Li, you may find an answer following a very usefull discussion on ¨What does "affine in the control" mean when used to describe a system?¨ Find it, very interesting.

A nonlinear system in which the input appears linearly, considering following examples:

1. dx/dt = x^2+2u (affine)

2. dx/dt=x^2+u^2 (nonaffine)

Hi, a very usefull discussion in the following link

https://www.researchgate.net/post/What_does_affine_in_the_control_mean_when_used_to_describe_a_system

Please refer to this article:

https://www.researchgate.net/publication/324609461_A_novel_second-order_nonlinear_differentiator_with_application_to_active_disturbance_rejection_control