Using event function in ode45

Question

g618034 2011-6-16

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https://ww2.mathworks.cn/matlabcentral/answers/9603-using-event-function-in-ode45

回答： Richard Buckalew 2016-2-29

Hi Everyone!

I'm solving a physical pendulum using ode45. I only want to solve it for one period, and I've chosen to use the event function to complete this task. In here a(2) would be the angular velocity of the pendulum. Right at the moment that the pendulum completes one period, the angular velocity would be zero. However, it would be the second zero point of angular velocity, while the integration stops at the first zero point.

Is there any way I could make the event occur twice before it stops? Here's what I've tried--but it's not working right.

        function[value, isterminal, direction] = event(t,a)
        persistent count;
        value = a(2);
        direction = 0;
        while count<=2
        value = a(1);
        direction = 0;
        count = count+1;        
        isterminal = 0;
        end
        isterminal = 1;
        end

Any suggestions would be appreciated. Thank you!

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Answer 1

Richard Buckalew 2016-2-29

2
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https://ww2.mathworks.cn/matlabcentral/answers/9603-using-event-function-in-ode45#answer_211716

The easiest solution is to integrate it, catch the event, and then let your end Y become a new initial condition and integrate again. The next time it stops will be the second time the event occurs.