{"id":21,"date":"2008-07-03T21:30:00","date_gmt":"2008-07-04T02:30:00","guid":{"rendered":"https:\/\/blogs.mathworks.com\/seth\/2008\/07\/03\/how-did-i-get-an-algebraic-loop-error-when-the-diagnostic-was-set-to-warning\/"},"modified":"2018-01-18T16:15:05","modified_gmt":"2018-01-18T21:15:05","slug":"how-did-i-get-an-algebraic-loop-error-when-the-diagnostic-was-set-to-warning","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/simulink\/2008\/07\/03\/how-did-i-get-an-algebraic-loop-error-when-the-diagnostic-was-set-to-warning\/","title":{"rendered":"How did I get an algebraic loop error, when the diagnostic was set to warning?"},"content":{"rendered":"

I once faced a problem where Simulink reported, \u201cCannot solve algebraic loops...\u201d<\/p>\r\n\r\n

\"Algebraic<\/p>\r\n\r\n

Algebraic\r\nloops<\/a> can be solved by Simulink, but often slow down the simulation. For\r\nthis reason, we have the algebraic\r\nloop diagnostic<\/a> that can be set to Error<\/em>, Warning<\/em> or None<\/em>. \r\nWhat was peculiar about this model was that the algebraic loop diagnostic was\r\nset to Warning<\/em>, yet the model would report an error, and not simulate! To\r\nmakes things more difficult, all I had to work with was a screen shot of the\r\nmodel, not the model it self. <\/p>\r\n\r\n

What are algebraic loops?<\/strong><\/p>\r\n\r\n

Algebraic loops exist when a variable shows up on both sides of the equation. For\r\nexample,<\/p>\r\n\r\n

\"y=y-2\"<\/p>\r\n\r\n

Algebraic loops generally occur in Simulink when there are un-modeled delays, for\r\nexample, sensors that just feed through a signal from input to output.<\/p>\r\n\r\n

Read the error message<\/strong><\/p>\r\n\r\n

The first step in debugging a problem like this is to read and understand the error\r\nmessage. Algebraic loop errors occur for different reasons. To understand the\r\ncause of the error I had to understand the message. <\/p>\r\n\r\n

Cannot solve algebraic loop involving 'model\/...\/Sensor System' because it consists of blocks that\r\n cannot be assigned algebraic variables, i.e., blocks with discrete-valued\r\n outputs, blocks with non-double or complex outputs, Stateflow blocks, or nonvirtual\r\n subsystems.<\/code>\r\n<\/p>\r\n\r\n

Picking apart this message, we learn that Simulink cannot solve algebraic loops unless\r\nit can assign an algebraic variable. The blocks that cannot be assigned\r\nalgebraic variables are listed in the error.<\/p>\r\n

\r\n