In sensor systems one of the most crucial issues is represented by the insensibility of the sensors with respect to the disturbances to obtain precision in the detected measurements. This paper proposes two different observers to identify an input voltage disturbance in sensors for ferromagnetic particle monitoring. The disturbance is identified together with the state variables which can be used for a controller. Because of a sinusoidal function being chosen to be the reference signal of the sensor, the repetitive controller is proposed to make robust this periodic work function of the sensor. The tracking of this working point is realised using a classical feed-forward control, which is made robust using a resonant control loop. The resonant control loop provides not only for the robustness but in the meantime minimises the energy needed for the sensor. The resonance is maintained by using the repetitive controller. In this sense the resonance region of the system guarantees a structural robustness and optimality. The repetitive control provides a tracking throughout the cycles. Computer simulations are devoted to compare the obtained results and conclusion closes the paper.