Delayed feedback control of periodic orbits in autonomous systems
  For controlling periodic orbits with delayed feedback methods the periodicity has to be known a priori. We propose a simple scheme, how to detect the period of orbits from properties of the control signal, at least if a periodic but nonvanishing signal is observed. We analytically derive a simple expression relating the delay, the control amplitude, and the unknown period. Thus, the latter can be computed from experimentally accessible quantities. Our findings are confirmed by numerical simulations and electronic circuit experiments 
Wolfram Just, Dirk Reckwerth, Johannes M\"ockel, Ekkehard Reibold, and   Hartmut Benner