Corner Matching in a Multi-RobotSewing

Corner Matching in a Multi-RobotSewing

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 presentedsewing case.The automation of sewing operations is a dif cult 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 feerate 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.VS Sewing Machines

 
The contribution of this paper is a new technique forcorner matching and experiments verifying the feasibility ofthe proposed method. Further, this paper includes a moredetailed presentation of the control mechanism, especiallywith focus on corner matching.Based on the experience from the past work, a new controlstrategy for sewing of two parts is proposed. This methodis based on stretching the material in order to align theend points of the edges. Technically, this method uses aleader/follower coordination strategy. The robot with thesmaller force vector in needle direction acts as leader andis in force control mode, while the robot with the largerforce is controlled to keep the same distance to the needleas the leader. The possible switching between leader andfollower is evaluated frequently, for example once a second.The force set point for the leader robot is set to2 N, whichprevents the parts from wrinkling. This strategy results ina sewing force larger than the ideal force set point, as atrade-off for sewing speed synchronization. This is differentfrom the theoretical approaches in [11], where the force isin uenced by pulses in the robot movement or by controllingthe feeding mechanism.The edge controllers are not in uenced by this techniqueand act independently for the two robots.To make this method work, several conditions have to bemet: The edge lengths of the two parts have to be similar.If the difference is too large, the parts cannot be sewnwhile keeping the sewing force within an appropriatelevel. The individual material characteristics of the two partshave to be equal within a certain range in order to obtainsimilar feed rates. Small differences are compensated bythe proposed control mechanism. The individual gripping points have to be in the pointsthat are to be matched, for example the corner of theparts. The seam segment has to be nearly straight so thegripping points are located nearly the extension of thesewing line. Curved parts can be sewn by partitioningthem into a sequence of nearly straight segments.When these conditions are not met, there is a risk of anincrease of the sewing force in one of the parts to a value thathas negative in uence on the sewing process. To prevent this,the sewing force of the distance-controlled robot is recorded,and in case a maximum force threshold is exceeded thesystem can be stopped for manual inspection of the errorcondition.
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Measurements and feature extraction in high-speed sewing

Measurements and feature extraction in high-speed sewing

 This paper presents the development of a signal acquisition and analysis equipment for measurement of sewing parameters on a high-speed overlock sewing machine. The objective of the work was to provide investigators of the textile area with hardware and software to ease the investigation on the dynamical behaviour of the following sewing parameters: force on needle bar, presser- foot and thread tensions. It should also have enough flexibility to incorporate further signal entries, and provide the user with tools to ease the gathering and analysis of tests on different materials, sewing speeds and machine configurations and settings. Outputs for actuators to implement closed-loop control strategies are also available. The paper will present an overview of the system, which is a development of earlier hardware and software and will focus on the results concerning the measurement of the force on the needle-bar, parameter is important to investigate on needle penetration force in fabrics during sewing. Several signal-processing implemented and tested algorithms, that aim to automate the detection of some characteristics, will be described. The purpose of this system, that implements some novel strategies, is to develop an add-on kit to apply to different sewing machines, but presently it has been implemented on a PC as a quality assessment system which will be used by textile technicians to build a quality database.
Present trends in the textile industry point to the reduction of the order sizes and to greater demands on shorter delivery times and higher quality. This factor leads apparel manufacturers to adopt new management strategies to enable a quick market response. On the other hand, in the sewing room, manufacturers must be able to quickly reconfigure their production system and equipments to cope with the rapidly changing needs of the materials that are being processed. The setting of sewing machines can, in this context, be a task that introduces si@icant time loss, as sewing technicians set the machines on base of experience and empirical methods. A scientifical approach to the behaviour of sewing parameters has been undertaken only in the last few years [l-41 and has revealed itself to be extremely difficult Objectives The present project aims to accomplish the following To ease the acquisition, storage and visualisation of the different signals acquired;To provide the user with analysis tools other than a simple time representation of the signals; To investigate on the usefulness of analysis tools used in other signal processing areas; To be itself a modular system, with expandable hardware so as to make it possible to accommodate other signal sources in addition to the ones considered at this moment, and also adding outputs for a control system based on the information collected in the first stages of this work. The-aim is to produce an add-on kit applicable to different sewing machines.VS Sewing Machines

 

Measured sewing parameters During high-speed sewing, several parameters are important for a good performance of the process. Measuring and analysing their behaviour during the sewing process can lead to a better understanding of the process, to the establishment of rules for correct setting of the machines and ultimately to a system which will be able to automatically set itself according to the different materials having the correct feedback to do so. In this work, sensors are used to measure the following parameters: IEEE Catalog Number: - 961 - Thread tension on the different sewing threads: the correct balance between them, as well as the correct amount of thread tension are very important for the stitch formation; Presser-foot force variations: One of the common sewing problems encountered in high-speed sewing is the bouncing of the presser-foot due to the feeding system, causing contact loss between the material and the presser-foot, which will result in an irregular stitch; Force on the needle bar: This parameter is of utmost importance, because this signal enables the determination of the necessary force for the needle to penetrate the material. Excessive needle penetration forces cause yam breaks, which will lower the seam's quality and also its strength. Due to the great variety of materials used presently, to the irregularity of some of their structures and difficult physical properties of some yams, it is very important for the apparel manufacturer to evaluate which needle or which limit to the sewing speed should be used. It is a task to be performed prior to the actual production. Two types of sensors were used to measure the above stated parameters: 0 Piezoelectric washers Nstler) were built directly into the needle and presser-foot bars to measure force on these components; 0 The sensing of thread tensions was achieved measuring the strain in cantilever beams put into the thread paths, as close as possible to the needle and loopers. Since quick variations on all of these parameters were expected, it was necessary to select components with a proper bandwidth. The piezoelectric sensors selected have a resonant frequency in the order of 200 KHZ, which lies very much above the maximum working frequency of the machine (7500 rpm = 125 Hz). In the case of the thread tension sensors, semiconductor strain gages were used to measure the strain produced in the beam. Calculation of the dimensions of a beam with a natural frequency of 5 KHz or above showed that the strain produced on its surface for the expected force values (max. 500 cN) would be too small to be picked up adequately by resistive strain gauges. Semiconductor strain gauges were used instead. This kind of strain gauge has a poorer performance concerning temperature drift and electromagnetic interference, but a much higher sensitivity. The sensor, manufactured by a Czech development institute, revealed to have very low temperature and mechanical drift, good frequency response and sensitivity. B. Data acquisition Hardware The hardware is divided into two blocks: 0 A Data Acquisition-board plugged in a PC 0 External signal conditioning hardware for the sensors. For the data acquisition, a LAB-PC+ board from National Instruments was selected, already tested in earlier experimentation. This board, although simple and economic, supports a sequential sampling of 8 analog channels at 83.33 KHz , features 3 parallel digital 1/0 ports and 2 analogue output channels. Concerning the signal conditioning circuitry, the following requirements should be met: 0 All signal amplifiers should be gain programmable, since significant variations of the signals' amplitudes are expected due to the testing at different sewing speeds and with different materials. Use of the LAB- PC's own gain programming is to be avoided because at higher gains settling times of the board's amplifiers and sample-and-hold circuits do not allow high sample rates.

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