Global variables to plot variables from ODE45 function

2023 9 7
1 个回答
更新时间：2023 9 14
4 次查看（30 天）

0 个投票

Hello,
the following code stands representative for a bigger code, in which i want to plot some variables over time that are calculated in the odefunction but are gonna be passed to the ODE solver. Is there any way to set these variables as global variables, so that I can plot them outside of the ode Function?
Like in this case to plot the variable "a", which is calculatet inside the function.
tspan= 0:.01:10;
x=1;
[t,x] = ode45(@(t,x) odefcn(x,t),tspan,x);
figure;
plot(t,a);
function [dxdt] = odefcn(x,t)
a=cos(t)*x;
dxdt = x*a;
end

请先登录，再回答此问题。

回答（1 个）

Torsten
Torsten 2023-9-7
编辑：Torsten 2023-9-7
Ran in:

0 个投票

Ran in:
t < pi/2 is necessary:
syms t x(t)
eqn = diff(x,t) == x^2*cos(t);
conds = x(0)==1;
dsolve(eqn,conds)
ans = 
tspan= 0:.01:10;
x=1;
[t,x] = ode45(@(t,x) odefcn(x,t),tspan,x);
Warning: Failure at t=1.562403e+00. Unable to meet integration tolerances without reducing the step size below the smallest value allowed (3.552714e-15) at time t.
a = zeros(size(t));
for i = 1:numel(t)
[~,a(i)] = odefcn(x(i),t(i));
end
figure;
plot(t,a);
function [dxdt,a] = odefcn(x,t)
a=cos(t)*x;
dxdt = x*a;
end

6 个评论
显示 4更早的评论 隐藏 4更早的评论

Marlon
Marlon 2023-9-8
编辑：Marlon 2023-9-8
Thank you for the answer.
I tried to implement this in the real code, however i always get the error
">> Zustandsraum_Pacjeka_App
Index exceeds the number of array elements. Index must not exceed 1.
Error in Zustandsraum_Pacjeka_App>odefcn (line 72)
rv = [x(4)-lv*x(6)*sin(x(3)); %x(3): PSI, x(4): XV', x(6): PSI'
Error in Zustandsraum_Pacjeka_App (line 26)
[~,a(i)] = odefcn(x(i),t(i));"
I tried to implement it like this, perhaps you could have a look at it :) :
(..)
[t,x] = ode45(@(t,x) odefcn(x,t) ,tspan,x );
a = zeros(size(t));
for i = 1:numel(t)
[~,a(i)] = odefcn(x(i),t(i));
figure;
plot(t,a);
end
function [dxdt,a] = odefcn(x,t)
(..)
vrv = [cos(x(3)+delta) sin(x(3)+delta) 0;
-sin(x(3)+delta) cos(x(3)+delta) 0;
0 0 1]*rv;
a=vrv(1);
Torsten
Torsten 2023-9-8
编辑：Torsten 2023-9-9
What is rv ? A column vector with three elements ?
Marlon
Marlon 2023-9-14
yes, exactly.
When I try to extract any variable that is dependent on state variables, I get the mentioned error message.
I noticed that It works to extract variables that are only dependent on the time and not on state variables, if I code it with "[~,a(i)] = odefcn(x,t(i));" and not "[~,a(i)] = odefcn(x(i),t(i));"
However I also need to extract variable that are calculated with state variables..
Torsten
Torsten 2023-9-14
If you want to extract vrv as a 3x1 vector, use
a = zeros(3,numel(t));
for i = 1:numel(t)
[~,a(:,i)] = odefcn(x(i,:),t(i));
end
Marlon
Marlon 2023-9-14
Thanks for the quick answer, but when I try to plot the first row of vrv vs time
I get following error:
Error using plot
Vectors must be the same length.
Error in Zustandsraum_lin (line 62)
plot(t,vrv(:,1))
Torsten
Torsten 2023-9-14
Change the code according to your needs:
t = 0:1:100;
x = [t.',t.^2',t.^3']
a = zeros(3,numel(t));
for i = 1:numel(t)
[~,a(:,i)] = odefcn(x(i,:),t(i));
end
plot(t,a(2,:))
function [dxdt,vrv] = odefcn(x,t)
rv = [1;1;1];
delta = 0;
vrv = [cos(x(3)+delta) sin(x(3)+delta) 0;
-sin(x(3)+delta) cos(x(3)+delta) 0;
0 0 1]*rv;
end

请先登录，再进行评论。

类别

在 帮助中心 和 File Exchange 中查找有关 Function Handles 的更多信息

产品

版本

R2022b

标签

提问:

Marlon
Marlon
2023-9-7

评论：

Torsten
Torsten
2023-9-14

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by