Low-frequency vibration monitoring system measuring the fringe shift by using linear Fresnel zone plates for shiny...

This paper presents a vibration monitoring system that measures the fringe shift from the Fresnel diffraction pattern for noncontact measurement of translational and angular vibrations. Conventional vibration systems have some major limitations in practical applications. For example, a laser Doppler vibrometer (LDV) fails to accurately measure large-amplitude vibrations of a mirrored object undergoing dynamic tilts or rotations. Additionally, achieving the accuracy required for micro/nanomeasurement in precision machines when measuring low-frequency vibrations (LFV) remains a significant challenge. In this study, the vibration measurement system utilizes a linear Fresnel zone plate with parallel strip lines to create the diffraction pattern. Surface vibrations cause shifts in these lines, and the resulting fringe pattern on the vibrating surface carries information about the local vibrational amplitude and frequency. To achieve this, the linear vibration and displacement of the object are accurately verified using a linear motion platform and a rotation stage, with accuracies of approximately 20 nm and 0.002°, respectively. Rotational vibration measurements utilize tracking beam deflection through the image fringe shift measurement method, while out-of-plane displacement is determined both from this method and from changes in the period density of diffraction patterns, which depend on the position of the patterns and the local spatial frequency. A CCD camera captures sequences of images of the deflected fringes due to vibrating object. The system can detect linear vibrations with frequencies above 251.91 µHz and has a displacement uncertainty of approximately 5.8 µm, achieving an angular resolution of 37.67 µrad. Compared to conventional LDV vibrometers and eddy current sensors, the proposed method offers an effective and accurate technique for measuring LFVs of shiny surfaces. Furthermore, it provides a significantly extended measurement range for both translation and rotational angles of objects in engineering applications.