Vibration-based plastic-gear crack detection system using a convolutional neural network - Robust evaluation and performance improvement by re-learning oleh Kien Huy BUI, Daisuke IBA, Yunosuke ISHII, Yusuke TSUTSUI, Nanako MIURA, Takashi IIZUKA, Arata MASUDA, Akira SONE, Ichiro MORIWAKI