Force Measurements in Magnetic Bearings Using Fiber Optic Strain Gauges

Abstract

A new method for measuring forces in magnetic bearings is presented. Fiber-optic strain gauges (FOSGs) mounted to the side of the magnet poles are used to detect the small levels of strain that the metal experiences as the bearing exerts a force. These strains can be converted into force components, providing measurements with a previously unattainable level of accuracy and precision. Tests were done using the Magnetic Bearing Test Rig at the Texas A&M University Turbomachinery Laboratory. Two FOSGs were placed approximately 90° apart on two separate poles of one of the bearings, and the strain levels for different load magnitudes and directions were measured. The raw signal has several undesirable attributes that prevent an accurate static measurement. However, dynamic measurements proved to be very effective in the frequency domain, as most of the noise in the signal is confined to frequencies below 1 Hz. Due to the raw signal characteristics, new techniques for load application and calibration were developed. By using these new approaches, an equation relating reaction force components and strain was generated. This equation provides precise knowledge of any force vector in the bearing. An uncertainty analysis was performed on the resulting equation, providing a measure of resolution and a reduction in error several times more precise than any previous result. As a result of these findings, magnetic bearings can now be used to perform precise diagnostic analysis, determine rotordynamic coefficients, and improve magnetic bearing design and performance.