Synchronization of two self-excited double pendula
  We consider the synchronization of two self-excited double pendula. We show that such pendula hanging on the same beam can have four different synchronous configurations. Our approximate analytical analysis allows us to derive the synchronization conditions and explain the observed types of synchronization. We consider an energy balance in the system and describe how the energy is transferred between the pendula via the oscillating beam, allowing thus the pendula synchronization. Changes and stability ranges of the obtained solutions with increasing and decreasing masses of the pendula are shown using path-following. 
Piotr Koluda and Przemyslaw Perlikowski and Krzysztof Czolczynski and   Tomasz Kapitaniak
physics.class-ph nlin.CD