Papers by Author: Jörg Wallaschek

Abstract: The harvesting of ambient energy has become more important over the last years. This paper will investigate an analytical effort to predict the Duffing parameters for a magnetoelastic cantilever structure. The modeling is compared to a nonlinear harvester with point dipoles. The system consists of a harmonic excited cantilever structure with a magnetic tip mass. The beam is firmly clamped to the host structure. A second oppositely poled permanent magnet is located near the free end of the beam. The system is a bistable nonlinear oscillator with two equilibrium positions. Several studies show the better performance of the setup. The approach is not limited for energy harvesting techniques. The setup is suitable for broadband oscillations and also to tune the resonant frequency closer to the excitation frequency.

Abstract: This publication presents a novel piezoelectric shunt damping circuit. It consists of a negative capacitance in parallel to a synchronized switch damping on inductor (SSDI) branch. This combination utilizes the increased piezoelectric coupling due to the negative capacitance together with the adaptive ability of the SSDI technique. This novel circuit is theoretically modelled using non-dimensional parameters, and optimum network parameters and the corresponding maximum damping is obtained. The increase in damping performance due to the negative capacitance is clearly highlighted. Theoretically, the energy dissipation can be increased unlimited when tuning the system at the stability boundary, given the system would still be excited to vibrations. However, due to imperfections and practical realization using operational amplifiers the increase is limited.

Abstract: Hybrid machining represents a possibility for technological progress in production. As a part of hybrid machining processes, ultrasonic-assisted machining is often used to manufacture materials that are difficult to machine since process forces can be significantly reduced and the material removal rate (MRR) can be increased. This paper describes an approach for a model for ultrasonic-assisted drilling with undefined cutting edges. The ultrasonic vibration can theoretically be applied in axial, tangential or radial direction or it can be superimposed. An axial excitation, parallel to the feed direction, is selected in the presented model. Since the drilling is superimposed with a high-frequency vibration, the trajectories of the grains are modified. Therefore, an analytical-kinematic model is established, which is characterised by a periodical contact loss of tool and workpiece. Due to the modified kinematics, process-specific parameters, such as impact velocity or the ratio between vibration and cutting speed, are important, in addition to conventional cutting parameters. Such process parameters are useful to describe dominant material removal mechanisms in ultrasonic-assisted machining. Moreover, two models on tool topography are presented in this paper. Based on an analytical approach, the material removal rate, established by adding up the individual grain removals, is calculated. The quality of the developed models is validated by the standard calculation of the material removal rate by feed rate and tool cross section. The results show, that it must be taken into account that the grains do not hit an even surface. The grain distribution is also an important aspect.

Abstract: Our contribution will describe the basic fundamentals of shape memory alloys. Emphasis will be given to specific characteristics for the use of shape memory wires in actuators. The investigation of shape memory wires in actuators includes qualitative and quantitative benchmarking based on measurements at different test beds. To display applicability of shape memory wires for different tasks, the main focus will be on the influence of different bias forces, the determination of performance, and the possibility of position control without position sensors.

Abstract: Rotary ultrasonic motors have found broad industrial application in camera lens drives and other systems. Linear ultrasonic motors in contrast have only found limited applications. The main reason for the limited range of application of these very attractive devices seems to be their small force and power range. Attempts to build linear ultrasonic motors for high forces and high power applications have not been truly successful yet. To achieve drives, larger force and higher power, and multiple miniaturized motors can be combined. This approach, however, is not as simple as it appears at first glance. The electromechanical behavior of individual motors differs slightly due to manufacturing and assembly tolerances. Individual motor characteristics are strongly dependent on the driving parameters (frequency, voltage, temperature, pre-stress, etc.) and the driven load and the collective behavior of the swarm of motors is not just the linear superposition of the individual drive’s forces.

Abstract: FEM is a very important tool for getting the numerical solution of many engineering problems. It has been widely used in solving structural, mechanical, heat transfer, and fluid dynamics’ problems as well as problems of other disciplines such as piezoelectricity. Since the complexity of many problems leads to the formation of models of large dimension, which are described with many degrees of freedom, their numeric solution makes for the highest demands. The number of degrees of freedom is often unnecessarily large to represent the structural response in a limited bandwidth and therefore, in such a case, a reduction should be conducted. The systematic reduction of complex FE models on simple electromechanical equivalent circuit models (with less mechanical degrees of freedom) is the subject of this paper. The aim of the work is to make the reduction in such a way that the characteristics of the systems input/output descriptions are approximated within a pre-selectable frequency range with sufficient accuracy. A transducerwedge- system is used as an example for the method, to extract a state space model of the systems piezoelectric input/output description and to derive the parameters of the electromechanical equivalent circuit directly from the modal finite element analysis. In the present paper we describe the general methodology as well as its application to piezoelectric transducers as used in ultrasonic engineering.