The application of robotics to the assembly of flexible parts by sewing
This thesis concerns the development of a robotic cell to perform assembly and handling operations on cloth.- A flexible automation approach was adopted, in which the robot was required to control the cloth panel during both handling and sewing operations, without the aid of hard automation attachments which might limit the flexibility of the system. The cell consisted of an adaptively controlled robot, a hierarchy of controllers, a conventional sewing machine, a two-fingered fabric steering end-effector, and several sensor systems. A technique was developed for producing a seam parallel to an edge of arbitrary contour, in which two cameras, a cloth tension sensor and the sewing machine's shaft encoder provided the sensory input. Two sensory servo control systems were required, one control system generated the robot's trajectory to maintain a small constant cloth tension, and the other directed the robot to manipulate the cloth panel to maintain a constant seam width. The design of the cloth tension control was based on the measured frequency response of the open loop system. The seam width control was designed using simulation studies, which accounted for the control transfer function, and nonlinearities such as camera pixel resolution, time delays and robot motion limitations. Several robotic handling techniques were developed, so that a cloth panel placed arbitrarily on the sewing table could be set up for an edge seaming operation, and the cloth could be rotated about the needle. The system's flexibility was demonstrated in the assembly of an irregularly shaped cloth panel, in which three adjacent sides were sewn up.