Geert Altena

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.

Design improvements for an electret-based MEMS vibrational electrostatic energy harvester

This paper presents several improvements to the design of an electret-based MEMS vibrational electrostatic energy harvester that have led to a two orders of magnitude increase in power compared to a previously presented device. The device in this paper has a footprint of approximately 1 cm2 and generated 175 μW. The following two improvements to the design are discussed: the electrical connection principle of the harvester and the electrode geometrical configuration. The measured performance of the device is compared with simulations. When exited by sinusoidal vibration, a device employing the two design improvements but with a higher resonance frequency and higher electret potential generated 495 μW AC power, which is the highest reported value for electret-based MEMS vibrational electrostatic energy harvesters with a similar footprint. This makes this device a promising candidate for the targeted application of wireless tire pressure monitoring systems (TPMS).

Design, modeling, fabrication and characterization of an electret-based MEMS electrostatic energy harvester

This paper reports on the design, modelling, fabrication and characterization of an electret-based MEMS electrostatic energy harvester with an elegant and robust process flow. The fabrication is based on a SOI wafer with self-aligned electrodes of the variable capacitor. The output current of the device shows distinct steps that are caused by strong lateral electrostatic forces. An electro-mechanical model was used to analyze and confirm the observed behaviour.

Modeling and characterization of electret based vibration energy harvesters in slot-effect configuration

The purpose of this article is to elaborate a model and the optimization guidelines for electret based harvesters with a specific electret/electrodes configuration, namely the slot-effect configuration. Slot-effect configured harvesters have been investigated experimentally by several research groups. A model describing their dynamic behavior has also been recently proposed in the literature. However, the simplifications used in the existing model can lead to inaccuracies and a refined analysis is elaborated in the present article. The model is compared with experimental measurements on MEMS fabricated devices with a corrugated electret. The electrodes dimensioning in the MEMS device are chosen so that the harvester behaves in a quasi-linear manner over its full range of displacement. This quasi-linearity simplifies greatly the device optimization. Indeed, the behavior of the developed electrostatic harvester is shown to be very comparable to that of piezoelectric harvesters, which are very well understood and documented. The influence of several design parameters on output power performance is investigated. As long as pull-in and breakdown voltage effects can be avoided, the electret surface potential should be maximized and the air gap minimized. We also investigate theoretically the influence of three types of electret on the generated power: planar, corrugated with partial charge coverage, and corrugated with full charge coverage. With the dimensions corresponding to our MEMS devices, the output power characteristics for the three types of electret are similar. However, it is shown that this is not always true. In some conditions, corrugated electrets with full charge coverage are detrimental for the generated power.

Solid state pH and chloride sensor with microfluidic reference electrode

We present for the first time a miniaturized multi ion-selective sensor to simultaneously determine pH and chloride (Cl−) levels in fluid. The sensor combines solid state iridium oxide (IrOx) and silver chloride (AgCl) electrodes fabricated on a Si substrate with a microfluidic reference electrode (RE). The drift of the RE is similar to a standard RE while the internal reservoir is orders or magnitude smaller. The drift rate depends on the RE geometry as predicted by a model based on diffusion. The sensitivity, response time and accuracy were determined using solution with known composition, household solutions and samples of sweat and saliva. Overall performance is similar or even exceeds standard, relatively large and expensive ion-selective sensors.

Mikrostrukturierte Energiewandler für energieautonome SensorenMicrostructured Energy Harvesters for Energy Autonomous Sensors

Zusammenfassung Diese Arbeit präsentiert mikrostrukturierte Energiewandler zur Stromversorgung energieautarker Sensoren. Vibrationsbasierte Energiewandler mit Betriebsfrequenzen von 100 bis 1000 Hz nutzen den piezoelektrischen oder den elektrostatischen Effekt. Wir beschreiben außerdem ein drahtloses energieautonomes Sensorsystem, welches von einem Vibrationswandler gespeist wird und alle 15 Sekunden Temperaturmessungen drahtlos überträgt. Mikrostrukturierte Thermoelemente werden darüber hinaus als kostengünstige Lösung für thermoelektrische Generatoren angesehen. Diese sollen bei geringen Temperaturunterschieden und schwachen Wärmeströmen arbeiten, die z. B. für menschliche Körperwärme typisch sind.