How to use if statement in ode45? With one value changing w.r.t time.

I have a 1st order differential equation in which 1 value is constant and 1 is changing w.r.t to time.

dp/dt= q/c

where c =55

q=600 for 0 to 1 second and q=0 for 1 to 2 second.Total time is 2 sec.

I think to use if statement but not sure how to implement it with time. I have made a ode45 function program in matlab but the result of graph is linear directly proportional.

function dq = ode45(t,y)

q=600;c=55;

dq=q/c;

end

Answer 1

[Jan]

No, do not integrate a non-smooth function, see http://www.mathworks.com/matlabcentral/answers/59582#answer_72047 .

Note: Do not call the function to be integrated "ode45", because this is the function name of the compiler.

A numerical correct approach:

q   = 600;
y0  = 3.1415;  % Your initial value
fcn = @(t,y) yourFcn(t, y, q);
[T1, Y1] = ode45(fcn, [0, 1], y0);

q   = 0;
y0  = Y1(end, :);
fcn = @(t,y) yourFcn(t, y, q);
[T2, Y2] = ode45(fcn, [1, 2], y0);

T = cat(1, T1(1:end-1),    T2);  % The time t=1 occurs twice
Y = cat(1, Y1(1:end-1, :), Y2);

And:

function dq = yourFcn(t,y,q)
c  = 55;
dq = q/c;
end

See also: http://www.mathworks.com/matlabcentral/answers/1971

[MG]

Thanks a lot . This algorithom solved my three DOF problem ( six first order ODEs) with three different conditions