We consider a piecewise smooth system in the neighborhood of a co-dimension 2 discontinuity manifold $Sigma$. Within the class of Filippov solutions, if $Sigma$ is attractive, one should expect solution trajectories to slide on $Sigma$. It is well known, however, that the classical Filippov convexification methodology is ambiguous on $Sigma$. The situation is further complicated by the possibility that, regardless of how sliding on $Sigma$ is taking place, during sliding motion a trajectory encounters so-called generic first order exit points, where $Sigma$ ceases to be attractive. In this work, we attempt to understand what behavior one should expect of a solution trajectory near $Sigma$ when $Sigma$ is attractive, what to expect when $Sigma$ ceases to be attractive (at least, at generic exit points), and finally we also contrast and compare the behavior of some regularizations proposed in the literature. Through analysis and experiments we will confirm some known facts, and provide some important insight: (i) when $Sigma$ is attractive, a solution trajectory indeed does remain near $Sigma$, viz. sliding on $Sigma$ is an appropriate idealization (of course, in general, one cannot predict which sliding vector field should be selected); (ii) when $Sigma$ loses attractivity (at first order exit conditions), a typical solution trajectory leaves a neighborhood of $Sigma$; (iii) there is no obvious way to regularize the system so that the regularized trajectory will remain near $Sigma$ as long as $Sigma$ is attractive, and so that it will be leaving (a neighborhood of) $Sigma$ when $Sigma$ looses attractivity. We reach the above conclusions by considering exclusively the given piecewise smooth system, without superimposing any assumption on what kind of dynamics near $Sigma$ (or sliding motion on $Sigma$) should have been taking place.
Download