algebraic loop warning despite using algebraic constraint block

I have a model that unfortunately has an algebraic equation, which would like to an algebraic constraint. I don't really want to add an artificial transfer function since it reduces the simulation speed. I also have tricky issues with initial conditions, etc.

I tried using an algebraic constraint block, but the warning persists. In desperation, I copied the model from the Matlab documentation http://www.mathworks.com/help/simulink/slref/algebraicconstraint.html to see any example working.

To my surprise, I still get a warning. I am using Matlab R2014b. Does anyone have insight why the system is still giving me a warning?

Many thanks, Josh

Answer 1

[Robert]

That warning is simply telling you that you have an algebraic loop. I imagine that algebraic loops are created more often by accident than on purpose. It sounds like you know what you are doing and have put the algebraic loop there on purpose. You can silence the warning with

set_param(myModel,'AlgebraicLoopMsg','none') % or 'warning' or 'error'

[Joshua]

In the original model, perhaps, I would tolerate an algebraic loop, but the run time can be higher and unstable. In any case, my question is that the algebraic constraint block is supposed to eliminate the algebraic loop and solve it correctly/efficiently. Unfortunately, this is not the case, even when using Matlab's example.

[Robert]

By my understanding, the Algebraic Constraint block does not (and is not supposed to) eliminate algebraic loops. Instead, it exists to help the algebraic loop solver to converge more quickly on a solution in the case of a real, intentional algebraic loop.

Admittedly, this is a bit confused by the documentation's arrangement, in which the brief discussion of the Algebraic Constraint block appears in the Remove Algebraic Loops section; however, the content does not relate to removing algebraic loops but offers the Algebraic Constraint block as an alternative to removing the loop, "when you think the loop is legitimate."