“…Unlike other feature extract methods, it can not only guarantee the global optimal solution but also obtain a series of features with varying rate from small to large. Owing to an input signal x ( t ) = [ x 1 ( t ), x 2 ( t ), ⋯ x m ( t )] T with m dimension, the SFA aims to determine a function g ( x ) = [ g 1 ( x ), g 2 ( x ), ⋯ g m ( x )] T , in which the feature s ( t ) = g( x ( t )) varies as slowly as possible, that is, to minimize under the constraints where 〈•〉 t and represent mean and the derivative of s with respect to time, respectively. Constraints exclude the constant solution and ensure that the solutions are independent.…”