中文摘要：

首先按运动功能对两自由度空间并联机构进行分类，得到了空间2Ryz型（绕 y轴和z 轴转动）和1Rz 1T z 型（绕z轴转动和沿z 轴移动）的两自由度机构；给出了两类并联机构的分支约束螺旋系和机构约束螺旋系，以及保证所有分支约束螺旋的合成是期望的机构约束螺旋系的几何条件。对这两种空间并联机构运用约束螺旋综合理论进行系统的型综合，得到了可实现连续运动的具有绕定平台平面内两轴转动的并联机构以及沿着定平台法线方向移动和绕其转动的两自由度并联机构；得到了两自由度并联机构各种类型的分支链结构，以及两类机构的结构约束特性。将少自由度对称并联机构类型由原来的9类扩充到11类，丰富了机构类型。

英文摘要：

The spatial symmetrical parallel mechanisms with 2-DOF were sorted into two classes firstly ,i .e .Type 2Ryz 2-DOF parallel mechanism with two rotations about y-and z-axes in the plane of fixed platform and Type 1Rz1Tz 2-DOF parallel mechanism with one rotation about z-axis and one translation along z-axis .The limb constraint system and the mechanism constraint system of the two kinds of parallel mechanism were enumerated with the geometrical conditions which guaranteed the combined effects of all the limb constraint screws equal to the desired mechanism constraint system . Then ,the type synthesis for the two classes of 2-DOF spatial symmetrical parallel mechanisms were performed systematically with the constraint screw synthesis theory ,and the structure of the mecha-nisms were obtained ,which could continuously realize the required rotations about the two axes in the base or rotation about the normal of the base plane and translation along it .The structures of limbs of the mechanisms as well as their constraint characteristics were gained also .The types of lower-mobili-ty symmetrical parallel mechanism are expanded into 11 from 9 ,and their synthesis methodologies are also provided ;all of these makes the type synthesis perfect .