FFT frequency does not correspond to my sensor frequency

Hello,

I'm trying to use matlab in order to retrieve the tremor fundamental frequency of a IMU sensor.

I use FFT and PSD to get my funcdamental frequency.

To perform measurements, i calibrated a 3D printer stepper motor. It manage to create rotational vibration around the Z axis of the IMU at a 2.01Hz. However, when i process the data into matlab, i retrieve a 1.3Hz fundamental frequency...

I don't know if my sensor values are incorrect or if my processing is incorrect.

Something to know : my signal is triangle waves due to the stepper motor.

Please find here the matlab code and a sample of data as attachment.

MATLAB :

if true
%import csv file
M = csvread('3.csv');

 %select the gx column
gx = M(:,1);
%select the gy column
gy = M(:,2);
%select the gz column
gz = M(:,3);

 %size of M (same size for each column)
s = size(gx, 1);
 %because of loop, need s/2
s2 = fix(s/2);

 % computes the fast fourier transform of M for gx
Ygx = fft(gx, s);
% computes the fast fourier transform of M for gy
Ygy = fft(gy, s);
% computes the fast fourier transform of M for gz
Ygz = fft(gz, s);

 %Compute the power spectral density for gx
PSDYgx = Ygx.*conj(Ygx)/s;
%Compute the power spectral density for gy
PSDYgy = Ygy.*conj(Ygy)/s;
%Compute the power spectral density for gz
PSDYgz = Ygz.*conj(Ygz)/s;

 %acquisition frequency
Hz = 30;
%calculate X axis (frequency)
f = Hz/s*(0:s2);

 %find peaks of the frequency sampl1e for gx
[pksgx,locsgx] = findpeaks(PSDYgx(1:s2+1), 'SortStr', 'descend', 'NPeaks', 1);
%find peaks of the frequency sample for gy
[pksgy,locsgy] = findpeaks(PSDYgy(1:s2+1), 'SortStr', 'descend', 'NPeaks', 1);
%find peaks of the frequency sample for gz
[pksgz,locsgz] = findpeaks(PSDYgz(1:s2+1), 'SortStr', 'descend', 'NPeaks', 1);

 figure 
 color = {'b', 'k', 'r'};
 PSDYg = {PSDYgx(1:s2+1), PSDYgy(1:s2+1), PSDYgz(1:s2+1)};
 pksg = {pksgx, pksgy, pksgz};
 locsg = {locsgx, locsgy, locsgz};

 linevgxx = [f(locsgx), f(locsgx)];
linevgxy = [pksgx, 0];
linehgxx = [0, f(locsgx)];
linehgxy = [pksgx, pksgx];

 linevgyx = [f(locsgy), f(locsgy)];
linevgyy = [pksgy, 0];
linehgyx = [0, f(locsgy)];
linehgyy = [pksgy, pksgy];

 linevgzx = [f(locsgz), f(locsgz)];
linevgzy = [pksgz, 0];
linehgzx = [0, f(locsgz)];
linehgzy = [pksgz, pksgz];

 linevgx = {linevgxx, linevgyx, linevgzx};
linevgy = {linevgxy, linevgyy, linevgzy};
linehgx = {linehgxx, linehgyx, linehgzx};
linehgy = {linehgxy, linehgyy, linehgzy};

 for i = 1:3
    hold on
    plot(f, PSDYg{i}, 'Color', color{i})
    plot(f(locsg{i}), pksg{i}, 'Color', color{i}, 'MarkerFaceColor', color{i}, 'Marker' , 'v')
    plot(linevgx{i}, linevgy{i}, color{i})
    plot(linehgx{i}, linehgy{i}, color{i})
    str = sprintf('%0.3e', f(locsg{i}));
    str2 = strcat(num2str(str), ' Hz');
    h = text(f(locsg{i}), -0.1 ,str2);
    set(h, 'HorizontalAlignment', 'center', 'VerticalAlignment', 'top', 'Color', color{i});
    str = sprintf('%0.3e', pksg{i});
    str2 = strcat(num2str(str), ' g²/Hz');
    h = text(-0.5, pksg{i}, str2);
    set(h, 'HorizontalAlignment', 'right', 'color', color{i});

 end

 title('Power Spcetral Density for gyroscope')
xlabel('Frequency (Hz)')
ylabel('Power (g²/Hz)')
hold off
  end

Thanks for your help !