# parameter optimization in function

3 views (last 30 days)
Elon Hendriksen on 22 Jul 2022
Answered: Rajiv Singh on 9 Aug 2022
Hi!
I have written a code in which a nonlinear pendulum is modelled. Now with measured data from a pendulum I would like to optimize my damping coefficient so that the model in matlab fits the measured data. Since the damping coeficient is in a separate function file I do not know how to tackle this. See script in attachment.
In F_nonlin the damping coefficient is currently 0.000035 and fits the curve quite well but as mentioned above I would like matlab to optimize this value automattically.
Thanks in advance for the help!
Kind regards, Elon

Alan Weiss on 22 Jul 2022
Try putting the following code at the end of yours:
[dfinal,resnorm] = fminbnd(@(damping)trytofit(damping,x0,Acc_clean),0.00002,0.00005)
function delta = trytofit(damping,x0,Acc_clean)
tspan = Acc_clean(:,1)/1000-5.017;
[~, S] = ode45(@(t,y) F_nonlin(t,y,damping),tspan,x0);
delta = sum((S - Acc_clean(:,3)).^2,"all");
end
I changed the F_nonlin code as follows:
function dx = F_nonlin(~,x,Bfactor)
% Derivative function for a nonlinear pendulum model.
%
% States:
% x(1): theta
% x(2): d theta/dt
% system parameters:
g = 9.81; % gravitational constant (m/s^2)
m = 1.042; % mass pendulum
L = 0.210; % length pendulum
M = m*L;
K = m*g; %iets met g
B = Bfactor * 2 * sqrt(M*K); %0.000035 for fitted line
dx(1,1) = x(2);
dx(2,1) = -( K*sin(x(1)) + B*x(2)*abs(x(2)) ) / M;
I get the result dfinal = 3.1459e-05, pretty close to what you have.
My code searches for a minimum of the sum of squares of differences between the simulated pendulum and the data you have. The only thing it varies is the damping (Bfactor in F_nonlin, a scalar), so I use fminbnd to get the minimum.
Of course, it is possible I made an error somewhere, but I think that you see how I put the parameter Bfactor in the simulation to give something that a solver can vary to search for a minimum.
Alan Weiss
MATLAB mathematical toolbox documentation
Torsten on 22 Jul 2022
In your code, L is an input variable to F_nonlin and you immediately overwrite that input by setting L to 0.21. One of these settings is superfluous.

Rajiv Singh on 9 Aug 2022

R2020b

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