Resolving problems with Algebraic Loops in SIMULINK models

Question

MY PROBLEM

I have a SIMULINK model that has got a feeback loop aka Algebraic Loops that is causing error in the simulation. My original solver configuration was Fixed-Step with Dormand-Prince (Order 5). I also tried Fixed-Step with Order 3 (Runga-Kutta). It still has not resolved the issue.

MY QUESTION

Is there any way to resolve algebraic loop without altering the original performance or design of the circuit too much?

CLARIFICATION FOR THE QUESTION

I am sure that there will be a way to solve this. However, I don't want it to compromise the original performance of the circuit. Moreover, it is a customer-supplied data and it seems that they have managed to work this out as fine. I simply require somebody to point me to the right direction regarding how to solve this.

MY Approach so far

I tried to break the loop using:

i) Switch (if ip = 0, op = 0; if ip = 1, op = 1) I know it is stupid but it is a different block that breaks the loop.

ii) Logic gate (XORing the feedback signal with 0).

Unfortunately, I don't know how to do a zero-order hold unit delay loop that seems to be another commonly prescribed solution for this kind of problem. But I believe that may cause issues with my model's originally intended performance.

I have posted a query on Mathworks website, but no response so far. SO I thought...why not stackoverflow? Below is the image.

Answer 1

Add a unit delay block on the feed back signal (from Logical Operator1 to Logical Operator4).

The unit delay provides the previous value of the output signal - this won't affect most of the circuitry in this scenario.

Answer 2

There isn't a "one size fits all" answer when it comes to algebraic loops. Here are a few resources about algebraic loops:

• What are algebraic loops in Simulink and how do I solve them?
• How can I resolve algebraic loops in my Simulink model in Simulink 6.5 (R2006b)?
• Algebraic Loops in the Simulink documentation
• and there are many others...

In your case, I would suggest highlighting the algebraic loop (as per the doc in the hyperlink above), and try inserting a unit delay in the loop. The doc shows how to do this. Other suggestions would be to try the algebraic loop solver or model parameters related to algebraic loops, or placing an IC or Algebraic Constraint block in the loop. Again, refer to the doc in the hyperlink above for details. I assume you are constrained to use a fixed-step solver and cannot switch to a variable-step solver.