# What are Bond Graphs?

Bond graphs are a graphical notation of physical systems. They are composed of half arrows (called bonds) representing the flow of energy flow between elements (Se, C, 0, R). These element respresent the generation, storage, distribution and dissipation of enery. Bond graphs are used to model and simulate physical systems.

If you look at the car suspension (left picture), we can represent the car physics (middle picture) as a mass (car body) connected with a spring/damper (suspension), mass (tyre mass), spring (tyre flexibility) and displacement (surface). In a bond graph notation (right picture), the masses are represented by the symbol **I**, the springs are represented by a **C**, the damper by an **R** and the surface by **Sf**. The symbols **1** and **0** represent the distribution of energy.

#### What are Bond Graphs used for?

Bond graphs are used to represent the physics of systems.In the pre-computer age they were used to derive mathematical equations and do hand calculations. Nowadays their main use is to model physical systems and use computer programs like 20-sim to perform simulations.

#### What is the advantage of Bond Graphs?

- Bond graphs are based on a formal notation, well founded in mathematics. They can be constructucted following logical steps, which makes them suited very well for modeling complex physics.
- Bond graphs models can by simulated using software packages like 20-sim.
- Bond graphs are domain neutral. The same elements are used for every physical domain (mechanics, hydraulics, ..). This makes bond graphs excellent for modeling multi-domain systems.
- Bond graphs use a small set of basic elements that have the same meaning in every country on our planet. This makes bond graph models crystal clear.

#### What is the disadvantage of Bond Graphs?

- Bond graphs are not part of the standard courses in polytechnics and universities. This means few people understand them.
- Bond graphs are difficult to learn.
- Bond graphs are difficult to interpret because of the same symbols that are used in every domain.

#### What are Bond Graphs suited for?

Because of their formal mathematical definition, bond graphs can be derived for every physical system, following the same logical steps. Because bond graphs are based on energy flows, bond graph models can be verified using energy balances. This makes them beloved by scientist and engineers who have to model complex fysical processes and components.