[R-sig-dyn-mod] Smooth transitions

Hi,
I am looking for a simple way to simulate 
smooth transitions of weather a positive or 
negative absolute rate:

Between two discrete time points a defined
amount (e.g. food, nutritions etc.) has to go into
the system or leave the system (ODE). Preferable 
this transition should have a sigmoidal shape.

In the end I would like to write a root function
triggering the start of each particular transition.

Thanks,
Simeon