]. These obtained image data are frequently composed of the medium, significant surface movements, and all of the prominent vibration signals
Summary
Because of the rapid development of Web sites and computer software and hardware, peripheral devices such as electret condenser microphones [1] and webcams are becoming increasingly popular. Any small region of a series of resulting image frames involves the following temporal information: intensity variations of all incident light rays reaching the point of interest, the surface motion of the target, and the motion of the camera [7,8,9] These obtained image data are frequently composed of the medium, significant surface movements, and all of the prominent vibration signals. A single pixel can resolve only 0 to gray levels of red, green, and blue, separately [6,7,8,9,10] Such a low degree of resolution is sufficient for the human eye to detect image information.
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