The Role of Testing and Characterization in the Methodical Development of Sensor-Integrating Bolts for Multi-Axial Force Measurement.

Sensor-integrating bolts offer a promising solution for in situ data acquisition by integrating sensory functions into existing machine elements. The standardized mechanical interfaces and performance must be maintained in order to ensure widespread use and retrofitting, which leads to a conflict of objectives between mechanical and sensory performance. The state of the art lacks multi-axial force-measuring bolts that meet these objectives. Therefore, we present a methodical approach that develops evolving prototypes of sensor-integrating bolts and applies testing activities to characterize the bolts' performance and gain knowledge to further improve the prototypes. Strain gauges and a specifically developed electronics platform for data acquisition with a BLE interface are used. The prototypes are tested for axial and bending loads. Mechanical and sensory performance indicators are investigated, such as von Mises stress increase, linearity errors, hysteresis errors and sensitivity. The characterization results of the testing activities and the knowledge extracted from each iteration concerning aspects like strain gauge type and position or sensor body integration and their influence on the performance indicators are shown. Overall, the prototypes improved the linearity error from 3.3% to 1% (axial) and 3.7% to 0.1% (bending). The error is below 6% and sensitivities are around 10 µV/V/kN (axial) and 1 µV/V/Nm (bending). The integration of the measurement chain reduces the mechanical performance within a range of 1.3% to 24.6%, underlining the conflict of objectives. Measures for compensating the reduction need to be taken, such as increasing the bolt's strength. Moreover, the measurements indicate that bending torques are superposed, even in axial load cases, making investigations of the effect on loadability necessary.