DETECTION OF PISTACHIO NUTS WITH CLOSED SHELLS USING IMPACT ACOUSTICS

Abstract

An acoustical sorting system was developed to separate pistachio nuts with closed shells from those with openshells. The system includes a microphone, digital signal processing hardware, material handling equipment, and an air rejectmechanism. It was found that upon impact with a steel plate, nuts with closed shells emit sound with higher signal magnitudesfor the first 0.33 ms than do nuts with open shells. After this interval, nuts with closed shells emit sounds with lower signalmagnitudes than those with open shells. Linear discriminant analysis was used to classify nuts using three features extractedfrom the microphone signal during the first 1.4 ms after impact. One of the discriminant features is the integrated absolutevalue of microphone output signal during the first 0.11 ms after impact. The other two features are the number of data pointsin the digitized microphone signal, between 0.6 and 1.4 ms after impact, that have a slope and signal magnitude below presetthreshold 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 pistachiodefects while throughput is comparable. Currently, closedshell pistachio nuts are removed by mechanical devices. Thesedevices have a lower classification accuracy (95%) and damage kernels in openshell pistachios by pricking them witha needle. The needle hole can give the appearance of an insect tunnel and cause rejection by the consumer. The newlydeveloped 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.