Event Detecting in Event-based ODE

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Mostafa Nazmi 2021-6-11

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https://ww2.mathworks.cn/matlabcentral/answers/853625-event-detecting-in-event-based-ode

评论： Bjorn Gustavsson 2021-6-14

Greetings. I'm trying to detect an event of a variable value crossing tha value of zero. The problem I'm facing is that the ode solver is detecting the value at 0.1*(1.0e-14) instead of zero. I'm using ode45 and the odeset options below:

ODEopts = odeset('Events', @event, 'Refine', 8, 'RelTol',...
    10^-6, 'AbsTol', 10^-6); 

Can I solve this issue by changing the options or the solver?

Thanks in advance

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Answer 1

Bjorn Gustavsson 2021-6-11

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https://ww2.mathworks.cn/matlabcentral/answers/853625-event-detecting-in-event-based-ode#answer_722385

In order to advice the contents of the event-function is necessary. The only thing to point out is that the ode-solvers use numerical precision of 32-bit(?) doubles - and 1e-14 look like a "reasonable default" setting of zero-detection. If your use-case is something on atomic scales and you use metres as units for your spatial coordinates (convert to what is sensible to your case) then 1e-14 m is clearly not a good enough precision for an event, but ode45 wouldn't know. Your easiest way forward is then to rescale the ODEs to use Å for your spatial coordinates.

HTH