Advantages of fiber Bragg gratings for measuring electric motor loadings in aerospace application.

Abstract

Electric motors are being investigated in-depth for their application in aerospace. Part of this investigation is the characterization of the loadings from the propulsion devices, in a stationary setup, usually accomplished through the utilization of load cells. The majority of the load cells used in this application are designed around a resistance-based strain gauge. However, electric motors radiate electromagnetic interference (EMI) when in operation, which degrades the signal retrieved through the strain gauge, due to the gauge s metallic construction acting as an antenna for the EMI. To demonstrate the advantage of fiber Bragg gratings (FBGs), with their immunity to EMI, a load cell implementing both sensor technologies was designed and subjected to the same mechanical loading and EMI, with a flywheel coupled to a brushless DC motor. The load cell had a sensitivity of 8.59 ± 0.18 N and 2.49 ± 2.49 N through the strain gauge and FBG system, respectively. The strain gauge signal contained the mechanical loading signal embedded in wideband noise and spikes (that increased linearly with motor angular velocity), while the FBG signal did not, with little noise. The raw strain gauge signal, at a maximum, had a signal power ratio (mechanical signal power divided by the overall signal power mean) of 21.06 at 104.72 rad/s; the FBG signal, at a minimum, had a signal power ratio of 40.09 at 52.36 rad/s. Therefore, on the basis of the mechanical tests performed in this work, the recommended sensor of choice for electric propulsion in aerospace applications is the FBG.