Optical encoder readhead chip

Optical encoders are pervasive in almost all sectors of industry including metrology, motion systems, electronics, medical, scanning/ printing, scientific instruments, space applications and specialist machine tools. The precision of automated manufacture and assembly has been revolutionised by the adoption of optical diffractive measurement methods. Today's optical encoders comprise discrete components: light source(s), reference and analyser gratings, and a photodiode array to utilise diffractive optic methods to achieve high resolution. However the critical alignment requirements between the optical gratings and the photodiode array, the bulky nature of the encoder devices and subsequent packaging mean that optical encoders can be prohibitively expensive for many applications and unsuitable for others. We report here a novel high resolution optical encoder readhead chip, which will initially be employed with Renishaw high precision metrology systems. Microsystems manufacturing techniques have allowed us to monolithically integrate the traditional key components of the encoder onto a single compound semiconductor chip. Fabrication of the gratings at the wafer level, by standard photo-lithography, allows for the simultaneous alignment of many devices in a single process step. This development, coupled with a new photodiode configuration, not only facilitates increased performance but also significantly improves the alignment tolerances in both manufacture and set-up, simplifying thereby the installation process. Operating in photoconductive mode a National Research and Development Corporation type optical encoder readhead chip has been successfully demonstrated under test conditions on 20µm, 8µm and 4µm pitch scale. DC to AC ratios of the order of 7:1, signal-to-noise ratios greater than 50:1 and Lissajous curves with maximum peak-to-peak voltages reaching the operating limits of the test set-up have been achieved. Various reference schemes with in-scale reference mark features have also been trialled. ©2008 IEEE.