MATLAB: What does the algebraic variable mean within the algebraic loop warning

Question

When running a Simulink model, I receive a warning that the diagram contains an algebraic loop which consists of about 50 blocks. Five of those blocks have the statement "(algebraic variable)" following the block name. What does this mean?

Answer 1

This is best explained through an example. Create the following simple model:

sine ------> sum ------> gain of 1 ----->

              |                                                    | 
              ----------------------------------------

where the output of the gain is fed back into the sum block. If you run this model, you will get the following warning:

Warning: Block diagram 'test' contains 1 algebraic loop(s).

Found algebraic loop containing block(s):

'test/Gain'
'test/Sum' (algebraic variable)

The "algebraic variable" next to the sum block indicates that this is the offending block creating the algebraic loop. The gain block will not affect the algebraic loop in the least. However, it is listed so as to help the user trace the path of the algebraic loop.