Detection of pistachio nuts with closed-shells using impact acoustics.

An acoustical sorting system was developed to separate pistachio nuts with closed shells from those with open shells. The system includes a microphone, digital signal processing hardware, material handling equipment, and an air reject mechanism. It was found that upon impact with a steel plate, nuts with closed shells emit sound with higher signal magnitudes for the first 0.33 ms than do nuts with open shells. After this interval, nuts with closed shells emit sounds with lower signal magnitudes than those with open shells. Linear discriminant analysis was used to classify nuts using three features extracted from the microphone signal during the first 1.4 ms after impact. One of the discriminant features is the integrated absolute value of microphone output signal during the first 0.11 ms after impact. The other two features are the number of data points in the digitized microphone signal, between 0.6 and 1.4 ms after impact, that have a slope and signal magnitude below preset threshold levels. The classification accuracy of this system is approximately 97%. Throughput rate is approximately 40 nuts/s. Cost is about $7,000 to $10,000 per channel. This cost is much lower than that of color sorters used to remove other pistachio defects while throughput is comparable. Currently, closed–shell pistachio nuts are removed by mechanical devices. These devices have a lower classification accuracy (95%) and damage kernels in open–shell pistachios by “pricking” them with a needle. The needle hole can give the appearance of an insect tunnel and cause rejection by the consumer. The newly developed system does not cause such damage. Increased sorting accuracy of the acoustic sorter, coupled with low cost, enables a payback period of less than one year.