Absolute Electromagnetic Encoders with 5-mm Resolution and 12 Bits per Position Based on Phase and Frequency Modulation

This paper proposes electromagnetic encoders based on two chains of transversely-oriented metallic linear strips etched on a dielectric substrate, able to provide the absolute position of the target with a resolution of 5 mm (encoder period). Encoding is achieved through the transverse position of the linear strips in the chain and through their size. Particularly, in the reported prototype, 16 different positions (4 bits) and 4 different sizes (2 bits) provide 6 bits per position for each chain and, hence, a total of 12 bits per encoder position, an unprecedented value in this type of encoders. This means that 2¹² = 4096 different positions can be discerned, providing a dynamic range of 20.48 m (= 5 mm × 4096) for linear motion sensing. The encoder reader consists of a pair of one-port microstrip lines sharing the same axis and terminated with a matched load. Such lines are fed by 4 harmonic signals, necessary to distinguish the 4 different strip sizes (frequency modulation). The phase of the reflection coefficient at each frequency is used to determine the transverse position (among 16) of the strips in the rows (phase modulation) as the encoder displaces on top of the reader at short distance, with the strips parallel-oriented to the axis of the reader lines.