Forrester diagrams and continuous Petri nets: a comparative view

Forrester diagrams (FD) and Petri nets (PN) are formalisms introduced in the sixties to model complex systems. This paper explores similarities and differences between FDs and the continuous relaxation of the originally discrete PNs. Historically speaking, the approaches were quite different: the PNs paradigm was introduced at a very abstract level, without timing interpretation, while FDs led to a modelling methodology where the systematic simulation of a set of differential equations was the goal. Strict flow conservation around valves, non explicit fork and join operations, separation of information and material flows, are peculiarities of FDs. In PN models the existence of global conservation laws is a potential for structural analysis.