Method for placing a rope in a target shape and its application to a clove hitch

This paper describes a method for generating hand trajectories for a robot to enable it to place a rope on a table in a target shape. In many aspects of rope-work, e.g., making knots, hitches, and bends, it is important to control not only the topological relations (e.g., a tangle of string), but also to control the shape (e.g., the position, size, and so on). The shape of the rope on a table is the starting stage of the rope-work in that it affects the result of the rope-work, e.g., the position and size of a knot. Thus, it is necessary to generate hand trajectories automatically according to the target rope-shape when executing rope-work with a robot arm. To generate trajectories from a tabletop target shape, it is necessary to consider a positional relationship between the robot hand and the touchdown point, which is the point that the rope is placed on the table. In this study we propose a rope-shape model which can be used in limited situations, and derive hand trajectories from the model in a simple manner. The parameter needed for the model is easily identified. An experiment of making a loop was conducted to verify the effectiveness and limitations of the proposed method. As further verification, the method was applied to making a clove hitch using a dual-arm robot. In the starting stage of tying a clove hitch, two loops were created in a specified shape, and the clove hitch was successfully tied.