Nondestructive Detection of Pear Firmness Using a Laser Doppler Vibrometer

摘要

Mechanical vibration characteristics of fruits were correlated with their firmness. In this paper, the possibility of nondestructively detecting pear firmness by a laser Doppler vibrometer (LDV) was investigated. Pears were placed on a designed stagemounted on the top of vibration generator driven by swept sine wave signals ranging from 100 to 2000 Hz. The excitation and response signals were collected by an accelerometer and the laser Doppler vibrometer, respectively, and FFT algorithm was used toextract the vibration parameters, including phase shift values at 400, 800, 1200 and 1600 Hz, the second resonance frequency (f_2), and the peak height at f_2. The mass (m) and the firmness values near skin, in the medium and near core of each pear weremeasured. In addition, the elasticity index (El) which was defined as f_2~(2-) m was calculated. The results showed the firmness maintained at a relatively stable level, the f_2 and El showed a quasi-exponential decline, peak height at f_2 decreased andphase shift values at all selected frequencies increased gradually before day 24. All the vibration parameters didn't show obvious relationships with firmness in this period. After day 24, the pear firmness declined gradually, while the vibration parameters exhibited an opposite trend except the peak height compared with that before day 24. The firmness values showed linear correlation with vibration parameters except peak height. The second resonance frequency had highest correlation coefficient with firmness near skin (-0.7656), in the medium (-0.6390), near core (-0.2035) than other vibration parameters.