What does this ODE error mean?

Hello,

I don't understand why I am getting the following error:

Error using odearguments (line 87)
The entries in tspan must strictly increase or decrease.

Error in ode15s (line 150)
    odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in trajecttest (line 39)
		[T,S] = ode15s(@bdipuniodefun, tspan, icv);

For this for loop:

spart = [0.05 0.05 0.];
vpart = [-1.7585E+7 -1.7585E+7 0.];

tstep = 1E-7; %Defining time step
tfin = 1.1E-6; %Defining final time
intspan = [0:tstep:tfin]; %Total time span
[introw,intcol] = size(intspan);
W = zeros((3*intcol),6); %Generates matrix of zeros that the trajectory solver will populate later


for t = 0:1:intcol/2-1
    
    x = spart(1);
	y = spart(2);
	z = spart(3);
	vx = vpart(1);
	vy = vpart(2);
	vz = vpart(3);
    
		icv = [x; y; z; vx; vy; vz];
		%Time span
        tspan = [intspan(2*t+1) intspan(2*t+2)/2 intspan(2*t+2)];
		[T,S] = ode15s(@bdipuniodefun, tspan, icv);
		[rownum,colnum] = size(S);
        W((1+3*t):(3+3*t),(1:6)) = S;
    
    vparts(1) = S(rownum,4);
	vparts(2) = S(rownum,5);
	vparts(3) = S(rownum,6);
	sparts(1) = S(rownum,1);
	sparts(2) = S(rownum,2);
	sparts(3) = S(rownum,3);
    
end

The ODE solver accepts the inputs if there is no for loop invovled like this:

x = 0.05;
y = 0.05;
z = 0.;
vx = -1.7585E+7;
vy = -1.7585E+7;
vz = 0.;

tstep = 0.5E-8; %Defining time step
tfin = 1E-8; %Defining final time
intspan = [0:tstep:tfin]; %Total time span
[introw,intcol] = size(intspan);

icv = [x; y; z; vx; vy; vz]; %Format: [x; y; z; vz; vy; vz]

%Time Span (sec)
tspan = [intspan];

[T,W] = ode15s(@bdipuniodefun, tspan, icv);
[rownum,colnum] = size(W);

So why is it that adding the for loop causes this error? I know that the numbers that the "tspan" array is taking in are exactly the same since I tested it in the command window.

[madhan ravi]

upload bdipuniodefun

Answer 1

[Stephan]

Hi,

the message is clear. See what happens if you inspect some lines of your code - therefore lets assume t=1 which is the second step of your loop in:

for t = 0:1:intcol/2-1    

So with t=1 what do we get as tspan?

t = 1
tstep = 1E-7;
tfin = 1.1E-6;
intspan = [0:tstep:tfin]; 
tspan = [intspan(2.*t+1) intspan(2.*t+2)/2 intspan(2.*t+2)]

And the result is:

tspan =

   1.0e-06 *

    0.2000    0.1500    0.3000
    

The error message tells us:

    Error using odearguments (line 87)
The entries in tspan must strictly increase or decrease.

Since your values first decrease and then increase you get this error.

Best regards

Stephan

[Tom Keaton]

Thank you!