Reduction in Cross-Talk Errors in a Six-Degree-of-Freedom Surface Encoder

This paper presents the reduction in cross-talk errors in the angular outputs existing in the previously designed six-degree-of-freedom (DOF) surface encoder for nanopositioning and nanometrology. The six-DOF surface encoder is composed of a planar scale grating and an optical sensor head with a reference grating, a displacement assembly and an angle assembly. The diffracted beams from both the scale and the reference gratings are received by the displacement assembly for measurement of the primary XYZ translational motions. The angle assembly only receives the diffracted beams from the scale grating for measurement of the secondary θ X θ Y θ Z angular motions. In this paper, at first, the cross-talk errors in the angular measurement results of the previous surface encoder are identified to be caused by the diffracted beams from the reference grating leaking into the angle assembly due to the imperfection of the polarization components of the sensor head. An improved design of the sensor head is then carried out to reduce the cross-talk errors by changing the position of the angle assembly in the sensor head. The sensor head is further optimized by replacing the beam splitter located in front of the angle assembly from a cube type to a plate type. Experimental results have demonstrated that the cross-talk errors were reduced from 3.2 arc-seconds to 0.02 arc-second.