Sensors 2017, 17, 789 7 of 13 Sdenesfoorrsm20a17ti,o1n7, 7a8n9d friction signal resides. Therefore, in the following work, the EMD is employ7eodf 1to3 extract the signals with frequencies higher than 90 kHz. FFiigguurree 44.. DDee--nnooiissiinngg rreessuullttss bbyy eemmppiirri...