Pavel Kejik

Biography

Pavel Kejik received the diploma degree in 1994 and the Ph.D. degree in 1999 at the Czech Technical University of Prague. In 1999, he joined the Institute of Microelectronics and Microsystems at the EPFL to work on Institute's circuit design and testing. His research interests include fluxgate magnetometry and micro-Hall sensors combined with mixed-signal IC design and low-noise circuit design for industrial applications. Since 2014, Pavel is with Monolithic Power Systems company (the EPFL spin-off company Sensima Technology SA before acquisition) actively working on industrialization of magnetic sensors. He is inventor or co-inventor of several patents related to novel magnetic sensing structures and methods in the domain of contactless current measurement, angular sensing and non-destructive testing. He is giving a lecture devoted to recent developments in the field of smart Hall microsystems within the frame of a yearly Europractice course Smart sensor systems .

Official source

https://people.epfl.ch/pavel.kejik

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It is important to reliably characterize cracks in metallic mechanical parts in order to assess the serenity of damage due to fatigue. The most important parameter for this is the crack depth. This is notoriously difficult with standard Eddy current techniques. We show here that this can be achieved by measuring the local magnetic field induced by Eddy currents flowing around the crack. This is done by using a high-density array of micro-Hall sensors integrated in a single CMOS chip with a spatial resolution of 10 mu m. We present the dependence of the signal on varying crack depths, liftoff and yaw angle. Finally, we study the response of the sensor to cracks in a DC magnetic field, i.e. the flux leakage due to the presence of a crack. This is especially relevant to cases where cracks appear predominantly close to edges with complex geometries for which it is difficult to induce clean Eddy currents.

Ios Press2014

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Eddy current techniques are often used to detect and, in some advanced applications, to characterize fatigue surface-breaking cracks appearing in mechanical parts. One of the major difficulties in this approach is to reliably separate the crack contribution to the signal from the others (liftoff, edge effects, material property variations). This is particularly prominent in weld inspection. We present here a novel sensor consisting in an array of 200 micro-Hall sensors on a CMOS chip aligned over a length of 2 mm with a 10 mu m pitch. This sensor, which we call a "high-resolution magnetic camera", gives a direct measurement of the perturbation of the local magnetic field caused by the modification of the Eddy currents flow due to the surface-breaking crack. We show that using this technique, a crack has a very distinctive signature which can be easily distinguished from all other contributions. Finite element simulations are used to understand the signal response of different cracks. In addition, this system can perform "micro-magnetoscopy" on magnetic materials by giving a fine picture of the local variations of the magnetic field caused by the presence of a defect.

Ios Press2014

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A new offset reduction strategy for CMOS Hall devices is proposed. The novelty is to fragment the Hall device into multiple Hall blocks, distributed over the silicon area and easy to interconnect. The suitable number of Hall blocks and the bias current level in each block can be adjusted according to the requirements in terms of offset, offset drift and signal to noise ratio. A chip was fabricated in 0.35 µm CMOS standard technology to demonstrate the potential of this architecture. The chip shows promising results, and in particular, a very low offset drift was observed at the front-end output stage (of the order of 10 nT/°C).

Institute of Electrical and Electronics Engineers2013