# How to find and fit the objective function for a damped oscillation?

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Piyumal Samarathunga on 28 Nov 2022
Commented: Star Strider on 2 Dec 2022
The excel file contains force data of a damped oscillation system. In order to find the natural frequency of the system I am trying to fit the graph with an objective function as follows. I have defined the function of the osillation.
function F = forcesim(t,A,omega,phi,C,K)
F=A*cos(omega*t+phi)*exp(-t*C) + K;
end
I have tried with a code to find the estimated frequency as follows.
Force = dataset.Force2;
Time = dataset.Time2;
plot(Time,Force)
xlabel("Time (s)")
ylabel("Force (N)")
grid on
ind = Time>0.3 & Time<1.3;
[pks,locs] = findpeaks(Force(ind),Time(ind),'MinPeakProminence',1);
peaks = [pks locs]
% visualizse the selected peaks and label them.
hold on
plot(locs,pks,'v')
text(locs+.02,pks,num2str((1:numel(pks))'))
hold off
% calculate the approximate frequency
meanCycle = length(pks)/(locs(end)-locs(1))
Hence, that frequency is an estimated one, and therefore I am planning to find the frequency of the graph by matching the objective function mentioned above.
It would be really appreciated if you could tell me how to do that and find the frequency ω.

Star Strider on 29 Nov 2022
Using fminsearch
T1 = 3001×2 table
Time2 Force2 _____ _______ 0 0.07471 0.001 0.05859 0.002 0.06006 0.003 0.04541 0.004 0.09375 0.005 0.07178 0.006 0.05713 0.007 0.05859 0.008 0.07031 0.009 0.06006 0.01 0.04541 0.011 0.0293 0.012 0.07031 0.013 0.06299 0.014 0.06299 0.015 0.07031
x = T1.Time2;
y = T1.Force2;
start = find(y > 1, 1, 'first') % Find Offset
start = 160
y = y(start:end);
x = x(start:end);
y = detrend(y,1); % Remove Linear Trend
yu = max(y);
yl = min(y);
yr = (yu-yl); % Range of ‘y’
yz = y-yu+(yr/2);
zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0); % Returns Approximate Zero-Crossing Indices Of Argument Vector
zt = x(zci(y));
per = 2*mean(diff(zt)); % Estimate period
ym = mean(y); % Estimate offset
fit = @(b,x) b(1) .* exp(b(2).*x) .* (sin(2*pi*x./b(3) + 2*pi/b(4))) + b(5); % Objective Function to fit
fcn = @(b) norm(fit(b,x) - y); % Least-Squares cost function
[s,nmrs] = fminsearch(fcn, [yr; -10; per; -1; ym]) % Minimise Least-Squares
s = 5×1
155.8223 -6.4293 0.0335 -1.4713 0.0000
nmrs = 61.2464
xp = linspace(min(x),max(x), numel(x)*10);
figure(3)
plot(x,y,'b', 'LineWidth',1.5)
hold on
plot(xp,fit(s,xp), '-r')
hold off
grid
xlabel('Time')
ylabel('Amplitude')
legend('Original Data', 'Fitted Curve')
xlim([min(x) 1])
text(0.3*max(xlim),0.7*min(ylim), sprintf('$y = %.3f\\cdot e^{%.1f\\cdot x}\\cdot sin(2\\pi\\cdot x\\cdot %.0f%.3f)%+.1f$', [s(1:2); 1./s(3:4); s(5)]), 'Interpreter','latex')
format short eng
grid mdl = fitnlm(x(:),y(:),fit,s) % Get Statistics
mdl =
Nonlinear regression model: y ~ b1*exp(b2*x)*(sin(2*pi*x/b3 + 2*pi/b4)) + b5 Estimated Coefficients: Estimate SE tStat pValue __________ __________ __________ ______ b1 155.82 1.6153 96.468 0 b2 -6.4293 0.041868 -153.56 0 b3 0.033533 7.6151e-06 4403.5 0 b4 -1.4713 0.0036676 -401.16 0 b5 2.2889e-16 0.03774 6.0647e-15 1 Number of observations: 2842, Error degrees of freedom: 2837 Root Mean Squared Error: 1.15 R-Squared: 0.971, Adjusted R-Squared 0.971 F-statistic vs. constant model: 2.37e+04, p-value = 0
.
##### 2 CommentsShowHide 1 older comment
Star Strider on 2 Dec 2022
As always, my pleasure!

Matt J on 28 Nov 2022
Edited: Matt J on 28 Nov 2022
Using fminspleas from the File Exchange,
region=t>=0.2 &t<=0.8;
F=F(region);
t=t(region);
flist={@(p,x)cos(p(1)*t+p(2)).*exp(-t*p(3)),1 };
p0=[0.15,0,1];
[p,AK]=fminspleas(flist,p0,t,F);
Warning: Rank deficient, rank = 1, tol = 3.271538e-12.
Warning: Rank deficient, rank = 1, tol = 3.271538e-12.
Warning: Rank deficient, rank = 1, tol = 3.271538e-12.
Warning: Rank deficient, rank = 1, tol = 3.271538e-12.
f=@(t) AK(1)*cos(p(1)*t+p(2)).*exp(-t*p(3)) +AK(2);
plot(t,F,'o',t,f(t)); legend('Data','Fit') Piyumal Samarathunga on 2 Dec 2022

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