Tom Sterken

Comparative modelling for vibration scavengers [MEMS energy scavengers]

Conversion of the mechanical energy stored in vibrations into useful electrical energy is possible using three principles: electromagnetic, electrostatic and piezoelectric conversion. In order to build the appropriate device for a given application, a unifying model is proposed for these three principles. A methodology for comparison is presented based on this model in order to match the generator type to the input specifications (frequency, amplitude) and to the requirements at the electrical output, such as load impedance and voltage.

Harvesting Energy from Vibrations by a Micromachined Electret Generator

The development of low power autonomous systems has triggered the research for waste energy recuperation. A micromachined generator has been designed to extract energy from vibrations in the ambient. It consists of an electrostatic transducer which is continuously polarized by an electret. The design of the generator has been optimized towards miniaturization. A prototype has been fabricated.

An electret-based electrostatic /spl mu/-generator

This paper proposes a new approach towards mechanical waste energy scavenging, using a micromachined electrostatic converter. The device consists of a vibration sensitive variable capacitor polarized by an electret. The variation of the capacitance results in a current through a load circuit. A physical model of the device is described; in order to obtain a closed form expression for the converted power a linear model is presented. This model shows that 50 /spl mu/W can be generated with a 5 /spl mu/m vibration. A first prototype design based on SOI MPW-service is proposed.

Power Processing Circuits for Piezoelectric Vibration-Based Energy Harvesters

The behavior of a piezoelectric vibration-driven energy harvester with different power processing circuits is evaluated. Two load types are considered: a resistive load and an ac-dc rectifier load. An optimal resistive and optimal dc-voltage load for the harvester is analytically calculated. The difference between the optimal output power flow from the harvester to both load circuits depends on the coupling coefficient of the harvester. Two power processing circuits are designed and built, the first emulating a resistive input impedance and the second with a constant input voltage. It is shown that, in order to design an optimal harvesting system, the combination of both the ability of the circuit to harvest the optimal harvester power and the processing circuit efficiency needs to be considered and optimized. Simulations and experimental validation using a custom-made piezoelectric harvester show that the efficiency of the overall system is 64% with a buck converter as a power processing circuit, whereas an efficiency of only 40% is reached using a resistor-emulating approach.

Piezoelectric Harvesters and MEMS Technology: Fabrication, Modeling and Measurements

Piezoelectric converters designed for harvesting energy from mechanical vibrations have been fabricated by micromachining technologies. The manufactured piezoelectric energy harvesters have been characterized by applying a sinusoidal oscillation as mechanical input and by using a simple resistive load to measure the output power of the system. A maximum output power of 40 muW has been measured for an input vibration having a frequency of 1.8 kHz and an amplitude of 180 nm. A model aimed at estimating the output power of the fabricated piezoelectric structures has been developed and theoretical estimations have been compared with experimental results.

Scavenging energy from human body: design of a piezoelectric transducer

The present work concerns the concept and the modeling of a device for transforming a part of the mechanical energy originating from human limb motion into electrical energy. The device is formed by a frame containing a free moving mass. Useful electrical power is generated by the impact of this mass on piezoelectric cantilevers positioned at the end of the frame. In order to evaluate device performances a detailed analysis of the dynamics of the system is coupled to a model of the piezoelectric transducer. It is found that an optimized device, when mounted on the human wrist, can theoretically generate up to 40 /spl mu/W/cm/sup 3/.

An electrostatic energy harvester with electret bias

We have designed, fabricated and characterized a MEMS electrostatic energy harvester using an electret as internal bias. The device operates in continuous mode and features a high voltage output, a large travelling distance of a big mass within a compact design using full bulk silicon thickness. The output power is about 1μW at an acceleration power spectral density of 0.03g2/Hz.

Cyclic endurance reliability of stretchable electronic substrates

Abstract Stretchable electronic circuit boards have been developed based on three different technologies. Such substrates serve to connect rigid interposers or electronic components. The conducting traces have a meandering shape and consist of Cu-foil or screen-printed Ag-paste. These conducting traces are attached to or embedded in polyurethane, polydimethylsiloxane, or breathable non-woven stretchable substrate material. The long-term endurance behavior of this novel type of boards is studied by cyclic elongation at strain ranges of up to 20% and monitoring the electrical connectivity. The main failure mode in the Cu-foil based technologies is fatigue of the conducting traces and can be described in terms of the Manson–Coffin relation. Indications for high-cycle fatigue were found. The screen-printed conductors on non-woven substrates fail by breaking of the connection between the metallic grains. The application areas are electronic monitoring systems that need to be placed directly on the skin, or conformable systems for curved surfaces.

Fabrication and Characterization of Flexible Ultrathin Chip Package Using Photosensitive Polyimide

Assembly of thinned die has become common practice to meet the demand for smaller and lighter electronic products. One way to achieve this goal is to embed the thinned die into two dielectric films, which results in a flexible ultrathin chip package (UTCP). This paper describes a new UTCP process flow with microvia formation by standard UV lithography through photosensitive polyimide (PSPI). Such microvia-formation method proved to be more reliable than laser drilling techniques and simpler than a dry etching process. Since the used PSPI is self-priming, a thin layer of potassium chloride was introduced as a release layer. In the end, the polyimide encapsulation of the thinned die can be released from the carrier substrate and becomes a flexible chip package with a total thickness of around 50 μm. Daisy-chain test dies were encapsulated inside spin-coated polyimide films. Excellent chip-to-package interconnection was demonstrated by electrical daisy chain and contact resistance measurements. Bending tests and thermal cycling tests were also performed on the daisy-chain test vehicles. Desired flexibility and reliability of UTCPs was observed.

Energy Scavengers : Modeling and Behavior with Different Load Circuits

This contribution describes the modeling of vibration-driven energy scavengers, either based on electrostatic, electromagnetic or piezoelectric principles. Subsequently, the behavior of the scavenger model is tested with two different types of load: a resistive load and a rectifier with a fixed voltage at the output. Optimal power output of the scavenger is calculated for both load-cases. It is shown that a resistance is not in all cases the most optimal load to maximize the power output of a scavenger. This conclusion has a big influence on the design of the power management circuit, needed to transform the scavenger output voltages to an appropriate shape for powering an electronic load.