Laser beam welding is a necessary and helpful tool in modern production technology. It provides low and located heat input, narrow weld widths, high welding speeds and weld depths. Nevertheless, in the weld metal and the surrounding area the microstructure and the mechanical characteristics can be changed afterwards. A decrease of strength and fatigue life is a possible result. To realize a manipulation or control of the weld metal’s microstructure during the welding process is a great challenge. Improving the strength as well as the homogeneity of mechanical properties and chemical composition are the aims of this approach. With indirect introduced ultrasonic amplitudes, the weld pool dynamics and the solidification are affected. The investigation focusses on the effects in the microstructure of high power (8 kW) laser beam welded stainless steel (AISI 304) with weld depths up to 15 mm. For two different amplitudes (3 and 6 μm) and three different positions of the weld pool in the vibration distribution (antinode, centered and node position) the weld metal is evaluated with metallographic cross sections. The types and the amount of microstructures are analyzed. The solidification of the weld metal is influenced by the vibration. Thus, the orientation, size and growth of the grains as well as the growth direction are changed. Furthermore, the weld characteristics (weld depth, weld width, weld area) are compared to the previously considered aspects.
Ultrasonic mechanical vibrations in solids are widely used in non-destructive testing, and high-power applications such as ultrasonic welding or soldering. The visualization of ultrasonic wave propagation in transparent solids is helpful for understanding the ultrasonic behaviours. The classical method of photoelasticity allows the visualization of the static stress distribution in birefringent materials. Utilizing recent high-power LEDs in the photoelasticity allows to capture dynamic stresses by high frequency stroboscopic light. High frequency stationary and transient oscillation processes in elastic solids can be visualized with this method. The designed LED array in this paper has a dimension of 210 mm_300 mm, and every LED has distance of 38mm to each other, and the light intensity has a homogeneity value. The temporal and spatial resolution of stress-optic systems depends mainly on the dynamic properties of the lighting technology used. The high speed synchronization of the stroboscopic light sources results in a high temporal resolution of the photoelasticity analyses. This enables the photoelastic investigation of highly dynamic load conditions, such as longitudinal waves and transverse waves.
KEYWORDS: Transducers, Ultrasonics, Systems modeling, Feedback control, Control systems design, Control systems, Velocity measurements, Sensors, Modeling, Magnetism
In this paper the modelling and feedback control of non-contact ultrasonic squeeze film levitation bearings are presented. Simple models are employed to describe the levitation effect in order to make it accessible to a wide range of applications. The ultrasonic transducer acts as the dominating lag element in the ultrasonic levitation system. Thus the transient behavior of the ultrasonic transducer is investigated by averaging method in order to analyse the dynamic behavior. Finally the design of the overall feedback control system is presented. This is applied to a linear squeeze film levitation bearing. For the system at hand the theoretical model is validated experimentally. It turns out that the theoretical model is in good agreement with the obtained experimental results.
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.
Vibro-Impcact harvesting devices are one concept to increase the bandwidth of resonant operated piezoelectric vibration generators. The fundamental setup is a piezoelectric bending element, where the deflection is limited by two stoppers. After starting the system in resonance operation the bandwidth increases towards higher frequencies as soon the deflection reach the stopper. If the stoppers are rigid, the frequency response gives constant amplitude for increasing frequencies, as long the system is treated as ideal one-DOF system with symmetric stoppers. In consequence, the bandwidth is theoretically unlimited large. However, such a system also has two major drawbacks, firstly the complicated startup mechanism and secondly the tendency to drop from the high constant branch in the frequency response on the much smaller linear branch if the system is disturbed. Nevertheless, the system has its application wherever the startup problem can be solved. Most modeling approaches utilize modal one-DOF models to describe the systems behavior and do not tread the higher harmonics of the beam. This work investigates the effects of the stoppers on the vibration shape of the piezoelectric beam, wherefore a finite element model is used. The used elements are one-dimensional two node elements based on the Timoshenko-beam theory. The finite element code is implemented in Matlab. The model is calculated utilizing time step integration for simulation, to reduce the computation time an auto-resonant calculation method is implemented. A control loop including positive feedback and saturation is used to create a self-excited system. Therefore, the system is always operated in resonance (on the backbone curve) and the frequency is a direct result of the computation. In this case tip velocity is used as feedback. This technique allows effective parametric studies. Investigated parameters include gap, excitation amplitude, tip mass as well as the stiffness of the stopper. The stress and strain distribution as well as the generated electrical power is analyzed with respect to the proper operation range.
