中文摘要：

再次论述基于约束螺旋原理的机构的自由度分析方法。这是一个很重要的机构学的理论问题，持续己经有150年了。本文首先简介背景和这个方法。然后集中分析一些反常的机构，包括著名的Bennett机构、Tsai机构等。结果证明对于这个困难的问题这个分析过程十分简单方便，并易于为广大机械工程师所掌握。对这个复杂的问题仅仅用一支铅笔、一张纸，花几分钟就可以解决。

英文摘要：

The paper once again deals with the mobility analysis methodology of mechanisms based on constraint screw principle. It is a very important subject of mechanism theory lasting about 150 year. The background and the methodology is introduced. The paper focuses on analyzes some ＂paradoxical mechanisms＂ including classical mechanisms and modem parallel mechanisms, such as Bennett mechanism, Delta robot, Tsai＇s CPM mechanism and so on. The result proves that for the difficult mobility problem the analysis procedures here are quite simple and convenient, and easy to hold for numerous mechanical engineers. The complicated problem is solved only by using a pencil, a paper and just for a few minutes.