Out of Sight: Line-scan camera technology aids solar glass inspection and drives down costs

What you can’t see sometimes can hurt you – and your bottom line. This is evidenced by the precise manufacturing processes required in the solar-cell market, poised to experience exceptional growth.

This market’s continued success will require manufacturers to drive production costs down. Automated optical inspection using machine-vision systems will play a key role in this cost reduction by speeding production and reducing waste. The challenge is to have the right vision system for each inspection task.

The solar cell industry has already seen a 35-percent compound annual growth rate (CAGR) since 1998 and is projected to remain at a 20 to 30-percent CAGR through 2011. A key factor that could limit this growth, however, is the cost of photovoltaic energy. Sunlight is free, so the solar panels’ price will be the dominant factor in setting energy cost. Panel prices must drop significantly in order for photovoltaic energy to be competitive with traditional alternatives.

Given the political climate, lower panel costs will increase market growth, even without cost parity.

Advantages of automated optical inspection
To reduce panel costs as well as meet growing demands, manufacturers will need to increase production efficiency. Automated optical inspection (AOI) can help in several ways. Understandably, it’s faster than manual inspection, allowing developers to speed their production process by removing the inspection bottleneck. It also offers high reliability, precision and accuracy. This permits detection of errors earlier in the production process, reducing waste. It also allows safe handling of thinner wafers while reducing production delays caused by wafer breakage and subsequent cleanup. The vision systems performing the inspections can provide immediate quantitative feedback on the location, type and quantity of errors detected to support continuous process monitoring and improvement.

One company leading the charge in this area is Laval, Que.-based SynergX. The company’s SGI Glass-Scan system provides solar-glass producers and solar-panel manufacturers with a turnkey yield management and quality control system for the inspection of both continuous float glass (web inspection) and cut sheet glass.

Founded in 2004 by Ken Wawrew, president and CEO, and Stéphane Lemieux, general manager and vice-president, SynergX is an industrial-technology company with its core competency being AOI and machine vision. The company designs and manufactures application-specific machine-vision systems, specializing in two industries: inspection for the glass industry (automotive glass and solar panels) and the inspection of bottling applications for the beverage industry. SynergX uses DALSA’s line-scan camera technology in its SGI Glass-Scan system to increase productivity, reduce the need for human inspection and guarantee the quality of the solar panel product by inspecting the glass prior to its lamination to the solar panel.

"With the ongoing interest in alternative energy supplies, such as solar power, the use of solar glass is rising," Wawrew states. "Solar-cell manufacturers are under immense pressure to drive down costs in order to have solar energy become roughly equivalent to the cost of conventional power; this is called ‘grid parity.’ Automated optical inspection is one way to both increase quality and drive down costs."

How the system works
The solar-glass market is one where quality is paramount and a major differentiator. It is important that the glass being produced is monitored for both process control and quality. Solar-panel manufacturers typically provide a warranty of 25 years with their panels, and poor quality glass could cause a failure in the field. The Glass-Scan system helps the producer guarantee quality by detecting and classifying defects in the glass, such as black stones, white stones, closed and open bubbles. The system also helps the solar-glass producer optimize the cutting of the glass into sheets by identifying and marking defects that are out of spec and then cutting around these defects, ensuring only the highest quality glass is shipped to customers. Productivity is further increased by replacing a large number of human inspectors previously used for this task.

Solar glass is fabricated on a continuous web. In a typical inspection, the glass sheets pass through the SGI inspection system at variable speeds where the cameras acquire an image at a frame rate relative to the actual speed of the glass web. As the images are acquired, the signals from each camera are sent to front-end processing boards where raw signal is filtered to isolate areas suspected to have a glass defect. These suspected areas are then sent to the post-processing boards, where they are automatically analyzed and classified. The defects are sent to the supervisory control board where the decision to reject or not to reject will be taken according to the settings establish by the line operators. Finally, the position of the out-of-specs defects will be sent to a glass-cutting system with the objective of optimizing glass cutting.

As the glass is produced, the SGI Glass-Scan system has a number of line-scan cameras covering the width of the web or glass production line. When a defect in the glass is detected, the system communicates with the line-control system and the defect location is marked. The manufacturer uses this information in the glass-cutting process by cutting around defects to ensure that defective glass is not shipped to the customer.

For the solar-panel manufacturer, the system integrates fully into the production line, providing an in-process inspection point after washing and drying, before the glass panel is laminated to the solar panel. The system provides yield management, ensuring no defective glass panels are laminated to a solar panel and thus wasting a significant amount of production material and capacity.

Image acquisition: the key factor
An integral part of the Glass-Scan system is the image-acquisition portion, which is managed by DALSA’s line-scan cameras. Currently, more than 200 systems are installed and working in the field.

"Over the years, DALSA’s camera technology has proven to be highly reliable and highly effective in meeting our application needs," Lemieux says. "Our systems’ success is dependent on camera speed, sensitivity and throughput, and the DALSA cameras deliver on these attributes.

"The nature of glass inspection requires a well-developed line-scan platform that can be adapted to both continuous glass web inspection and cut sheet glass inspection," he adds. "In some cases, the edge of the glass is ground, which can also require inspection. The DALSA line-scan camera has formed the basis of the SynergX Glass-Scan systems, meeting all the requirements for both surface scanning and edge inspection. And, as higher speed manufacturing and tighter defect specifications necessitate faster and higher resolution systems, these cameras have kept pace with the all our technological requirements. As a result, SynergX is able to deliver reliable, high-performance glass-inspection systems covering a wide range of glass tints and textures."

The solar cell market faces an exceptional opportunity for growth, but production costs will set the pace and extent of that growth. Increasing production speeds and yields, with corresponding reduction in cost, will require use of automated inspection methods at a variety of points in the fabrication process. Inspection systems such as the SynergX Glass-Scan are assisting in the adoption in solar power, with advances in automated inspection.

Philip Colet is the vice-president of sales and marketing at DALSA in Montreal. He can be reached by email at philip.colet@dalsa.com or by telephone at (514) 333-1301.