Near-common-optical-path two-axis surface encoder with a three-layer gratings interference method

A near-common-optical-path two-axis surface encoder design is proposed for in-plane and out-of-plane wide-range displacement measurement. A three-layer gratings (TLGs) interference method is newly adopted in the optical design, contributing to a near-common-optical-path for interference beams to improve robustness against environmental disturbances, which had not been realized by conventional surface encoders in out-of-plane Z-axis displacement measurement based on the Michelson interference method. An incident beam is divided into two beams in positive and negative directions at the first layer of transparent grating. At both the positive and negative directions of the second layer of transparent grating, two diffracted beams are generated. One of the diffracted beams incidents on the third layer of scale grating perpendicularly, and the other incidents on the third layer of scale grating at a Littrow angle. The two diffracted beams in each direction then diffracted back to the second layer of transparent grating along the same optical path and superimposed with each other, generating interference signals at each direction, which are received within the sensor head. By calculating the interference signals, two-axis displacement can be obtained simultaneously. A prototype surface encoder is constructed to demonstrate the effectiveness of the proposed TLGs interference method.