An Ultra-Precision Absolute-Type Multi-Degree-of-Freedom Grating Encoder

• Published in: Sensors (Basel, Switzerland) Vol. 22; no. 23; p. 9047
• Main Authors: Wang, Shengtong, Luo, Linbin, Zhu, Junhao, Shi, Ningning, Li, Xinghui
• Format: Journal Article
• Language: English
• Published: Switzerland MDPI AG 22.11.2022; MDPI
• Subjects: absolute measurement; Accuracy; Decoupling; Degrees of freedom; Experiments; grating encoder; laser autocollimation; Lasers; Light; Light beams; multi-degree-of-freedom; Position measurement; precision positioning; Pulse compression; Stitching; synthetic aperture optics; Synthetic apertures; Three axis; laser autocollimation; grating encoder; precision positioning; synthetic aperture optics; absolute measurement; multi-degree-of-freedom
• ISSN: 1424-8220; 1424-8220
• DOI: 10.3390/s22239047

An absolute-type four-degree-of-freedom (four-DOF) grating encoder that can simultaneously measure the three-axis pose ( , , ) and one-axis out-of-plane position ( ) of an object with high accuracy is demonstrated for the first time in this research. This grating encoder is composed of a stationary reading head and a movable grating reflector. A light beam from the reading head is projected onto the grating, and three diffracted beams (0th-, +1st-, and -1st-order) are generated, collimated, and received by three separate quadrant photodetectors (QPDs). The information of , , , and is coded into spot positions of these three diffracted beams on the QPDs. Thus, the modeling and decoupling algorithms were investigated, and an independent calculation of these four-DOF absolute positions was theoretically guaranteed. A prototype was then designed, constructed, and evaluated. Experimental results verified that the proposed grating encoder could achieve the absolute measurement of four-DOF , , , and with an accuracy of sub-arcseconds and sub-micrometers. To the best of our knowledge, the proposed encoder in this research is the first one to achieve absolute simultaneous measurements of four-DOF position and pose with a large measurement range. The success of this new grating encoder can benefit various multi-DOF positioning applications, especially for large-scale synthetic aperture optics (SAO), including stitching off-axis parabolic mirrors and pulse compression grating.