Giovanni Biancuzzi

Detailed study of a micro heat engine for thermal energy harvesting

This paper presents a micro heat engine fabricated in silicon micro technology. Its operation principle is based on a cavity filled with a liquid?gas phase-change working fluid that performs a self-controlled reciprocating motion between a heat source and a heat sink. A bistable buckling membrane generates the respective upward and downward driving forces upon expansion and contraction of the working fluid. For prediction of the engine performance a hybrid model is developed. This model predicts an operation frequency of 0.72?Hz and a mechanical output power of 1.29??W at a temperature difference of 37 K. Loss mechanisms are theoretically explored and ways to enhance the overall engine efficiency are discussed. To verify this model, a functional demonstrator is fabricated. In the experiments, an operation frequency of 0.71?Hz is found at a temperature difference of 37 K.

Fabrication of a normally-closed microvalve utilizing lithographically defined silicone micro O-rings

The focus of this work is on the development of a simple and variable process chain for the integration of flexible silicone material into silicon-based microfluidic devices. A normally-closed microvalve is chosen as a demonstrator device, as it combines features that are not easily obtained from silicon devices alone, especially, a high leak tightness of up to 1 bar pressure difference in the closed state and a high forward flow of several mL in the open state. For this purpose, a photopatternable silicone is used as a deformable circular valve lip between a piezoelectrically actuated membrane and a valve seat, similar to a micro O-ring with a width of 50 µm. The microvalve is piezo actuated by monolayer piezo actuators with a peak-to-peak driving voltage of = 200 V. The micro O-ring is pre-deformed by 2.8 µm during the valve fabrication process to yield the normally-closed behavior. A dry film resist lamination technology is developed for this critical process step to mate the two silicon wafers with the actuation membrane, the valve seat and the silicone O-ring in between at a well-defined distance. The dry film resist is used in a multifunctional way, not only to pre-deform the valve lip, but also to define the geometry of the valve chamber and to ensure a leak-tight connection of both wafers. Altogether, a peak value for the on- to off-ratio of the normally-closed microvalve higher than 30 000 is measured. This opens a wide range of potential applications, e.g. in micro-dosing, drug delivery, μ-TAS and microfluidics for biological or chemical applications in general.

Design and simulation of advanced charge recovery piezoactuator drivers

The German Artificial Sphincter System project aims at the development of an implantable sphincter prosthesis driven by a piezoelectrically actuated micropump. The system has been designed to be fully implantable, i.e. the power supply is provided by a rechargeable lithium polymer battery. In order to provide sufficient battery duration and to limit battery dimensions, special effort has to be made to minimize power consumption of the whole system and, in particular, of the piezoactuator driver circuitry. Inductive charge recovery can be used to recover part of the charge stored within the actuator. We are going to present a simplified inductor-based circuit capable of voltage inversion across the actuator without the need of an additional negative voltage source. The dimension of the inductors required for such a concept is nevertheless significant. We therefore present a novel alternative concept, called direct switching, where the equivalent capacitance of the actuator is charged directly by a step-up converter and discharged by a step-down converter. We achieved superior performance compared to a simple inductor-based driver with the advantage of using small-size chip inductors. As a term of comparison, the performance of the aforementioned drivers is compared to a conventional driver that does not implement any charge recovery technique. With our design we have been able to achieve more than 50% reduction in power consumption compared to the simplest conventional driver. The new direct switching driver performs 15% better than an inductor-based driver. A novel, whole-system SPICE simulation is presented, where both the driving circuit and the piezoactuator are modeled making use of advanced nonlinear models. Such a simulation is a precious tool to design and optimize piezoactuator drivers.

An energy-autonomous self-tunable piezoelectric vibration energy harvesting system

A vibration energy harvesting system is presented, which is equipped with a smart resonance-frequency adaptation. The system is able to react to environmental frequency shifts in a completely autonomous way and can be excited in a resonant mode over a large frequency range (150 to 190 Hz). The device was programmed to readjust its resonance frequency every 22 s, and even for large adjustment steps, the recovery time is less than 10 s at an acceleration amplitude of 0,6 g. The effective output-power ranges between 30 and 45 µW, depending on the ambient vibrational frequency. The control-unit in use relies on an ultra-low-power microcontroller, equipped with a look-up table and a learning algorithm. The look-up table allows a very fast and hence energy saving frequency adjustment. With the learning algorithm the control unit is able to keep the look-up table up to date. It can therefore correct inaccurate factory settings and deal with hysteresis, temperature or ageing effects.

