Bilateral signal flow

From the fact that one of the variables on a port is an independent variable of the constitutive relation and the other one a dependent variable, it follows that the computational direction of the two conjugate variables at a bond is always bi-directional and can in principle be expanded into two opposite signals, in other words, a bond can be considered a bilateral signal flow. The effort is the input of the multiport node at the one side and the output of the node at the other side. Only after a particular choice is made about the two possibilities of these directions, this can be represented by causally augmenting the (multi)bond with a so-called causal stroke, a little line drawn orthogonal to the bond at the end of the bond where the effort serves as an output of the bond from a computational point of view. This implicates that it serves as an input for the port that is connected to that side of the bond. Similarly the conjugate flow at that side of the bond is an output of this port and an input to... 

In the first case the effort and flow can be considered bilateral signals at the port (cf. Fig. 1.4). Consequently, the bond connected to that port represents a bilateral signal flow and at the port at the other side of the bond, the roles of effort and flow in the constitutive relation of that port are reversed. 

Qiu, H.-H. and Sommerfeld, M., The impact of signal processing on the accuracy of phase-Doppler measurements. Proc. 6th Workshop on Two-Phase Flow Predictions, Erlangen 1992, (Ed. Sommerfeld, M ), Bilateral Seminars of the International Bureau Forschungszentrum Julich, 421-430 (1993)... 

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