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|Fully Integrated Sensor Electronics for Inductive Proximity Switches Operating up to 250 °C|
|Keywords: Proximity Switch, Inductive Sensor, SOI CMOS|
|State of the art sensors comprise extensive signal conditioning and processing circuitry realized as mixed signal integrated circuit. Standard electronic components, however, are not suitable for use at high temperatures. In order to extend the operational temperature range of integrated circuits, Fraunhofer IMS provides a 0.35 µm high temperature SOI CMOS technology allowing circuits to work up to 250 °C. This technology is the base for the development described in this paper. Inductive proximity switches are widely used in industry and process automation to monitor the position of tools and components due to its contactless sensing principle which is insensitive against environmental conditions like dust and dirt. Although the robustness and reliability of these kind of sensors has led to a very wide spread of inductive proximity switches, the area of high environmental temperatures up to 250 °C has not been solved satisfactory yet. In this paper we will present the design, simulation and measurement results of a fully integrated inductive proximity switch electronics which requires very few external components and can operate under ambient temperatures up to 250 °C. The sensor system is based on the fore mentioned technology which was specifically developed for high temperature operation. The core of the circuit is built by an oscillator which is integrated together with rectifier and readout electronics for threshold detection, references and voltage regulators to provide the necessary internal voltages as well as extensive trimming capabilities to compensate for temperature effects. The circuit can be operated from a single dc-voltage supply from 12 to 35 Volts. Calibration data can be stored in an internal EEPROM. Switching distance and hysteresis are also programmable for adapting the circuit to a wide range of different detector coils. Two output signals are provided that can be independently set to function as push/pull or single polarity switches with programmable polarity. The only external components required are a blocking capacitor for supply voltage stabilization and the LC resonator. Reverse polarity protection and special high temperature ESD and clamping structures are also fully integrated on the silicon die. We will elaborate in detail on the ASIC implementation and design considerations that allow operation in the wide temperature range from -40 °C to 250 °C. Simulation and measurement results of the main features will be presented. The ASIC was fabricated, assembled and functional as well as first performance measurements were done. Finally we will elaborate on possible improvements and give an outlook on future work for this kind of sensors.|
|Sebastian Braun, Scientific associate
Fraunhofer Institute for Microelectronic Circuits and Systems IMS