ODE45 & heaviside- [Inputs must be floats, namely single or double.]

Question

baney acosta 2018-5-9

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/399948-ode45-heaviside-inputs-must-be-floats-namely-single-or-double

编辑： baney acosta 2018-5-10

采纳的回答： Steven Lord

在 MATLAB Online 中打开

Hi!

I'm having problems with the code when solving differential equations by ode45.

The point is that I must enter singular functions from GUIDE:

u(t) -> Step. " ONLY THE HEAVISIDE FUNCTION ALLOWS ME"

Code:

vin=get(handles.editE,'String');
vin=strrep(vin,'u','heaviside');
t=(str2double(get(handles.editI,'String')):0.001:str2double(get(handles.editF,'String')));
tspan = [0 9];
vs=(str2double(get(handles.editPVI,'String')));
R=(str2double(get(handles.editR,'String')));
C=(str2double(get(handles.editC,'String')));
tau = R*C;
syms t;

NOW HERE I TRIED:

vin = eval (vin) -> not working.
vin = symfun (vin, t) -> not working.
syms vin -> not working.

.

f =@(t,v) vin/tau-v/tau; v0=0;
% works like this (literally the function):
% f =@(t,v) heaviside(t)/tau-v/tau; v0=0;
% [t,v] = ode45(f,tspan,v0)

But I can not put this function as such, since vin can be a step, pulse, ramp, an exponential function, etc ...

[t,v] = ode45(f,tspan,v0);
axes(handles.axesVc);
grid on
plot(t,v);

Somebody help me :(

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Steven Lord 2018-5-9

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/399948-ode45-heaviside-inputs-must-be-floats-namely-single-or-double#answer_319459

The ode45 function cannot accept functions that return a sym object as the first input to ode45. You could use symbolic calculations inside the ODE function so long as what it returns is a column vector of data type single or double.

For two other approaches, you could use dsolve to try to solve the system of ODEs symbolically or you could use the odeFunction function to try to convert a symbolic expression into a function handle that is compatible with the numeric ODE solvers like ode45. See this page in the documentation for more information.

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

baney acosta 2018-5-10

编辑：baney acosta 2018-5-10

在 MATLAB Online 中打开

Hi.

In fact, I chose ode45 since the dsolve function gives me a strange result.

This is the result that should give me this equation:

And this is the result that dsolve gives me:

sign(t)/2 - exp(-t)/2 + 1/2

I really do not know why this is.

vin=get(handles.editE,'String');
vin=strrep(vin,'u','heaviside');
ti=str2double(get(handles.editI,'String'));
tf=str2double(get(handles.editF,'String'));
tspan =[ti:0.01:tf];
vs=(str2double(get(handles.editPVI,'String')));
R=(str2double(get(handles.editR,'String')));
C=(str2double(get(handles.editC,'String')));
tau = num2str(R*C);
syms t;
vc = dsolve(['Dv+v/',tau,'=',vin,'/',tau],'v(0)=0');
disp(vc)