Why do I get an "odezero: an event disappeared (internal error)" error when using an ODE events function in MATLAB 7.4 (R2007a)?

I am trying to use an events function to in my ODE function. However, when I run the attached file 'ode_events_error.m' I am getting the following error message:

 ??? Error using ==> funfun\private\odezero at 50

 odezero: an event disappeared (internal error)

 Error in ==> ode45 at 419

     [te,ye,ie,valt,stop] = ...

 Error in ==> mycode at 10

 [r,state,re,statee,ie] = ode45(@(r,state)prbode(r,state,tauc),rspan,stateinit,options);

Answer 1

[MathWorks Support Team]

This is a zero-crossing events error. The events locator is incapable of capturing a zero crossing for complex signals.

You can see this by adding the following code to the 'events' subfunction in 'ODE_events_error.m'function:

if ~isreal(state(2))

      error('Imaginary Events signal (%g +i%g) at r=%.16f.',real(state(2)),imag(state(2)),r)

end

You will see that during the simulation the value state(2) becomes complex, and the following error message is returned:

 ??? Error using ==> mycode>events at 37

 Imaginary Events signal (4.02767e-009 +i7.46735e-027) at r=2.7392559940601449.