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The key to ‘printing’ CIGS: tight tolerance controlBy applying copper indium gallium (di)selenide onto a web of thin foil in aprocess that resembles offset printing, themanufacturer can produce CIGS solar cellsthat are much less expensive than siliconwafer cells.chronize multiple axes of the rolls in onelong production line. As the web moves>100 feet down the line through differentprocesses, the axes have to align the foilmaterial in proper position for the nextstep. From step to step, we need tight tolerancecontrol so what happens in one operationlines up with the next. If the layersdo not match up, then the solar cell mustbe scrapped, resulting in wasted materials.Consequently, the machinery has to startwith tight tolerances, then maintain it atthe next step so we don’t have to make a lotof adjustments to the web’s tension, speedand position.”In conventional printing, rotary pressesoften use mechanical shafts and gears,but they do not come close to providingthe accuracy required in this application.Neither can stepper motors, a solution thatNorthfield Automation Systems has traditionallyemployed in their web-handlingmachinery.Motion Tech Automation, a local distributorof Bosch Rexroth motion controlproducts, assisted Northfield in finding asuperior alternative—the Rexroth SYNAX200 shaftless drive system. With theSYNAX 200 system, all the machines’ axesare electronically synchronized so whenthe line speed increases or decreases, theaxes ramp up or down together to maintainprecise web position.To maintain accuracy in thousandthsThe modularity inherent in the SYNAX 200 system made it possible to develop modular machinedesigns, which simplifies shipping and provides flexibility in configuring the line to meetvarious requirements of the solar-cell production process.of millimeters along a 100-foot line,Northfield specified Bosch Rexroth’sSYNAX 200 control platform along withIndraDrive intelligent servo drives usingSERCOS III industrial Ethernet communication.Designed for the web-handling industry,Rexroth’s SYNAX 200 is a control anddrive solution that provides tight control ofweb positions by making minute changesin speed to maintain registration of thelayers. Instead of mechanical cam shaftsand gears, tightly synchronized digitalservo drives and dynamic servo motorsrun off a standardized control platform tocreate virtual drives, an approach knownas Electronic Line Shafting. The primaryshaft is a virtual master axis. A programmableelectronic gearbox ratio simulatesthe mechanical gearbox between themaster axis and the drive. The master axismaintains a fixed relationship between itsposition and other virtual slave axes toachieve positioning accuracy that cannotbe obtained with mechanical gearboxes—or even stepper motors.Web handling can be done with steppermotors, however the positioning accuracyof stepper motors is between 500 and50,000 steps per revolution. That may seemhigh, but a servomotor using sine/cosineencoders for position monitoring providesover one million counts per revolution.That improves resolution and accuracy byseveral orders of magnitude.The customer was not previouslyfamiliar with Electronic Line Shafting.According to Stotz, “They knew what servomotorswere capable of, but we showedthem how one single controller could runall those axes and how the architecturecould achieve the precision they wanted.”For this application, the SYNAX platformincluded a machine vision camerathat focuses on registration marks onthe web. Position inputs are translated bya PLC that sends instructions to a rackmounted PPC multiaxis controller. Thecontroller communicates with the RexrothIndradrive C Converters powering RexrothMSK synchronous servo motors, whichmake the appropriate position and speedadjustments as required. Each drive functionsas a stand-alone device with its ownpower supply. Data is transmitted betweenthe motion controller and drives in realtime over SERCOS III industrial Ethernetthat provides noise immunity.The servomotors adjust speed to varyweb tension—and use speed to vary printlocation. Speed and tension can be adjustedby 1 percent increments as needed.With the SYNAX 200, achieving preContinued on page 11www.globalsolartechnology.comGlobal Solar Technology – June 2011 – 9