{"id":7774,"date":"2018-07-09T16:03:37","date_gmt":"2018-07-09T21:03:37","guid":{"rendered":"https:\/\/blogs.mathworks.com\/simulink\/?p=7774"},"modified":"2018-07-09T16:03:37","modified_gmt":"2018-07-09T21:03:37","slug":"visualizing-fixed-point-algebraic-loops","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/simulink\/2018\/07\/09\/visualizing-fixed-point-algebraic-loops\/","title":{"rendered":"Visualizing Fixed-Point Algebraic Loops"},"content":{"rendered":"

A few days ago I had to debug a very large model generating an error similar to the following:<\/p>\n

\"Algebraic<\/p>\n

In my case, the loop involved more than a hundred blocks and signals spread everywhere in the model and combined in buses, which was making it complex to analyze.<\/p>\n

ashow (And why you don't need it anymore!)<\/strong><\/p>\n

For many years, I have been using the ashow<\/tt><\/a> function to highlight algebraic loops. Here is an example of how it works for a simple model:<\/p>\n

\"Algebraic<\/p>\n

However, in addition to a few usability challenges, one major difficulty with ashow<\/tt> is that it can only highlight a subset of algebraic loops - and, of course, it cannot highlight the one I needed to solve.<\/p>\n

The limitation affecting me was that the algebraic loop was made of fixed-point signals. This led to a sort of chicken-and-egg problem. For ashow<\/tt> to work, it needs the model to be compiled. But since the loop is made of fixed-point signals, the Simulink engine knows that it cannot solve the loop, and errors out during model compilation, making it impossible to use ashow<\/tt>.<\/p>\n

That's when I remembered a relatively new function that I tend to forget: Simulink.BlockDiagram.getAlgebraicLoops<\/tt><\/p>\n

Simulink.BlockDiagram.getAlgebraicLoops<\/strong><\/p>\n

If you read the documentation page on algebraic Loops<\/a> attentively, you might have noticed that Simulink now offers the function Simulink.BlockDiagram.getAlgebraicLoops<\/tt>, a more modern and powerful way to highlight algebraic loops.<\/p>\n

For the same example as above, here is what it looks like:<\/p>\n

\"Loop<\/p>\n

This function was introduced in R2015a, and in R2017b it was enhanced to be able to find and highlight more types of algebraic loops. In my case, it allowed me to highlight a loop made of fixed-point signals. Seeing the loop allowed me to figure out the best place to introduce a delay and break the loop.<\/p>\n

A few things to note:<\/p>\n