50 punches crash onto the sheet metal of the car body and punch small holes into it which will later be used for mounting other components. However, if any holes are missing it can be costly, as the component either has to be scrapped or reworked. The result is downtime. Researchers at the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig have developed a sensor which continuously monitors the punches in real time. If holes are omitted or tools are blunt the sensor registers it, so that machine operators can quickly deal with the problem and exchange blunt tools before they break or the material is damaged. What is special about the sensors is that they are only two millimeters thick and have a diameter of only twenty to twenty-five millimeters – just ten percent of the size of commercially available sensors. They are therefore also suitable for use in large machine tools where many punches are mounted close together and conventional sensors would be too bulky. “Also, because of the low costs involved, you can afford to monitor every punch in a large machine,” explains Martin Weber, project manager at the IST. “When our sensors are in mass production, they will be up to ninety percent cheaper than models which are currently available.” The sensor consists of a layer of carbon which the researchers apply to a thin metal disk. “This is the first time established vacuum coating processes have been used to produce carbon coatings for a sensor,” says Saskia Biehl, group manager for micro and sensor technology at the IST. If the pressure in such a carbon coating increases, its electrical resistance sinks. These changes make it possible to establish how much force the punch exerts on the sheet metal. The researchers and their colleagues at the Fraunhofer Institute for Machine Tools and Forming Technology IWU have already tested prototypes of the sensors, with satisfactory results. Now they are optimizing the technology still further so that it can be reliably mass-produced. The researchers will be presenting their work at the Hannover Messe (Hall 6, Stand C2) which runs from April 16 to 20. Another application for the sensor, and one that will also be on display in Hanover, is the ‘intelligent washer’, a sensor system which measures the clamping force of bolted connections and simultaneously detects heat.
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