Table 4.

The features used for the identification of collision direction.

Features	Function Equation
Geometric Appearance	$q_{M2},q_{M3}$
Vibration Frequencies	${\widehat{\omega }}_{Di}(i=1,2,\cdots ,m)$
Relative modal between test direction of contact position	${\widehat{\varphi }}_{cx,j,i}/{\widehat{\varphi }}_{cy,j,i},{\widehat{\varphi }}_{cx,j,i}/{\widehat{\varphi }}_{cz,j,i},(i=1,2,\cdots ,m)$