I am using the system identification tool box to create a greybox model. My error is The sizes of the matrices returned by the ODE function must be consistent with the input/o

[Rahul]

Hi @Yilin, can you share your code of how you're using the output from "GreyBox18" function to construct a grebox model? That'll help in better understanding the issue.

[Yilin]

Sure, the codes are in the picture attach below.

[Torsten]

Shouldn't K be 3x1 instead of 2x1 ?

[Sam Chak]

Hi @Yilin

We don't understand your grey model. However, typical linear state-space models have this form:

state equation:

,

output equation:

.

If input vector is given by

,

can you 'reverse engineer' to determine the size of matrix ?

[Torsten]

@Sam Chak

The model is

x(t)=Ax(t)+Bu(t)+Ke(t)

y(t)=Cx(t)+Du(t)+e(t)

e(t) is 1x1, so K should be 3x1, shouldn't it ?

[Sam Chak]

Thanks for info, @Torsten. I've found the Grey box model in this link:

https://www.mathworks.com/help/ident/ug/estimating-linear-grey-box-models.html

[Yilin]

@Torsten Got it, Thanks for your help!!

Answer 1

[Rahul]

Hi @Yilin,

From what I understand, you are trying to create a linear ODE grey-box model using the state space matrices returned from your ODE function “GreyBox18”.

For a generic state space system, the state space equations are represented as follows:

Hence, as pointed out by @Torsten, since LHS of eq (1) has dimensions (3,1), the resultant of matrix multiplication of ‘K.*e(t)’ should also have (3,1) as its dim. So, it might be that the matrix ‘K’ needs to be initialized with size (3, 1), since error matrix ‘e(t)’ is a scalar. Here is how you structure your ODE function:

function [A,B,C,D,K] = GreyBox18(C_n, C_en, C_m, R_en, R_mn, R_oen, ~[AJ1] ) 
A = [(-1/(C_n*R_en)-1/(C_n*R_mn)), 1/(C_n*R_en), 1/(C_n*R_mn);     
    1/(C_en*R_en), -1/(C_en*R_oen)-1/(C_en*R_en), 0; 1/(C_m*R_mn), 0, -1/(C_m*R_mn)];    
B = [1/C_n, 1/C_n, 0, 0, 0, -1/C_n;       
    0, 0, 1/(C_en*R_oen), 1/C_en, 0, 0;     
    0, 0, 0, 0, 1/C_m, 0];            
C = [1,0,0];          
D = zeros(1,6);         
K = zeros(3,1); % Change size of K to (3,1)      
end