# Error in ODE arguments

2 views (last 30 days)
Alexander Salas on 7 Dec 2021
Edited: Jan on 7 Dec 2021
Hello,
I am a newbie at MATLAB and I am trying to get a forcing function to work. I can get it to work without the F(t)/M but once I add it, it says error in ODE arguments:
clear
close all
% System parameters
m1 = 2000; Icg = 2500; % kg
c1 = 3000; c2 = 3000; % kg/s
k1 = 30000; k2 = 30000; % N/m
l1 = 1; l2 = 1.5;
M = [m1 0
0 Icg];
C = [(c1+c2) (c1*l1-c2*l2)
(c1*l1-c2*l2) ((c2*l2^2)+(c1*l1^2))];
K = [(k1+k2) (k1*l1-k2*l2)
(k1*l1-k2*l2) ((k2*l2^2)+(k1*l1^2))];
Br = [k1 k2
k1*l1 -k2*l2];
Brdot = [c1 c2
c1*l1 -c2*l2];
r = [.015 .015]';
r1 = [.086 .086]';
F = @(t) Br*r + Brdot*r1;
% Time grid
t0 = 0; tf = 10; dt = 0.01; t = t0:dt:tf;
% Set initial state and integrate equations of motion
s0 = [1 1 0 0]';
f = @(t,s) [s(3);s(4);(F(t)/M)*-C/M*[s(3) s(4)]'-K/M*[s(1) s(2)]'];
[t,s] = ode45(f,t,s0);
% Plot system motion
figure
subplot(211),plot(t,s(:,1),t,s(:,2),'LineWidth',2)
grid minor
legend('x1','x2')
xlabel('Time (s)')
ylabel('Displacements (m)')
title('System Dynamic Response')
subplot(212),plot(t,s(:,3),t,s(:,4),'LineWidth',2)
grid minor
legend('v1','v2')
xlabel('Time (s)')
ylabel('Velocity (m/s)')
Jan on 7 Dec 2021
This is no valid Matlab syntax:
M = [m1 0
0 Icg];
The blank line in the code is not allowed. Therefore I asked you to remove the blank lines.

Alan Stevens on 7 Dec 2021
I suspect you mean like this (notice the way M divides, using the back-slash):
% Numerical solution of IVP
% M*xddot + C*xdot + K*x = F(t) with x(0) = x0 and xdot(0) = v0
% System parameters
m1 = 2000; Icg = 2500; % kg
c1 = 3000; c2 = 3000; % kg/s
k1 = 30000; k2 = 30000; % N/m
l1 = 1; l2 = 1.5;
M = [m1 0
0 Icg];
C = [(c1+c2) (c1*l1-c2*l2)
(c1*l1-c2*l2) ((c2*l2^2)+(c1*l1^2))];
K = [(k1+k2) (k1*l1-k2*l2)
(k1*l1-k2*l2) ((k2*l2^2)+(k1*l1^2))];
Br = [k1 k2
k1*l1 -k2*l2];
Brdot = [c1 c2
c1*l1 -c2*l2];
r = [.015 .015]';
r1 = [.086 .086]';
F = @(t) (Br*r) + (Brdot*r1);
% Time grid
t0 = 0; tf = 10; dt = 0.01; t = t0:dt:tf;
% Set initial state and integrate equations of motion
s0 = [1 1 0 0]';
f = @(t,s) [s(3);s(4);M\F(t)-M\C*[s(3) s(4)]'-M\K*[s(1) s(2)]']; %%%%%%%%%%%%%%
[t,s] = ode45(f,t,s0);
% Plot system motion
figure
subplot(211),plot(t,s(:,1),t,s(:,2),'LineWidth',2)
grid minor
legend('x1','x2')
xlabel('Time (s)')
ylabel('Displacements (m)')
title('System Dynamic Response')
subplot(212),plot(t,s(:,3),t,s(:,4),'LineWidth',2)
grid minor
legend('v1','v2')
xlabel('Time (s)')
ylabel('Velocity (m/s)')
Jan on 7 Dec 2021
A further simplification:
f = @(t,s) [s(3); s(4); M \ F(t) - M \ C * s(3:4) - M \ K * s(1:2)];

R2020a

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