Network


Latest external collaboration on country level. Dive into details by clicking on the dots.

Hotspot


Dive into the research topics where Jens Twiefel is active.

Publication


Featured researches published by Jens Twiefel.


Smart Structures and Materials 2006: Damping and Isolation | 2006

Model-based design of piezoelectric energy harvesting systems

Jens Twiefel; Björn Richter; Tobias Hemsel; Jörg Wallaschek

In the design process of energy harvesting systems based on piezoelectric elements, achievable energy output is the most interesting factor. To estimate this amount a priori manufacturing of prototypes a mathematical model is very helpful. Within this contribution we will introduce a model based on electro-mechanical circuit theory. Its parameters are identified by measurements and the model is validated by comparison to experimental results. The model is designed to support the development-engineer in the dimensioning of energy harvesting units to specific application demands. Two main challenges in device design are investigated with the mathematical model: influence of the ambient excitation frequency, and influence of the load impedance. Typically, the equivalent model approach delivers models for piezoelectric elements that are driven in resonance by electrical excitation. In the case of energy harvesting the piezoelectric elements are excited mechanically and most often non-resonant. Thus, we first set up a mechanical equivalent model for base excited systems. In first approximation it represents an energy harvesting unit around one resonance frequency. The model is expandable for a wider frequency range using the superpositioning of multiple circuits. From the viewpoint of optimum energy transformation between mechanical and electrical energy it is favorable to drive piezoelectric elements at resonance or anti-resonance. Thus, an energy harvesting system should be tuned to the excitation frequency.


Production Engineering | 2011

Ultrasonic-assisted machining of stone

Uwe Heisel; Rocco Eisseler; Rainer Eber; Jörg Wallaschek; Jens Twiefel; Minghui Huang

Hybrid machining processes represent a potential approach to meeting the constantly increasing demands on cutting. In ultrasonic-assisted cutting, as a part of hybrid machining, the machining process is superimposed with a high-frequency vibration of small amplitude. This paper presents investigations on the drilling of stone materials, i.e. different granites and marble, in which this process is applied. It could be observed that the resultant forces and torques were reduced, which had already been discovered in investigations of various other materials. The influence of different parameters on the force reduction is shown, which is basically similar to the theoretical reduction of friction by ultrasound for small speed ratios δ, but is considerably increased. This increase must be due to other effects. Besides the force reduction, a reduction of cratering at the drill exit can be observed. Based on the present results, definite conclusions on tool wear cannot be drawn yet.


The 15th International Symposium on: Smart Structures and Materials & Nondestructive Evaluation and Health Monitoring | 2008

Digital signal processing for an adaptive phase-locked loop controller

Jens Twiefel; Martin Klubal; Carlos Paiz; Sebastian Mojrzisch; Holger Krüger

Many piezoelectric systems are operated in resonance, requiring some sort of control. Especially weakly damped systems need a control algorithm to hold the system in resonance. There are many factors, which can change the systems resonant frequency during operation. The two most important factors are load effects and temperature effects. A common algorithm to drive a piezoelectric system in its eigenfrequency is the PLL (phase-locked-loop) controller - well known from communication technologies - including some adaptive variantions of the PLL. Beside a brief introduction into the APLL (adaptive PLL, this paper concentrates on one of the main components of the (A)PLL, the phase detector. It investigates and compares different types of phase detectors with a focus on the implantation on a digital control system.


Proceedings of SPIE | 2013

Highly-integrated energy harvesting device for rotational applications utilizing quasi-static piezoelectric and electromagnetic generators

Marc Christopher Wurz; Gleb Kleyman; Jens Twiefel

This work addresses the design of an integrated energy harvesting system under production viewpoints. The system is developed to harvest energy from rotational movements. Therefore, a piezoelectric bending element – mounted on the rotational part - is actuated by magnetic force introduced by hard magnets installed in the fixed frame. This work concentrates on a high integration, the energy harvesting circuit, including rectifier, power management and storage is integrated in the structure of the bending harvester. Further the soft magnetic tip mass is equipped with a coil for electromagnetic energy harvesting; the necessary electronic is also integrated in the structure. The paper addresses the special systems demands for large scale production. The production technology for a small series of prototypes is explained in detail. Performance tests of the device conclude this study.


