It's probably not even the pattern, just the drive frequency. The system is operated just off resonance, so getting the frequency right is really important. You

It's probably not even the pattern, just the drive frequency. The system is operated just off resonance, so getting the frequency right is really important. You can do this easily with feedback from the drive assembly and no NFC, or you can do with with NFC to tell you what's on the other end and just do it open-loop. The feedback method is cheaper (I think), so it's what was done previously. But if you want brush head ID, you go the other route....