Reflective Optical Encoder Capitalizing on an Index Grating Imbedded in a Compact Smart Frame

Abstract

A reflective optical encoder has been proposed and realized capitalizing on a miniaturized sensing head, which integrates both a pair of index gratings and a beam router in a plastic smart frame. The index gratings play a role in selectively transmitting the incident beam carrying the periodic grating pattern of a code scale. The beam router, consisting of a collimating lens in conjunction with a prism, is used to collimate and steer the beam originating from a vertical-cavity surface-emitting laser (VCSEL) toward the code scale. The proposed encoder is rigorously designed by ray optic simulations and manufactured by passively aligning a smart frame, which is produced by plastic injection molding, with a VCSEL at λ = 850 nm and two semicircular photodetectors. The index grating is created by forming an Al grating pattern of a 10-μm pitch on a silica substrate. Two output signals provide sinusoidal signals, which are 90° out of phase from each other. The period of the signals is 4 °s at a frequency of 250 kHz, which is equivalent to a period of 10 μm. As expected, the relative phase relationship is reversed by altering the direction of rotation of the code scale. By examining either of the two output signals, we can simply demonstrate positional and angular resolutions of about 10 μm and 0.04°, respectively. The attained resolutions can be readily enhanced to 2.5 μm and 0.01°, by simultaneously taking into account the two signals with a distinct 90° phase difference.