Joachim Bös

Vibration-based energy harvesting with stacked piezoelectrets

Vibration-based energy harvesters with multi-layer piezoelectrets (ferroelectrets) are presented. Using a simple setup with nine layers and a seismic mass of 8 g, it is possible to generate a power up to 1.3 µW at 140 Hz with an input acceleration of 1g. With better coupling between seismic mass and piezoelectret, and thus reduced damping, the power output of a single-layer system is increased to 5 µW at 700 Hz. Simulations indicate that for such improved setups with 10-layer stacks, utilizing seismic masses of 80 g, power levels of 0.1 to 1 mW can be expected below 100 Hz.

Ductile reinforcements for enhancing fracture resistance in composite materials

Abstract Ductile reinforcements can supply fracture toughness to a polymer matrix by pulling out and by plastically deforming. In the case of metal reinforcements that are not in a toughened condition, there may be more toughening to be gained when the fibers remain in the matrix and plastically deform rather than pulling out. These fibers can be said to have an unused plastic potential . When these fibers bridge a crack, their plastic deformation causes a rise in the force which is trying to pull out the fiber. Because of this, the shape of the fiber must be adjusted along its length if it is to remain anchored and contribute its plastic work. The use of anchored, ductile fibers provides a new design axis that brings new possibilities not achievable by the current research focus on the fiber–matrix interface. This paper describes the experimental pullout of aligned ductile fibers from a polymer matrix, and indicates the effect of the shape and embedded length of the fiber on the toughness increase of the composite. Anchored, plastically deforming fibers are shown to provide a major improvement to the toughening. Even for unoptimized ductile fibers, the calculated toughening improvement equals or exceeds the toughening available from current short glass or graphite fibers. In addition, pullout values are obtained for fibers that are embedded at an angle, simulating fiber bridging from fibers not perpendicular to the crack surface. These results further demonstrate the toughening efficiency of ductile fibers.

Energy harvesting with single-layer and stacked piezoelectret films

Vibration-based energy harvesting with single- or multiple-layer (stacked) piezoelectret films is discussed. First, a mechanical and an electrical model for this kind of energy harvesting is established and equations for the generated charge, current, voltage, and power are derived. In particular, the case where the electrical corner frequency equals the mechanical resonance frequency of the film stack combined with the seismic mass is of importance since harvesting at this frequency results in maximal generated power. Then, experimental results for harvesters with single layers and with stacks of piezoelectret films are presented. The data confirm the predicted dependence of the generated power on terminating resistance, frequency, seismic mass, acceleration, and number of film layers in the stack. If the stress on the piezoelectret layers, generated by the accelerated seismic mass, is larger than 3 kPa, a nonlinear increase of the generated power with stress is observed. The maximum power harvested with a single-layer piezoelectret is 18 μW for an acceleration of 9.81 m/s2 and for a seismic mass of 40 g. A three-layer harvester yields an increase of the maximum power close to a factor of √3.

Compact electret energy harvester with high power output

Compact electret energy harvesters, based on a design recently introduced, are presented. Using electret surface potentials in the 400 V regime and a seismic mass of 10 g, it was possible to generate output power up to 0.6 mW at 36 Hz for an input acceleration of 1 g. Following the presentation of an analytical model allowing for the calculation of the power generated in a load resistance at the resonance frequency of the harvesters, experimental results are shown and compared to theoretical predictions. Finally, the performance of the electret harvesters is assessed using a figure of merit.

Experimental sensitivity analysis for robustness studies of a controlled system

Active and adaptive systems consist of various components with different functionalities. As the complexity of the systems increases, reliability and robustness studies become a more complicated task. Sensitivity analysis helps system designers to understand interactions between the system components and to identify the important parameters with significant overall influences on the system performance. To analyze the complex interactions of the components and parameters of an active system with respect to system performance, a framework structure with active vibration damping in a lab scale test rig was set up. With this test rig an experimental sensitivity analysis was performed to investigate the influences of the system components and parameters on the vibration reduction. A higher robustness of the active system was achieved by using adaptive control.