An Efficient Low-voltage Micropump For An Implantable Sphincter System

We present our current developments in the GASS project. GASS is an implantable sphincter prosthesis driven by a micropump. The aim of the previous stages of this project was to demonstrate the feasibility of the approach. Currently we focus on the safety and efficiency of the micropump. A low-voltage multilayer actuator was developed to reduce the driving voltage of the micropump from approximately 300 Vpp to 30 Vpp. A miniaturized driving circuit with charge recovery is currently developed. The overall efficiency of the micropump system is in the range of 2% depending on the hydraulic conditions. Different approaches for the energy management of the prosthesis and the resulting size, weight and lifetime are presented.

Design and characterization of a low-voltage piezoelectrically actuated polymer membrane

This publication presents the design, fabrication and characterization of a novel piezo-polymer technology, as a main building block for polymer-based MEMS actuators and devices. A high-precision hot embossing process has been developed to fabricate mechanically robust polycarbonate membranes, with a high flatness, large size and small, precise adjustable thickness. The mounting of a low-voltage multilayer piezoactuator was performed with an optimized gluing process to provide a thin interface layer with a high mechanical strength. By applying a voltage of 40 V to the actuator, displacements up to 18 µm were achieved without fatigue after 10 000 cycles. Up to an isostatic pressure of 50 kPa the membrane shows a linear mechanical deflection without any mechanical hysteresis or bulging.

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.

A high performance bidirectional micropump utilizing advanced low voltage piezo multilayer actuator technology for a novel artificial sphincter system

We present our latest developments for a novel artificial sphincter system. This system had been continuously developed over several years within the GASS project. Main part of the system is a high performance bidirectional silicon micropump driven by piezo actuators. The driving voltage of the newly developed system had been reduced from 300 V to 40 V peak-to-peak by using advanced multilayer piezo actuators. A peak flowrate of 4.36 ml/min had been measured at a driving frequency of 35 Hz. We have measured a maximum backpressure capability of 505 mbar with a square wave driving signal at Vpp=45 V. The whole system is designed to work as a self-sustaining battery driven, remote controlled system.

Low Power Electronics for Square-Wave Piezoactuator Driving

The German Artificial Sphincter System (GASS) project aims at the development of an implantable sphincter prosthesis driven by a micropump. During the last few years the feasibility of the concept has been proven. At present our team’s effort is focused on the compliance to safety regulations and on a very low power consumption of the system as a whole. Therefore a low-voltage multilayer piezoactuator has been developed to reduce the driving voltage of the micropump from approximately 300 Vpp to 40 Vpp. Doing so, the driving voltage is within the limits set by the regulations for active implants. The operation of the micropump at lower voltages, achieved using multilayer piezoactuators, has already resulted in a much better power efficiency. Nevertheless, in order to further reduce power consumption, we have also developed an innovative driving technique that we are going to describe and compare to other driving systems. A direct switching circuit has been developed where the buffer capacitor of the step-up converter has been replaced by the equivalent capacitance of the actuator itself. This avoids the switching of the buffer capacitor to the actuator, which would result in a very low efficiency. Usually, a piezoactuator needs a bipolar voltage drive to achieve maximum displacement. In our concept, the voltage inversion across the actuator is done using an h-bridge circuit, allowing the employment of one step-up converter only. The charge stored in the actuator is then partially recovered by means of a step-down converter which stores back the energy at the battery voltage level. The power consumption measurements of our concept are compared to a conventional driving output stage and also with inductive charge recovery circuits. In particular, the main advantage, compared to the latter systems, consists in the small inductors needed for the power converter. Other charge recovery techniques require very big inductors in order to have a significant power reduction with the capacitive loads we use in our application. With our design we will be able to achieve approximately 55% reduction in power consumption compared to the simplest conventional driver and 15% reduction compared to a charge recovery driver.Copyright