Fig. 1(a) illustrates the causal loop diagram of the proposed control
mechanism. In SD discipline, causal loop diagrams serve as
preliminary sketches of causal hypotheses, simplify the representation
of a model and represent the major feedback mechanisms
(Sterman, 2000). These mechanisms are either negative feedback
(balancing) or positive feedback (reinforcing) loops. A negative
feedback loop exhibits goal-seeking behavior; after a disturbance,
the system seeks to return to an equilibrium situation. In a positive
feedback loop an initial disturbance leads to further change,
suggesting the presence of an unstable equilibrium. To improve