Fig. 1: Silicon strain gauge / Fig. 2: Simulation of the pretensioning force acting on the bolt head
At the CiS Research Institute, new types of silicon-based MEMS sensors are currently being developed to check safety-relevant bolt connections. Such special bolts are used, for example, in mechanical engineering, conveyor technology and wind turbines.
The new sensor is applied to the bolt head and measures its deformation due to the bolt pre-tensioning force. The bolt preload force – not to be confused with the torque – acts between the thread and the bolt head on the workpieces to be connected. It causes elastic deformation in all parts of the bolt. This can be easily measured at the bolt head. Until now, the elongation of the screw could only be measured by measuring the length of the bolt or by strain gauges attached to the bolt shank. These solutions are especially suitable for the evaluation of individual bolts. The MEMS sensor developed by the CiS Research Institute consists of four piezoresistive strain-sensitive measuring resistors, which are connected to form a Wheatstone measuring bridge. The active area without the electrical connections is only (200 x 200) µm², with a thickness of 10 µm (Fig. 1). The sensor is joined to the bolt head by means of glass solder. The most suitable position on the bolt head was localised by an FEM analysis (Fig. 2).
The pretensioning force can be measured directly with the screwdriving tool while the bolt is being tightened or alternatively, e.g. for the safety-related monitoring of screw connections, contact-free via an additionally integrated RFID transponder.
With the new sensor technology, the user can not only improve reliability and safety in the technical system, but also save material and costs.
Visit us for a presentation of the development results:
Hannover Messe, 24-28 April 2017, Hannover, Hall 4 Stand F34.
SMT Hybrid Packaging, 16-18 May 2017, Nuremberg, Hall 4, Stand 239B
SENSOR+TEST, 30 May – 1 June 2017, Nuremberg, Hall 1 Stand 1-150