Vision-based modal testing system for hyper-nyquist frequency range using external triggering

Abstract

Modal analysis is used to describe the dynamic characteristics of structures using the modal parameters. Modal testing system can be utilized in a variety of applications and ways, depending on the sensor that measures vibration. Recently, numerous studies in the field of non-contact measurement techniques have been carried out. Among the development of image processing techniques, vibration measurement using vision information becomes popular. Vision-based vibration measurement has important advantages. Displacement response of a entire surface can be obtained by single measurement and mass of the sensor has no effect on a system. However, using camera vision information directly for the modal analysis, there are critical problems. Because the sampling rate is too insufficient as compared with the conventional contact type sensor. The sampling rate is determined by the limited frame rate of the camera hardware. Therefore, Higher-order modes cannot be measured when using a limited frame rate according to the Nyquist theorem. In order to solve this problem, we proposed a new measurement method using an external triggering signal obtained with time-phase transformation. A feature of this method is to measure response of a structure according to phase of excitation signal. To implement the proposed method in hardware, we used a FPGA board which is widely used in various fields. Internal logic of FPGA board can be designed in parallel, making it suitable for time critical systems. In this modal testing system, all signals are acquired using phase-based event triggering of excitation signals. And it can be used with any image processing that extracts displacement from vision information. As a simple experiment on the cantilever beam to validate the proposed system, the modal analysis of the hyper-Nyquist frequency range was performed.