A Linear Direct-Digital Converter for Sinusoidal Encoders

This paper proposes a novel linear direct-digital converter for sinusoidal encoders (LDCS) to measure the angular position of a rotating shaft. The outputs of a sinusoidal encoder vary as sine and cosine functions of the shaft angle. The proposed LDCS, which has a low component count, processes these outputs and renders a linear accurate direct-digital output for full-circle range. The LDCS is suitable for both static and dynamic measurements of the shaft angular position. The working principle of the LDCS is mathematically derived and explained in this paper. An elaborate evaluation of non-ideal parameters affecting the LDCS operation is carried out and reported. Furthermore, the static and dynamic performances of the LDCS are verified using simulation and emulation studies. The output nonlinearity obtained during emulation studies, for static measurement, is 0.027%. A close agreement between simulation and emulation results is observed during these studies. Later, a tunneling magnetoresistance (TMR) angle sensing unit which acts as a representative sinusoidal encoder is fabricated. The LDCS is interfaced with the sensing unit, and several experimental tests are carried out. The worst-case nonlinearity in the static measurement of shaft angle is 0.29%. Additional tests which study the dynamic performance and effect of mechanical misalignments of the combined TMR-LDCS system is also revealed in this paper.