Simulation in time domain allows predicting the behaviour of a system, and nowadays it is not only necessary but also essential if a good design is to be accomplished at reduced cost and time. Nevertheless, simulation in time domain acts as reaction: it answers the question "How will the system behave in other working conditions?", and the answer to this question is essential for a good design, but sometimes is not enough for optimising its behaviour. For optimising the response, working in the frequency domain yields very useful results, so that expressions that allow studying the influence of the design parameters are obtained. In this paper, a method for obtaining symbolically the transfer functions of a model, once its Bond Graph representation has been obtained, is presented. The method is based in what was initially presented by Rosenberg and Karnopp, (Rosenberg, 1983), and contributions by Redfield and Krishnan (Redfield, 1990 and Redfield, 1992) and Granda and Reus (Granda, 1997).
