Problems fitting couple differential equations to experimental data

I'm trying to fit experimental data with a system of differential equations. So far, I've followed what was done here, as it closely resembles what I'm trying to accomplish: https://www.mathworks.com/matlabcentral/answers/254566-how-do-i-fit-coupled-differential-equations-to-experimental-data

The equations are:

dP(t)/dt = kn*(10-M(t))^nc + km*M(t) - ka*P(t)^2

dM(t)/dt = kp*(10 - M(t))*P(t)

P(0) = M(0) = 0

My data consists of time, t, and fiber mass, M(t), so essentially I want to solve for M(t) to fit the data. If it helps, the M(t) can generally be fit by a sigmoidal function, but I'm using these equations because I would like to obtain the constants kn, nc, km, ka, and kp. I know there are a lot of parameters which makes fitting harder... Here is the code:

function [x] = knfit(t,f)

% t = time
% f = raw data

function S = prefun(B,t)
    % variables: x(1) = P = P(t) x(2) = M = M(t)
    % parameters:
            % B(1) = kn
            % B(2) = nc
            % B(3) = km
            % B(4) = ka
            % B(5) = kp;
    x0 = [0 0];
    [~,Sv] = ode45(@DiffEq,t,x0);
    
    function dS = DiffEq(t,x)
        dS(1,:) = B(1)*(10-x(2))^B(2) + B(3)*x(2) - B(4)*x(1)^2;
        dS(2,:) = B(5)*(10-x(2))*x(1);
    end
    
S = Sv(:,1);
end

objfcn = @(B,t) prefun(B,t);

B0 = [0.001, 2, 0.0001, 1.1, 0.00002]; % initial values

x = lsqcurvefit(objfcn,B0,t,f);
            
end

The errors that I get right now are:

Matrix dimensions must agree.

Error in lsqcurvefit/objective (line 279)
         F = F - YDATA;

I'm super confused because the x and y data definitely are the same dimensions. Does B0 have to be something different?

I'm pretty new at this so go easy on me! :) Thanks!

Answer 1

[Alex Sha]

Hi, if possible, upload your data (t, P and M) please

Answer 2

[Star Strider]

Running your code with random data ran without error (R2020a):

t = (0:20)';
f = rand(size(t));

function [x] = knfit(t,f)
% t = time
% f = raw data
function S = prefun(B,t)
    % variables: x(1) = P = P(t) x(2) = M = M(t)
    % parameters:
            % B(1) = kn
            % B(2) = nc
            % B(3) = km
            % B(4) = ka
            % B(5) = kp;
    x0 = [0 0];
    [~,Sv] = ode45(@DiffEq,t,x0);
    
    function dS = DiffEq(t,x)
        dS(1,:) = B(1)*(10-x(2))^B(2) + B(3)*x(2) - B(4)*x(1)^2;
        dS(2,:) = B(5)*(10-x(2))*x(1);
    end
    
S = Sv(:,1);
end
objfcn = @(B,t) prefun(B,t);
B0 = [0.001, 2, 0.0001, 1.1, 0.00002]; % initial values
x = lsqcurvefit(objfcn,B0,t,f);
            
end

x = knfit(t,f)

I am not exactly certain what the problem could be, other than it is possible that ode45 encountered a singularity (infinite value) and stopped integrating. The vectors would then not be the same lengths, however that would likely throw a different error.