Weakly coupled limit cycle oscillators can be reduced into a phase model using phase reduction approach, and the phase model itself is helpful to analyze a synchronization. For example, phase model of two oscillators is one-dimensional differential equation for the evolution of a phase difference, and an existence of fixed points determines frequency-locking solutions. By treating each oscillator as a black-box possessing a single-input single-output one can investigate various control algorithms to change the synchronization of the oscillators. In particular, we are interested in a delayed feedback control algorithm, which applied to oscillator after the phase reduction gives the same phase model as of the control-free case, yet a coupling strength is rescaled. The conventional delayed feedback control is limited to change a magnitude but not a sign of the coupling strength. In this work we present modification of the delayed feedback algorithm supplemented by an additional unstable degree of freedom, which is able to change the sign of the coupling strength. Various numerical calculations performed with Landau-Stuart and FitzHugh-Nagumo oscillators show successful switching between an in-phase and an anti-phase synchronization using provided control algorithm. Additionally we show that the control force becomes non-invasive if our objective is a stabilization of an unstable phase difference for two coupled oscillators.
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