Enforcing values in ODE solver

I am trying to solve a stiff ode with ode15s. There is a variable for which there exists a physical constraint due to which it cannot exceed a certain maximum (the allowed maximum also changes with other variables).

I could use events to stop the ode integration and reset the variable but there is a regime where the event persists for some time so, I don't know if exiting the solver each time after just one iteration is efficient.

I also tried using global variables, so that I can always manually control it but then it becomes unphysical when solver rejects a step and goes back.

Is there a way to apply the constraint within the solver? Is there some other solver that could help or would it be better to write one by myself?

Thanks!

[Torsten]

I could use events to stop the ode integration and reset the variable but there is a regime where the event persists for some time so, I don't know if exiting the solver each time after just one iteration is efficient.

This is only possible if you define the event as a discontinuous function, and this is not allowed. An allowed event function has a zero crossing without remaining in the state.

Answer 1

[Walter Roberson]

Use events, and remember that events are only detected when a value crosses the trigger, and are not detected when the system starts in that state.

[Pulkit]

Oh in that case, I don't think i can make use of built in solvers. The variable reaching a maximum is not a one-time event but a whole regime that would persist for some time. Thanks for the reply.