This paper presents a linear, decoupled, robust output trajectory tracking controller, addressed as flat filtering controller (FFC), for a multiple input multiple output (MIMO) nonlinear differentialy flat systems. The flat filter only requires the outputs to be regulated with the help of a well tuned filter and a suitable linear combinations of the internal states of such filter. To illustrate the proposal, the Permanent Magnet Synchronous Motor (PMSM), which is a flat MIMO system with the angular position and the id current as flat outputs, is considered. Although the control inputs are coupled, the controller is designed on the basis of a drastic simplification to a first and third order pure integration systems respectively with additive disturbances to be compensated. The robustness of the proposed scheme, with respect to ignored nonlinear state-dependent, endogenous, disturbances and, also, to independent unstructured exogenous disturbances, is verified via computer simulations as well as experimental results.