The firmness of watermelon can be nondestructively evaluated by measuring the resonant frequency of the watermelon. The watermelon was excited by a wooden pendulum ball and the response signal was captured by an accelerometer. The FE models of watermelons were established and the optimum location of the force excitation and the suitable response measurement sensor for measuring the resonant frequency of watermelon were determined. The middle part on the fruit surface was suggested to be used while estimating the fruit firmness by the resonant frequency of the firsttype longitudinal mode. To measure the flexural mode, the directions of both the exciting force and sensory axis of the response measurements sensor should be kept in tangent to the fruit surface. The comparison between experimental results and the finite element estimated results confirms that the model is in reasonable agreement with real fruit. It was found that the Young's modulus was an adequate indicator for firmness, and the high correlation between the resonant frequency and Young's modulus existed, R2 = 0. 90. The results of this study may be helpful to develop the non-destructive firmness sensor for spherical fruits.%西瓜的硬度可以通过检测其固有频率得到.试验时,采用一个摆动的木球激励被测西瓜,产生的信号由加速度传感器采集,利用ANSYS 7.0建立了西瓜的有限元模型,并确定最佳激励点及适于测量其固有频率的传感器.试验结果表明,西瓜的杨氏模量与其固有频率之间具有较高的相关性,相关系数高达0.90,西瓜的硬度可以由其杨氏模量来表征,研究结果为进一步研究西瓜硬度无损检测装置打下了基础.
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