Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany. klamt@mpi-magdeburg.mpg.de|Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany. klamt@mpi-magdeburg.mpg.de|Max Planck Institute for Dynamics of Complex Technical Systems, Magdeburg, Germany. klamt@mpi-magdeburg.mpg.de