Error using ode45 in matlab

Question

tyler brecht 2014-10-23

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评论： Torsten 2014-10-24

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.

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Torsten 2014-10-24

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.

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Answer 1

Yu Jiang 2014-10-23

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在 MATLAB Online 中打开

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?

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tyler brecht 2014-10-23

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 2014-10-24

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.

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Error using ode45 in matlab

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