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Error in using ode45 for solution of nonlinear system of three equations which are interlinked to each other

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FAIZ UL HASSAN
FAIZ UL HASSAN 2023-4-12
评论： FAIZ UL HASSAN 2023-4-12
% Define the ODE system
f = @(t,x) [-(p1 + x(2)) * x(1) + p1 * Gb + D; -p2 * x(2) + p3 .* x(3) + p3.* Ib; -n * (x(3) - Ib) + u];
% Solve the ODE system
[t,x] = ode45(f, tspan, [x1; x2; x3]);
Error using vertcat
Dimensions of arrays being concatenated are not consistent.
Error in UpdatedTrialsmc>@(t,x)[-(p1+x(2))*x(1)+p1*Gb+D;-p2*x(2)+p3.*x(3)+p3.*Ib;-n*(x(3)-Ib)+u] (line 51)
f = @(t,x) [-(p1 + x(2)) * x(1) + p1 * Gb + D; -p2 * x(2) + p3 .* x(3) + p3.* Ib; -n * (x(3) - Ib) + u];
Error in UpdatedTrialsmc (line 54)
[t,x] = ode45(f, tspan, [x1; x2; x3]);

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回答（3 个）

Torsten
Torsten 2023-4-12
移动：Torsten 2023-4-12
Check
size(p1)
size(Gb)
size(D)
size(p2)
size(p3)
size(Ib)
size(n)
size(u)
If one of the sizes is not 1x1 (a scalar), your code won't work.
Bjorn Gustavsson
Bjorn Gustavsson 2023-4-12
Perhaps you need to check the size of each component in your ode-function:
f = @(t,x) [-(p1 + x(2)) * x(1) + p1 * Gb + D; -p2 * x(2) + p3 .* x(3) + p3.* Ib; -n * (x(3) - Ib) + u];
What are the sizes of p1, Gb, D, p2, p3, Ib, n, Ib and u? The error-message you get is what I typically get when I try to concatenate arrays of incompatible sizes.
HTH
Sam Chak
Sam Chak 2023-4-12
编辑：Sam Chak 2023-4-12
Ran in:
Hi @FAIZ UL HASSAN
Try this:
% Define the time-varying Disturbance
D = @(t) sin(pi/10*t);
% Define the scalar parameters
Gb = 1;
n = 1;
p1 = 1;
p2 = 1;
p3 = 1;
Ib = 1;
% Define the feedback input u
u = @(x) - x(1) - 2*x(2);
% Define the system dynamics
f = @(t, x) [-(p1 + x(2))*x(1) + p1*Gb + D(t);
- p2*x(2) + p3*x(3) + p3*Ib;
- n*(x(3) - Ib) + u(x)];
% Define the initial conditions
x0 = [1; 0; 0];
% Define the time interval
tspan = [0 20];
% Solve the system using the ode45 solver
[t, x] = ode45(f, tspan, x0);
% Plot the result
plot(t, x); grid on
xlabel('t')
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Sam Chak
Sam Chak 2023-4-12
编辑：Sam Chak 2023-4-12
Ran in:
Thanks @FAIZ UL HASSAN. It looks a little complicated to read the equation this way without breaking up the terms. But some parameters are unavailable. Try simplifying u by making up u1, u2, u3, ... This helps troubleshooting later. I guess that the error "Dimensions of arrays being concatenated are not consistent" is caused by u.
Edit: On a second look, I'm just guessing...💡 that u is some kind of a sliding mode forcing input structure and e, ed, edd refers to , respectively. S must be the sliding surface then.
Besides, check if sign(S) is a discontinuous function or just a scalar like this
sign(3.14159)
ans = 1
The ode45() solver does not work well with discontinuous function when .

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