Find angle of rotation matrix in MATLAB

Question

Kevin Maresca 2020-11-22

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https://ww2.mathworks.cn/matlabcentral/answers/657948-find-angle-of-rotation-matrix-in-matlab

回答： Asad (Mehrzad) Khoddam 2020-11-22

Hey everyone,

I'll preface by letting you know that I am a novice at MATLAB so I'm sure the code will need plenty of help.

I'd like to find the angle at which a matrix needs to be rotated about the y-axis to reach given coordinates. 'I' is the given matrix, I'd like to rotate it by 't' to get 'Iwant'. I'd also like to print out which value of 't' got the desired matrix.

As far as I can tell, the while loop runs forever. What am I doing wrong? Is this a completely wrong approach?

Thank you.

I = [11755 0 4821; 0 15000 0; 4821 0 23245]

Iwant = [10000 0 0; 0 15000 0; 0 0 25000]

t = 0;

while t < 3.14

R = [cos(t) 0 sin(t); 0 1 0; -sin(t) 0 cos(t)];

I1 = R*I*R';

if I1 == Iwant

fprintf(t)

break

t = t+0.2;

end

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Answer 1

Bruno Luong 2020-11-22

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编辑：Bruno Luong 2020-11-22

Yes

the approach is wrong
your code has plenty of bugs*
you need to learn how to debug
you need to pratice MATLAB onramp

On (*) is fixed

I = [11755 0 4821; 0 15000 0; 4821 0 23245]
Iwant = [10000 0 0; 0 15000 0; 0 0 25000]
t = 0;
while t < 2*pi
    R = [cos(t) 0 sin(t); 0 1 0; -sin(t) 0 cos(t)];
    I1 = R*I*R';
    if norm(I1-Iwant,'Inf') < 10
        fprintf('t=%g\n', t)
        break
    end
    t = t+1e-3;
end

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Answer 2

Asad (Mehrzad) Khoddam 2020-11-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/657948-find-angle-of-rotation-matrix-in-matlab#answer_552683

I = [11755 0 4821; 0 15000 0; 4821 0 23245];
Iwant = [10000 0 0; 0 15000 0; 0 0 25000];
t = 0;
i=1;
tList = 0:.02:3.14;
dList = NaN(size(tList));
for t = tList
    R = [cos(t) 0 sin(t); 0 1 0; -sin(t) 0 cos(t)];
    I1 = R*I*R';
    d = I1-Iwant;
    dList(i)= sum(d(:).^2);
    i = i+1;
end
[~, index] = min(abs(dList));
disp(tList(index))