Non‐contact method based on intensity modulation of light for measurement of vibration of a thin cantilever beam

Abstract

A non-contact method based on intensity modulation of light for measurement of vibration of a thin cantilever beam (CB) is presented in this study. The system consists of a laser source, a photo-detector-based signal conditioning and comparator circuit, an ATmega8 microcontroller based data acquisition system and an interface with personal computer. The sensing principle is based on modulation of intensity of light falling laterally on a vibrating CB. The photo-detector detects the modulated light and generates pulses, which is processed by signal conditioning and comparator circuit. The output of the comparator consists of alternative high and low transistor–transistor logic pulses, interfaced to two pins of the microcontroller simultaneously; one to external event counter and the other to input wave capture. The microcontroller is used to measure the frequency, width of high and low pulses. The system is calibrated using a Tektronix AFG3251C Arbitrary Function Generator. The % error in measurement of width of pulses is found to be within the range +0.12 to −0.41%, whereas 0% measurement error is observed in the measurement of frequency. Experimental results obtained under free vibration tests of two samples of thin CB of 0.4 and 0.5 mm thickness are also presented.