Frank Goldschmidtboeing

Characterization of different beam shapes for piezoelectric energy harvesting

This paper deals with the analysis of different beam shapes for piezoelectric energy harvesters. The theory is based on the well-established Rayleigh–Ritz method for piezoelectric compound structures. It is validated by experiments with triangular-shaped and rectangular-shaped beams. It turns out that triangular-shaped beams are more effective than rectangular-shaped ones in terms of curvature homogeneity independent of the proof mass. This effect is opposed by the adverse mass distribution and the increased stiffness of triangular-shaped beams. Therefore, the overall efficiency is only weakly influenced by the beam shape. Nevertheless triangular-shaped beams drastically outperform rectangular ones in terms of tolerable excitation amplitude and maximum output power.

Bidirectional frequency tuning of a piezoelectric energy converter based on a cantilever beam

A piezoelectric energy converter is presented, whose resonance frequency can be tuned by applying mechanical stress to its structure. The converter consists of a piezo-polymer cantilever beam with two additional thin arms, which are used to apply an axial preload to the tip of the beam. The compressive or tensile prestress applied through the arms leads to a shift of the beams resonance frequency. Experiments with this structure indicate a high potential: the resonance frequency of a harvester to which a compressive preload was applied could be altered from 380 Hz to 292 Hz. In another experiment, a harvester with stiffened arms was tuned from 440 Hz to 460 Hz by applying a tensile preload. In combination with automatic control of the applied force, this type of structure could be used to enhance the performance of energy harvesters in vibrating environments with occasional shifts of the vibrational frequency.

Electromagnetic vibration harvester with piezoelectrically tunable resonance frequency

This paper presents an electromagnetic vibration scavenger that exhibits a tunable eigenfrequency. By applying a static electrical field the eigenfrequency can be shifted. This feature is originated from exploiting the elastostriction of the utilized piezoelectric bimorph suspension. It is demonstrated that in the tuning operation mode more than 50 µW are scavenged continuously across the feasible frequency range of 20 Hz.

A novel two-stage backpressure-independent micropump: modeling and characterization

A novel design of a piezoelectric silicon micropump is proposed, which provides a constant flow rate over a wide backpressure range of up to 30 kPa. This highly appreciable feature is based on a new serial arrangement of two active valves and relies on both an appropriate electrical actuation sequence of the piezo-actuators and an immanent limitation of the membrane deflection by the valve seats. The design is optimized for the low flow regime ranging from 0.1 to 50 µl min−1. A detailed lumped-parameter model is derived in order to reveal the physics behind this pumping principle and to identify the optimum control scheme. For the fabrication of our device, a comparably simple and robust 2-wafer process is utilized. A thorough experimental investigation demonstrates the high performance of the micropump. The backpressure independence of the flow rate enables high-resolution volumetric dosing within the aforementioned flow range. The stroke volume and hence the resolution of the micropump is adjustable via the upstroke voltage of the actuator between 50 and 200 nl. Depending on this setting typical actuation frequencies range from 0.05 to 5 Hz and the flow rate scales proportional to the frequency within that frequency range.

Strategies for void-free liquid filling of micro cavities

We deal with the problem of void-free capillary filling of micro cavities. Special emphasis is given to the effect of critical edges. Analytical, numerical and experimental methods are used to examine the capillary filling of micro cavities. The advantages and disadvantages of these methods are discussed in detail. In summary, a brief theory of the capillary dynamics in critical edges is established. Strategies are introduced to conduct a design for void-free filling of microfluidic devices on the basis of critical edges and pinning barriers.

Highly elastic conductive polymeric MEMS

Abstract Polymeric structures with integrated, functional microelectrical mechanical systems (MEMS) elements are increasingly important in various applications such as biomedical systems or wearable smart devices. These applications require highly flexible and elastic polymers with good conductivity, which can be embedded into a matrix that undergoes large deformations. Conductive polydimethylsiloxane (PDMS) is a suitable candidate but is still challenging to fabricate. Conductivity is achieved by filling a nonconductive PDMS matrix with conductive particles. In this work, we present an approach that uses new mixing techniques to fabricate conductive PDMS with different fillers such as carbon black, silver particles, and multiwalled carbon nanotubes. Additionally, the electrical properties of all three composites are examined under continuous mechanical stress. Furthermore, we present a novel, low-cost, simple three-step molding process that transfers a micro patterned silicon master into a polystyrene (PS) polytetrafluoroethylene (PTFE) replica with improved release features. This PS/PTFE mold is used for subsequent structuring of conductive PDMS with high accuracy. The non sticking characteristics enable the fabrication of delicate structures using a very soft PDMS, which is usually hard to release from conventional molds. Moreover, the process can also be applied to polyurethanes and various other material combinations.

