Martin Wischke

Electromagnetic vibration harvester with piezoelectrically tunable resonance frequency

This paper presents an electromagnetic vibration scavenger that exhibits a tunable eigenfrequency. By applying a static electrical field the eigenfrequency can be shifted. This feature is originated from exploiting the elastostriction of the utilized piezoelectric bimorph suspension. It is demonstrated that in the tuning operation mode more than 50 µW are scavenged continuously across the feasible frequency range of 20 Hz.

Vibration harvesting in traffic tunnels to power wireless sensor nodes

Monitoring the traffic and the structural health of traffic tunnels requires numerous sensors. Powering these remote and partially embedded sensors from ambient energies will reduce maintenance costs, and improve the sensor network performance. This work reports on vibration levels detected in railway and road tunnels as a potential energy source for embedded sensors. The measurement results showed that the vibrations at any location in the road tunnel and at the wall in the railway tunnel are too small for useful vibration harvesting. In contrast, the railway sleeper features usable vibrations and sufficient mounting space. For this application site, a robust piezoelectric vibration harvester was designed and equipped with a power interface circuit. Within the field test, it is demonstrated that sufficient energy is harvested to supply a microcontroller with a radio frequency (RF) interface.

Parameter identification for resonant piezoelectric energy harvesters in the low- and high-coupling regimes

We demonstrate a method for identifying the parameters of piezoelectric harvesters by simple electrical measurements. The basic equations are derived from a simplified yet accurate model. The accuracy of the model is proven by comparison with the full model. Two beam harvesters, one with a unimorph beam and one with a bimorph beam, have been fabricated and characterized. The results coincide well with proposed theory and demonstrate the differences between harvesters with low and high coupling constants. It turns out that four parameters are sufficient to completely describe the electrical behavior of a harvester with a low coupling constant. For harvesters with high coupling constants, five parameters are needed.

A hybrid generator for virbration energy harvesting applications

As vibrations are present almost everywhere in urban infrastructure, they are an excellent source to power wireless sensors. Based on our previous clamped-free cantilever, we have developed a beam-type vibratory energy harvester that is suspended on both sides. In the generator, a piezoelectric and an electromagnetic transducer are merged to increase the power output. Different arrangements of magnet and inductor have been tested. At resonance 300 µW and 120 µW can be drawn with 10m/s2 stimulation and optimal load conditions from the piezoelectric and electromagnetic generator part, respectively.

Resonance tunability of clamped–free piezoelectric cantilevers

Piezoelectric cantilevers often form a crucial part in actuating and sensing devices. Many dynamic applications require them to operate at the basic resonance. To ensure optimal performance, the eigenfrequency has to be adjustable to match varying application demands. Within this publication, it is demonstrated that the resonance frequency of a piezoelectric bimorph cantilever can be tuned about 20% by an applied static electrical field. This beneficial feature is explained by an extended analytical model including nonlinear properties of piezoelectric materials.

Assessing the elastostriction and the electrostriction parameter of bulk PZT ceramics

For high electrical loads, the electromechanical characteristics of PZT actuators are impacted by nonlinearities. This work presents a procedure, derived from nonlinear analysis, to assess separately the electrostriction and the elastostriction parameters of bulk PZT ceramics. A detailed investigation has shown that the electrostriction is dominant in actuator applications, while the elastostriction affects sensor applications. Further, the relationship between electrostriction and polarization in PZT material is discussed.

A Low Cost Generator Concept for Energy Harvesting Applications

This paper examines piezoelectric disc benders as energy generators for self-powered systems. PZT-steel composites are sensitive to low frequency stimuli and furthermore to aperiodic excitation e.g. by pressure pulses. A coupled-field FE analysis was carried out to evaluate the electrical output for different disc sizes. The numerical model serves as a tool to specify the properties of the disc benders for different load applications. Beside a replacement of passive damper systems and a harvesting of dissipated energy, piezoelectric disc benders can retrieve information from their ambience and hence operate as self-sufficient sensors. Low cost parts and the simple mechanical bearing were chosen to maximize the output of the generator and especially to push energy harvesting devices towards practical applications.

Piezoelectrically tunable electromagnetic vibration harvester

The presented electromagnetic vibration harvester consists of NdFeB magnets fixed to an elastic suspension in close vicinity to the coil. The suspension is realized with a piezoelectric bimorph cantilever with oppositely polarized layers. If an electric field is applied to that configuration the elastostriction alters the cantilevers bending stiffness. As a consequence the eigenfrequency of the harvester can be tuned up to 20%. This beneficial effect is explained with an analytical model including nonlinear piezoelectric properties. Under tuning operation mode, more than 50 µW are scavenged continuously across the entire frequency range.

Design of Micro Diaphragm Pumps With Active Valves

This paper deals with the theory, fabrication and characterization of micro diaphragm pumps with active valves. Three types of micropumps with piezoelectric actuation are presented. Special emphasis is given on the accordance of theory and experiment. The theory is based on a lumped-element approach that is reduced to its basics to enable a reasonable accuracy with a minimized set of conceptional parameters. The experimental results fit well to the theory. The fabrication technology of the micropumps comprises of a silicon bulk micromechanics process in combination with a back-end gluing process of piezoelectric PZT-disks (PZT lead-circonat-titanat) to the silicon diaphragms. The micropumps were developed for different applications. The three- and the four-diaphragm micropumps were designed as high performance drivers for an artificial sphincter prosthesis. They show a maximum flowrate of 4 ml/min and a maximum sustainable backpressure of up to 70 kPa. The two-diaphragm micropump was engineered for an implantable drug delivery device and features a pressure independent dosing for backpressures up to 20 kPa.Copyright

A dynamic linearization concept for piezoelectric actuators

We present a linearization circuit based on a capacitive Wheatstone bridge that is able to set a desired polarization in a piezoactuator. The system is meant to be used for dynamic actuation in a broad frequency range. A general nonlinear model for piezoactuators is presented in which two nonlinear sub-systems are cascaded: the electric-field-to-polarization (E-P) and the polarization-to-strain (P-x) blocks. The inversion of the latter sub-system in combination with the linearization bridge results in a reduction of up to 19 dB of the harmonic distortion of the actuators mechanical displacement.