Using a PCB Layout Tool to Create Embroidered ... - Semantic Scholar

Using a PCB Layout Tool to Create Embroidered Circuits George F. Eichinger III, Kara Baumann, Thomas Martin, Mark Jones E-textiles Lab, Virginia Polytechnic and State University { georgefe, kbaumann, tlmartin, mtj } gvt.edu Abstract

Embroidery using conductive threads is an appealing method for creating circuits on fabric, with the embroidered threads forming traces and pads, and ICs and PCBs attached via several diferent methods including epoxy, snaps and stitches. However, embroidery software was designed for the look and feel of the stitches, not the electrical properties and The manufacturability of embroidered circuits. process remains largely an art form where one must trace lines, determine stitch density, freehand designs and hope the software places stitches in the right place. The goal of this project is to automate this process by using PCB design tools and custom software to convert the layout to an embroidery pattern. By automating the tool-flowfrom PCB layout to embroidery, we have improved the quality of the embroidered circuits, reduced the burden on the designer to have extensive embroidery knowledge, and taken first steps towards automated assembly of embroidered circuits.

1. Introduction

One of the goals of electronic textiles is to stay as close to existing manufacturing processes in both the textile and electronic industries, in order to leverage off the economies of scale of both industries, Embroidery is one of several textile techniques that are appealing for creating circuits on cloth. Embroidering circuits, or e-broidery, was first described in 2000 [1]. Since then, several different research areas have spawned from embroidering conductive thread. At Virginia Tech, we have been experimenting with varied e-broidery applications ranging from capacitive sensing and variable resistors to attaching standard PCB packages. Thus far, research papers on the subject have focused on the result of the embroidery and have not provided indepth descriptions of the steps used to embroider the circuits [2, 3, 4]. The common thread in these papers is that a machine friendly yarn with conductive properties is used to embroider patterns onto fabric. In our experimentation, for all but the simplest circuits (e.g. wide spacing between traces, right angles), there was significant work required to use an embroidery CAD software package to create valid and repeatable

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circuits on the sewing machine. Considerable effort was spent manually designing circuits and figuring out the proper scaling to create pads on fabric that match up with packages. The embroidery software often placed stitches in such a way that it hurt the electrical behavior of the traces, e.g. long jumps between stitches and shorted adjacent traces.

2. Tool-flow Our goal is a fully automated tool-flow from circuit layout to component placement on the fabric. Furthermore, we aim to use freely available tools wherever possible so that the tool-flow can be used by a wider audience. This method gets rid of expensive embroidery software, eliminates the guesswork in designing embroidery and creates an easy to use interface for the engineer who is already accustomed to PCB design. The original tool-flow involved 11 steps and included expensive 3D CAD packages as well as expensive embroidery software. This is real hindrance to the engineer both in the time it takes to master all of the software and the start up expense of

getting into e-broidery. Even with the proper software,

it can be difficult to navigate through all of the embroidery choices of what stitches to use, what stitch density and how far away traces should be so there are no shorts. This project moves away from these problems and adopts a standard PCB design tool, Eagle, to layout the e-broidery circuit as a single layer circuit and a custom written C# program to convert the Eagle output to sewing machine format. Eagle allows us to utilize PCB rule sets, existing part libraries and auto routing. The custom software allows the use of predetermined stitches and conversion to our specific machine. Eagle is a good choice because the existing rule sets allows the definition of trace width and the distance between traces. Also, if the end result is the placement of a PCB, IC or other designed part on the fabric, Eagle performs well because the layout of these other parts is already present in the software. No additional work is needed to use its large set of existing and custom libraries. Lastly, auto routing eliminates the arduous task of laying out the traces. Once the Eagle design is done, it passes it off to our C# program via Gerber files. While we used Eagle because it is free and works for the typical complexity of our circuits, it should be noted that any

PCB design software that outputs RS-274X Gerber files should work. Gerber files were originally created to control photo plotters and the format stores commands in an ASCII format that gives information such as the type of aperture to draw a line with and what the absolute position of that line is. This converted nicely to sewing data. The Husqvama file format stores all of the stitches in an array using relative positioning [5]. To convert the Gerber files to Husqvama files, we wrote a series of functions to

interpret the Gerber command set and produce corresponding stitches. Presently, the program is limited to line conversion but almost all of a PCB's design can be interpreted as a series of lines. The .VP3 is currently the only embroidery format that is implemented but commercial software can convert this format to work on any embroidery machine. We have done some initial characterization of the electrical properties of traces using different stitch sizes and densities and trace widths. For the Bekaert BK 50/2 we fo eac that placings sttchread aruirtang, haveng stitch 1.5 mm apart w and tracing all of the lines four

times is a good compromise between electrical conductivity (about 2.6 Q/cm) and physical properties (e.g. look and feel) of the traces. However, anyone using our tool-flow will want to perform there own tests to determine the stitch width and placement that provides adequate electrical and physical properties for their choice of thread and fabric.

custom software have been removed. If your circuit passes the design checks in the PCB CAD software, it can be embroidered. We intend to expand the flow to include subsequently automatically placing components onto the fabric. Acknowledgment: This material is based upon work supported by the National Science Foundation under Grant No. CCR-0219809, CNS-0447741, and CNS0454195.

References [1] E. Post, M. Orth, P. R. Russo, and N. Gershenfeld, "E-

Broidery: Design and fabrication of textile-based computing", IBMASystems Journal, 2000, 39:3-4. [2] T. Linz, C. Kallmayer, R. Aschenbrenner, and H. Reichl, "Fully Integrated EKG Shirt based on Embroidered Electrical Interconnections with Conductive Yarn and Miniaturized Flexible Electronics", IEEE BSN, 2006, pp. 23-30. [3] T. Linz, C. Kallmayer, R. Aschenbrenner, and H. Reichl, "Embroidering Electrical Interconnects with Conductive

Yarn for The Integration of Flexible Electronic Modules into

Fabric", IEEE ISWC, 2005, pp. 86-89. [4] L. Buechley, N. Elumeze, C. Dodson and M. Eisenberg, "Quilt Snaps: A Fabric Based Computational Construction Kit", IEEE WMTE, 2005, pp. 219-221. [5] J. Weiler, "VP3 File Format," [Online Document], Jan. 2007, Available at HTTP:

http://wwwjasonweiler.com/VP3FileFormatInfo.html

3. Results

One of the applications we have explored with ebroidery is capacitive sensing. Our capacitive sensing board was designed with pie-shaped points for mating the PCB to the fabric, providing easy attachment to fabric and sufficient surface area contact. Figure 1 shows the pie shaped connection points routed to the desired capacitive sensing segment. The goal was to attach the capacitive thread sensor to the board. If this were to be done without the exact layout in Eagle, it would be difficult because of the specific placement and angle of the pie shaped elements. Figure 2 shows the final embroidered product on woven cloth with

Figure 1 - Eagle Embroidery Design

This paper has described some of the difficulties of

using embroidery software to create e-textile circuits

and outlined our e-broidery tool-flow based upon a standard PCB layout tool. Our previous tool-flow involved coercing the software into creating_ something we could use. With our tool-flow, anyone llll.li who can design a circuit in PCB software can design a 111 .. fabric circuit. Many of the complicated decisions andFiue2-EbodrdDsg

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