how can make a nonlinear model?

hi! i want to design active suspension system, but i dont have any idea to make nonlinear model. i have th and following dynamic equation. x1=xs-xsr, x2=d/dtxs, x3=xu-xur, x4=d/dtxu d/dtx1=x2; d/dtx2=f(x,t)+g(t)*u; d/dtx3=x4; d/dtx4=-ms(t)/mu*(f(x,t)+gr)-1/mu*(u+kt(x3+xur-z)+ct(x4-d/dtz)+mugr); where f(x,t)=1/ms(t)[-k1s(x1-x3+delo)-k2s(x1-x3+delo)^3]+1/ms(t)[-cs(t)(x2-x4)-ms(t)*gr]; g(t)=1/ms(t); ms=sprung mass; mu=unsprung mass;ks= spring coefficient; cs(t)= damper coefficint;kt and ct are respectively spring and damper coefficient of tyre model. u is the control force genreted by actuator, z denotes road disturbence, xs and xu is the vertical displacement of sprung and unsprung mass, gr is acceleration due to gravity.

[Oleg Komarov]

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