With the continued advancement in electronics the power requirement for micro-sensors has been decreasing
opening the possibility for incorporating on-board energy harvesting devices to create self-powered sensors. The
requirement for the energy harvesters are small size, light weight and the possibility of a low-budget mass
production. In this study, we focus on developing an energy harvester for powering a pulse rate sensor. We propose
to integrate an inductive energy harvester within a commonly available pen to harvest vibration energy from normal
human motions like jogging and jumping. An existing prototype was reviewed which consists of a magnet wedged
between two mechanical springs housed within a cylindrical shell. A single copper coil surrounds the cylindrical
shell which harvests energy through Faraday's effect during magnet oscillation. This study reports a design change
to the previous prototype providing a significant reduction in the device foot print without causing major losses in
power generation. By breaking the single coil in the previous prototype into three separate coils an increase in power
density was achieved. Several pulse rate sensors were evaluated to determine a target power requirement of 0.3 mW.
To evaluate the prototype as a potential solution, the harvester was excited at various frequencies and accelerations
typically produced through jogging and jumping motion. The improved prototype generated 0.043 mW at 0.56 grms
and 3 Hz; and 0.13 mW at 1.14 grms at 5 Hz. The design change allowed reduction in total volume from 8.59 cm3 to
1.31 cm3 without significant losses in power generation.
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.
KEYWORDS: Transducers, Spindles, Ultrasonics, Ceramics, Systems modeling, Finite element methods, Aerodynamics, Acoustics, Electromagnetism, Control systems
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.
KEYWORDS: Control systems, Field programmable gate arrays, Signal processing, Digital signal processing, Sensors, Actuators, Signal detection, Phase measurement, Analog electronics, Demodulation
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
system's 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.
KEYWORDS: Resistance, Mathematical modeling, Chemical elements, Data modeling, Energy harvesting, Systems modeling, Mechanics, Capacitance, Dielectrics, Transducers
The increased demand for mobile systems using low-power electronics leads to a need for new power sources.
Using batteries as power source may be inapplicable in distributed systems like wireless sensor networks because
the batteries have to be exchanged frequently. Energy Harvesting systems are one possible energy source for
such systems exploiting environmental energy like mechanical vibrations. One good solution to convert vibration
energy is the use of piezoelectric generators usually realised as piezoelectric bending beams.
The generators convert mechanical energy to electrical energy due to resulting strain of the element. However,
the power output of piezoelectric generators is a challenging task even if low-power applications have to be driven.
Due to the low electric power output of piezoelectric generators, it is an important task to obtain a suitable
geometric design of the transducer element. Beside the element dimensions the electric power output depends
on the input excitation as well as on the electric load to be powered.
To analyse the system behaviour, input variables and the generator itself have to be described in a mathematical
model. This enables the calculation of optimal elements in principle. A modal electro-mechanical model
of the piezoelectric element assuming to be base-excited is used in this paper. Although the modal model is very
helpful to analyse the system, it cannot be easy used to determine a proper design of the piezoelectric elements.
The problem is that the parameters of the model do not show any apparent relations to geometric dimensions
or material data. Therefore, a mathematical method to obtain the parameters from the physical properties of
a piezoelectric bending element is briefly described. The knowledge of the link between physical and modal
parameters allows the usage of the mathematical model as a qualified design method. The input parameters of
the linked model are the material data which can be found on data sheets. Additionally, boundary conditions
of the environment like the impedance of the driven load and the vibration excitation has to be specified. The
linked model shows the influences on power output to connected electric loads. The given power demands of
applications which have to be satisfied yields in a design space of suitable elements. The design method enables
the development engineer to select piezoelectric generator elements.
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.
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