We come across screws everywhere. For example, on cranes, scaffolding, high-rise buildings, bridges, wind turbines and machinery of all sizes as well as in production facilities. However, wear and the impact of phenomena such as temperature variations or vibrations can lead to the loosening or even the complete loosening of one or more screws. This can have fatal repercussions. Therefore, regular inspections are necessary for safety-critical structures. A team of researchers from the Fraunhofer Cluster of Excellence Cognitive Internet Technologies CCIT has now developed a solution called “Smart Screw Connection” which combines sensors and radio technology to provide a reliable means of remotely monitoring screw connections – not to mention that it is autonomous -powered, informs the Fraunhofer Institute in a press release.
The Smart Screw Connection comes with a screw fitted with a washer which is fitted with a piezoresistive DiaForce® thin film. When the screw is tightened, the pressure-sensitive sensors register the preload force at three points. Any change in the preload force changes the electrical resistance of the DiaForce® thin film. “When a screw loosens, the resulting change in resistance is signaled to a radio module on the head of the screw. The radio module, in turn, sends the data to a base station, which collects the information from all relevant screws in the object,” says Dr. Peter Spies, project manager and group leader Integrated Energy Supplies at the ‘Fraunhofer Institute for Integrated Circuits IIS. .
Reliable data transmission via the mioty® protocol
DiaForce® thin film was developed by the Fraunhofer Institute for Surface Engineering and Thin Films IST. In terms of radio technology, Fraunhofer IIS has contributed to the mioty® Low Power Wide Area Network (LPWAN) protocol. This technology is able to send small packets of data over long distances, consuming minimal power, from more than 100,000 sensors via a single base station. The base station could be located on the edge of a wind farm, that is to say several hundred meters or even a few kilometers away. A software program then displays the data for each individual screw in a graphical overview. Depending on the configuration and application, the status of the bolted joints is transmitted continuously, on an event basis or at specified intervals.
“This remote monitoring system allows us, for the first time, to keep an eye on the stability of safety-critical infrastructures at all times, even remotely, and to carry out an appropriate check on each relevant screw. This is a major asset in terms of security. When inspecting a bridge or a wind turbine, no engineer has to be on site and check each screw individually, because all data is transmitted by radio to the service station,” says Spies. .
Adaptable for multiple applications
The smart screw connection can be adapted to a wide variety of applications. Whether flange joints in industry, bolts in steel beams of high-rise buildings, load-bearing parts of bridges or fastening of rotors on wind turbines, the system can be configured individually for each scenario and adapted to the corresponding load profile.
The researchers also solved the energy demand problem in a resource-efficient way. The system operates on the principle of energy recovery. It involves the use of heat or light to generate electricity. In this system, for example, a thermoelectric generator generates electricity from minute temperature differences between the screw head and the environment. It is also possible to generate electricity using solar cells. Energy harvesting makes the system self-powered.
Encryption protects against hacker attacks
Safety is of paramount importance to Fraunhofer researchers. During installation, each individual screw, as well as the sensor and the radio module, are placed in a shielded programming box. The housing gives the screw an individual identifier via short-range RFID, as well as its requirement profile and an individual encryption key. The radio link between the screws and the base station is also encrypted during data transmission. “This is how we prevent criminals or hackers from sabotaging the system. Technical staff monitoring a wind turbine, for example, can really rely on the data,” says Spies.
Fraunhofer IST, Fraunhofer IIS, as well as the Fraunhofer Institute for Structural Durability and System Reliability LBF and the Fraunhofer Institute for Applied and Integrated Security AISEC are involved in the project led by the Fraunhofer Cluster of Excellence Cognitive Internet Technologies CCIT.
The technology is designed for commercially available DIN screws. The system is ready to use for M18 screws, and versions will soon be available for M20 and M36 screws. A demo version will be exhibited at Hannover Messe 2022 (May 30 to June 2) (Hall 5, Stand A06).