A real-time processing system for dual-channel six-degree-of-freedom grating ruler based on FPGA

Abstract

Six-degree-of-freedom (6-DOF) grating encoders have a wide prospect of application. Aiming at the requirement of realtime detection of 6-DOF grating encoders, this study designs and builds a real-time calculation system platform based on field-programmable gate array (FPGA). We realized a real-time parallel calculation of 16-path displacement signal and 24path angular displacement signal, respectively. Specifically, the optical interference signals, generated by the translation and rotation of the motion stages, are firstly shaped by the front-end analog circuit. We further sampled the front-end analog circuit into an FPGA through an analog-to-digital convertor (ADC) for the realization of the digital filtering, amplitude normalization, phase correction, and phase-information calculation. Thus, the calculated signals on the 6-DOF motions can be displayed in real time. The established system was evaluated with the experimental parameters in terms of the translation with a 50 μm/s moving speed and an 18 mm stroke and the rotation with a frequency of 0.5 Hz, a step length of 100 micro-rad, and within a reciprocating rotation of 24 s. Finally, a linear-displacement error of <1 nm and an angle displacement error of <0.9 micro-rad were achieved, respectively. Furthermore, the system delay of <15 ms is obtained, exhibiting a high performance for the real-time measurement and high integration in the practical application.