Traceability for Control

In the coming years, photovoltaic solar power will account for a significant portion of the European electricity market. The European Photovoltaic Industry Association (EPIA) came to this conclusion after studying the market trend in Europe, which showed that photovoltaics (PV) – under appropriate general conditions – could by 2020 supply 12 percent of the electricity required in the EU, thus becoming one of the fastest growing renewable energy technologies. According to the EPIA, PV systems with a capacity of roughly 4.5 GW were installed throughout Europe in 2008. Market observers expect that PV manufacturers will greatly expand their output as a result of the rapid pace of technological progress.

The prices of solar cells and modules, as technology-oriented products, decline quickly as a result of rapid technological developments. Photovoltaic manufacturers are therefore stepping up their efforts to lower production costs, while simultaneously also increasing product quality and improving the economic efficiency of their plants through optimized production processes. Mass production of solar cells – in particular thin-film modules – is a complex process, whose reproducible product quality depends on systematic monitoring of the individual processing steps. Complete traceability – provided by suitable identification technologies – enables weak points in the process quality to be detected early and corrected as quickly as possible; in addition, the production yield is increased.

Reproducible product quality – from the start

A number of factors can affect the quality of the solar modules during each of the various processing steps. It is therefore important to perform a quality inspection not just at the end of the production process, but to acquire quality-relevant data in real-time at every production phase. Only in this manner can error sources be precisely determined. The most widely used identification method in the solar industry is the data matrix code (DMC). On the one hand, data matrix codes offer sufficient reserves for applications in the solar industry – both optically and with regard to data capacity. On the other hand, the products can be marked very well despite the often limited available area. The DMC also ensures a high data integrity, making wrong readings virtually impossible.

Code reading systems are used for decoding the DMC. They offer outstanding data integrity and are optimally suited for reliable identification, even in harsh industrial operating conditions. Automation obviates the need for manual acquisition of the product and production data. Reading of the DMC is accomplished by placing the object (or its DMC) within the field of view of the reader. Reflections, insufficient lighting, or discolorations of the data field can be eliminated by selecting an appropriate sensor, lens, and illumination. Provided that no more than 25 percent of the code area is affected, it is even possible to compensate for dirt on or damage to the data field. The DMC is characterized by very high storage capacities and can contain over 3116 digits or 2335 characters for mapping the product data.

Code reading systems for stationary and mobile use

Intelligent code reading systems are available to the various users for industrial identification purposes. For example, the stationary code reading systems of the Simatic MV420 and MV440 series as well as the handheld Simatic Hawkeye not only ensure speed and ease of use, but also are highly flexible with regard to interfacing. For communication with the process control, they feature interfaces such as Profibus, Profinet, Ethernet, RS232, as well as digital inputs and outputs. Fast and reliable transfer of the code reading results is thus guaranteed.

The Simatic MV440 – the flagship among the code reading systems – can be flexibly and individually configured by the user. It features powerful illumination in red or white that permits operation at a distance of up to 80 cm between the code surface and the optics. In conjunction with an external illumination system from Siemens, reading distances of up to 300 cm can be achieved. The decoding algorithms reliably detect and read printed, lasered, drilled, punched, etched, or dot-peened codes – regardless of their rotational orientation. Codes can be on a wide range of substrates (metal, glass, plastic, etc.) and on moving or non-moving objects.

Fast deployment of the OCR solution

Another feature of the code reader is the possibility of system expansion for multicode reading – the simultaneous reading and comparison of OCR plain text (optical character recognition) and machine-readable 1D and 2D codes in a single image field. Specifically in the solar industry, the use of OCR alongside DMC markings is becoming increasingly prelevant. The solar cells, for example, are marked by a data matrix code containing the identification number stored on the production computer. Depending on the application, the serial numbers may be engraved in plaint text on the silicon or thin-film cells so that in addition to the automated identification, a visual check without code reading system is possible.

The OCR software is easy to operate without a learning curve. By selecting the appropriate presettings, the reader is parameterized in a few minutes. When employed in high-speed applications, the code reading system can accomplish up to 1000 readings per minute. Five image regions with independent settings are read in parallel. No additional software is necessary for commissioning and network integration. With the Simatic MV400 products, the user interface through a web server is located on the code readers. Except for an HTML browser – such as Internet Explorer with Java Runtime Environment (JRE) – no other software is needed.

In addition to optical identification with DMC or plain text, radio-based solutions with radio frequency identification (RFID) can also be employed. The main advantage of the RFID method is that the identification occurs without visual contact of the marked object. The systems consist of mobile data media with integrated antenna (transponders) and a reader. Passive transponders receive the energy required for data transfer from the reader, whereas active transponders have a built-in battery. Different radio frequencies are available for RFID data transfer. With HF technology, distances from a few centimeters to about one meter can be bridged. Systems operating with UHF frequencies can have a range up to ten meters.

Effective quality management prevents production downtimes

The most cost-intensive part of solar cell production is at the beginning of the process – effective quality management is thus of utmost importance in the early stages. Only in this way is it possible to detect faulty cells early and sort them out. When employing a vacuum coating process, the system – once restarted after an outage – may require several hours before again reaching full efficiency. Any malfunction will incur considerable costs. Companies thus have a huge interest in maintaining defect rates that are as low as possible . Optical code reading systems permit attainment of the high quality standards demanded – the output of solar cell production increases and process costs decrease. The higher the quality of the solar cells, the higher the energy yield of the finished modules will ultimately be.

The requirements of photovoltaic manufacturers vary according to the respective production processes and products. Manufacturers with global sites use, for example, the Asian market to source their wafers. Manufacturing of solar cells and module assembly, in turn, is located in EU countries. In this case, the blanks marked with DMC or plain text are checked by readers upon delivery, and the code quality is verified according to established standards. If, for example, glass breakage frequently occurs during further processing, the cause of the error can be precisely located (in manufacturing, final assembly, etc.).

The use of code reading systems thus also prevents products from going through the entire production process just to be sorted out as defective in the final processing step. Quality assurance, however, does not end with the shipping of the products. Photovoltaic manufacturers usually offer a performance guarantee for their modules, which stipulates a nominal output of 80 percent for a duration of 25 years. If modules do not achieve the specified output or yield, manufacturers are obligated to provide replacement modules or make up for the missing output. Because of this guarantee, PV manufacturers generate measurement reports during the manufacturing that allow precise conclusions to be drawn on the production and performance data of the solar modules.

Integrated automation solution saves costs

For manufacturers of solar cells, the challenge often lies in selecting identification systems that are suitable for the production processes involved. When planning to use the above-mentioned products, Siemens consultants – together with the users – determine the individual expectations and requirements in order to develop a tailor-made solution. The combination of technical know-how and project experience in the solar industry contributes greatly to the successful implementation of the called-for solution. The portfolio offered ranges from the delivery of high-performance devices for the reading and verification of machine-readable 1D/2D codes and plain text to the development of turnkey automation solutions.

System integration according to the principle of “Totally Integrated Automation (TIA)” ensures an integrated solution with interference-free availability and superior productivity. Furthermore, TIA establishes conditions for optimized production processes. The products and systems of the portfolio are optimally coordinated and characterized by unique transparency and interoperability. With reduced interfacing work, they provide full transparency across all levels – from the field level via the production management level to the ERP/MES level. Companies benefit from TIA throughout the life cycle of their plants. Last but not least, the seamless interaction of all components reduces the engineering work required for automation solutions and offers greater diagnostic options for the entire plant. n

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