A six-degree-of-freedom surface encoder for precision positioning of a planar motion stage

This paper presents a multi-axis surface encoder that can measure six-degree-of-freedom (six-DOF) translational displacement motions and angular motions of a planar motion stage. The six-DOF surface encoder is composed of a planar scale grating and an optical sensor head. A blue laser diode with a wavelength of 405 nm and an output power of 25 mW was employed as the light source of the sensor head. The light rays from the laser diode were collimated to a parallel beam with a diameter of 1.8 mm. The collimated beam was divided by a beam splitter into two beams, which were projected onto the scale grating and a reference grating with an identical grating period of 0.57 μm, respectively. The three-DOF translational displacement motions of the scale grating with respect to the sensor head along the X-, Y- and Z-directions were detected from the interference signals generated by superimposition of the first-order diffraction beams from the two gratings. A part of the zeroth-order and the negative first-order diffraction beams from the scale grating were employed for detection of the three-DOF angular motions about the X-, Y- and Z-axes. The sensor head was designed to have a dimension of 95 mm (X) × 90 mm (Y) × 25 mm (Z) so that it can be mounted on a previously developed planar motion stage. The grating area of the scale grating was designed to be 60 mm (X) × 60 mm (Y), which was larger than the stage moving ranges of 40 mm (X) × 40 mm (Y). Experiments were carried out to test the basic performances of the surface encoder.