Not Sure How to Call a Norm from Within a Function

Hello,

I'm not sure how to make a plot of a norm that I found inside a function.

Here is my code, which will best help me explain:

clc; clear all;

ti = 0;  
tf = 15; 

global I11 I22 I33 Mx My Mz w10 w20 w30 eps10 eps20 eps30 eps40 IC K0

I11 = 160;  
I22 = 400;
I33 = 400;

Mx = 0;
My = 0;
Mz = 45;

w10 = 2;
w20 = -1;
w30 = 1;
   
eps10 = 0;
eps20 = 0;
eps30 = 0;
eps40 = 1;
K0 = eps10^2+eps20^2+eps30^2+eps40^2;

IC = [w10 w20 w30 eps10 eps20 eps30 eps40];

[t, y] = ode45(@(t,y) DynEqn1(t,y,I11,I22,I33,Mx,My,Mz), [ti tf], IC);


figure (1)
plot(t,y(:,1:3))

figure (2)
plot(t,y(:,4:7))

figure (3)
plot(t,(norm(y(:,4:7))-K0))

function soln = DynEqn1(t,y,I11,I22,I33,Mx,My,Mz)
w1 = y(1);
w2 = y(2);
w3 = y(3);
eps1 = y(4);
eps2 = y(5);
eps3 = y(6);
eps4 = y(7);
disp(norm(y(4:7)))

w1_dot = (Mx - w2*w3*(I33-I22))/I11;
w2_dot = (My - w1*w3*(I11-I33))/I22;
w3_dot = (Mz - w1*w2*(I22-I11))/I33;

eps1_dot = .5*(w1*eps4-w2*eps3+w3*eps2);
eps2_dot = .5*(w1*eps3+w2*eps4-w3*eps1);
eps3_dot = .5*(-w1*eps2+w2*eps1+w3*eps4);
eps4_dot = -.5*(w1*eps1+w2*eps2+w3*eps3);

soln = [w1_dot; w2_dot; w3_dot; eps1_dot; eps2_dot; eps3_dot; eps4_dot];

end

Basically, if I just display the norm from within the function, I get the values. But I would like to plot those values (minus a set value) versus time, but can't get it to work.

I tried defining the norm from within the function as a variable, K, but that didn't work either.

Any help is appreciated.

Thanks,

Jon

Answer 1

[Walter Roberson]

N = sqrt(sum(y(:,4:7).^2,2));
plot(t, N)

[Jon Stapchuck]

Thank you.

Why do you add the ',2'? What does it represent?

[Walter Roberson]

sum(EXPRESSION,2) is summation along the second dimension -- along the rows in other words. In this case equivalent to

N = sqrt(y(:,4).^2+y(:,5).^2+y(:,6).^2+y(:,7).^2);

[Jon Stapchuck]

Oh I see, thank you for the explaination