This paper proposes an original approach for modelling ferromagnetic sheet in inverse problem. It is realised with layers of tangential dipoles. This distribution is built thanks to measurements on sensors located in the interior of the device. The problem being ill-posed, we propose a new regularisation method to obtain a physical solution by injection of direct problem (DPI). It's then easy to compute the field everywhere around the device where sensors can't be placed. Moreover, the permanent magnetisation of the shell can be identified and systems with fewer equations than unknowns can be solved.