Defect detection using eddy-currents and Hall effect sensor arrays

This article describes a new probe technology with enhanced flaw detection capability, which through the use of multiple magnetic field sensing elements, can also reduce inspection time. The probe contains a linear array of high-sensitivity Hall sensors and a racetrack coil. Instead of using individual Hall sensors to form the array, sensor arrays were fabricated directly on a wafer using photolithography techniques and then mounted in their unencapsulated form. The array dimensions are chosen to achieve high spatial resolution and to limit the overall probe size. Individual active elements have a minimum separation of 400 μm and a sensitive region 50 μm across. Electronic hardware and custom software have been developed to interface the new probes to a computer. Modulation of Hall current was introduced to discriminate the Hall signal from the effects of inductive pick-up and to improve signal to noise ratio. INTRODUCTION Conventional eddy current inspection uses induction coils to detect cracks and other flaws but it has been suggested recently that inductive probes are reaching their development limits and that new sensors are needed to push back the present boundaries of flaw detection [1]. For subsurface flaws such as cracks under fasteners in aircraft skins, the performance of inductive 69 sensors is limited by the need to use low frequencies to achieve significant penetration. Induction coil signals, being dependent on the rate of change of flux linkage, are less effective at low frequencies whereas typical solid state sensors do not suffer a diminished sensitivity in the low frequency regime. In a driver pick-up probe configuration, a relatively large coil can be used as a driver and high spatial resolution plus good low frequency performance achieved by using small solid state devices as sensors. Because the solid state sensors are easily fabricated as arrays, we can take advantage of this to produce probes that facilitate faster inspections. Although coil arrays have been successfully produced and tested, additional turns of wire are needed for subsurface flaw detection to improve the low frequency performance. However, this increases their size and reduces the spatial resolution of the probe [2]. Arrays made with individual Hall sensor integrated circuits (ICs) and a rectangular coil have been produced [3] but the integrated circuit package is much bigger than the actual Hall element therefore the spacial separation of the devices is limited by the packaging. The probes described in this article overcome these difficulties. We also describe electronic hardware and custom software developed to interface the probes to a computer. DESIGN OF THE HALL SENSOR ARRAYS The Hall sensors, type P2, grown by a molecular beam epitaxy (MBE) system on a 2 inch wafer, were obtained from the University of Manchester Institute of Science and Technology in the United Kingdom. They are new two-dimensional electron gas Hall devices which were designed using the AlGaAs-InGaAs structure shown in Fig A.1. The sensors are designed for low magnetic field measurement of the order 1 μT with a magnetic field amplitude resolution of a few hundred nanoTesla [4]. The characteristics of the P2 Hall sensor are given in Table A.1 [4]. Two types of arrays were fabricated on a single wafer: one with the devices connected in series and the other with them connected in parallel. While the series arrays are suitable for constant current drive, its input resistance is very high. The arrays in parallel are suitable for constant voltage drive and have low input resistance which requires a low supply voltage.