ODE45: IF/loop function
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Hello,
I would like to know if its correct to write my differential equations as follow in the 'if' command in the function file. If not, please advice. I am trying to model a simple damper.
Thanks.
function [dy] = SDOF2(t, u);
dy = zeros(2,1);
m=1000; %Mass (lb.sec^2/in.)
k =100000; %Stiffness (lb/in.)
omega = sqrt(k/m); %Natural Frequency
c=2000;%Damping coefficient (lb.sec/in.)
g =386;%Acceleration of gravity (in./sec^2)
c_cr=2*m*omega; %Critical damping coefficient
xi = c/c_cr;%Damping ratio
%%Define the forcing function
% if t<=0.5
% F = sin(4*pi*t);
% else
% F =0;
F = 0;
% b=1*m*(((-omega*omega*u(1)-2*xi*omega*u(2)+F))/u(2)>0);
% end %%%%-ESTIMATION-%%%%
if ((((-omega*omega*u(1))-(2*xi*omega*u(2))+F/m)/u(2))>0)
dy(1) = u(2);
dy(2)= -omega*omega*u(1)-2*xi*omega*u(2)+F/m-0.5*m*((-omega*omega*u(1))-(2*xi*omega*u(2))+F/m);
else
dy(1) = u(2);
dy(2)= -omega*omega*u(1)-2*xi*omega*u(2)+F/m;
end
% dy(2)= -omega*omega*u(1)-2*xi*omega*u(2)+F/m;
end
7 个评论
Walter Roberson
2020-1-9
You need to switch to using event functions to terminate the ode45 call and then restart it.
ode45 requires that the function be differentiable. When you have an if in the function it is rarely differentiable. You need to stop at the discontinuity and restart.
Komal Rajana
2020-1-10
编辑：Komal Rajana
2020-1-10
Walter Roberson
2020-1-10
Strictly speaking, no. However you can subtract eps(realmin) from the value as the result will be negative if the value was exactly 0.
Aquatris
2020-1-10
It might be easier to write your own RK method for this problem.
Komal Rajana
2020-1-10
编辑：Komal Rajana
2020-1-10
Meg Noah
2020-1-10
There's a solution here on the web:
Also, I've implemented a Runge-Kutta to solve baseball motion under atmosphere drag and lift:
It's a 3-D version.
Aquatris
2020-1-10
There are a lot of examples online, here is on of them.
采纳的回答
Jyothis Gireesh
2020-2-12
0 个投票
It is my understanding that the differential equation is a function of “u(1)” and “u(2)”. So it is safe to assume that “u(1)” and “u(2)” are symbolic variables (or can be defined as symbolic). In this case, it may be better to use the “piecewise” function which allows conditionally defined expressions or functions.
Please refer to the following documentation link to get information on “piecewise” function
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