Non-contact sensors measure thickness and flatness of solar glass panels

Thin film is the manufacturing technology used to produce thin solar cells, which are deposited directly onto a low cost carrier material, typically glass, metal foil or plastic film. The advantages of using thin film technology are the material and energy savings made during the production process, good conductivity and the ability to produce large area solar cells. The glass panes are delivered with a conductive layer in order to optimise the production yield of the thin film solar cells. Here, one of the most important criteria is the flatness of the glass panes. This must be accurate to a just few millimetres across the complete surface of the glass. This means they need to be measured to an accuracy of microns. Other inspection criteria are thickness, length, width and squareness of the glass. The edges must also be inspected for cracks, shell defects or breakouts. For these inspection tasks, Micro-Epsilon employs two different optical measurement methods: confocal chromatic sensors and the light intersection method. The glass pane, which is lifted onto the conveyor belt by a robot, is placed on a solid block of granite in order to have a defined, flat reference surface. Six optoNCDT 2401 confocal chromatic sensors are positioned on a traversing beam directly above the pane, which measure the thickness and planarity in six planes. In this way, the glass pane can show a target thickness of between 2mm and 4mm. The desired distance or thickness information for the confocal principle is obtained from polychromatic white light. A conventional LED provides the light source. The sensors have a measuring range of 10mm and only measure the thickness from one side. During the traversing process, a Micro-Epsilon scanCONTROL laser scanner circumnavigates the glass pane. Therefore, the edge of the pane is simultaneously inspected for damage and its geometric values measured.