Non-destructive detection of Korla pear stiffness based on acoustic vibration measurement

Abstract: Korla pear is one of the characteristic fruits in Xinjiang, and it has a wide international market. The quality of pear is related to its economic value. An effective and inexpensive non-destructive detection method is needed to reliably evaluate the internal quality of Korla pear. Stiffness is one of the most important indices to indicate fruit internal quality. Traditionally, Korla pear stiffness is quantitatively measured with the method of M-T test. This method is not suitable for commercial application because it can destroy the fruit when measuring. Thus, this study presents a measuring device based on the acoustic impulse response of Korla pear for non-destructively determining its stiffness. The measuring device consists of vibration control and dynamic signal analyzer, voltage amplifier, software system of vibration measurement and analysis, and test bench installed with 2 piezo beam transducers (one operates as an actuator, and the other as a sensor). In order to simulate hammer tapping signal, a positive half-sinusoid pulse with a peak amplitude of 2.5 V was generated by the vibration control and dynamic signal analyzer. This output signal was amplified by the voltage amplifier, which offered an impulsive excitation with an 80 V peak amplitude to the actuator, which was in contact with pear. Then the sensor, which was positioned at the opposite side of the actuator and also in contact with pear, detected the response signal of fruit. Both the excitation and response signal were acquired by the signal analyzer. The resonance frequency was obtained by the FFT of the response signal with the software system. The sound propagation velocity was calculated from the distance of the 2 contact points between pear and sensors divided by the lag time between excitation and response signal, which was determined by the cross correlation analysis. The fruit stiffness could be measured by the resonance frequency or sound propagation velocity. Different stiffness detection models were acquired by the linear regression analysis of the relationships between the stiffness obtained by resonance frequency and sound propagation velocity detection method and the stiffness measured with M-T method. The sensitivities of the different stiffness detection models were tested. Also, the discriminating rates of normal pear and rough-skinned pear by different stiffness detection models were compared. The results of repeated excitation at the same point showed that the acquired response signal from the system was stable and reliable. The differences of the resonance frequency and sound propagation velocity detected from different sensing points at the pear equator were not significant (P>0.05). All the stiffness detection models showed a good correlation with high correlation coefficient. The correlation coefficient (r=0.938) of the model based on resonance frequency detection stiffness combined with sound propagation velocity detection stiffness was significantly higher than the model based on resonance frequency detection stiffness (r=0.841) or sound propagation velocity detection stiffness (r=0.877), and its sensitivity was also the highest among the 3 models, which reached 67.30% and was very similar to that of the M-T method (68.49%). When the normal pear and rough-skinned pear were detected with this model, the discriminating rate was 86.7%, which was higher than that of the model based on resonance frequency detection stiffness (78.3%) or sound propagation velocity detection stiffness (80.0%). The results have proved that the acoustic vibration measurement is feasible for non-destructive detection of Korla pear stiffness, and can provide theoretical support for the commercial application of the new detection method.