Felix Herbst

Piezoelectric sensors convert electric charge in response to mechanical loading and are widely studied for their high sensitivity. In particular, ferroelectret sensors provide the flexibility and material system design that enable biocompatibility. Insole-based gait and pressure sensing remain active research topics, yet most published electronics are limited to conceptual diagrams without practical strategies for noise mitigation or robustness against electromagnetic interference (EMI). Therefore, this work presents a compact, four-channel portable measurement system specifically designed for charge-based force sensing with piezoelectric sensors, called ChrisBox. A comprehensive shielding strategy, including both the circuit board and the sensor connection, via cost-effective USB Type-C cables is implemented. Performance was validated by comparing the output of the developed electronics with a calibrated electrometer under defined mechanical loads applied to a shielded ferroelectret sensor. The system measured the charge with a relative deviation of 8.2%, partly attributed to variability in the verification setup, with a high linearity of R² > 0.99. In controlled EMI testing within a transverse electromagnetic (TEM) cell, noise levels remained below 0.1 pC RMS, enabling a signal-to-noise ratio (SNR) of 53 dB for a measured charge of 43 pC RMS during human pulse measurement with a ferroelectret. The results confirm the system’s suitability for portable, high-resolution ferroelectret sensing in EMI-prone environments and provide a foundation for further application-specific development.

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.

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.

Piezoelectric sensors convert electric charge in response to mechanical loading and are widely studied for their high sensitivity. In particular, ferroelectret sensors provide the flexibility and material system design that enable biocompatibility. Insole-based gait and pressure sensing remain active research topics, yet most published electronics are limited to conceptual diagrams without practical strategies for noise mitigation or robustness against electromagnetic interference (EMI). Therefore, this work presents a compact, four-channel portable measurement system specifically designed for charge-based force sensing with piezoelectric sensors, called ChrisBox. A comprehensive shielding strategy, including both the circuit board and the sensor connection, via cost-effective USB Type-C cables is implemented. Performance was validated by comparing the output of the developed electronics with a calibrated electrometer under defined mechanical loads applied to a shielded ferroelectret sensor. The system measured the charge with a relative deviation of 8.2%, partly attributed to variability in the verification setup, with a high linearity of R2 > 0.99. In controlled EMI testing within a transverse electromagnetic (TEM) cell, noise levels remained below 0.1 pC RMS, enabling a signal-to-noise ratio (SNR) of 53 dB for a measured charge of 43 pC RMS during human pulse measurement with a ferroelectret. The results confirm the system’s suitability for portable, high-resolution ferroelectret sensing in EMI-prone environments and provide a foundation for further application-specific development.

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.

Strain gauges are regarded as the gold standard in various force sensing applications, including minimally invasive surgical devices, due to their high sensitivity and ability for miniaturization at millimeter scale. However, further reduction in size and improvement concerning the limit of detection are desirable to enable seamless integration into highly constrained environments, such as needle tips, without compromising performance. Capacitive micromachined ultrasonic transducers (CMUTs), though not previously applied to direct force measurement, offer significant potential as high-sensitivity alternatives when their resonant characteristics are effectively harnessed. In this work, we present a piston-structured CMUT with a thinned back electrode, configured as a force-sensing element mounted on a deflecting cantilever. The system is evaluated using both numerical simulation and experimental analysis. The numerical simulation is verified analytically and validated by measuring the deflection of the top plate. As a proof of concept, the performance of the CMUT is characterized via impedance-based frequency tracking, avoiding full circuit integration. We demonstrate the feasibility of CMUTs as force-sensitive resonators, achieving sensitivities of 0.80 Hz/mN under tensile loading and 0.76 Hz/mN under compression, with a linearity error of 2.69% and an initial response threshold of 20.5 mN. These results establish the functionality of CMUTs as force-sensing elements and lay the groundwork for further optimization toward a new class of highly integrable force sensors.

Electromyography (EMG) is an important technique for motion detection and diagnostics, particularly in applications such as prosthetics and wearable devices. Traditional EMG electrodes face challenges regarding miniaturization, adaptability to complex geometries, and seamless integration. Addressing these limitations, we utilize an additive manufacturing approach for fabricating integrated, customizable 3D-printed dry EMG electrodes using dual-material printing with a conductive filament. In this work, we fabricate electrodes in the same form factor as commercial ones, which enables a straightforward comparison of their performance. The method involves a disruption during the manufacturing process, wherein a preamplifier is embedded. A conductive filament, together with silver ink, is then used to establish electrical connections, creating functional EMG electrodes fully integrated into the printed structure. Investigations focused on measuring the EMG performance on the musculus flexor carpi ulnaris during grip strength exercises with 50 kg as well as durability tests. In our experimental setting, commercial electrodes achieved an SNR of 43.7$\pm$7.8 dB, computed from preprocessed EMG signals, while the 3D-printed electrodes reached an average SNR of 37.5$\pm$4.7 dB - a difference of approximately 6 dB. Durability testing through immersion in a saline solution over five days resulted in a failure of one out of three electrodes, whereas the SNR of the remaining electrodes remained stable. While the integrated 3D-printed electrodes exhibit reduced performance compared to the commercial reference, their integrability makes them a promising option for embedded EMG applications.

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.

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.

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.