Hidden strange nonchaotic attractors in a piecewise-smooth system without equilibrium points

The origin and generation mechanisms of hidden strange nonchaotic attractors (HSNAs) in non-smooth systems remain an open research question. This paper reports the emergence of HSNAs in a quasiperiodically forced piecewise-smooth system without equilibrium points. It is demonstrated that the truncation of bi-period-doubling bifurcations serves as the underlying mechanism generating distinct types of HSNAs. Three principal formation routes are identified and characterized: the Heagy-Hammel route, the fractalization route, and the intermittency route. Within two key tongue-shaped regions in the parameter space, different HSNA formation mechanisms are investigated and verified. The identified HSNAs exhibit characteristic non-positive Lyapunov exponents and significant phase sensitivity. Furthermore, these different types of HSNAs are also characterized by singular continuous spectrum, Fourier transform and distributions of finite-time Lyapunov exponents. This work advances the understanding of HSNAs formation in non-smooth systems and provides a theoretical basis for analyzing hidden complex dynamics in engineering applications subject to discontinuous forcing.