Step by step, the German state aid for photovoltaic systems will be reduced. In order to compensate this and to remain competitive, solar manufacturers have to optimize their production processes. The quality of solar cells and therefore their efficiency, as well as the throughput, have to be increased. In reaching this goal, manufacturers are supported by machine vision specialists offering fast solutions for quality inspection.

Solar technology has received a substantial boost within the last years, since rising energy prices cause an exceptionally high demand for photovoltaic systems which transform solar energy into electric power. In Germany, this trend was supported by a number of different economic stimulus packages, which have recently suffered drastic cuts: until December 12, 2012, funding for solar power will be cut back by up to 50% in four steps. In order to secure a continuously high demand as well as their market shares, German solar manufacturers are now facing the challenge of minimizing production costs for solar cells and other components faster than before while simultaneously increasing product quality and efficiency. Fully automated production facilities, which make the handling of the extremely fragile components easier and improve productivity, will therefore gain increasing importance in the future. Providing high-performance machine vision components, machine vision expert Vision Components enables users in the solar industry to design reliable and cost-efficient quality control solutions.

Inspection Tasks in Solar Cell Production
Electrically connected solar cells composed of monocrystalline or polycrystalline silicon wafers form the core of photovoltaic modules which directly transform solar energy into electric power. Solar plant operators aim for maximum efficiency, which means that the silicone wafers, which are approximately 0.2 mm thick, are manufactured with high demands on purity and precision. Defective parts must be rejected as early as possible to avoid cost-intensive further processing, and in order to exclude faulty components from impairing the energy balance of finished photovoltaic modules. Inspection tasks in solar cell production include detecting wafer positions and geometric attributes, monitoring wafer thickness and curvature as well as detecting fissures and damaged edges. Therefore, machine vision solutions must span a large field of view and ensure a fast image transfer. Real-time systems that enable the parallel processing of several frames help increase the process speed since they can carry out several inspection tasks simultaneously.

Custom-tailored Software Library
Vision Components has developed the VC Solar Solution software library which can be combined with VC‘s intelligent cameras, forming a complete solution for camera-guided production monitoring and quality control in solar cell production. No separate PCs are required since the real-time cameras are designed as compact, stand-alone vision systems. The library provides optimized positioning functions for this task, turning cameras into a comprehensive robot guidance solution which transfers all detected positioning data to the assembly robot via a communication line. This allows the robot to carry out all necessary adjustments or to divert defective wafers before the next production step.

Wide Range of Cameras
Vision Components offers a wide range of performance-graded cameras for the software library. The VC4067/NIR model, for instance, has been especially developed for electroluminescence-based quality control which enables users to look inside solar wafers. Thanks to a unique sensor operation mode, the photosensitive technology provides highly precise images at wavelengths up to 1,200 nm, thus ensuring very precise image recording even in low-light conditions. Featuring an integrated 400 MHz processor from Texas Instruments, the VC4067/NIR has a computing power of 3,200 MIPS. The 2/3" CCD sensor has a 1,280 x 1,024 pixel resolution and provides a maximum frame rate of 14 fps. A new model, the VC4467/NIR which provides increased computing power thanks to a 1 GHz processor, has been recently introduced.
VC‘s nano cameras with a board that is populated on both sides are another recent development. Available as a single board camera or with a protective housing, both types are extremely compact and provide a performance equaling that of much larger PC systems. Suitable for use as high-performance vision sensors, they can execute many tasks in photovoltaics' production, such as the mea-suring, position detection and handling of solar wafers. The VC6210 nano camera measures 80 x 45 x 20 mm, while its sister model, the single board camera VCSBC6210 nano, measures 40 x 65 mm. The units, which are equipped with 32 MB Flash and 128 MB DRAM, have more memory space than all other VC camera models. Featuring a 700 MHz processor and 5,600 MIPS, they provide a high computing power. Images are recorded by means of a global shutter CMOS sensor whose performance equals that of a CCD sensor, providing clear, high-resolution pictures even in extremely fast applications.

Application: Testing Facility for Solar Modules
VC cameras have already proven themselves in a wide range of solar technology applications. Buchanan Systems, a manufacturer of measuring systems for the solar industry, has for instance developed a tried-and-tested electroluminescence imaging system for the company Bosch Solar Modules. The EL-MES module, which is based on two VC4067/NIR cameras, is suitable for solar panels measuring up to 2,200 x 1,200 mm. Images are recorded by a an overview camera, which is installed in a fixed position above the panel, and a detail camera, which can be moved by means of a manually adjustable linear axis system. Users can adapt the measuring parameters to specific control tasks by means of specially developed software. The system ensures recording times as low as 200 to 500 ms. Image recording is triggered by means of a connected PC; moreover, images can be stored and analyzed.

Real-time Laser Scribing
Another application is the SolarEye system from the company EVT which consists of a VC4002L line scan camera and application-specific software. The camera controls laser scribing of thin-film solar cells, i.e. the separation of conducting paths. The cells‘ substrate material, e.g. glass, is coated with a thin silicon film which is then structured by a laser. The conducting paths run in parallel and as close as possible without touching. The monitoring system controls the laser in real-time with a precision of 1 µm. The laser line position is scanned at a 5 ms rate, providing 200 measurements per second. All data can be read out directly from the camera via an Ether-net or RS232 interface. The camera can transmit the analyzed data and scan results to an optional display terminal and a computer in order to immediately display any error sources. Containing all required analysis electronics, the robust camera autonomously executes all image processing processes. It needs to be connected to a PC only for parameterization. The device supports three integration modes: autonomous operation or external triggering with a constant exposure time or with a triggered exposure.