Development of a four-degrees-of-freedom diffraction sensor

The development of a diffraction-type four-degrees-of-freedom sensor based on the three-dimensional diffraction method and collimation method is described in this paper. This sensor is designed to be integrated with a linear laser encoder to allow five-degrees-of-freedom measurement. It is composed of a miniature collimation-type sensor, a reflective diffraction grating and a quadrant detector to simultaneously measure a straightness error and three rotational angles. Based on the diffraction method, the reflective diffraction grating reflects the incident laser from the collimation-type sensor to several diffractive laser rays and only the 0 and +1-order diffractive laser rays are detected by the collimation-type sensor and a quadrant detector respectively. According to the changed spot positions of the diffraction laser lights on the collimation-type sensor and the quadrant detector, a straightness error and the three rotational errors can be solved simultaneously.