Velocity Coordination in a Multi-RobotSewing Cell
Velocity Coordination in a Multi-RobotSewing Cell
Automated sewing is a complicated task in man-ufacturing. Due to the non-rigid work pieces and variationsin the material characteristics, sensor-based control has to beused to accomplish the sewing operation. This paper presentsa strategy for velocity synchronization and corner matching inan automated sewing cell based on two industrial manipulatorsand a sewing machine. A hybrid force/motion control schemeis adopted using feedback from force/torque sensors for tensioncontrol and optical sensors to control the seam position. Thestrategy is based on switching between force control and dis-placement control using a leader/follower coordination scheme.This addresses the problem of corner mismatch occurring whentwo independent force controllers are used for controlling thetwo robots. Experiments verify that the proposed method gives asatisfactory corner matching, which is crucial for the presented sewing VS Enterprises
The automation of sewing operations is a difcult taskin manufacturing. Several challenges arise from the non-rigidity and the large variations and uncertain material char-acteristics of the processed materials. This makes the designand implementation of both material handling and controlduring the sewing applications a complex task compared tohandling rigid materials. Nevertheless, there is a demandfrom the industry in high-cost countries to automate thesewing process.In the past decades, several research groups have workedin the eld of automated sewing and the handling of non-rigid materials.An automated sewing cell consisting of one robot and asewing machine is presented in [1], [2]. Both the tensionin the work piece and the seam allowance are controlled inreal-time.In [3], a device for handling curved fabrics during a sewingoperation is presented. It is based on rollers in front ofand behind the needle. Different feeding speeds allow foradapting to different seam lengths.In [4], an overview of the challenge of automated sewingis presented, especially the sewing of 3D-shaped products.The focus is on material handling. Adaptive control strategiesbased on measurements of the seam allowance and the feedrate during the sewing operation are suggested.A system based on two robots is presented in [5]. Therobots work together to handle a single piece of fabric duringthe sewing operation. The system includes controllers forpressing force and tension in the fabric. The position iscontrolled by a visual tracking of the fabric.Another sewing cell demonstrator based on a single robotand a sewing machine is presented in [6]. The task is to sewan assembly of two similarily shaped parts. A triangulation-based sensor is used for edge detection while optical motionsensors are used for measurement of the sewing speed.A demonstrator based on a sewing machine with a servo-controlled feeding mechanism is presented in [7]. Two parts,that are separated by a thin plate, are controlled indepen-dently by the servo mechanism. An open loop path controlis used which utilizes recognition of patterns on the fabric.The need for sensor-based feedback control is emphasizedby the authors.In [8], a different approach for the control of sewingoperation is used. The presented work demonstrates thesewing of a single work piece using a controller based onneural networks.This paper presents a sewing cell based on a sewingmachine and a multi-robot system. The demonstrator isbuilt in cooperation with a furniture manufacturer and anautomation supplier. The objective is to automate parts ofthe sewing process of recliners as a step towards a fullyautomated production. The specic case is to sew four leatherparts into an assembly for a footstool cover. These partshave different curvatures that have to be matched duringthe sewing operation. This results in a non-at shape of thenished product. A cover that has been sewn in the presentedsewing cell is shown in Fig. 1. The control methods usedfor edge control and force control are presented in [9], [10].The presented experiments demonstrated the feasibility of theproposed control methods for force and edge control. How-ever, the experiments also showed that the independent forcecontrol of the two parts leads to a longitudinal displacementof the corner at the end of the seam as well as of referencepoints along the seam. The gure of the assembly shows aseam with an independent force controller with a set pointof2 N. On the top left seam and the bottom right seam, itis possible to see an offset between the corners, .There are different possible reasons for this offset:Slightly diffent length in the parts to be sewn together.Different material characteristics such as differentstretch factors.Difference in the effective feed rate between the upperand lower part, e.g. caused by mechanical reasons.In [11], different techniques utilizing changes in the forceset point in order to synchronize the robot movements areproposed.
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