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The power of a bond is positive when both effort and flow have a positive value or both effort and flow have a negative value (power = effort × flow).

 

To denote the orientation, i.e. the direction of positive power, we use the harpoon at the end of a bond. An element with an inward bond connected, consumes power when the product of effort and flow is positive.

 

bond

A bond as power connection.

When two elements are connected by a bond, one element will always determine the effort, while the opposite element will always determine the flow. The element that determines the effort, gets an enforced flow from the other element.

 

We can therefore also see a bond as a bilateral signal connection (effort-signal and flow-signal), of which the directions are opposite to each other.

 

With direction we mean the direction of the flow of information, just like in a block diagram. In a block diagram the effort and flow variables, which together form the flow of power, are not shown as a couple. This breaks up the symmetry between the physical system and the the structure of the model.

 

BilateralSignalBondGraph

A bond as a bilateral signal flow.

The interpretation of a bond as a bilateral signal flow, does not fix the individual direction of the effort and the flow. It only means the direction of the effort and flow are opposite. For the derivation of a simulation model out of a bond graph however, the individual directions are of importance.

 

To indicate the individual direction of the effort and flow, we use a small stroke (causal stroke) perpendicular to the bond. This stroke indicates the direction of the effort. The direction of the flow is opposite.

 

The choice of the direction of signals, also known as causality, depends on the element that is connected to the bond.