How to make ode15s not to cross zero?

Here is the solution to a differential equation by simulink(ode15s, orange) and matlab(ode15s, blue). The solution mustn't cross Zero, how to prevent that in matlab ode15s?

Answer 1

[Steven Lord]

Does your problem involve a mass matrix? If not, use the NonNegative option as shown in the Knee Problem example on this documentation page.

If your problem does involve a mass matrix, use an Events function as shown in the bouncing ball example and the three body problem example on this documentation page. In this case you'll need to stop the solver, recompute what the new "initial" conditions should be, determine if the differential equations themselves change, and restart the solver. In the case of the bouncing ball, the ballode.m file recomputes the initial conditions at each bounce to reflect the ball rebounding from the floor.

[Akbar]

Thank you for interesting solution. I am not using mass Matrix, and don't know if i Need it at all. I just know that the differential eq's is stiff, so i am using stiff solver, ode15s.

These are the Options i tries, and Matlab solution still crosses Zero. Seems like nonnegative thing didn't work. I wrote ones(1,14) because all 14 states should be nonnegative. I mean I have a 14 dimensional System.

options = odeset('RelTol',1e-6,'AbsTol',1e-6,'maxstep',0.2,'nonnegative',ones(1,14));
[t{ind},X{ind}] = ode15s(func,0:.01:170,X0,options);

[Steven Lord]

Specify 1:14 as the value for the NonNegative option.

Answer 2

[Torsten]

Tighten the tolerances RelTol and AbsTol, e.g.

opts = odeset('RelTol',1e-8,'AbsTol',1e-8);

Best wishes

Torsten.

[Akbar]

That didn't help, I tried, thanks though.

[Torsten]

What is the differential equation ?