A Long-Stroke Nanopositioning Control System of the Coplanar Stage

With the continuing trend toward device miniaturization in many engineering and scientific fields, the need to accomplish highly precise measurements at the micro- or nanoscale has emerged as a critical concern. This paper presents a high-precision motion control system for the nanopositioning of a coplanar X-Y stage driven by two commercial ultrasonic motors. The motor drive provides three main driving modes, namely ac, Gate, and dc, for millimeter, micrometer, and nanometer displacements, respectively. The displacement of each axis stage is sensed using a linear diffraction grating interferometer (LDGI) with a nanometer resolution. To compensate for the effects of the variable friction force during stage motion, the gains of the proportional-integral-derivative controller used to regulate the stage motion are tuned adaptively by a back propagation neural network (BPNN) based on the feedback signals provided by the LDGI. Furthermore, to obtain a high-accuracy positional motion, the error compensation strategy is implemented to eliminate the systematic errors of the stage with error budget. The error budget is obtained by positioning error calibration using a laser interferometer, which optical axis is detected by a quadrant photodetector (QPD) to ensure no cosine error. The positioning accuracy of the proposed system is evaluated by performing a series of contouring experiments. The results demonstrate that the system achieves a nanometer level of accuracy and resolution and is, therefore, a suitable solution for micro-coordinate measuring machine, microlithography, and micromachining applications.