Silicon wafers for solar industry
Source: CAN Newsletter June 2012
It is really fun to watch the two carbon-fiber jib axes working „hand in hand“ without interfering with each other. In order to take care of their sensitive freight, they work with a steady acceleration curve without jerks. Nevertheless their velocity can make oneself getting dizzy.
The article describes the new plant of a well-known manufacturer of photovoltaic cells and the observing of a so-called wafer transfer system at its work in the manufacturing line. The described handling system may also be used in many other applications where high throughput, jerk-free motion, minimized downtimes and maintainability are required. The complete wafer transfer system has an overall axis number of 15. As network system between the drive units and the higher-level NC controller CANopen is used.
Wafer transfer system
The task of the wafer transfer system is to bundle the wafers (156 mm x 156 mm), which are coming in multiple tracks from the cleaning system to one track. Subsequently, the wafers are fed into the inspection system. Broken wafers or wafers lying partly and/or completely upon each other have to be fed aside carefully.
Regarding the handling in a manufacturing line the silicon wafers used in photovoltaic cells are rather challenging because the material is brittle and may easily break if handled roughly. In the described wafer handling application a double xyz-axis system of Jenaer Antriebstechnik with two jib axes made of carbon-fiber material replaces a robot with delta kinematics. The integration of the system into the manufacturing line has been carried out by SIM Automatisierungssysteme and Rex & Schley Automatisierungtechnik.
In five tracks the wafers run continuously through an optical image evaluation where the geometry is checked and defective wafers are sorted out. The double xyz-axis system takes over the wafer transfer from five tracks to one track (see Figure 1) and the automatic buffering. Currently a maximum throughput of 4800 wafers per hour can be reached, i.e. a complete handling sequence is carried out in only 0,75 s.
Compared to the single picker unit of the robot with delta kinematics the two picker units of the double xyz-axis system can run a steady acceleration curve and have a higher wafer throughput. The advantage of avoiding jerks is the significantly lower breakage rate of the wafers. Moreover, the reduction of vibrations has the additional effect that a disturbance of the measurements in the subsequent wafer inspection system is avoided.
