对一种三自由度支链嵌套并联机构支链划分的合理性判断

摘要

三自由度支链嵌套机构是一种视觉上各支链间存嵌套的新型机构,该机构活动空间大,安装空间小.但由于各支链存嵌套,彼此之间并不是完全分离,使得其支链划分方式不唯一,其中的不合理的支链划分方式会导致自由度计算错误,无法正确识别机构的主螺旋等问题.针对这一问题提出了一种判断支链划分合理性的方法.该方法以欠秩空间并联机构输入选取理论作为判断依据,首先求取各个不同支链划分方式的结构约束反螺旋的最大线性无关组,然后计算机构自由度,最后求刚化机构主动副后映射到动平台的反螺旋系,并判断其线性相关性,从而确定支链划分的输入选取的合理性,其中满足输入选取理论的支链划分方式即为合理的支链划分方式.%A three degrees of freedom (DOFs) parallel mechanism with limbs of embedding structures is a new type of parallel mechanism whose moving platform can continuously rotate for 360°,so this mechanism has a larger workspace.Comparing with the traditional mechanisms,the biggest difference among them is to share joint axes for each limb,which makes the structure more compact.However,this special mechanism leads to non-unique limb division method.Based on the above method,a method of reasonably dividing limbs is proposed.This method takes the degenerate-rank spatial parallel mechanism input selection theory as the basis of judgment.According to this theory,not all the limb division methods are correct.In judging,the first is to correctly calculate the freedom by using the existing three limb division method,the second is to calculate the structure constraint reciprocal screw and the rigid constraint reciprocal screw,finally the maximum linear independence group of the structure constraint reciprocal screw and the rigid constraint reciprocal screw is counted.So the input selection reasonableness of the limb division method can be verified,and the one which meets the input selection theory is the reasonable limb division method.