An integrated inductive proximity sensor

Inductive proximity sensors are widely used for the contactless measurement of object or target displacement and position in numerous technical products and systems. They are found in various application domains such as transportation, robots, assembly lines, telecommunication or security. The inductive sensing principle is known for its robustness, high precision and low sensitivity to environmental conditions as well as to extreme working conditions like cryogenic temperatures. The typical measuring range of inductive proximity sensors is between 0.1 cm and 5 cm. The overall dimensions of the smallest inductive proximity sensors, including the sensing coil and the electronic circuit interface, is rarely under some cubic centimeters, due to the high number of components of which such sensors are composed. Although the electronic interface circuitry may be integrated in an "application specific integrated circuit" (ASIC), the traditional fabrication processes of the inductive primary transducer (the coil and the flux concentrator) set the limit to the scaling down. The introduction of the inductive principle for proximity sensors into new technical mechatronic systems requires a small size and ease of fabrication (i.e. a low cost). The challenge of this thesis work was to develop such a sensor by using new technologies for the integration of microsystems. In this work, we successfully demonstrate the integration of an inductive proximity sensor on a small-size chip, and its usefulness as a key component in new sensing and mechatronic applications. To the best of our knowledge, we have realized the first fully integrated inductive proximity sensor ever reported. This challenge has been taken up by first studying the effect of the miniaturization on the basic behavior of sensing coils, and then by determining the scaling down laws as well as the limitations due to the parasitics, especially the increase of the inductor series resistance. Considering these first results, we developed a new and simple electronic interface, namely the differential relaxation oscillator. This highly sensitive, self oscillating circuit has only few components and the output signal is digital compatible. This readout principle is well adapted for miniaturized coils with low Q factor. A comparator has been designed and integrated with the two relaxation branches, forming the oscillator, on an ASIC. The miniaturized flat coil has been integrated on top of this ASIC using photolithography and electrodeposition compatible with the standard processes. The connections between the coil and the electronic circuit interface is realized through vias. In its smaller version, the integrated sensor chip size is of 1.5 x 2 mm2 with a square coil of 1 x 1 mm2 on top. This miniaturized flat coil has an inductance of 75 nH, a serial resistance of 6.2 Ω and a resonance frequency of 315 MHz. Its excitation frequency is close to 10 MHz. No external component is needed for this microsensor basic functioning, and the connections with the external world are limited to power supply and output signal. This integrated sensor is fabricated using the 1 µm 3V CMOS technology and the standard and compatible gold bumping layer to form the coil. This new device has been successfully tested. In the measuring range from 50 µm up to 150 µm, it shows a high sensitivity (24 kHz/µm) and a submicrometric resolution. The temperature behavior of integrated coils, of the electronic circuit principle and of their combination as a integrated sensor, has been studied. A simple temperature compensation scheme using one NTC resistor, compatible with the sensor integration, has been developed and tested successfully. Using this method, a temperature independence better than ±100 ppm/°C between -20 °C and +80 °C has been achieved with the 3.8 mm side flat coil version of the integrated inductive proximity sensor. New application have been demonstrated, using the good performances of the developed inductive proximity sensor microsystems. Metallic profile and inductive coin imaging have been successfully recorded. The angular position control of a watch motor has been evidenced with an angular precision of 2 DEG (limited by the step motor). The angular speed and position sensing of a non-ferromagnetic toothed wheel has been sensed up to at least 7800 rpm (corresponding to an excitation of 13 kHz). The application and the promising perspective of our integrated inductive proximity sensor into active magnetic bearings for hard disk drives has been presented. This multidisciplinary thesis work has been realized with emphasis on using simple elements and on obtaining a simple overall behavior. The device has only one coil, it uses only one comparator, it is made of only one chip, and moreover it can be fabricated in only one wafer foundry using standard processes. By focusing on the optimization of the global sensor system, good performances have been achieved and the integration of an inductive proximity sensor has been proven.


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