An Integrated Microtransformer System For Displacement Measurement

The paper discusses the design and prototype evaluation of an integrated inductive microsystem used for displacement measurement, realized in the form of an ASIC. The advantage of the system is in its ability to operate without external stimulation such as a magnetic field source or an optical source used in conventional position encoders. The system comprises multiple microtransformers with corresponding analog front-end electronic circuits and is fabricated in an unmodified 350 nm CMOS process. The microtransformers consist of two concentric microcoils with their magnetic coupling dependent on the position of an external metal object, i.e. a target or a scale. The primary microtransformer winding is excited by an AC signal with a frequency of several MHz. This signal induces the secondary voltage, which has its amplitude and phase modulated by a moving metal scale. The modulation is then measured by the analog front-end as a differential output signal of two series of adjacent microtransformers geometrically shifted by a half of the scale period. The paper introduces the design and the construction of the microsystem, presents the results of the microtransformer characterization with a resolution of 100 mu m, and explains the output characteristics shape due to the asymmetry in the microtransformer pair.