Characterisation of an Electrostatic Vibration Harvester

CHARACTERISATION OF AN ELECTROSTATIC VIBRATION HARVESTER

T. Sterken1,2, G. Altena3, P.Fiorini1, R. Puers2

1 IMEC, Leuven, Belgium, 2 K.U.Leuven, Leuven, Belgium 3 IMEC – NL/Holst Centre, Eindhoven, The Netherlands Tom.Sterken@imec.be

ABSTRACT

Harvesting energy from ambient vibration is proposed as an alternative to storage based power supplies for autonomous systems. The system presented converts the mechanical energy of a vibration into electrical energy by means of a variable capacitor, which is polarized by an electret. A lumped element model is used to study the generator and design a prototype. The device has been micromachined in silicon, based on a two-wafer process. The prototype was successfully tested, both using an external polarization source and an electret.

1. INTRODUCTION

Autonomy, mobility and lifetime are important characteristics of state-of-the-art electronic systems. The autonomy of these systems is assured by the use of primary or secondary batteries; the lifetime of storage based energy supplies depends on the ratio between the amount of energy stored in the battery to the power consumption of the electronic device. As the storage capacity is limited by the size (and weight) of the device, designers of electronic circuits make strong efforts to decrease the power consumption of the device.
An alternative approach is presented by ambient energy harvesters. These devices extract and convert energy from the environment. In this way the theoretical lifetime of autonomous systems is not limited by the size of the device, but depends on the availability of the ambient energy source. Low power design is still a key parameter in the autonomy of the system, as the power level of ambient energy supplies tends to vary in the lower microwatt ranges.
The feasibility of vibration powered systems depends on the ambient conditions, the envisioned application and boundary conditions such as maximum size, maximum weight and cost issues [1].
The generator presented in this contribution extracts energy from vibrations to which the autonomous device is subjected. The vibrations are characterised by small amplitudes of displacement in the micrometer scale and by relatively large frequencies, in the 100 - 3000 Hz range. These vibrations can be found in industrial environments. The generator design as presented is not suited for large amplitude motion scavenging, characterised by lower frequency vibrations. These are often related to human activities. Vibration harvesting generators consist of a seismic mass whose motion is coupled either to an electromagnetic, a piezoelectric or an electrostatic transducer. The inertia of the mass acts as an artificial reference to the vibration.
The mass is often suspended to reduce unwanted damping losses. At the same time the suspension can be designed to allow resonance at the main working frequency of the vibration. This approach is favourable for vibrations at higher frequencies with lower amplitudes: resonance converts the small amplitude of the package into an internal motion with a larger amplitude. The work produced by the relative motion between the package of the device and the mass is converted to electrical energy by a transducer. In this work an electrostatic transducer is designed and fabricated using micromachining technology (Fig. 1). The transducer consists of a variable capacitor that is polarised. As the capacitance changes, a current is forced between the electrodes of the capacitance, through a load circuit. The current performs work in the electrical system, while the mechanical motion is damped by the associated electromechanical forces. The polarisation source can either be an external voltage source or a built-in electret.

Suspension Damping Mass Internal Motion Input Vibration load

Polarisation Source Suspension Damping Mass Internal Motion Input Vibration load

Polarisation Source

Figure 1. Schematic overview of an electrostatic harvester for vibrations.

©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3

T. Sterken et al. Characterisation of an Electrostatic Vibration Harvester

2. DESIGN

The design of the prototype is based on the analysis of a lumped element model of the generator. This type of modeling converts the mechanical behavior to an electrical circuit, based on the equivalences between the behavior of masses, springs and dampers to inductances, capacitors and resistors respectively. The transducer then connects the mechanical equivalent circuit to the electrical load circuit. The equivalent model for the transducer is based on the relations between the forces, voltages, charges and geometry of the capacitance. As these relations are non-linear, a linearised small-signal model is used. The equivalent model consists of a trans

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