Development of Picometer Positioning System and its Application for Ultraprecision Machining

针对下一代超精密机床，提出了一种三模皮米定位系统。定位系统有三种定位模式，即粗定位、精定位和超精定位。通过扭辊摩擦驱动（TFD）机构和普通交流伺服电机执行长行程纳米定位的粗调模式。用于亚纳米定位的精细模式也由扭辊摩擦驱动机构执行，但是该机构由压电致动器驱动。该定位系统采用主动固有限流器（AIR）的超精密定位模式实现皮米级定位，两个三模定位单元垂直布置，每个模式由带微处理器的DSP（运动控制器）控制。在粗调模式下测得的定位分辨率为5nm，在精调模式下测得的定位分辨率为0.2nm。在超精细模式下，目前的定位分辨率为25 pm。为了提高三模式皮米定位系统的性能，设计了一种主动式空气静压耦合器，通过主动式固有节流器（AIRs）控制推力轴承表面的气膜厚度，轴承的推力刚度可以无限大。因此，在将横向振动与TFD隔离的同时，主动空气静力联轴器作为刚性联轴器操作。主动耦合器还可以作为精密定位装置，期望在适当的时候实现皮米级的定位分辨率，并为提高超精密机床的定位技术做出贡献。

For ultraprecision machine tool of the next generation, a tri-mode picometer positioning system is proposed. The positioning system has three positioning modes, namely coarse, fine and ultra-fine mode. The coarse mode for nanometer positioning with long stroke is executed by a twist-roller friction drive (TFD) mechanism and an ordinary AC servomotor. The fine mode for sub-nanometer positioning is also executed by the twist-roller friction drive mechanism, however the mechanism is driven by a piezoelectric actuator. The ultra-fine mode using an active inherent restrictor (AIR) incorporated into the aerostatic guideway realizes the picometer positioning.Two tri-mode positioning units are arranged in perpendicular, and each mode of the positioning system is controlled by a DSP (motion controller) with a microcomputer. Measured positioning resolution in the coarse mode is 5nm and that in the fine mode is 0.2nm. In the ultra-fine mode, current positioning resolution is 25pm.For improving the performance of the tri-mode picometer positioning system, we devise an active aerostatic coupling, where the air film thickness on the thrust bearing surface is controlled by the active inherent restrictors (AIRs) and the thrust stiffness of the bearing can be infinite. Therefore, while isolating the lateral vibration from the TFD, the active aerostatic coupling operates as a rigid coupling. The active coupling can also be used as a fine positioning device.To conclude our research, we expect that the picometer order of positioning resolution will be realized in due course, and the technological knowledge obtained in this research will contribute to the improvement of the positioning technology for ultraprecision machine tools.