How to make ode15s not to cross zero?

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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?

Accepted Answer

Steven Lord
Steven Lord on 6 Jul 2018
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.
  2 Comments
Steven Lord
Steven Lord on 6 Jul 2018
Specify 1:14 as the value for the NonNegative option.

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More Answers (1)

Torsten
Torsten on 6 Jul 2018
Tighten the tolerances RelTol and AbsTol, e.g.
opts = odeset('RelTol',1e-8,'AbsTol',1e-8);
Best wishes
Torsten.
  2 Comments
Torsten
Torsten on 6 Jul 2018
What is the differential equation ?

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