Scan of the rebuilt model and original product model is one of the key steps for the registration when scanning the prod -uct by the industrial CT .The problem of the uncertain main direction method existed in the rough registration algorithm for Princi-pal Component Analysis (PCA),which could result in the registration failure.Proposed the main direction method to solve the problem.Firstly a registration result was worked out by PCA ,then calculated the relative vector from model centroid to geometric center according to analyze the projection characteristics of the vector in each reverse situation ,determined whether there was the opposite problem or not for the registration result ,and the correct direction was worked out by the rotation axis .The experiment software is developed for the rough registration algorithm and the improved method is used to make experiment .It shows that the method can effectively solve the opposite direction problem of the model by PCA .%在利用工业CT对产品进行检测过程中，扫描重建模型并与产品原始模型进行配准是其中一个关键步骤。经典的主成分分析方法在粗配准中存在主方向不确定的问题，可能导致配准失败，为此，提出了主方向的修正方法。首先利用经典主成分分析方法得到配准结果，再求出两模型形心与质心的位置关系矢量，通过分析各种反向情况中该矢量的投影特征，判断配准结果是否存在反向问题，并通过计算旋转轴线来修正模型方向。开发了粗配准实验软件，对改进后的方法进行了实验，实际应用验证了该方法可以有效解决使用主成分分析法配准时产生的模型反向问题。
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