Simulink model of RLC circuit (2nd order differential equations)

Hello guys,

I have those differential equations, that describe currents in RLC circuit:

Then i created a model of those equations in Simulink:

I set values of R,L,C like this:

R = 1000

L = 1.0000e-06

C = 2.0000e-12

and my problem is that I am getting error message when I try to run the Simulink model.

Does anyone, please, know where the problem is?

[Sam Chak]

Hi @Techie

Can we have a closure on this original question? The suggested solution using the State-space block can definitely solve the original question in Simulink because it is a Linear System.

https://www.mathworks.com/matlabcentral/answers/1748460-model-of-two-differential-equations-of-rlc-circuit-simulink

It was later (48 hours after original question is posted), then you posted a reply stating that you wanted the model to be specifically constructed in fundamental math blocks. Thank you for your consideration on this matter.

In your new question, it seems that a singularity has occurred during the simulation in Simulink.

[Tech]

Hello,

I will close my previous question as anwered.

I want to you those math blocks, because i believe that if i will go that way i will be able to get graphs of current or voltage from one part of the RLC circuit (for example i could get graph of current on resistor or something like that).

Do you know how could i solve my problem with that error message in this simulation?

Thank you.

Answer 1

[Sam Chak]

Hi @Tech

The first impression is that the singularity might be caused by the "Division by zero" issue.

Looking at your differential equations, four terms have divisions by .

Note that and are both very small numbers.

In fact, is even smaller than eps, the floating-point relative accuracy.

L = 1e-6;
C = 2e-12;
Test = L*C < eps  % 1 is True, 0 is False

Test = logical
   1

[Torsten]

@Sam Chak

Just out of interest (because I don't have any experience with SIMULINK):

Is it really necessary to define such a complicated and confusing SIMULINK model to solve these 3 differential equations ?

[Sam Chak]

Hi @Torsten,

I definitely won't solve it that way, or at least not making the Simulink model looks like a bowl of spaghetti.

STEM students taking the "System Dynamics" course without coding experience are generally encouraged to model and design systems in the block diagram environment (like Simulink), so that they can understand the system better by seeing how the signals flow between each block, something that behaves like .

Some lecturers with the "You Sink Or Swim" mentality generally won't care about how students learn the software, because teaching how to use a software to solve problems is not part of the intended learning outcomes. Unomotivated students are left to explore the software by themselves and they will usually stuck in a loop of frustration when nobody volunteers to help/teach them.

If I have to present the solution of this problem in Simulink, then I'll make a model like this. Contrary to standard ODE, the system outputs are generally placed at the right side of the block diagram. Hope @Techie can benefit from this.

[Tech]

Hello @Sam Chak

Your point that there is division by zero make sense. Do you have any idea how can I fix this problem? Because values of C and L are normally simillarly small.

I start to thinking that I must be doing something wrong. What I want to do is to make a Simulink model of those differential equations and then be able to, for example get graph of voltage or current from one capacitor or resistor.

Do you have any more advices for me, please?

[Tech]

New Update of my situation.

I have found some mathematical mistakes I have made and i set values of L and C to bigger one and it is somehow working.

I will now mark this question as anwered and if i will have any more problems I will create a new article.

Thank you very much.