Error using ode45 in matlab

Hi guys

I have a block and spring system with no damping. I'm given the equation x'' = -k*(x1 - L1)/m + k*(x2-x1-w1-L2)/m. Where k = spring stiffness; x1 = initial position of block; L1 = length of spring; x2 = initial position of the end of block; w1 = width of block; L2 = length of second block.

I have converted the second order ode into 2 first order odes.

u1' = x1' = u2

u2' = -k*(u1 - L1)/m + k*(u3-u1-w1-L2)/m

This is my function

function ydot = jipo1(t,y)

m = 2; % mass of the block

k = 5; % Spring stiffness

L1 = 2; % length of unstretched spring

k2 = 5; % Spring stiffness

L2 = 2; % length of unstretched spring

w1 = 5; % length of block

yd1 = y(2);

yd2 = -k*(y(1)-L1)/m + k2*(y(3)-y(1))/m

ydot = [yd1;yd2];

end

This is my ode45 script

clc

m1 = 2; % mass of the block

k = 5; % Spring stiffness

k2 = 5; % Spring stiffness

wn1 = sqrt(k/m1); % Natural frequency

t_final = 20; % Calculation time

L1 = 2; % length of unstretched spring

L2 = 2; % length of unstretched spring

w1 = 5; % length of blockk

x1_0 = 2; % Initial displacement

x_dot_0 = 0; % initial velocity

X_0 = [x1_0,x_dot_0]; % form a vector (array) of initial conditions

[t,y] = ode45(@jipo1,[0,t_final],X_0);

When I run it Matlab gives me this error.

Error in ode45 (line 114)

[neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...

Error in jipo (line 18)

[t,y] = ode45(@jipo1,[0,t_final],X_0);

I would really appreciate any help that will help me understand what I'm doing wrong.

[Torsten]

You use the expression

yd2 = -k*(y(1)-L1)/m + k2*(y(3)-y(1))/m

although you only have y(1) and y(2).

Best wishes

Torsten.

Answer 1

[Yu Jiang]

In this line

yd2 = -k*(y(1)-L1)/m + k2*(y(3)-y(1))/m

you are using y(3)

However, both y and ydot should only have 2 elements.

What is the physical meaning of y(3), is it noise or disturbance?

[tyler brecht]

I have no idea why I did put y(3). I have been searching the internet for example codes and most put in a y(3). I have no idea on how to put my ODES into my function that incorporates my initial conditions

[Yu Jiang]

I guess you were using piece of code where the system has multiple mass. Do you have one spring or two springs? If you only need one, you can remove the entire term

+ k2*(y(3)-y(1))/m

[tyler brecht]

I have two masses and two springs which are attached to a wall (No damping). The equation for the 1st mass is x'' = -k*(x1 - L1)/m + k*(x2-x1-w1-L2)/m (This equation obeys newtons 2nd law of motion). it looks something like this -

http://nbviewer.ipython.org/github/pv/SciPy-CookBook/blob/master/ipython/attachments/CoupledSpringMassSystem/two_springs_diagram.png

[Yu Jiang]

In this example you need four states, namely, x1, x2, x1dot, x2dot. You need to simulate them together, instead of only simulating the first mass (the one on the left). In your code you only used 2 states, x1 and x1dot. The equations for x2 and x2dot are missing.