Logarithmic Decrement and Damp Ratio

I have a velocity vs time input data on which I had to apply butterworth filter but now I have issues with application of the log decrement method for damping calculation. The log decrement formula is depicted in the picture and I have implemented it as following:

%INPUTS%
%Veloc - Veloc vector
%n_peaks - No. of peaks of interest
%Time - Actual time series 
%OUTPUTS%
%damp_ratio_logdec - Damping ratio 
z=importdata('P1veloc.txt'); 
Time = z(:,1);  
Veloc = z(:,2);  
n_peaks=10  
%Vector length of interest 
t_new = Time(imax(1:n_peaks));  
y_new = ymax(1:n_peaks);  
t_new(1)=[];%Remove first point - Due to impact  
y_new(1)=[];   
%---Calculate Logarithmic Decrement 
Log_Dec = zeros(length(n_peaks));  
for nn = 1:n_peaks-2 %We go to '-2' because length(n_peaks) = since we ignore first one
Log_Dec(nn)= log(y_new(nn)/y_new(nn+1));  
end  
Mean_dec = mean(Log_Dec); %Calculate Average Logarithmic Decrement  
%Damping  
damp_ratio_logdec = 1/sqrt(1+((2*pi/(Mean_dec))^2)); %Assesses Damping Constant

I have also put a .txt file with velocity and time columns. I started the code but I am not doing well so I would appreciate any help, since I just started using Matlab as part of my thesis and I have to process data in this way.

[Agus Danish]

Hello, the error said "Index in position 2 exceeds array bounds. Index must not exceed 1.". What does it means?

[Mathieu NOE]

hello

to which code / data are you refering ?

this was my suggestion (see below) :

clc
clear all
close all
z=importdata('P1veloc.txt'); 
ind = 1:100;
Time = z(ind,1);  
Veloc = z(ind,2);  
ind = 1:20;
[Ypk,Xpk,Wpk,Ppk] = findpeaks(Veloc(ind));
plot(Time,Veloc,Time(Xpk),Ypk,'dr');
n_peaks=numel(Xpk);  
%Vector length of interest 
t_new = Time(Xpk);  
y_new = Ypk;  
%---Calculate Logarithmic Decrement,  undamped natural frequency, damped natural frequency, damping ratio
Log_Dec = zeros(length(n_peaks));  
for nn = 1:n_peaks-1 %
    Log_Dec(nn)= log(y_new(nn)/y_new(nn+1));  % computed with n = 1 periods apart
end  
Mean_dec = mean(Log_Dec); %Calculate Average Logarithmic Decrement  
%Damping  
damp_ratio_logdec = 1/sqrt(1+((2*pi/(Mean_dec))^2)); %Assesses Damping Constant 

Answer 1

[A K M Kamrul Hasan]

Can you please explain, what is imax and ymax?

Answer 2

[Mathieu NOE]

hello

a very late answer to this question - siply because this code was reused in another post

FWIW, this is the answer I already provided

clc
clear all
close all
% z = csvread('G03.csv');
z=importdata('P1veloc.txt'); 

ind = 1:100;
Time = z(ind,1);  
Veloc = z(ind,2);  

ind = 1:20;
[Ypk,Xpk,Wpk,Ppk] = findpeaks(Veloc(ind));
plot(Time,Veloc,Time(Xpk),Ypk,'dr');
n_peaks=numel(Xpk);  

%Vector length of interest 
t_new = Time(Xpk);  
y_new = Ypk;  
%---Calculate Logarithmic Decrement,  undamped natural frequency, damped natural frequency, damping ratio
Log_Dec = zeros(length(n_peaks));  
for nn = 1:n_peaks-1 %
    Log_Dec(nn)= log(y_new(nn)/y_new(nn+1));  % computed with n = 1 periods apart
end  
Mean_dec = mean(Log_Dec); %Calculate Average Logarithmic Decrement  
%Damping  
damp_ratio_logdec = 1/sqrt(1+((2*pi/(Mean_dec))^2)); %Assesses Damping Constant