Which nonlinear Observer to study to estimate the speed of the Plant?

Question

I have designed the mathematical model of the plant with nonlinear hystersis function $f(x_1)$ and is validated using simulation. Now I want to design the nonlinear observer to estimate the speed (that is $x_2$). Not that I have also modeled the nonlinear function $f(x_1)$ in the model. My state space model of the plant is $$\begin{aligned} \dot{x}_1 & = x_2;\\ \dot{x}_2 & = q*x_1 + c*x_2 + f(x_1) + u. \end{aligned} $$ Please suggest suitable observer with known nonlinearity to estimate the angular speed $x_2$. $x_1$ is the angular position of the plant. $q$ and $c$ are constants.