Accelerance FRF with modal FRF?
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Hello, I'm trying to compute the accelerance FRF based on acceleration and modal hammer force signal.
I use the script of Manuel Lozano (script link) and the matlab modalFRF function, but I have diferences in magnitude and shape.
clc; clear; close all;
a = readtable ('aceleracion.csv').Var2; %Aceleration in [g]
f = readtable ('fuerza.csv').Var2; %Force in [N]
%% Using the code of https://la.mathworks.com/matlabcentral/answers/530968-what-s-wrong-with-my-frf
N = length(a); %length of signal
fs = 10000; %sampling frequency [Hz]
df = fs/N; %frequency resolution
fn = fs/2; %nyquist frequency
frequencies = linspace(0,fn,N/2+1);
Y = fft(a);
X = fft (f);
FRF = (Y./X); %Accelerance
Ymag = abs(FRF/N);
Ymag = Ymag(1:N/2+1);
Ymag(2:end-1) = 2*Ymag(2:end-1); % double magnitudes except c0
%% Using modalfrf function
winlen = size(a,1);
[FRF,f] = modalfrf(f, a, fs, winlen);
FRF = abs(FRF);
plot(frequencies,Ymag, f, FRF)
legend ('Using FFT', 'Using Matlab ModalFRF');
I have also calculated the accelerance function using commercial software based on LabVIEW language. It can be seen that there is no coincidence either.
Does anyone done this comparison? can someone tell me where is the error?
Thanks in advance.
Paul on 2 Aug 2022
Edited: Paul on 2 Aug 2022
The two blue curves look pretty close, perhaps just off by a scale factor (except for the dip at ~600 Hz?). Those curves are the accelerance at least according to how Ymag is computed and the statement about what LabView computes. However, modalfrf always computes the receptance. So perhaps the output of modalfrf needs to be multplied by -omega^2 (omega in rad/sec) to get the accelerance? Worth a try anyway to see if that better aligns the curves.
Then still need to figure out the scale factor.. To sort that out, I think you'll really need to understand more precisely what LabView and modalfrf are really computing.