Hidden Degeneracies in Piecewise Smooth Dynamical Systems

When a flow suffers a discontinuity in its vector field at some switching surface, the flow can cross through or slide along the surface. Sliding can be understood as the flow along an invariant manifold inside a switching layer. It turns out that the usual method for finding sliding modes – the Filippov convex combination or Utkin equivalent control – results in a degeneracy in the switching layer whenever the flow is tangent to the switching surface from both sides. We derive the general result and analyse the simplest case here, where the flow curves parabolically on either side of the switching surface (the so-called fold-fold or two-fold singularities). The result is a set of zeros of the fast switching flow inside the layer, which is structurally unstable to perturbation by terms nonlinear in the switching parameter, terms such as (signx) [where the superscript does mean “squared”]. We provide structurally stable forms, and show that in this form the layer system is equivalent to a generic singularity of a two timescale system. Finally we show that the same degeneracy arises when a discontinuity is smoothed using the standard regularization methods.