报告人：Dr. Jens Twiefel （Leibniz Universit?t Hannover）
报告题目1：Ultrasonic systems and processes @ IDS
The presentation will provide an overview on the topics and recent findings of the Piezoelectric and Ultrasonic Technology Lab of the Institute of Dynamics and Vibration Research at the Leibniz Universit?t Hannover. The lab is working a lot on power ultrasonic systems for actuation in the frequency range from 20 kHz to 100 kHz. The ultrasonic systems are covered from the power plug to the process, both theoretically and practically/experimentally. Currently, the lab is focusing on four main areas/groups. First, ultrasonic wave propagation in solids and fluids. The topics include ultrasonic assisted casting and laser welding but also the characterization of wave fields for sensing applications. The second main topic is the area of electromechanically coupled structural dynamics covering the optimization and evaluation of complex transducer structures and systems under various time dependent boundary conditions. Additionally, broadband vibration energy harvesting techniques are under investigation. The third group focuses on the control of dynamic systems. Therefore, a ?C-based standalone control and measurement unit has been developed and implemented, that is capable of measuring the instable branches in the frequency response of non-linear transducers. The fourth area works on various interface and vibro-impact processes including the ultrasonic transportation and drives, interface processes in ultrasonic wire bonding as well as a novel tactile display providing a tactile impression of virtual surfaces.
报告题目2：Single piezoelectric energy harvesters compared to piezoelectric arrays under volume aspects
Energy converters for autonomous systems are a current research field. In particular, the class of piezoelectric converters is important due to its high application potential especially for use in industrial environments. This is mainly due to two major environmental influences, the presence of relatively predictable vibrations and the absence of direct sunlight. Although solar cells are the most promising candidates for autonomous systems due to their specific energy density, this requires sufficiently strong light irradiation, which cannot usually be found in an industrial environment. Therefore, vibrations as energy source is most interesting in this application field. Among the possible transducer principles used to convert mechanical energy into electrical energy, piezoelectric transducers have the highest power density and, for small systems, the highest efficiency. In the field of energy generation from vibrations, there is the fundamental challenge that for an efficient design of the transducer, one of its resonances should be exploited in order to generate a large strain amplitude in the piezoelectric material of the transducer even with small input amplitudes. The strain present in the active material is crucial for the energy conversion in the piezoelectric material. Due to the generally weak damping in piezoelectric transducers, amplitude increases by a factor of 20 and more can easily be achieved, resulting in high strain and thus also high electrical voltage. However, the resonant design has the major disadvantage that the usable frequency bandwidth is very narrow and the generator is therefore very sensitive to changes in the vibration frequencies from the environment. The presentation provides a comprehensive overview on the design aspects of classical piezoelectric bending transducer for energy harvesting applications, considering the transducer volume. Further, the broadband characteristic of the bending transducer is compared to an electrically coupled piezoelectric array utilizing volume aspects.
报告题目3：Experimental observation of ultrasonic processes
Ultrasonic technologies can enhance or even enable many applications in various fields. Power ultrasonics in the frequency range from approx. 20 kHz to 100 kHz are primarily used in industry. Typical applications utilizing structural-borne ultrasound are ultrasonic joining, such as ultrasonic plastic welding or ultrasonic wire bonding, ultrasonic supported machining in grinding, milling or turning of metals and brittle materials or ultrasonic processing of fluids utilizing acoustic cavitation, to name only a few. Other applications utilize air-borne ultrasound such as ultrasonic levitation or sensing. All ultrasonic processes have the high frequency range and the rather tiny amplitudes in common. However, for a deep process knowledge, measuring the processes is essential for the validation of hypothesizes and models.
This presentation shows the optical real-time observation of wire bonding and ultrasonic machining as well as the observation of an ultrasonic air field in a sensing application.
Dr. Jens Twiefel graduated in computer engineering with major in mechanical engineering from Universit?t Paderborn. Back then he started to work in the area of ultrasonic systems. In his doctoral thesis he was working on piezoelectric standing wave motors, with this graduated at Leibniz Universit?t Hannover. He was a visiting scholar at Virginia Tech, USA and a visiting researcher at The Tokyo University. Today, he is in charge of the Piezoelectric and Ultrasonic Technology Lab of the Institute of Dynamics and Vibration Research at the Leibniz Universit?t Hannover since 2008. Within his group various topics in the fields of power ultrasonics, non-linear vibration, vibration energy harvesting, wave propagation and control of dynamic systems are covered. Currently 13 PhD candidates are working and studying in his lab under his co-supervision.