I assume your system is nonlinear. The elastic element may present two

different behaviors, linearization (for small deviations) and asymptituc expansion (for large deviations). Then you might work with two different approximations, depending on the case, using some scheduling scheme to switch between them.

In fact, if you assume small deviations from equilibrium then linearization

should work all the time. Namely, you design your controller for the linear approximation of your system.