The effect of surface roughness variations to eddy current displacement measurement

Eddy current sensors are widely used in various industrial applications. A common application where high resolution and repeatability is required is vibration monitoring in rotating machinery. Eddy current displacement sensors are used to detect vibrations by measuring very small movements of the rotating shaft. However, the electromagnetic properties of the target affect directly to the strength of the eddy currents in similar manner as the displacement change. Thus, the sensor may interpret the changes in the surface properties as changes in displacement causing measurement errors. Surface roughness causes similar errors in the eddy current measurements since the roughness variations on the surface also affect the eddy current flow. The goal of this study was to investigate the relationship between surface roughness and measurement errors in eddy current displacement measurement. Surface roughness modifications were performed to a round test piece using an abrasive filament brush. The runout of the test piece was measured before and after with eddy current sensor to detect changes caused by the brushing. Tactile surface roughness measurements were performed to evaluate the change in the surface roughness. The results showed that surface roughness variations cause significant changes to the eddy current measurements. Largest measured changes caused by the surface roughness variations were over 10 μm, The corresponding root mean square surface roughness decreased by 0.11 μm in the longitudinal direction and increased 0.06 μm in the tangential direction of the test piece. It was concluded that surface roughness variations can cause significant errors in the eddy current displacement measurement.