How can I stop ode45 when certain derivative is negative?

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Juan Manuel Hussein Belda
评论： Juan Manuel Hussein Belda 2021-11-30
I have a set of differential equations which I am trying to integrate.
I have created this function in order for the integration to stop when a value is reached:
function [value, isterminal, direction] = myEventFunction(t, Y, c0_CO)
tol = 0.001;
value = Y(1) - 0.001*c0_CO;
isterminal = 1; % Stop the integration
direction = 0;
end
However, I would like to add another condition for value, and it is
value = (Y(1) - 0.001*c0_cO) || (dYdt(6) < 0)
I have been trying many things but I am failing at making dYdt an input for this function, and I do not know how to check it otherwise and stop it.
If anyone could help me with this I would really appreciate it :)

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回答（1 个）

Steven Lord
Steven Lord 2021-11-30
Your events function can return a vector of values and detect an event if any of the elements of that vector crosses zero. See the advanced example on this documentation page for an illustration of this technique.
edit orbitode
  2 个评论
Juan Manuel Hussein Belda
Juan Manuel Hussein Belda 2021-11-30
I have been looking at the example and I am not sure I understand it right. So, in my case, if I want it to stop when one of the derivatives of Y (integrated in ode45), then I should copy in the event function the formula for such derivative?
Juan Manuel Hussein Belda
Juan Manuel Hussein Belda 2021-11-30
The following happens to me:
Thus, I would like the integration around time 3e-3 seconds, because then it starts oscillate. I do not want to remove, however, the steady state for the temperature. Thus, I wanted to stop whenever a negative slope was reached (which I think leads to the oscillatory behaviour in Figure 3). I am somehow confused because I copied the formula for dTdt and either it never stops or it never reaches a negative derivative (which is counterintuitive from Figure 3).
Thank you!

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