在机构中，有些运动副的约束与另外的运动副的约束可能是重复的。因而，这些约束对于机构的运动实际上并没有起到约束的作用。故称这类约束为虚约束。
本文论述了机构中存在虚约束的充分必要条件及虚约束的判定准则。
The paper discusses the criterion for deciding virtual constraint and the sufficient and necessary conditions for the constraint existing in mechanisms.
平面机构中虚约束的存在，给制造加工和机构运动分析带来了诸多不便。
Virtual restraint in planar mechanism brings about many problems to machining and mechanism movement analysis.
通过引入虚任务区概念，将三维约束降为二维约束，有效地降低了决策变量的复杂性。
The virtual mission area concept was advanced to transform the constraints of three dimensions to those of two dimensions to decrease the decision-variable complexities.
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