How to correct resonance phase plot

2021 11 5
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更新时间：2021 11 8
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Hello,
I am trying to implement the resonance response at different damping ratios. I dont know why the phase plot looks like this
In litteratures, it is supposed to look like this, (Shifted),

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Bjorn Gustavsson
Bjorn Gustavsson 2021-11-5

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You could quick-fix this by plotting the phase-angle phi like:
plot(unwrap(2*phi)/2)
But you might have to check that you calculate the phase-angle correctly (right sign relative to the driving force and whatnot...)
HTH

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Derek Cooper
Derek Cooper 2021-11-5
Hello,
That was unfortunately not correct. The equation that generates the phase angle is
for i = 1:length(xi)
alfa(i,:) = atand((2*xi(i).*r)./(1-r.^2));
end
where r is the frequency ratio and xi is the damping ratio.
Bjorn Gustavsson
Bjorn Gustavsson 2021-11-5
编辑：Bjorn Gustavsson 2021-11-5
@Derek Cooper: Transfer to work with angles in radians now, only when plotting you convert to degrees (sooner rather than later you will start to look at the mechanics of rotating bodies and whatnot, then it will become more of a nuisance to work with angles in degrees). If you change your code to:
for i = 1:length(xi)
alfa(i,:) = atan((2*xi(i).*r)./(1-r.^2));
end
plot(180/pi*unwrap(alfa*2)/2)
it should work.
Derek Cooper
Derek Cooper 2021-11-5
@Bjorn Gustavsson Unfortunately, that doesnt work too.
Bjorn Gustavsson
Bjorn Gustavsson 2021-11-5
Sure does - if you had manage to replace the obvious typo...
Derek Cooper
Derek Cooper 2021-11-5
This is what I am getting with your suggestion
Bjorn Gustavsson
Bjorn Gustavsson 2021-11-5
Chek the size of your alfa-variable. From the help of unwrap you see that it unwraps along the first (non-singleton) dimension, and you would most likely want the unwrapping along the second dimension:
plot(r,180/pi*unwrap(2*alfa,[],2)/2)
Derek Cooper
Derek Cooper 2021-11-8
Now it worked, thank you very much.

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