how to modify plot range from (-200 to 200)

% Define the mass, damping, and stiffness matrices
load C K
load M.mat

% Define the input force as a sinusoidal function of frequency w in Hz
f = linspace(0, 40, 100); % linearly spaced frequency vector in Hz
w = 2*pi*f; % angular frequency vector in rad/s
F = @(t) [sin(2*pi*f(1)*t); zeros(3, 1)];

% Define the initial conditions
x0 = zeros(4, 1);
v0 = zeros(4, 1);
y0 = [x0; v0];

% Define the time span
tspan = [0 20];

% Preallocate arrays to store the magnitude and phase of the response
mag = zeros(size(w));
phase = zeros(size(w));

% Loop over the frequency vector and compute the response at each frequency
for i = 1:length(w)
    % Define the input force at the current frequency
    F = @(t) [sin(w(i)*t); zeros(3, 1)];
    
    % Solve the differential equation to obtain the steady-state response
    [~, y] = ode45(@(t, y) vibration_equation(t, y, M, C, K, F), tspan, y0);
    x = y(end, 1:4)';
    
    % Compute the magnitude and phase of the response at the current frequency
    mag(i) = abs(x(1));
    phase(i) = angle(x(1)) * 180/pi;
end

% Plot the magnitude and phase of the FRF in Hz
figure;
subplot(2, 1, 1);
plot(f, mag);
xlabel('Frequency (Hz)');
ylabel('Magnitude');
title('Frequency Response Function');
grid on;

subplot(2, 1, 2);
plot(f, mod(phase+180,360)-180);
xlabel('Frequency (Hz)');
ylabel('Phase (deg)');
ylim([-180, 180]);
grid on;

function dydt = vibration_equation(t, y, M, C, K, F)
% Compute the acceleration
x = y(1:4);
v = y(5:8);
a = M \ (F(t) - C*v - K*x);

% Compute the derivative of the state vector
dydt = [v; a];
end

I want something like in shown in photo. from -200 to 200 range for phase graph.

Answer 1

[Cris LaPierre]

The function angle already returns the phase angle on the interval of [,π], which you convert to degrees, or [-180,180]. For some reason, your angles are only negative. So I would first look into why the angles are between 0 and -180 before I'd modify the phase plot and data.

[Cris LaPierre]

Investigating a little more, you position information is not complex, so the following algorithm is used to determine phase angle using the angle function.

• When the elements of z are non-negative real numbers, angle returns 0. When the elements of z are negative real numbers, angle returns π.

[AL]

thank you so much for your effort and give me inside behind what is happning. is there anyway we can normalize this and get both positive and negative values?

[Cris LaPierre]

I don't think that is the correct approach. Consider looking at the bottom of this page to see what they do with a system with 4 degrees of freedom:

https://www.brown.edu/Departments/Engineering/Courses/En4/Notes/vibrations_mdof/vibrations_mdof.htm