(8.5) Implementation of a linear optical encoder for high precision in line position referencing of plastic film in a Roll-to-Roll system Kai Hollstein,1 Daniel O’Connor,2,* Christopher W. Jones,2 Paul Morantz1 and Paul Comley1 1
Cranfield University, College Rd, Cranfield, Bedford MK43 0AL National Physical Laboratory, Teddington, Middlesex TW11 0LW * Corresponding author: daniel.o-connor@npl.co.uk 2
Keywords Flexible substrate alignment; Optical encoder; Dimensional metrology; Testing and system integration challenges. Abstract
The EPSRC Centre for Innovative Manufacturing in Ultra Precision is developing a large scale Reel-to-Reel (R2R) film fabrication research platform. This R2R research platform is aimed at processing 1.2 m wide films at accuracy levels and processing speeds previously not achieved. Novel means of measuring and controlling the flexible substrate’s position and shape state are required to enable closed-loop web steering systems for the control of fabrication processes. For some processes it may be sufficient to infer the position of the web by tracking the motion of the rollers using rotary encoders since the web nominally follows this motion; however, the translation, slippage and stretch of the web material in two dimensions over the rollers prevents truly high precision control. Direct monitoring of the web is therefore required. We present the concept and experimental realisation of a linear optical encoder head adapted to track the in-plane motion of surface relief gratings embossed directly into a polyethylene terephthalate (PET) substrate. This is achieved by extracting signals in quadrature from a fringe pattern created by projecting coherent light through the grating. Figures 1(a, b) show an example fringe pattern and the associated Lissajous figure captured from the encoder. Figures 1(c-e) show the sub-micrometre measurement performance achieved by the developed encoder as determined by comparison with a reference encoder that is embedded in the stage used to simulate the linear motion.
Figure 3 Tracking of a 16 µm period phase grating moving over a distance of 130 µm. Here, the encoder is triggered by a signal from a reference encoder every 500 nm. a) Example fringe pattern. b) Extracted Lissajous figure. c) Measured versus reference displacement. d) Fringe periodic displacement residuals. e) Displacement residuals.
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