Jonas Groten

In recent decades, energy harvesting in rotating tires has been an active field of research as this could avoid the use of toxic batteries, which are currently used to power pressure monitoring systems installed in millions of cars worldwide. However, due to the harsh mechanical conditions in a tire and the economic competition with low-cost and widely spread batteries, self-powered tire pressure monitoring systems have not yet entered the market. In this work we present a fully integrated and self-powered bicycle tube pressure monitoring system using a multilayer piezoelectric energy harvester based on P(VDF-TrFE) fabricated using low-cost and scalable screen-printing. The harvester design was optimised to increase the power output. In addition, the long-term durability of the system up to 5000 km was investigated. The durability tests show that for a real-world application, a compromise must be found between energy output and durability since both factors depend in opposite ways on the mechanical stress in the system.

A vibration energy harvesting system based on fully printed piezoelectric transducers is realized. The transducers have a butterfly‐like architecture based on two single‐optimized cantilevers with a resonance frequency tuned to 49.5 Hz and can be easily mounted to an industrial engine. By comparing single‐ and multistack configurations of the piezoelectric layers combined with full‐wave or voltage doubler rectifiers, the power transfer characteristics and impedance can be matched to the electrical requirements of the sensing circuitry. A single stack with 21 μm thickness results in the maximum power output of 14.4 μW at a vibration velocity of 11.5 mm s⁻¹, typical for industrial engines. The system is used to power a wireless sensor node on a 1 kW rotary pump in normal operation. The system can harvest up to 138 mJ within 24 h, sufficient for daily remote monitoring of the engine's vibration spectrum and temperature state. The system is thus suitable as a low‐cost, ecofriendly power source for industrial IoT applications.

The interactions with touchscreens rely heavily on vision: The virtual buttons and virtual sliders on a touchscreen provide no mechanical sense of the object they seek to represent. This work presents PopTouch: a 500 µm thick flexible haptic display that creates pressable physical buttons on demand. PopTouch can be mounted directly on touchscreens or any other smooth surface, flat, or curved. The buttons of PopTouch are independently controlled hydraulically amplified electrostatic zipping taxels (tactile pixels) that generate 1.5 mm of out of plane displacement. When pressed by the user, the buttons provide intuitive mechanical feedback thanks to a snap‐through characteristic in their force‐displacement profile. The snap‐through threshold can be as high as 4 N, and is tuned by design and actuation parameters. This work presents two versions of PopTouch: a transparent PopTouch for integration on Touchscreens with built‐in touch sensing, such as smartphones and a sensorized PopTouch, with embedded thin‐film piezoelectric sensors on each taxel, for integration on substrates without built‐in touch sensing, such as a steering wheel. PopTouch adds static and vibrating button‐like haptics to any device thanks to its thin profile, flexibility, low power consumption (6 mW per button), rapid refresh rate (2 Hz), and freely configured array format.

Measuring the shear viscosity of polymeric melts is an extensive effort frequently performed in high-pressure capillary rheometers, where the pressures required to push the melt through a capillary at various temperatures and volumetric flow rates are recorded. Then, the viscosity values are obtained through Bagley and Weissenberg–Rabinowitsch corrections involving parameter fitting. However, uncertainties in those conversions due to pressure variations and measurement inaccuracies (random errors) affect the accuracy of the consequently calculated viscosities. This paper proposes quantifying them through a propagation of uncertainties calculation. This has been experimentally demonstrated for a polycarbonate melt. In addition, the derived viscosity uncertainties were used for the weighted residual sum of squares parameter estimation of the Cross-WLF viscosity model and compared with the coefficients obtained using the standard residual sum of squares minimization approach. The motivation was that, by comparison, individual poorly measured viscosity values should have a less negative impact on the overall fit quality of the former. For validation, the rheometer measurements were numerically simulated with both fits. The simulations based on the Cross-WLF fit, including the derived viscosity uncertainties, matched the measured pressures ~16% more closely for shear rates below 1500 1/s. Considering the uncertainties led to more precise coefficients. However, both fits showed substantial deviations at higher shear rates, probably due to substantial non-isothermal flow conditions that prevailed during these measurements. A capillary rheometer experiment was also simulated using arbitrarily chosen Cross-WLF parameters to exclude such systematic errors. A normally distributed error was then applied to the simulated pressures before re-fitting the parameters. Again, taking advantage of the derived viscosity uncertainties, the fit could recover the initial parameters better.

The integration of assembled foils in injection-molded parts is a challenging step. Such assembled foils typically comprise a plastic foil on which a circuit board is printed and electronic components are mounted. Those components can detach during overmolding when high pressures and shear stresses prevail due to the injected viscous thermoplastic melt. Hence, the molding settings significantly impact such parts’ successful, damage-free manufacturing. In this paper, a virtual parameter study was performed using injection molding software in which 1206-sized components were overmolded in a plate mold using polycarbonate (PC). In addition, experimental injection molding tests of that design and shear and peel tests were made. The simulated forces increased with decreasing mold thickness and melt temperature and increasing injection speed. The calculated tangential forces in the initial stage of overmolding ranged from 1.3 N to 7.3 N, depending on the setting used. However, the experimental at room temperature-obtained shear forces at break were at least 22 N. Yet, detached components were present in most of the experimentally overmolded foils. Hence, the shear tests performed at room temperature can only provide limited information. In addition, there might be a peel-like load case during overmolding where the flexible foil might bend during overmolding.

Pressure sensors integrated in surfaces, such as the floor, can enable movement, event, and object detection with relatively little effort and without raising privacy concerns, such as video surveillance. Usually, this requires a distributed array of sensor pixels, whose design must be optimized according to the expected use case to reduce implementation costs while providing sufficient sensitivity. In this work, we present an unobtrusive smart floor concept based on floor tiles equipped with a printed piezoelectric sensor matrix. The sensor element adds less than 130 µm in thickness to the floor tile and offers a pressure sensitivity of 36 pC/N for a 1 cm² pixel size. A floor model was established to simulate how the localized pressure excitation acting on the floor spreads into the sensor layer, where the error is only 1.5%. The model is valuable for optimizing the pixel density and arrangement for event and object detection while considering the smart floor implementation in buildings. Finally, a demonstration, including wireless connection to the computer, is presented, showing the viability of the tile to detect finger touch or movement of a metallic rod.

The integration of structural electronics in injection-molded parts is a challenging step. The films—comprising of laminated stacks with electronics—are exposed to shear stresses and elevated temperatures by the molten thermoplastic. Hence, molding settings have a significant impact on the successful, damage-free manufacturing of such parts. In this paper, test films with polycarbonate (PC) sheets as outer and two different thermoplastic polyurethanes (TPUs) as middle layers incorporating conductive tracks on a flexible printed circuit board (flexPCB) are manufactured and overmolded with PC. Parameter studies investigating the influence of the melt temperature, mold temperature, injection speed and used TPU layer were performed. The molded parts were inspected visually and compared with a numerical simulation using injection molding software. A shear distortion factor for the TPU layer was derived based on the simulations that linked the shear stresses with the injection time and the softening (melting) of the TPU. The distortion of the films was found to reduce with higher melt temperature, lower mold temperature and faster injection speed. Films using the TPU with the higher melting temperature yielded significantly better results. Moreover, distortion on the films reduced with the increasing distance to the gate and a larger cavity thickness was found to be beneficial. All those relations could be correlated with the shear distortion factor.