Null controllability for distributed systems with time-varying constraint and applications to parabolic-like equations

We consider the null controllability problem fo linear systems of the form $ y'(t) = Ay(t)+Bu(t) $ on a Hilbert space $ Y $. We suppose that the control operator $ B $ is bounded from the control space $ U $ to a larger extrapolation space containing $ Y $. The control $ u $ is constrained to lie in a time-varying bounded subset $ \Gamma(t) \subset U $. From a general existence result based on a selection theorem, we obtain various properties on local and global constrained null controllability. The existence of the time optimal control is established in a general framework. When the constraint set $ \Gamma (t) $ contains the origin in its interior at each $ t>0 $, the local constrained property turns out to be equivalent to a weighted dual observability inequality of $ L^{1} $ type with respect to the time variable. We treat also the problem of determining a steering control for general constraint sets $ \Gamma (t) $ in nonsmooth convex analysis context. Applications to the heat equation are treated for distributed and boundary controls under the assumptions that $ \Gamma (t) $ is a closed ball centered at the origin and its radius is time-varying.