The following error occurred converting from sym to double: Unable to convert expression into double array. Error in PartB (line 21) p_hat(i) = P(i)+(a1*u​(i-1))+(a2​*du(i-1))+​(a3*ddu(i-​1));

Can somone help me figure out why this doesn't run? It worked several times and now doesn't run and I havent changed aything.

figure 
hold on 
 
for m = 1000;
Z = 0.05; 
wn = 0.5/Z; 
k =(wn^2)*m;
G = 0.5; 
B = (1/6); 
wd = wn*(sqrt(1-(Z^2))); 
delta_t = 0.01;
c = 2*Z*sqrt(k*m); 
P = (100*sin(0.5*t))-(150*cos(0.5*t));
a1 = (m/(B*delta_t^2))+(G*c/(B*delta_t));
a2 = (m/(B*delta_t))+((G/B)-1)*c;
a3 = ((1/(2*B))-1)*m+delta_t*((G/(2*B))-1)*c;
k_hat = k + a1;
t = 0:0.01:10;
for i = 2:1001
p_hat(i) = P(i)+(a1*u(i-1))+(a2*du(i-1))+(a3*ddu(i-1));
u(i) = p_hat(i)/k_hat;
du(i) = ((G/(B*delta_t))*(u(i)-u(i-1)))+((1-(G/B))*(du(i-1)))+(delta_t*(1-(G/(2*B)))*ddu(i-1)); 
ddu(i) = ((1/(B*(delta_t^2)))*(u(i)-u(i-1)))-((1/(B*delta_t))*du(i-1))-(((1/(2*B))-1)*ddu(i-1));
end 
plot(u)
xlabel('Time (sec)')
ylabel('Displacement (m)')
title('Time vs Displacement (Newmark Method)')
grid on
end

Answer 1

[Guru Mohanty]

Hi, when I executed your code and my understanding is that the error is due to it is trying to access an data which is not computed yet.

for i = 2:1001
    p_hat(i) = P(i)+(a1*u(i-1))+(a2*du(i-1))+(a3*ddu(i-1));
    u(i) = p_hat(i)/k_hat;
    du(i) = ((G/(B*delta_t))*(u(i)-u(i-1)))+((1-(G/B))*(du(i-1)))+(delta_t*(1-(G/(2*B)))*ddu(i-1)); 
    ddu(i) = ((1/(B*(delta_t^2)))*(u(i)-u(i-1)))-((1/(B*delta_t))*du(i-1))-(((1/(2*B))-1)*ddu(i-1));
end 

Here in this loop for the computation of “P_hat”, “u ” is needed which is evaluated in next expression.

However, you can initialize the values to get the plot.

figure 
hold on  
for m = 1000;
Z = 0.05; 
t = 0:0.01:10;
wn = 0.5/Z; 
k =(wn^2)*m;
G = 0.5; 
B = (1/6); 
wd = wn*(sqrt(1-(Z^2))); 
delta_t = 0.01;
c = 2*Z*sqrt(k*m); 
P = (100*sin(0.5*t))-(150*cos(0.5*t));
a1 = (m/(B*delta_t^2))+(G*c/(B*delta_t));
a2 = (m/(B*delta_t))+((G/B)-1)*c;
a3 = ((1/(2*B))-1)*m+delta_t*((G/(2*B))-1)*c;
k_hat = k + a1;
u=zeros(1,1000);
du=zeros(1,1000);
ddu=zeros(1,1000);
for i = 2:1001
    p_hat(i) = P(i)+(a1*u(i-1))+(a2*du(i-1))+(a3*ddu(i-1));
    u(i) = p_hat(i)/k_hat;
    du(i) = ((G/(B*delta_t))*(u(i)-u(i-1)))+((1-(G/B))*(du(i-1)))+(delta_t*(1-(G/(2*B)))*ddu(i-1)); 
    ddu(i) = ((1/(B*(delta_t^2)))*(u(i)-u(i-1)))-((1/(B*delta_t))*du(i-1))-(((1/(2*B))-1)*ddu(i-1));
end 
plot(u)
xlabel('Time (sec)')
ylabel('Displacement (m)')
title('Time vs Displacement (Newmark Method)')
grid on
end