Design and implementation of a new high-accuracy interpolation encoder IC for magneto-resistive sensors

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TECHNICAL PAPER

Design and implementation of a new high-accuracy interpolation encoder IC for magneto-resistive sensors Wen-Yu Chen1 • I-Feng Chang1 • Paul C.-P. Chao1 • Smriti Thakur1 • Tse-Yi Tu1 Received: 15 December 2019 / Accepted: 1 July 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract A new algorithm named the eight-section (ES) method for carrying out the interpolation for a magneto-resistive (MR) encoder is successfully developed and fabricated in a digital chip. It is known that a conventional magneto-resistive (MR) encoder employs the interpolation method, which converts incoming front-end analog signals in harmonics in sinusoids/cosinusoids to moving displacement via calculating arc-tangents. This conventional interpolation requires divisions to be carried out for the displacement, which often leads to large noises while conducting digital computation, eventually undermining significantly the accuracy of the MR sensor. The proposed interpolation of eight-section (ES) is designed specially without divisions in the digital computation, leading to higher precision than the conventional interpolation conducting the computation of arc-tangents. The digital computation chip designed by this study consists of a cycle counter, two decimators for incoming analog signals of the MR sensor, a correcting circuit, and the proposal ES interpolation unit. The designed chip is successfully fabricated by TSMC 0.18-lm CMOS process, the area of which is 1643 9 1676 lm. The chip is than calibrated by a reference interferometer by experiments for further improving the measurement accuracy. The precision finally results in measuring displacement reaches as accurate as within 1.065 lm, which is much favorable to the existing performance around 2 lm by the conventional interpolation.

1 Introduction The encoders for a magneto-resistive (MR) encoder are commonly classified into two different types, linear and rotary encoders. Linear encoders have advantages like high accuracy and are economical for long travel length whereas rotary encoders are more cost-effective. On the other hand, the encoders can also generally be classified as optical and magnetic encoders also. Optical encoders are used in industry for a very long time, but now the focus is shifting towards magnetic encoders due to their superior features like cost-effectiveness and ability to work in harsh environments and many more (Le et al. 2008). Accuracy is very important for better control and hence many attempts are done to improve this. Some of them are a magnetic resistive encoder with MR sensors (Campbell 2002), electromagnetic couple with optical tracking (Gao et al. 2018), a

& Paul C.-P. Chao [email protected] 1

highly sensitive and stable rotatory magnetic encoder (Wang 2018). The circuit designed in this work is for the magnetoresistive (MR) encoder. Figure 1 shows the architecture of a typical MR encoder. The MR encoder system consists of a magnetic grid, magnetic head and dete