Close-to-process machine vision in the photovoltaic industry
1 Jun 2010
Machine vision technology has become established in almost every stage of the photovoltaic production process, in spite of being a comparitively new element of automation technology. However, the ability to supervise and control manufacturing processes in real time by means of the image data acquired is still too rarely used.
The procedure should not be used just to check the quality at the end of individual process steps in order to find out that the process is not perfect, as is often the case with machine vision systems. Machine vision is actually an important tool that can provide a better understanding of the process itself and can be used to control the process to the best levels possible.
"We have found that machine vision systems are becoming more and more integrated into the machines and we are convinced that eventually they will become just another part of the machines“, says Dr. Johannes Stelter, Head of the Intralogistics and Machine vision Division at ECKELMANN AG. According to him, machine manufacturers will need to give careful consideration to the capabilities of machine vision technology to increase the efficiency of their machines and systems.
The design and development of a process-oriented machine vision system requires early clarification of factors such as the amount of space available, data integration and the mode of operation. This makes it possible to implement a custom-built system, adapted to the specific needs, that provides not only process-related advantages but also synergistic effects, such as reduced space requirements.
Machine vision as a key technology
ECKELMANN has always developed control systems for industrial automation. The company identified machine vision as a new key technology in automation at an early stage, and in 2005 created a dedicated division to focus on this subject. This early adoption of machine vision has allowed ECKELMANN to make its mark as a supplier of high quality machine vision solutions.
ECKELMANN already uses machine vision systems in various applications in photovoltaic production. Some of the examples given by Dr. Stelter are edge isolation systems, wafer classification or the inspection of solar modules. According to him, the common ground for all of the applications is the level of integration with the resulting technical and economic advantages. According to Dr. Stelter's experience, some of the recurring tasks in machine vision in the photovoltaic industry are the positioning, accurate measurement of geometric properties such as edge and chamfer lengths of square and pseudo- square wafers and the identification of material faults, e.g. microcracks or chips.
Fast and highly accurate laser edge isolation
A system for laser edge isolation has been designed by ECKELMANN for the ASYS group. ASYS is a leading manufacturer of handling systems, process machines and special machines for the electronic and solar industries. Dr. Stelter uses the example of the ASYS system to demonstrate the advantages resulting from a sophisticated system designed in close cooperation with the respective experts. Edge isolation is used to provide electrical separation between the active front side of a solar cell and the rear side. In the edge isolation process, a laser cuts a small groove along the cell edges, the depth of the groove depending on the cell doping. The difficulty lies in positioning the groove as close as possible to the outer contour of the cell in order to maximize the active surface and thus the efficiency.
To cope with this problem, ECKELMANN has designed an machine vision system based on line scan cameras that is able to measure the outer contours with high precision and to transfer the contour data to the control system of a connected laser with deflection mirror. Edge damage is detected and, if the damage is within tolerance levels, the laser will ignore it and proceed with the cutting process.
"Image acquisition and analysis take place in a record time of only 800 ms“, declares Dr. Stelter: "The resolution of the system makes it possible to ensure that the distance to the edge during laser cutting is clearly below 100 µm. Another advantage of the system is that the calibration and qualification of the laser and camera have been automated. This process determines both the individual characteristic of the laser deflection unit and the inner and outer camera parameters. Based on this data, the system calculates a compensation map which will be used for error correction in the control system of the laser deflection unit. This avoids time-consuming qualification of the camera and laser coordinate systems. The system can thus be easily commissioned or recalibrated after maintenance work“, declares Dr. Stelter.
Even errors in the transport system of the machine can be diagnosed by means of the machine vision system, thus allowing for remedial action to be taken immediately.
Choosing the right components is vital
ECKELMANN worked in close co-operation with STEMMER IMAGING, located at Puchheim, to select the best machine vision components for the current application. "In the past, we have gained good experience from our cooperation with STEMMER IMAGING whose machine vision specialists impressed us with their large range of components and their service“, affirms Dr. Stelter. "While designing the edge isolation system, we have been assisted by STEMMER IMAGING who provided intensive feasibility studies and invaluable advice on the machine vision components to be used for this application.“
The control system for laser edge isolation, for example, features a line scan camera with 4096 pixels from the Canadian manufacturer Dalsa, optics from ZEISS and customized LED illumination. Image analysis is performed in the context of the machine control and is therefore completely incorporated in the machine's infrastructure. "Without doubt, our focus is on process-integrated machine vision“, declares Dr. Stelter: "As system integrators, we are responsible not only for the products and services we provide, but also share responsibility for the entire machining or manufacturing process based on our system. Being specialised in automation solutions for almost 40 years and having a thorough knowledge and understanding of many sectors, we have the necessary experience and competence to meet these requirements."
"We rely on the know-how of the customer to provide us with detailed information on the manufacturing process and material properties", declares Dr. Stelter. A thorough understanding of the properties to be classified and/or the parameters to be measured is, according to him, the basis for choosing the right machine vision components, and it is these components that make it possible to simplify the software algorithms needed for adequate interpretation of pixel information.
ECKELMANN and ASYS
ECKELMANN AG (www.eckelmann.de) is a medium-sized automation partner and supplier for the machine and plant manufacturing sector. Since its foundation in 1970, the company has always focused on the development and implementation of electronic control systems for machines based on modern microprocessors, customized to meet the individual functional requirements and adapted to the machine environment. It offers a wide spectrum of solutions ranging from the compact machine controller to the complete automation of large and complex industrial plants.
The ASYS group of companies (www.asys.de) is a globally acting technology company and leading manufacturer of handling systems, process machines and special machines for the electronic and solar industries. ASYS Automatisierungssysteme GmbH is the lead company for the activities of the entire group of companies. It maintains an efficient infrastructure: all sectors ranging from metal and housing construction to surface refinement, and including the machine capacities are located in Dornstadt. This is where the company develops and produces handling systems, process machines for marking and blanking and metallization lines for the solar industry.
STEMMER IMAGING has been one of the leaders in the machine vision market since 1987. It is one of Europe's largest technology providers in this field. In 1997 STEMMER IMAGING presented Common Vision Blox (CVB), a powerful programming library for fast and reliable development and implementation of vision solutions, which has been deployed successfully throughout the world in more than 80,000 imaging applications in various industries.