<p>FMI/Model-exchange is targeted at the distribution of models (systems of differential algebraic equations), whereas FMI/Co-Simulation targets the distribution of models along with an appropriate solver.</p>

<p>Due to the many challenges in coding solvers with an appropriate support of rollback, it is hard to come by exported FMUs that can be used in a strongly coupled co-simulation.</p>

<p>So, to answer your question: it depends on the scenario. If you wish to simulate a strongly coupled physical system using FMI/Co-simulation, and you wish to do so with multiple FMUs, it better be that these support rollback, to avoid stability issues. If you have, for example, a scenario where one FMU simulates the physical system, and another FMU simulates a controller, then you may do well with a loose coupling approach.</p>

<p>It is hard to pinpoint exactly how strongly coupled two FMUs need to be before you need to apply a stabilization technique.
Have a look at the following experiment, which compares a strong coupling master with a loose coupling one.
Both master are used for the co-simulation of a strongly coupled mechanical system:
<a href="https://github.com/into-cps/case-study_mass-springer-damper" rel="nofollow noreferrer">https://github.com/into-cps/case-study_mass-springer-damper</a></p>

<p>Also, see the following report (disclosure: I contributed to it :) ) for an introduction to these concepts:
<a href="https://arxiv.org/pdf/1702.00686v1" rel="nofollow noreferrer">https://arxiv.org/pdf/1702.00686v1</a></p>
Functional Mockup Interface (FMI): Loose vs. strong coupling
<p>I am new to the topic of co-simulation. I am familiar with the definitions (based on Trcka "COMPARISON OF CO-SIMULATIONAPPROACHES FOR BUILDING ANDHVAC/R SYSTEM SIMULATION "):</p>

<ul>
<li>Quasi-dynamic coupling, also called loose coupling,
orping-pongcoupling, where distributed models run in sequence, and one
model uses the known output values, based on the values at the previous
time steps, of the coupled model.</li>
<li>Fully-dynamic coupling, also called strong coupling, oronion coupling,
where distributed models iterate withineach time step until the error
estimate falls within a predefined tolerance.</li>
</ul>

<p>My question: Is FMI/co-simulation a loose coupling method? What is FMI/model-exchange? From my understanding, it is not a strong coupling method. Am I understanding it correct that in model-exchange, the tool that imports the FMU is collecting all ODE and algebraic equations and the tool solve the entire system with a single solver. So it is more a standard to describe models in a unified way so that they can be integrated in different simulation environments?</p>

<p>Thank you very much for your help</p>