Etching-Based Measurement Error Compensation for Eddy Current Displacement Measurement

Eddy current sensors have an important role in various applications, such as displacement measurement. A common use case is the vibration measurement in rotating machinery. The vibrations can be detected by measuring changes in relative displacement of the shaft during the machine operation. The eddy current sensors measure the displacement indirectly by inducing a small electric current on the surface of the shaft. The displacement can be measured as the intensity of the induced currents vary relative to the displacement. However, the intensity of the induced currents are also affected by variations in material properties around the shaft surface. The sensor may detect the material property variations as a displacement change and thus a measurement error is introduced. The goal of the present study is to demonstrate a method for mitigation of these measurement errors. This method uses localized chemical etching treatments on the surface of a shaft. The treatment produces a local passivated layer on the treated surface, which can be used to compensate the existing measurement errors. The response of the eddy current sensor can be controlled by varying the duration of the etching treatment. In the present study, localized etching treatments were performed on a round test piece. The treatment duration was varied and an eddy current sensor was used to measure the runout of the test piece was measured before and after the treatments. The largest measured changes caused by the etching treatment were approximately 12 um, This study also demonstrated how this treatment can be applied to compensate the measurement errors in practice. The presented compensation method can be especially useful in cases where the relevant standards restrict the use of conventional measurement error compensation methods.