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Analysis and improvement of a magneto-resistive position measurement circuit

Helmut Wernick

This student project deals with a position measurement circuit, which was designed within a diploma thesis in collaboration with Leotec GmbH. The measurement principle of this circuit is based on the magneto-resistive effect, which causes a change of resistance depending on variable magnetic fields.

Structure of the hardware: above the PCB there is a carriage with a permanent magnet. This carriage is moveable over a distance of 100 mm, where 8 magneto-resistive sensors are placed. The signals of the sensors are amplified by instrumentation amplifiers and flow through low pass filters and unity gain buffers to four AD-converters. These converters have a sampling rate of 1 MSPS and a resolution of 16 bits. Following the digitalization the AD-converters get sequentially prompted by an FPGA-Micromodul "Spartan 3" of the company Trenz Electronic to transmit the signals via a 16 bit wide parallel bus. The data can then be fetched into a PC via a serial interface.

The aim of the diploma thesis was to create a hardware which has a resolution of 5 µmm, which means a digital resolution of at least 12 bit. Sadly this target was not reached because of disturbances caused by insufficient parts, external coupling from the supply and PCB layout mistakes. Accumulating these, the maximum resolution was determind to be 9 bit.

The objective of this student project was to find the causes of the disturbances, isolate and eliminate them. To reach this target, a special measurement device, which is able to gauge voltages in the range of µV, was developed. After detailed analyses and some changes in the schematic, a resolution of 13-14 bits can be expected.

Figure 1: PCB and carriage

Figure 1: PCB and carriage

August 31, 2007