RLC Circuit Equation Implementation-Runge Kutta

Hello, I need to convert an RLC equation to work inside the functions I have wirtten. Currently MatLab is telling me there are too many variables inside my equation for it to work. Here is the code of the function I am trying to work with

function [derriv_value] = RLC(y)

%Function that contains the derrivative value

%RLC Implementation: The equation is given by L*Q'' + R*Q' + (1/C)*Q = E(t)

% E(t) is the sum of total voltage drop across the circuit.

% R is resistance, L is indunctance, and C is capacitance. Q Is the charge

% in coloumbs. This equation is currently in a second order form.

% In most cases, the voltage in the system is known, it is usually the

% current (I) that is unknown.

% If we differentiate both sides and subsitute Q for I, we get,

% L*I'' + R*I' + (1/C)*I = E'(t)

% Since this is the more pratical case, we will use this format.

% Here are some values that could be used to simulate this.

% These above are some example values.

R = 10; % Resistance (Series RL Branch), Ohms..

L = 100e-3; % Inductance (Series RL Branch), H..

C = 50e-6; % Capacitor (Series RLC Branch), F

derriv_value = L*y(3)+R*y(2)+(1/C)*y;

end

And this function will be called in a Runge Kutta implementation file. I'll attach it. I have another file called FunctionC that has a format that matlab accepts. It works with the Runge Kutta file.

[darova]

Read this

[OvercookedRamen]

Yeah I already looked over that. I have the equation converted fine on paper, the issue is trying to set up a system of 3 first order ODEs in a format that will run like the equation in FunctionC.

Answer 1

[Daniel M]

Instead of this

derriv_value = L*y(3)+R*y(2)+(1/C)*y;

Do you mean this?

derriv_value = L*y(3)+R*y(2)+(1/C)*y(1);

[OvercookedRamen]

Yes, I meant to use y(1), I've corrected it in my current copy of the code, but it still does not work. MatLab says the equation I am trying to use has too many input arguments for MyVec_Function2

[Daniel M]

Can you show exactly how you are calling these three functions in a script and the exact error message. If you're passing RLC as the input F into MyVec_Function2, then your error is the fact that you're trying to access F(x(i), y(i)), but RLC only has one input RLC(y). So, yes, you are putting in too many input arguments. You need to adapt either MyVec_Function2, or RLC.

It would be fairly straightforeward to turn RLC into an anonymous function of x and y, which you could just put somewhere in the script that calls MyVec_Function2.

% set the constants
R = 10;
L = 100e-3;
C = 50e-6;
RLC_anon = @(x,y) L*y(3) + R*y(2) + (1/C)*y(1);
% renamed it RLC_anon, since you probably still have the original RLC() in the same directory.

% now pass RLC as the first input into MyVec_Function2

[OvercookedRamen]

[x,y] = MyVec_Function2(@RLC,0.01,1,20,[0;1])

This is how I call RLC into MyVec_Function2

[OvercookedRamen]

Oh, and here is the error,

Error using RLC

Too many input arguments.

Error in MyVec_Function2 (line 9)

k1 = F( x(i) , y(:,i) ); % Altered

[Daniel M]

Right. So you are passing in x(i), which is a single element, and y(:,i) which is a 2x1 array, into RLC which only takes a single input that is [1x3] or [3x1].

Do you intend x(i) to be the value for y(1), and y be the values for y(2) and y(3)? If so, you can adapt RLC like this:

function [derriv_value] = RLC2(y1,y23)
R = 10;
L = 100e-3; 
C = 50e-6; 
derriv_value = L*y23(2)+R*y23(1)+(1/C)*y1;
end

Otherwise.... you're just calling the function wrong.

[darova]

THe main equation is here i think

[OvercookedRamen]

That seemed to work, that last format you posted there. I think that is what the problem was. Setting the xi values up for the function. Thanks!

[OvercookedRamen]

How would you get this equation into a system of 3 first order ODEs?

[darova]

Your equation is of second order

% L*Q'' + R*Q' + (1/C)*Q = E(t)
% Q'' = 1/L*( E(t) - R*Q' - Q/C );
f = @(t,y) [y(2)
            1/L*(E - R*y(2) - y(1)/C)];
[t,y] = ode45(f,ts,y0);