Looking to plot yaw motion of an aircraft but get "Array indices must be positive integers or logical values." Error

clear all

clc

N_phidot = -0.36752;

N_phi = -8.3569;

N_delta_r = -3.8313;

delta_R = -10;

sigma = 1/2.*N_phi;

omega_d = (-N_phi-1/4.*(-N_phidot)^2)^(1/2);

xi = 1/2.*(-N_phidot/-N_phi^(1/2));

omega_n = (-N_phi)^(1/2);

beta = (N_delta_r/N_phi).*delta_R;

phi = atan(sigma/omega_d);

alpha = -1/2.*beta.*(omega_n/omega_d);

Tau = 2*3.14/omega_d;

xiomega_n = 1/Tau;

t = [0:0.01:10];

Phi(t) = beta.*(1-(omega_n/omega_d.*exp(-sigma.*t).*cos((omega_d.*t)-phi)));

Phidot(t) = diff(Phi(t));

figure(1)

plot(t,phidot)

title('Phidot vs time')

xlabel('seconds')

ylabel('degrees')

figure(2)

plot(t, phi)

title('Phi vs time')

xlabel('seconds')

ylabel('degrees')

Error Messages

Array indices must be positive integers or logical values.

Error in HW1MAE503Q2 (line 22)

Phi(t) = beta.*(1-(omega_n/omega_d.*exp(-sigma.*t).*cos((omega_d.*t)-phi)));

>>

Answer 1

[Harshavardhan]

Hi @David Scidmore

MATLAB expects indices to be positive integers or logical values. You're trying to assign values to “Phi(t)”, which is interpreted as indexing into “Phi” using the values of “t”, which are not valid indices.

To fix this:

• define“Phi”as a vector using element-wise operations.
• Use the “gradient” function to compute the numerical derivative for “Phidot”.

Here is the updated code for the definitions of “Phi” and “Phidot”:

% Phi and Phidot 
Phi = beta .* (1 - (omega_n / omega_d) .* exp(-sigma .* t) .* cos((omega_d .* t) - phi_val)); 
Phidot = gradient(Phi, t);% Numerical derivative

For more information on “gradient” refer to the link below:

https://www.mathworks.com/help/matlab/ref/gradient.html