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  • 2017 | Volume  | Number 5

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摘 要
目的:虚拟手抓取是虚拟交互中核心技术之一,直接影响沉浸感。已有的虚拟手抓取规则基于接触点法矢夹角和接触点与体心连线的夹角,但在部分形状上不能正确抓取。本文提出了一种适应物体形状的抓取规则。方法:针对长方体、球、圆柱这些构成物体的基本几何体,利用形状特征和手指抓握的接触关系制定抓取规则。1)长方体的规则是至少有三个不共线的接触点,并且一对接触点法矢夹角超过90°。2)球的规则要对接触点和球心的空间关系进行计算。3)圆柱的规则根据接触点是否在曲面上,使用长方体规则,或是仿照球的规则判断。结果:能正确抓取这些形状的物体。对于需要使用多个几何体描述外形的复杂物体,先按照这三种基本几何体分解、单独计算,然后筛选出其中不稳定状态的几何体,把整体的接触点位置、法矢信息集中在这些不稳定状态几何体上,用单一几何体的抓取规则计算整体的抓取。同时加入力矩平衡可能性的计算,抓取规则和力矩平衡二者都满足才判定抓取成功,进一步减少了错误判断的发生。利用unity3d软件和neuron数据手套实现结果表明,该方法可以正确处理类似水杯这样的多几何体组合物体的抓取判断。结论:本文提出了基于物体形状的抓取规则,和用简单基本形状分解复杂物体的计算方法,准确计算抓取,符合直观感受。
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Abstract
Abstract: Objective: Virtual hand grasping is one of the core topics/techniques in virtual interaction, and it strongly determines the immersion. Real-time force analysis is very complex, so more rules that meet the characteristics of hand grasping are formulated, instead of complex mechanics calculations. Some existing rules focus on the included angle of two contact points’ normal vector, or the included angle of two lines that connect contact point and object’s center, but they can''t deal with some shape correctly. This paper proposes a shape-adaptive grasping method. Method: This method uses shape features and contact points of basic geometries like cube, sphere and cylinder to design grasping rules. 1) For a cube, there must be at least three contact points on the surface that are not all collinear, and one of included angles of every two contact points’ normal vector is larger than 90 degrees: First getting the positions of all the contact points. Second according to the rotation of the cube, rotating the cube and all the points so that every edge of the cube parallels to the world axis which simplifies calculation. Third determining every point’s location whether it is on the plane, edge, or vertex of the cube by the scale of the cube. Fourth calculating the normal vector of these points based on their locations. The vector’s direction is perpendicular to the plane when the point is on the plane, or is the same as the moving direction of the point when the point is on the edge or vertex. At last calculating the included angles of every two normal vectors, and determining the grasping result. 2) For a sphere, there must be at least four contact points, and the spatial relationship between these points and sphere’s center should satisfy the proposition obtained from the analysis: First getting the position of sphere’s center and all the contact points. Second using the center and every two of these points to form a plane, and then calculating whether all the other points are on the same side of this plane. At last determining the grasping result with the calculation results. 3) For a cylinder, depending on whether the contact points are all on the curved surface, a rule like sphere rule is used or not: First getting the position of all the contact point and then rotating the cylinder and all the points so that the bottom plane of the cylinder parallels to the world axis. Second projecting these points to the bottom plane of the cylinder. Third using the circle center of the bottom plane and every projected point to form a diameter, calculating whether all other projected points are on the same side of the diameter. At last determining the grasping result with the calculation results. When there are points on the top or bottom plane, cube rule is used instead. Result: It can more realistically handle the grasping of an object with curved surface. For a complex object which can be composed by multiple basic geometries, our method divides the calculation into two steps. Firstly, the object is decomposed to these three basic geometries, and then each geometry is calculated separately. Some geometries nearly meet the grasping rule, called unstable state. Secondly, our method selects every one of these unstable state geometries, gathers the information like position and normal vector of all the points to it, and then makes decisions whether the whole object is caught or not with the basic geometry rules obtained before. The calculation of the possibility of moment balance further reduces the occurrence of erroneous determinations. All the normal vectors are the opposite direction of force, but their values are uncertain. According to the proposition obtained from the analysis, the moment may balance when the normal vectors meet a specific spatial relationship just like the sphere rule: First normalizing all normal vectors, and then projecting them to a unit sphere, so there are points on the surface. Second using the points to calculate the same proposition in sphere rule. At last determining whether the moment is balanced or not. Only both rules and moment balance are satisfied, the grasping will determine success. By using unity3d software and neuron data glove, the experimental results show that the method can effectively handle the grasp of objects with curved surfaces like a ball, and complex objects like a cup. When the virtual hand touches the upper hemisphere, the ball is not caught. Only if the figures ring around the ball, it will be caught. Casually grasping the cup will not catch it. Only when the gesture is correct and the moment is balanced, the cup can be caught. Conclusion: This paper proposes a Shape-adaptive grasping method, and a calculation method that decomposing complex objects into simple basic geometries. This method effectively handles the grasp effectively, and conforms to intuitive feel.
Keywords
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