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

Abstract

An absolute-type four-degree-of-freedom (four-DOF) grating encoder that can simultaneously measure the three-axis pose (θx, θy, θz) and one-axis out-of-plane position (Z) 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 θx, θy, θz, and Z 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 θx, θy, θz, and Z 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.

A Review: High-Precision Angle Measurement Technologies

“…Gratings are currently used in various fields due to their excellent structural dimensional consistency and structural stability thanks to increasingly advanced processing methods [ 180 , 181 , 182 , 183 , 184 , 185 , 186 , 187 , 188 , 189 , 190 , 191 , 192 , 193 ] as a standard device widely used in precision displacement [ 194 , 195 , 196 , 197 ] and orientation measurement [ 178 , 179 , 198 ]. The optical frequency comb output by the mode-locked femtosecond laser can be used to generate an ‘angle standard comb’ as an optical scale for angle measurement.…”

“…The detection units are CCDs (Charge Coupled Devices) [ 228 , 229 ], QPDs (quadrant photodiodes) [ 230 , 231 ], and PSDs (position-sensitive detectors) [ 64 , 232 , 233 ]. In multi-degree-of-freedom measurement, an autocollimator is often used for integrated design, with the help of PSDs [ 234 , 235 , 236 ] and QPDs [ 179 , 237 ]. The PSD also needs position correction at long distances and high-speed measurements [ 238 , 239 ].…”

Long binary coding design for absolute positioning using genetic algorithm

“…On the one hand, laser interferometers excel in controlled environments like laboratories, where the laser wavelength remains stable and measurement accuracy is exceptionally high [6][7][8][9][10][11][12]. On the other hand, grating interferometers are preferred in scenarios where environmental conditions cannot be precisely controlled [13][14][15][16]. For instance, in the case of precision machine tools, grating interferometers are commonly employed to measure workpiece processing [17][18][19].…”

“…The accuracy of heterodyne type is higher, which has great potential for application in the movement and alignment of the workpiece table in lithography machines [17,18,26]. Absolute position detection is one of the critical requirements for grating interferometers used in high-precision machine tools [27]. It enables the retrieval of the original working position in the event of unexpected downtime, thereby minimizing the production of scrap and defective products with a compact module [28] and ability of multiple-degree-of-freedom measurement [29,30].…”

Codes coupling optimization for absolute measurement

“…These advantages make grating interferometers widely applicable in various precision measurement fields, serving as displacement measurement or positioning feedback systems [18]. The research directions for grating interferometers primarily focus on integration [19][20][21][22], multi-degree of freedom [23][24][25][26][27][28][29][30][31], real-time signal processing [32][33][34][35], and absolute positioning research [36][37][38]. Among these, absolute research holds great significance, as traceability of the position helps equipment restore its status more efficiently.…”