Active vibration reduction applied to the compressor of an air‐conditioning unit for trams

Within the framework of the European Integrated Project InMAR (intelligent materials for active noise reduction) active vibration control and active structural acoustic control approaches are applied to an HVAC (heating, ventilation, and air conditioning) unit used to air‐condition the drivers cab of a tram. Measurements previously performed by the manufacturer of the unit indicated that annoying tonal noise in the 50 Hz and 100 Hz 1/3 octave bands inside the drivers cab is mainly caused by the vibration of the compressor mounted in the HVAC unit on the roof of the tram. Therefore, two different concepts for the design of active compressor mounts were developed that are used to reduce the vibration excitation of the HVAC units housing. The first one is an active tuned vibration absorber whose natural frequency can be adapted by means of piezoelectric patch actuators and which behaves as a vibration compensator at higher frequencies. The second one is an active mount based on four piezoelectric stack ac...

Investigation of structural intensity applied to carbon composites

Structures made from carbon composite materials are rapidly replacing metallic ones in the automotive industry because of their high strength to weight ratio. The goal of this study is to enhance acoustic comfort of cars made from carbon composites by comparing various carbon composites in order to find the most suitable composite in terms of mechanical and dynamic properties. In order to achieve this goal, the structural intensity method was implemented. This method can give information concerning the path of energy propagated through structures and the localization of vibration sources and sinks. The significance of the present research is that it takes into account the effect of the material damping on the dissipation of the energy in a structure. The damping of the composite is presented as a function of its micro and macro mechanical properties, frequency, geometry, and boundary conditions. The damping values were calculated by a 2D analytical multi-scale model based on the laminate theory. The benef...

Response surface methodologies for coupling factor exploration: an application example for noise reduction by means of smart structures

The development of smart structures requires methods to explore the behavior of electromechanical systems. Engineers must take into account various interactions between particular system parameters and the system’s responses. A variety of mathematical models is used to describe the system behavior depending on defined design variables. Based on the mathematical models, system analyses and optimization procedures can be performed. All suitable methods can be summarized as design exploration. This paper describes four mathematical models, their application in design exploration and the resulting differences. The mathematical models are linear interpolation, full second-order polynomials, Kriging interpolation and genetic programming. As an application example a smart thin curved plate is presented. The importance of the electromechanical coupling factor, which is used as the system response considered in this paper, for noise reduction by means of smart structures is pointed out.

Approaches to Sensitivity Analysis for System Reliability Study of Smart Structures for Active Vibration Reduction

The modern engineering products must fulfill the increasing requirements to the vibroacoustical behavior of the components and the system. For many applications such as automotive engineering, where light-weight design is desired, passive measures for noise and vibration reduction have reached certain limits. For this reason, active techniques for structural vibration reduction are becoming increasingly important in this field of applications. Commonly, actively or adaptively controlled structural systems consist of a large number of components with various functionalities. As the complexity of the systems increases, reliability and robustness studies become a more complicated task. The knowledge of parameter effects and their interactions is important for the reliability study and the design optimization of such systems. Sensitivity analysis can help the system designers to understand interactions between the system components and identify the important parameters with significant overall influences on the system performance. In this paper, several approaches to sensitivity analysis are applied for a smart structure system with active vibration control. Through these analyses, the influences of the system parameters and control algorithms on the performance of active vibration reduction are investigated. An improvement of the robustness of the active system by using adaptive control will be shown.

Advanced materials and technologies for reducing noise, vibration and harshness (NVH) in automobiles

Abstract: The automotive industry is facing the problem more and more of reducing the weight of vehicles but guaranteeing an equivalent level of comfort in terms of noise, vibration and harshness (NVH). To overcome these contradicting requirements traditional design and material choices must be revisited. Besides advanced passive material active systems or smart concepts are being increasingly considered for the NVH optimization of vehicles. Therefore, this chapter addresses different passive and active measures for NVH control with a focus on smart structures. After a general discussion of the NVH problems in automotive engineering, some principal passive and active measures are described. On this basis passive and active measures are presented in different depth from general aspects over specific concepts to some selected applications.