In permanent magnetic DC machines the nonlinearity due to the quadratic terms of the current makes difficulties in the control system. In order to cancel this nonlinearity, a special current controller is proposed in this paper. The decoupling of the state current variables in the physical system is shown as a structural property and a decoupling strategy is developed. The designed decoupling controller is also able to keep and to guarantee the observability of the whole system almost everywhere in the time domain. The robustness with respect to the external disturbances is achieved. Thanks to its simple structure the resulting observer-based control system guarantees real-time applicability. Simulation results using real data measurements are reported to validate the proposed technique.