Posture Determination

System Equation

We assign a virtual spring to each rotational degree of freedom of a joint(Figure \thefigure). State variables (angles of the joints) are determined by equilibrium between the elastic torque of virtual springs and the input torque.

Natural Posture

Since the fingers in a hand are connected with tendons, muscles, other tissues and covered by skin, the movements of them are not completely independent. There are some rules that determine the interactions of joint motions. We explore these rules, and build a model to describe them. From this model, we can tell whether a given posture is natural, and if the given posture is unnatural, we can estimate the degree of difference between the posture and the natural one. Then the natural posture can be generated by adjusting the given posture in a way that the unnaturalness of the result posture decreases.

In this research, the unnaturalness of a hand posture is modeled by a potential energy function. Thus, the goal of generating natural hand posture can be achieved by producing a hand posture of which the potential energy is zero, or as small as possible. The potential energy can give forces to decrease itself. We apply these forces to the skeletal model to adjust the joint angles. As a result, the hand posture will have less potential energy, and thus become more natural. We use these ``forces'' as natural motion constraints and call them ``inner forces''.

The posture determination system is composed of the skeletal model and the dynamic system. We put virtual springs on joints in the skeletal model to express the rigidity of the joints. Once the user input is given to the system, potential energy is changed and inner forces are generated. The output of the process is determined as a equilibrium state of the dynamic system. The computed posture is expressed by the joint angles and is used to generated a hand shape in the skin model.

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