Why wont the state-space block accept a mux input?

I have a state space block with equation modelling the beahvoiur of a buck converter given 2 inputs [Iref and Vin].

I have tried using a Mux, a matrix concatenate and a vector concatenate to pass the 2 signals without sucsess. I get the following errors.

Error in port widths or dimensions. Output port 1 of 'MUx1' is a one dimensional vector with 2 elements.

Error in port widths or dimensions. Invalid dimension has been specified for 'Input Port 1' of 'State-Space'.

I have ensured that the dimensions match, however everytime i do that the error chnages to something new. This also happened with matrix concatenate and vector concatenate blocks

Any help on what might be causing this would be greatly appreciated.

Answer 1

[Sam Chak]

Hi @ymps

Your given state-space model is

,

and you will obviously see that it takes only a SINGLE input u after doing the matrix multiplications.

If the input u is the Voltage input , a function of the reference , where you want to regulate as , then you should design something like this

and the mux is not needed in your case.

Here is a demo in MATLAB instead Simulink. Theoretically, it should work in Simulink as well.

tspan  = [0 10];
x0     = [1 0];
[t, x] = ode45(@odefcn, tspan, x0);
plot(t, x), grid on, xlabel('t')

y = x(:,1);
y(end)

ans = 0.2000

% State-space model
function xdot = odefcn(t, x)
    xdot = zeros(2, 1);	
    % parameters
    Rs1  = 1;
    Rs2  = 1;
    R    = 1;
    L    = 1;
    C    = 1;
    % elements of matrix A
    a11  = - (Rs1 + Rs2 + R)/L;
    a12  = - 1/L;
    a21  = - 1/C;
    a22  = - (Rs1 + Rs2)/C;
    % system matrices
    A    = [a11 a12; a21 a22];  % state matrix
    B    = [1/L; 0];            % input matrix
    % user-design stuff
    K    = [0.1918 -0.0937];    % user-designed matrix gain
    lref = 0.2;                 % reference
    Vin  = - K*x + 2.7385*lref; % voltage input
    % RLC system
    u    = Vin;                 % 
    xdot = A*x + B*u;           % state-space
end

Answer 2

[Paul]

The B, C, and D matrices of the state space block indicate the state space model is expecting a single input to generate a single output. At a minimum, the B and D matrices should have two columns if there will be two inputs.

Answer 3

[Walter Roberson]

A mux is not the same thing as a vector of values. If you need to build a vector of values use https://www.mathworks.com/help/simulink/slref/vectorconcatenate.html

[Paul]

I'm pretty sure there is no difference between the output of the Mux block and of the Vector Concatenate (default settings) block for scalar inputs.