A hole lot better

German collaboration develops a tiny, low-cost sensor designed to improve the accuracy of car body manufacture. Siobhan Wagner reports.


German researchers have developed a method to enable car manufacturers to detect immediately if errors have been made on the production line.

A team from the Fraunhofer Institute for Surface Engineering and Thin Films IST in Braunschweig has designed a tiny, low-cost sensor that will monitor the accuracy of machines used to punch holes into the sheet metal of car bodies. Those holes are used to mount other components of the car. If any holes are missing or misplaced, the entire component of the vehicle has to be either scrapped or reworked.

To punch a hole, the metal has to be positioned exactly over the hole of a die. The new sensor, which is integrated into the punch itself, will register if the piece slips before it is punched. That would result in the tool striking only the edge of the metal. The sensor will also be able to detect if the machine fails to make a punch, or if it is using tools that are too blunt. This would enable machine operators to exchange blunt tools before they break or the material is damaged.



Dramatic improvement

The sensors are claimed to be a dramatic improvement over those that are currently available. The devices are only two millimetres thick and have a diameter of just 20-25mm — 10 per cent of the size of current sensors. This makes them suitable for use in large machine tools where many punches are mounted close together and conventional sensors would be too bulky.

Fraunhofer researchers also claim that their sensors are cheaper to produce, meaning that manufacturers will be able to afford to monitor every punch in a large machine. When the sensors are in mass production, the team estimates they will be 90 per cent cheaper than models currently available.

The sensor consists of a layer of carbon, which the researchers apply to a thin metal disc, using a vacuum coating process. This, they claim, is the first time the process has been used to produce carbon coatings for a sensor.

‘This coating has two functions —tribological and piezoresistance,’ meaning protection from friction and monitoring capabilities, said Saskia Biehl, group manager for micro and sensor technology at IST. She said the sensors are able to establish how much force the punch exerts on the sheet metal by changes in pressure and electrical resistance. As pressure in the carbon coating increases, its electrical resistance reduces.

‘It is really simple to detect,’ she said. ‘If you use constant voltage, you just measure the voltage change, and then you get the resistance change.’

After several tests of prototypes of the sensors, the researchers are determined that they are sufficiently robust to handle the tough life of a system on the production line. They are now optimising the technology so that it can be reliably mass-produced.



Other applications

The researchers are also looking at other applications. One idea they have is an intelligent washer that would be created by being coated with the special piezoresistance layer. The washer would work as a sensor system that measures the clamping force of bolted connections and simultaneously detects heat.