Pre-filtered Hankel Total Least Squares method for condition monitoring of wet friction clutches

Wet friction clutches play a critical role in a transmission system. The clutches progressively degrade while the transmission is under operational condition. If the clutches unexpectedly fail, then the complete transmission will fail. To avoid unexpected failures, a proper condition monitoring tool extended with a lifetime prediction method must be implemented. In order to achieve this, appropriate parameters representing the degradation level must be determined. Our previous study reveals that tangential contact stiffness of the clutch in the post-lockup phase changes in function of the clutch degradation level. Accordingly, the degradation can be monitored via changes of the dynamic behaviour of a transmission which is expressed by changes of the torsional natural frequencies and damping ratios of the transmission. Things become more complicated by the fact that, since the applied normal load in the postlockup phase is not constant, the contact stiffness varies. As a consequence, the transmission behaves like a time-variant system. This behaviour is manifested by varying high torsional vibration natural frequencies. However, the dominant lowfrequency of the post-lockup torsional vibration signal is relatively constant. In this work, the dominant undamped natural frequency and the corresponding damping ratio are proposed as relevant features. The features are extracted by using pre-filtered Hankel Total Least Squares (HTLS) method. The results show that the proposed features exhibit clear trends related to the clutch degradation.