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Judging the ripeness of watermelons on the farm is difficult because the
outer skin looks much the same whether the fruit are ripe or not. Watermelons
don’t ripen much after picking, and hard green melons are picnic disasters. Now
four mechanical engineering students at the University of Delaware have designed
an automated sensor that spots ripe melons by thumping them with a
pendulum-mounted mallet. A laptop computer analyses the sounds, spotting echoes
at frequencies which indicate the inside is soft, sugary and ripe. A prototype
was able to spot a ripe watermelon accurately in 12 seconds.