Analytical model for nonlinear piezoelectric energy harvesting devices

In this work we propose analytical expressions for the jump-up and jump-down point of a nonlinear piezoelectric energy harvester. In addition, analytical expressions for the maximum power output at optimal resistive load and the 3 dB-bandwidth are derived. So far, only numerical models have been used to describe the physics of a piezoelectric energy harvester. However, this approach is not suitable to quickly evaluate different geometrical designs or piezoelectric materials in the harvester design process. In addition, the analytical expressions could be used to predict the jump-frequencies of a harvester during operation. In combination with a tuning mechanism, this would allow the design of an efficient control algorithm to ensure that the harvester is always working on the oscillatorʼs high energy attractor.

A New Approach to Determine Ligament Strain Using Polydimethylsiloxane Strain Gauges: Exemplary Measurements of the Anterolateral Ligament

A thorough understanding of ligament strains and behavior is necessary to create biomechanical models, comprehend trauma mechanisms, and surgically reconstruct those ligaments in a manner that restores a physiological performance. Measurement techniques and sensors are needed to conduct this data with high accuracy in an in vitro environment. In this work, we present a novel sensor device that is capable of continuously recording ligament strains with high resolution. The sensor principle of this biocompatible strain gauge may be used for in vitro measurements and can easily be applied to any ligament in the human body. The recently rediscovered anterolateral ligament (ALL) of the knee joint was chosen to display the capability of this novel sensor system. Three cadaver knees were tested to successfully demonstrate the concept of the sensor device and display first results regarding the elongation of the ALL during flexion/extension of the knee.

Fabrication of a normally-closed microvalve utilizing lithographically defined silicone micro O-rings

The focus of this work is on the development of a simple and variable process chain for the integration of flexible silicone material into silicon-based microfluidic devices. A normally-closed microvalve is chosen as a demonstrator device, as it combines features that are not easily obtained from silicon devices alone, especially, a high leak tightness of up to 1 bar pressure difference in the closed state and a high forward flow of several mL in the open state. For this purpose, a photopatternable silicone is used as a deformable circular valve lip between a piezoelectrically actuated membrane and a valve seat, similar to a micro O-ring with a width of 50 µm. The microvalve is piezo actuated by monolayer piezo actuators with a peak-to-peak driving voltage of = 200 V. The micro O-ring is pre-deformed by 2.8 µm during the valve fabrication process to yield the normally-closed behavior. A dry film resist lamination technology is developed for this critical process step to mate the two silicon wafers with the actuation membrane, the valve seat and the silicone O-ring in between at a well-defined distance. The dry film resist is used in a multifunctional way, not only to pre-deform the valve lip, but also to define the geometry of the valve chamber and to ensure a leak-tight connection of both wafers. Altogether, a peak value for the on- to off-ratio of the normally-closed microvalve higher than 30 000 is measured. This opens a wide range of potential applications, e.g. in micro-dosing, drug delivery, μ-TAS and microfluidics for biological or chemical applications in general.

Low temperature plasma-assisted wafer bonding and bond-interface stress characterization

This paper presents the development and characterization of a low temperature plasma-assisted direct wafer bonding process for structured silicon wafer pairs. We have achieved spontaneous bonding at room temperature with a surface energy of up to 1.2 J/m/sup 2/. It turned out that the bonding process is not deteriorated by the history of the wafers, even after etching for several hours. The yield of the process is 80-95%. A blister test was used to determine the bond strength and the failure distribution for different plasma gases and annealing treatments. Film stress of the bonded interfacial oxide was found to transfer to the bonding partner. Selective wafer bonding was made by structuring the interfacial oxide layer. Design rules for proper bonding are given.