Algebraic loops in the context of signal buses

Algebraic loop

Algebraic loops (for more information, refer to the algebraic loops documentation) are sometimes unavoidable in Simcenter Amesim, despite the best efforts of the engineers creating the models. There are specific methods that can be employed in order to break algebraic loops, and one of the essential tools for troubleshooting this type of issue is the ability to identify these loops in the first place.

In the legacy implementation of signal buses, a limitation exists which makes the management of algebraic loops very difficult. Indeed, any part of a sketch connected to signal bus components and containing an algebraic loop, will cause all of the signal bus components to be identified as having algebraic loops. This quickly becomes very troublesome in the presence of multiple algebraic loops connected to signal bus components.

Indeed, if N independent algebraic loops are present in a legacy sketch, all inputs and outputs involved in these N loops will be seen by Simcenter Amesim as a single loop, meaning that the very conservative assumption is made that all outputs in the looped circuit are dependent upon all inputs. This would make such a circuit very difficult to debug, and forces users to introduce loop-breaking measures (e.g. delays or first-order lags) in some places where they are not required.

This limitation no longer exists with the current implementation of signal buses. To illustrate this, consider the sketch below, containing two equivalent circuits: one constructed using legacy buses, and the other using the new implementation of signal buses.

Figure 414: Signal bus circuits containing algebraic loops: legacy vs. current

Here, the first input in each circuit is looped back into itself after passing through a gain component. Likewise, the second input is also looped in the same manner. The third input is simply a constant, which is extracted into a signal sink.

The two loops in each circuit are entirely independent. However, when compiling the model, the following algebraic loops highlighting is given for the legacy circuit:

Figure 415: Algebraic loop highlighting in legacy circuit

Note that despite being independent, both algebraic loops are grouped into one here. When the simulation code is generated, this visual loop is seen in the same way in terms of the dependencies between equations.

Observing the highlight for the new signal buses below, it is easy to see that each one is identified as a separate loop, and can thus be dealt with on an individual basis. This helps avoid costly measures for mitigating algebraic loop effects where not strictly necessary.

Figure 416: Algebraic loop highlighting in current circuit