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.

Robotic arms are used in a wide range of industrial and medical applications. However, for research and education, users often face a trade-off between costly commercial solutions with no adaptability and open-source alternatives that lack usability and functionality. In education, this problem is exacerbated by the prohibitive cost of commercial systems or simplifications that distort learning. Thus, we present HELENE, an open-source robot with six degrees of freedom, closed-loop position control, and robot operating system (ROS) integration. The modular design of the robot, printed on a commercial 3D printer, and its integrated custom electronics allow for easy customization for research purposes. The joints are driven by standard stepper motors with closed-loop position control using absolute encoders. The ROS integration guarantees widespread control options and integration into existing environments. Our prototype, tested in accordance with ISO 9283, has a small positional accuracy error of 8.4 mm and a repeatability error of only 0.87 mm with a load capacity of 500 g at a reach of 432 mm. Ten prototypes were built and used in various research and education applications, demonstrating the versatile applicability of this open-source robot, closing the gap between reliable commercial systems and flexible open-source solutions.

Plantar pressure measurement, or pedobarography, is an essential tool for analyzing human motion in healthy individuals and patients. Across the reviewed literature, sensor insoles are motivated as wearable, mobile solutions for assessing pressure distribution in applications including diabetic foot monitoring, rehabilitation guidance, assistive device control, and sports performance analysis. This review evaluates the current state of the art with particular attention to sensor technologies, sensor quantity and placement, participant cohorts, and reference standards. The focus lies on original works with innovative designs, preferably supported by ambulation experiments. The modalities covered include resistive, capacitive, inductive, piezoelectric, triboelectric, and optical sensing approaches. We identify a lack of proper sensor calibration, gait-based verification, and human study validation, and propose a gold standard based on testing machines and instrumented treadmills to ensure comparability across studies. The bidirectional interaction between insole insertion and foot-sole mechanics is examined, with tissue stiffness identified as a key source of uncertainty in sensor signals. Guidelines are provided for sensor dimensions and unobtrusive insole designs to foster natural gait. Finally, future directions include the development of multimodal sensors to compensate for limitations of individual modalities and the emerging trend of multiaxial sensing for capturing shear components in pressure distributions.

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.

Structurally integrated force measurements enable real-time monitoring of the overall system status. Existing sensor integration strategies in bolts often compromise mechanical integrity and deviate from standardized external dimensions, limiting their applicability. In this paper, a sensor is presented that is integrated into a M20 bolt to measure axial force and bending torques. The sensor is integrated directly into the bolt itself without altering its external dimensions. This integration method offers a more flexible solution compared to currently available solutions. Furthermore a method is presented that is extracted out of the development process. It includes the optimization of the design space for sensor and electronics as well as a general procedure based on the VDI 2206 and the methodology of testing. The sensor-integrating bolt retains 76.5% of the static load capacity of a standard bolt, which can be compensated by selecting a higher strength class. The sensor body consists of three vertical bending beams with applied strain gauges. The experimental findings have revealed a low linearity error of ±0.82% for axial load and ±0.11% for bending load. Linearity errors were measured at ±1.53% and ±0.52%, respectively. A flexible modular electronic platform has been developed, suited for complete integration into the bolt’s head. It allows experimentation with the sensors read-out electronics, and various modules for communication. The energy supply is evaluated using different energy harvesting and energy transmission concepts. The novel sensor integrating bolt can be used to measure all relevant loads on bolts and to indicate early failure like decrease of clamping force or opening joints. The method can be used to support future sensor integration projects.

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.