How to correct resonance phase plot

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),

Answer 1

[Bjorn Gustavsson]

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

[Derek Cooper]

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]

@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]

@Bjorn Gustavsson Unfortunately, that doesnt work too.

[Bjorn Gustavsson]

Sure does - if you had manage to replace the obvious typo...

[Derek Cooper]

This is what I am getting with your suggestion

[Bjorn Gustavsson]

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]

Now it worked, thank you very much.