Comparison of resistance-based strain gauges and fiber bragg gratings in the presence of electromagnetic interference emitted from an electric motor

Abstract

This thesis reports a performance analysis of resistance based strain gauges and fiber optic fiber Bragg gratings in an environment contaminated by high levels of electromagnetic interference. The obtained results are directly applicable to the development of aerospace vehicles propelled by electrical motors. An area of importance in this relatively new technology is characterizing the mechanical loadings coming off a propulsion device in a stationary setup. This characterization is usually accomplished through the utilization of load cells. The majority of the load cells used in such an application are based on measurements acquired through resistance strain gauges. However, electric motors are known to radiate electromagnetic interference (EMI), which in the case of brushless DC motors is pulsing, alternating, square waves. This EMI severely degrades the signal produced by the resistance strain gauge. This degradation is due to the gauge's metallic construction, acting as an antenna for the EMI. To evaluate the performance of alternative strain measuring methods, a load cell implementing both the resistance strain gauge and fiber Bragg grating sensor, the latter of which is immune to EMI, was designed as a test article. The load cell was calibrated and demonstrated a thrust load sensitivity of 1.93 ±0.04 lbf through the strain gauge system and 0.56 ±0.56 lbf through the fiber Bragg grating system. The device was subjected to both mechanical loading and EMI to quantify the effect of the EMI on the resistance strain gauge. Testing of the device included operating a brushless DC motor, with a coupled flywheel, attached to the load cell at a range of angular velocities from 500 to 2400 RPM. During laboratory testing the resistance strain gauge signal exhibited an important amount of signal spikes and electrical noise, introduced by the EMI contamination; the fiber Bragg grating did not. The spikes increased linearly with the speed of the motor. The electrical noise required bandpass filtering to extract the mechanical signal, which was obtained without noise in the fiber Bragg grating signal. The resistance strain gauge signal, at a maximum, had a signal to noise ratio of 0.0443; the fiber Bragg grating signal, at a minimum, had a signal to noise ratio of 2.0114. These results demonstrated the fiber Bragg grating is more applicable in an EMI contaminated environment.