Co-Simulation Fundamentals

Co-Simulation is when two or more modeling tools run concurrently, exchanging data between the tools. Co-Simulation is desirable when a single tool cannot achieve either the fidelity or execution speed required to model a given element.

Types of co-simulation

There are two primary types of co-simulation: imported and networked.

  • Imported: The “primary” tool incorporates the “secondary” models into its’ framework. The primary tool is responsible for the execution and timing of the secondary tool.
  • Networked: In the networks, case the tools execute independently with a secondary program providing the data exchange layer between the tools. The data exchange layer is responsible for matching the time stamps for the tools.

Why Imported?

If your primary tool has a method for importing third party executables, like S-Functions in Simulink, then this is generally the easiest method for performing co-simulation. Once the functional interface is defined, the simulation engine of the primary tool provides the full execution context.

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The downside of this approach is that it’s generally less accurate than the networked option. This is most acute when either of the tools requires a variable step solution as the incorporated tools are most often run at a fixed step or a variable step set by the primary tool.

Why Network?

In contrast, the networked approach allows you to run each tool with the optimal step size for the model; this results in high accuracy but is more often than not, much slower. The second issue with this approach is, how do you synchronize the tools?

In general a 2nd or 3rd order spline should be used to match the data points between the tools for the different time steps. This means that the integration tool may need to store large amounts of data and perform significant calculations at each data exchange.

Before you start

Co-Simulation, regardless of the approach is a significant investment of time. Before adopting this methodology determine if you need this level of fidelity or speed.

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