Proceedings of SPIE | 2009

Design and experimental investigations of high power piezoelectric transducers for a novel squeeze film journal bearing

Su Zhao; Jens Twiefel; Joerg Wallaschek

A novel active squeeze film journal air bearing actuated by high power piezoelectric transducers is presented. The proposed bearing uses in-air squeeze film levitation to suspend the rotating spindle without contact. Unlike conventional journal bearings, the presented bearing journal is formed by multiple independently vibrating surfaces driven individually by piezoelectric transducers. Langevin type piezoelectric transducers with a special radiation surface are developed. Detailed design procedures to develop the ultrasonic transducers are presented. A complete spindle-bearing system is constructed to test the proposed squeeze film bearing. Load carrying forces are measured at different vibration amplitude and compared with the calculated results. The proposed squeeze film journal bearing is operated in ultrasonic frequency range. The achieved load capacity is about 50N, which is five times of the load capacity achieved by the previous squeeze film bearings reported in the literatures.


internaltional ultrasonics symposium | 2012

Ultrasonic assisted simultaneous composite casting - A feasibility study

Kai-Alexander Saalbach; Patrick Freytag; Kai Kerber; Jens Twiefel

This article reports on the improvement of cohesive connections by ultrasound assistance during master forming. In an attempt to produce monolithic components of several metals by simultaneous composite casting quality of the resulting bond between the metals was insufficient. Impurities and oxide layers on the metal surface were determined as the reason for bad bonding. In order to achieve an improvement the authors proposed ultrasonic excitation of the separating metal plate, in order to clean the plates surface by occurring cavitation. To verify the effect of such excitation, a feasibility study was carried out and the results are presented here. An ultrasonic transducer which provided an opportunity of mounting metal disks at its tip was developed and manufactured for these experiments. Aluminum was melted and the transducers tip with mounted metal disk, representing the separating plate, was dipped into the melt for a defined period of time. In the experiments the vibration amplitude of the ultrasonic excitation was varied. As a result an amplitude-dependent wetting improvement could be noted, which can even have a destructive effect on the samples.


Proceedings of SPIE | 2009

Parameter identification of piezoelectric bimorphs for dynamic applications considering strain and velocity dependent effects

Björn Richter; Jens Twiefel

Piezoelectric bimorph elements are commonly used in a wide area of applications, among them various actuator applications in textile machines, applications in sensing like medical tissue identification, or the use in energy harvesting systems. Especially the last field may create a mass market for piezoelectric elements. Due to their easy use and low resonance frequency, bimorphs seem to fit energy harvesting demands quite well. To get the best possible power output, the element has to be designed as good as possible to fit the environmental excitation characteristics as excitation frequency and amplitude. Due to the need of a good understanding of the resulting system, a model based approach is desirable for the design of the used bimorphs. This is the case not only in Energy Harvesting systems but in most of the mentioned applications.


Ultrasonics | 2018

Capability evaluation of ultrasonic cavitation peening at different standoff distances

Fushi Bai; Kai-Alexander Saalbach; Yangyang Long; Jens Twiefel; Jörg Wallaschek

HighlightsThe effects of different standoff distances during ultrasonic cavitation peening are first investigated.The optimal standoff distance is deduced theoretically.The experimental optimal standoff distance increases with the increase of the vibration amplitude. ABSTRACT Ultrasonic cavitation peening is a novel surface treatment technology which utilizes the effect of cavitation bubble collapses to improve the properties of metal surfaces. In order to obtain high impact during ultrasonic cavitation peening, a small standoff distance between a sound radiator and a rigid reflector (the surface of treated specimen) is necessary. However, the effects of different standoff distances on the capability of ultrasonic cavitation peening are not yet clear. In this paper, a simplified model was developed to evaluate the cavitation capability at different standoff distances. Meanwhile, to validate the theoretical model, the plastic deformation or erosion on the peening surface before and after treatment were compared. It was found that at a very small standoff distance the impact pressure generated by cavitation bubbles did not cause much deformation or erosion, as the dynamics of cavitation bubbles was limited. At a large standoff distance, due to much attenuation of sound propagation in the bubbly liquid, little impact pressure was generated by the collapse of cavitation bubbles and reached the treated surface. A fixed vibration amplitude, however, corresponded to a standoff distance which caused the largest deformation or erosion on the treated surface.


