Fundamentally, sinusoidal encoders operate by encoding position information by providing a pair of quadrature sine and cosine signals as the shaft is rotated. The signals may be generated by optical or magnetic means and typically produce 512 or 1024 cycles per mechanical revolution. As such, they are indispensable and core sensors in leading-edge manufacturing equipment, motion control system and fault diagnosis systems. Recent technological developments in such fields require excellent system precision coupled with high-resolution sinusoidal encoders with sub-micro accuracy.
Currently, electronic interpolation technique provides an effective way and increasing possibility to further improve the resolution of the sinusoidal encoders. So far, much on the design of electronic interpolators has been done; where interpolation strategies have been classified into two: closed-loop strategy and open-loop strategy. Both strategies have been significantly explored with the latter being preferred. A focus on the open-loop strategy has revealed that due to the use of the ratio metric technique, the robustness of the linearization conversion to the signal imperfections generally need a dedicated compensation signal to further improve the linearity of the output linearized signals.
In a recent publication, Xi’an Jiaotong University scientists (Dr. Guoyong Ye, Zeze Wu, Zhengchen Xu, Yang Wang, Yongsheng Shi and Professor Hongzhong Liu) proposed a digital interpolation module for high-resolution sinusoidal encoders. Specifically, the research team from the State Key Laboratory for Manufacturing Systems Engineering put forward a strategy that could perform phase-shifting manipulation on the sinusoidal encoder signals, and takes advantage of the linear sections of the absolute values of the phase-shifted sinusoidal signals. Their work is currently published in the research journal, Sensors and Actuators A.
The authors considered an open-loop interpolation strategy based on linearization conversion technique, which only involved basic operations (+, -, ×) on the sinusoidal encoder signals and required no dedicated compensation signal. The strategy was used to perform phase-shifting manipulation on the sinusoidal encoder signals. The researcher also developed a digital interpolation module by applying the interpolation strategy in a FPGA. Lastly, simulations were carried out followed by detailed descriptions of the digital interpolation module and experimental results.
Simulation results showed that the theoretical error of the proposed interpolate strategy was within 0.072◦ over the 360◦ signal period and matched the interpolation error of 4 nm for a linear optical encoder with a period of 20 μm. In addition, the experimental results showed that the interpolation error and long-range error (over 100 mm travel range) were ± 0.205 μm and ± 0.991 μm respectively, which were comparable to that of a high-precision commercial iC-TW8 interpolation circuit.
In summary, Xi’an Jiaotong University researchers successfully presented a high-resolution digital interpolation module for sinusoidal encoders. Generally, the employed scheme incorporated the linear sections of the absolute values of the multiple phase-shifted signals derived from the sinusoidal encoder signals. Altogether, both theoretical and experimental results demonstrated the effectiveness of the proposed interpolation strategy.
Guoyong Ye, Zeze Wu, Zhengchen Xu, Yang Wang, Yongsheng Shi, Hongzhong Liu. Development of a digital interpolation module for high-resolution sinusoidal encoders. Sensors and Actuators A, volume 285 (2019) page 501–510.Go To Sensors and Actuators A