4. Mode coupling

This mechanism is based on mechanical modeling with 2 or more DDLs. Typically, the model has one degree of freedom in the tangential direction, as in the case of saccadic sliding or yawing described in the previous sections, plus at least one degree of freedom in the normal direction. If these degrees of freedom have identical natural frequencies, then mode coupling can come into play.

Before explaining the phenomenon of mode coupling on a minimalist 2-degree-of-freedom model, we'll look back at the phenomena that may be responsible for the degree of freedom in the normal direction.