A new method for identifing and controling, in transitional regime, an unknow system excited by an unknow strength is presented. This problem is particularly accurate for example for mechanical structure in water for which transfert functions are generally unknown (or too complex due to fluid/structure coupling). Furthermore, the ambiantal forces are often unknow or at least very instational. The originality of the method is in the identification phase because of the non-knowledge of both excitation and system. This identification seems impossible, the number of unknows (strength and system) being too high. In fact, for a one degree of freedom dynamic system excited by a monochromatic strength, we show that the problem could be solved. After having identified the system and the force by analysing the very beginning of the transitional response, under optimal control can be successfully applied far before the end of theoretical transitional regime. The response of the system is analysed both in time and frequencial domain. In this last one appear the caracteristic frequencies of the system and the strength which can be separated. Coming down to the time domain by an inverse FFT after filtering, the damping coefficient is evaluated. So, all the caracteristics of the system and the strength are determined and control can be applied. The quality of the identification and the control is studied in simulated cases. In most of them, the quality of the identification and of the control permits to reduce the response of more than 50 %.
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