Fabrication of higher precision planer multi-joint positioning mechanism using high-performance joint elements and research on its limit of positioning accuracy

虽然精密装配线上对平面多关节定位机构(SCARA型机械手)的需求很大，但目前机械手的定位精度远远低于笛卡儿型定位机构。然而，它们比工厂中的装配线具有更好的可用性，因为它们的工作范围如此之广，设置区域如此之小。预计它们将作为装配线上的坐标测量机使用。为实现SCARA型机械手在30 0×2 0 0 mm工作范围内具有1μm级的定位精度，研制了一种采用高性能关节元件的新型平面多关节机构。在严格标定连杆参数后，对SCARA型机构的定位精度极限进行了研究，研制出的机构臂长L_1=250 mm，L_2=147 mm。关节轴线采用精度为0.0 5μm的高精度空气轴承进行导向，并设计了摩擦驱动机构和直流伺服电机来驱动关节。设计了一种能够检测实际关节角度的超高分辨率旋转编码器。θ_1和θ_2的编码器分辨率分别为720×10~(-4)ppr和144×10~(^4)ppr。此外，对手臂结构进行了布置，使重心与关节轴线重合，并利用参考角度轨迹进行了关节角度控制实验。确定了将关节角度控制在分辨角度偏差范围内。测量的30次定位重复性的色散范围约为±3.5μm(3σ)，验证了色散椭圆的面积减小到常规机构的10%。链路参数的校准和定位精度的更详细评估计划在下一阶段执行。

Though planer multi-joint positioning mechanism (SCARA-type manipulators) are in great demand on precision assembly lines, the positioning accuracy of current manipulators is much lower than Cartesian-type positioning mechanism. However, they have superior availability to assembly lines in factories because they have such a wide work range for such a small setting area. And they are expected to work as coordinate measuring machines in assembly lines. This study develops an novel planer multi-joint mechanism using high-performance joint elements, so as to realize an SCARA-type manipulator with 1 μm level positioning accuracy over working range of 300 × 200 mm. Moreover, it aims at researching the limits of positioning accuracy by the SCARA-type mechanism after strict calibration of link parameters.The arm length of the developed mechanism is L_1=250 mm and L_2=147 mm. A high precision air bearing with accuracy of 0.05 μm is adopted to guide the joint axis, and a friction drive mechanism and a DC servomotor are devised to drive joint. And a rotary encoder with ultra-high resolution is devised that can detect the actual joint angle. The encoder resolution is 720×10^4 ppr for θ_1 and 144×10^4 ppr for θ_2. Moreover, the arm structure is arranged so that the center of gravity coincides with the joint axis.The experiments are performed to control the joint angle with reference angle trajectory. It is confirmed that the joint angles are controlled to settle within the deviations of resolution angle. The dispersion range of the measured repeatability of 30 times positioning is within about ±3.5 μm (3σ), and it is verified that the area of dispersion ellipse is decreased to 10 % of conventional mechanism. The calibration of link parameters and more detailed evaluation of positioning accuracy are scheduled to perform on the next stage.