Standardized fasteners such as bolts are particularly suitable for structurally integrated force measurements, as they are located directly in the force flow and therefore allow the monitoring of the overall system status in-situ via the multi-axis components of the acting forces. Available solutions are either tailored to specific applications or focus solely on measuring individual components such as the axial force. We present a sensor integrated bolt that is capable of measuring axial force and bending torques. Two cavities are designed within the bolt for the measurement and full integration of electronics, allowing for full encapsulation and autonomy of the sensor. Adjustments to the bolt, such as the cavity design, are optimized using a finite element analysis to minimize any weakening of its primary function. Losses in resilience correspond to the reduction of a strength class. The sensor incorporates three bending beams equipped with strain gauges benchmarked through a calibration setup, that demonstrates a low uncertainty of only 2.5% and the capability to decouple axial and bending loads from the sensor’s output signals. The sensor is integrated into a prototype M20 bolt by means of screwing and bonding connection. Measurements confirm a linear force transmission to the sensor with a linearity error of 1.0% for axial loading and a hysteresis error of 1.9 %. Experimental validation confirms the bolt’s capability to detect bending moments under axial pretension, thus allowing for capturing all acting forces on such bolted connections.

Comprehensive data on machines is essential for digitization in the industry. Since standardized machine elements are used in most machines, integrating them with sensors provides the opportunity to acquire data comprehensively from in-situ. The sensor integration must not change the element’s core function and the standardized mechanical interfaces. Hence, sensors must be fully integrated and self-sufficient. Regarding bolts as widely used machine element, a solution that combines all the aforementioned requirements does not yet exist. A main problem in developing sensor integrating machine elements and especially bolts is to overcome the conflicting objectives in defining design space for mechanical and sensory functions. There is a lack of an approach to model the effects of the design space parameters on the mechanical and sensory functions. This paper proposes an optimization function to aid mechanical and electrical engineers in resolving conflicting objectives by balancing function fulfillment when determining design space parameters. Therefore, the effects of the parameters on the mechanical and sensory functions are modelled using FE-analysis and composing an optimization function with weights. This function provides optimal design space parameters. With respect to mountability boundary conditions the optimum for equal weights is at diameters 13.2mm both, increasing v. Mises stress by 29% and strain at sensor position by 80 %. The location of the optimum is very dependent on proper weighting, which resembles a prioritization of the mechanical versus the sensory function fulfillment. This enables engineers to find optimal parameters by balancing the mechanical and the sensory function fulfillment.

Powering embedded sensors in metal-enclosed environments poses significant challenges, such as structural constraints, battery dependence, and electromagnetic shielding. This work investigates using acoustic energy transfer (AET) as a viable approach and explores key factors influencing performance. A parametric study examines various parameters affecting energy transfer, which is followed by an experimental demonstration in a sensor-integrated bolt. Results indicate that electrical impedance matching greatly enhances power transfer but also alters the optimal transmission frequency. Elevated temperatures reduce the transducers' resonance frequency. This potentially impacts performance if only a single transducer is subjected to a temperature gradient, which can be caused by nearby machinery, exposure to hot industrial processes, or self-heating due to inefficiencies. Pure sine wave excitation increases power transfer up to 2.5 times over pulsewidth modulation at the expense of increased circuit complexity. Given that multiple parameters influence not only peak output power but also its frequency, active resonance tracking is essential for optimal energy transfer. Ultrasonic communication enables receiver-side frequency tracking, improving efficiency and adaptability in dynamic conditions. The final bolt-integrated setup achieves an output power of $\text{0.38 mW}$, demonstrating the feasibility of AET while also revealing size-related inefficiencies in embedded sensors.

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.