Eddy-current sensor

Abstract: This paper reports the principle of displacement eddy current sensors and highlights the advantages compared with other non-contact sensors. Key factors that influence the sensor accuracy are presented. An experimental study has been undertaken into the effects of different target metals measured by the sensors. Based on a theoretical analysis, the methods to overcome the inhomogeneity (electrical run out) of eddy current sensors are discussed. Some initial results are presented to demonstrate the efficiency of the methods.

Abstract: An eddy current sensor (10) has a sensor coil (100) disposed near a conductive film (6) formed on a semiconductor wafer (W) and a signal source (124) configured to supply an AC signal to the sensor coil (100) to produce an eddy current in the conductive film (6). The eddy current sensor (10) includes a detection circuit operable to detect the eddy current produced in the conductive film (6). The detection circuit is connected to the sensor coil (100). The eddy current sensor (10) also includes a housing (200) made of a material having a high magnetic permeability. The housing (200) accommodates the sensor coil (100) therein. The housing (200) is configured so that the sensor coil (100) forms a path of a magnetic flux (MF) so as to effectively produce an eddy current in the conductive film (6).

Abstract: In this paper, we present a new integrated eddy current sensor for proximity sensing and for the detection of microcracks on the surface of metals. The device consists of two stacked planar coils fabricated onto a glass substrate and encapsulated on one side by a Ni/Fe permalloy magnetic core. Fabrication of the device is achieved by a UV–LIGA thick photoresist lithography process, which involves the lithographic patterning of 15–25 μm thick molds using AZ-4000 series photoresist. The introduction of the permalloy core coupled with the thick conductor lines produces a high inductance, low resistance device capable of generating large magnetic fields at low driving currents. The device has been tested in the frequency range of 10–500 kHz and has been shown to work as both a proximity sensor and crack detector at input powers of 30 mW or less. When used as a proximity sensor, the unamplified output voltage on the sensing coil changes by as much as 75 mV with an aluminum target placed at a distance of 400 μm from the coil. The device has also shown the capability of clearly detecting cracks with depths of as little as 8 mil (200 μm) in both aluminum and titanium. Results show an extremely linear relation between crack depth and output signal voltage with an unamplified signal strength of several millivolts.

Abstract: Pulsed eddy current (PEC) testing is a new emerging and effective electromagnetic non-destructive testing (NDT) technique. The main purpose of this study is to identify surface defects and sub-surface defects using features-based rectangular pulsed eddy current sensor. The further study of PEC rectangular sensor proposed in author's previous work has been made to classify the different types of defects in specimen. In different directions of sensor scanning, peak waves of pick-up coil are studied. We find that when sensor is on different position against the defect, peak waves of response signals present the same shape in direction of magnetic induction flux, while present different shapes in direction of exciting current. Experiment results have shown that the different classes of defects can be identified and classified effectively by selecting the rising time as the time domain feature in both directions. For improving the performance of defect classification, two new features from differential response signal are proposed to classify different types of defects combined with rising time. One is called as crossing time; the other is differential time to peak. The blind test is carried out and the results show that the new features are effective to classify the defects.