Energy Harvesting and Systems | 2017

A Study on Bandwidth and Performance Limitations of Array Vibration Harvester Configurations

Noha Aboulfotoh; Jens Twiefel

Abstract Many researchers introduced an array of generators for broadband energy harvesting. The array has been studied in comparison to a single element from this array, but never compared to a single reference harvester with same volume as the whole array. This paper presents a theoretical study of evaluating the performance of the array harvester in comparison to the reference harvester. Power from the reference harvester as well as from the array is analytically calculated. The array is compared to the reference harvester when loaded by their optimal resistances which provide maximum power capability. The comparison is divided into two sections: firstly when the elements of the array are tuned to resonate at matching frequencies and secondly when they are tuned to non-matching resonance frequencies. The comparisons lead to two significant limits of the working bandwidth of the array: the lower and the upper limit. Between the two limits, the power produced from the array is less than the reference harvester, but with a small additional bandwidth. Below the lower limit, the array has no advantage over the reference harvester. Above the upper limit, output power of the array is inconsistent. Hence, design guidelines for the array are provided.


Ultrasonics Sonochemistry | 2018

A control system for ultrasound devices utilized for inactivating E. coli in wastewater

Hendrik Ohrdes; I. Ille; Jens Twiefel; Jörg Wallaschek; R. Nogueira; Karl-Heinz Rosenwinkel

Sonochemical processes applied to wastewater treatment have an influence on the behavior of ultrasonic systems. This is especially due to the load characteristic of the sonochemical process itself and the temperature increase caused by internal damping within the converter. Hence, a controlling device is needed to guarantee the operation in resonance and to keep the vibration amplitude constant. This paper presents a digital control system for the operation of weak to strong damped ultrasonic devices and its application for inactivating Escherichia coli in wastewater. In an experimental investigation, the electric data during a sonochemical process to inactivate E. coli in wastewater is taken into account to analyze the efficacy of the treatment process and the reaction of the vibration system to the process. Frequency response measurements depict that the resonance frequency changes with the sonicated medium and the vibration amplitude decreases with driving current. In addition to a common continuous operation of the system, different pulsed modes are investigated. The experiments prove the common dependencies between inactivation and power level or treatment time. Additionally, it is pointed out that the control of the sonochemical device is of utmost importance to guarantee an efficient treatment of water, because fast process changes, especially in pulsed operation modes, need to be controlled to a steady state as fast as possible. Although a water treatment efficiency increase using pulsed modes was not proved, it is shown, that the performance of the control unit is capable of using different driving modes in water treatment.

Collaboration


Dive into the Jens Twiefel's collaboration.

Top Co-Authors

Avatar
Top Co-Authors

Avatar

Liang Wang

Nanjing University of Aeronautics and Astronautics

View shared research outputs
Top Co-Authors

Avatar

Jiamei Jin

Nanjing University of Aeronautics and Astronautics

View shared research outputs
Top Co-Authors

Avatar

Malte Krack

University of Stuttgart

View shared research outputs
Top Co-Authors

Avatar
Top Co-Authors

Avatar
Top Co-Authors

Avatar

Omid Majdani

Hannover Medical School

View shared research outputs
Top Co-Authors

Avatar

Rainer Eber

University of Stuttgart

View shared research outputs
Top Co-Authors

Avatar
Top Co-Authors

Avatar
Researchain Logo
Decentralizing Knowledge