Solar-powered microdrive

A hand-crafted, 18cm diameter solid silver bowl with a circular solar cell powers a tiny drive unit in the bowl's interior to rotate an ornament at its centre.

For the first visit by a Pope in the city's almost 900-year history, the city of Freiburg commissioned a very special and symbolic gift. Not only does the bowl depict city landmarks like the cathedral and the Black Forest, it also underscores the city's development model in the areas of environmental protection and renewable energies. The choice of material and the design at the same time evoke the gothic architecture of the cathedral and the silver deposits that bestowed prosperity and riches on the city in the Middle Ages.

The elaborate design and highly sophisticated mechanism of this unique piece was only possible as a result of active collaboration between several partners, including the Freiburg Fraunhofer Institute for Solar Energy Systems and the drive specialists from Faulhaber in Schönaich. The centerpiece of this artistically and technologically sophisticated work is a round, monocrystalline solar cell and a precious metal commutated microdrive. Unlike conventional solar cells, the energy-conducting "grids" were not given the usual lattice structure but were installed in the shape of a star. On completion of the cell, its measured power output was 5A short-circuit current.

The interesting question now was whether this amount of energy would also suffice to turn the ornament. This is where the technological benefits of Faulhaber's precious metal commutated microdrives came into play. Thanks to their commutation and coil technology, they can be operated at extremely low starting voltages and are therefore ideal for use in battery operation or with solar cells. A series 1516...S DC-Micromotor was chosen to turn the figure in the center of the silver bowl. This motor can run at a current intensity of just 10 mA, and with its continuous torque of up to 0.4mNm has enough power to rotate the ornament at a speed of up to 20 rpm,