# Bond Graphs

20-sim was the first commercially released software package to support bond graph modeling. The first version of 20-sim was released in 1995. Since then a continuous effort to improve bond graph modeling has made 20-sim the number one software package for bond graph modeling. Bond graphs are a network-like description of physical systems in terms of ideal physical processes. With the bond graph method, the system characteristics are split-up into an (imaginary) set of separate elements. Each element describes an idealized physical process. To facilitate drawing of bond graphs, the common elements are denoted by special symbols.

#### Library

20-sim has a large library containing all standard bond graph elements. Next to standard elements 20-sim supports custom user made bond graph models.

*From the Library Browser (left) you can drag and drop elements into the Editor (right).*

#### Ports and Multiports

The foundation of bond graph modeling is the use of ports. 20-sim allows you to create user defined models with an arbitrary number of power ports and signals. Ports sizes can be 1 (default) or larger (multiports). For every port you can specify the causality as fixed preferred, indifferent or depending on the causality of other ports.

#### Causality

Causal strokes indicate the direction of the efforts and flows in a bond graph model. In 20-sim, you only have to enter the equations in one of the possible causal forms. If causality is changed, the equations are rewritten automatically. 20-sim shows causal strokes in black color for preferred causality and in causal strokes in orange color for non-preferred causality. The causality of a complete model is derived automatically but can be changed manually.

#### Algebraic Loops and Differential Causality

Algebraic loops and differential causalities are traced automatically. If possible, 20-sim will rewrite the equations symbolically to remove algebraic loops and differential causalities.

#### Custom Made Models

In 20-sim you can create your own bond graph models and save them in your own model library. Models can have an arbitrary number of ports, input and output signals. A specialized drawing editor can be used to give the models any kind of representation.

#### State Space

Any bond graph can be transformed automatically to a linear state space model. The result is shown in the Linear Systems Editor where you can show the resulting Bode Plot or export the state space model as a Matlab M-file or S-function. If the bond graph model contains non-linear elements, the transformation is performed using linearization in a user defined working point.

#### Read More

More information on bond graphs can be found in the blog section and in the 20-sim webhelp.