S. Basrour

Wireless sensor network node with asynchronous architecture and vibration harvesting micro power generator

This paper presents recent advances in the development of a microsystem designed to be part of a wireless sensor network. This microsystem is developed with two particular technologies: asynchronous circuits and ambient energy harvesting power generator. Asynchronous technologies offer several advantages allowing a global decrease in the power consumption of the node. In addition, the presence of an ambient energy scavenger allows the system to power itself, thus reducing maintenance and increasing the lifetime of the node.

MEMS Vibration Energy Harvesting Devices With Passive Resonance Frequency Adaptation Capability

Further advancement of ambient mechanical vibration energy harvesting depends on finding a simple yet efficient method of tuning the resonance frequency of the harvester to match the one dominant in the environment. We propose an innovative approach to achieve a completely passive, wideband adaptive system by employing mechanical nonlinear strain stiffening. We present analytical analysis of the underlying idea as well as experimental results obtained with custom fabricated MEMS devices. Nonlinear behavior is obtained through high built-in stresses between layers in these devices. We report experimentally verified frequency adaptability of over 36% for a clamped-clamped beam device at 2 g input acceleration. We believe that the proposed solution is perfectly suited for autonomous industrial machinery surveillance systems, where high amplitude vibrations that are necessary for enabling this solution, are abundant.

Dielectric polymer: scavenging energy from human motion

More and more sensors are embedded in human body for medical applications, for sport. The short lifetime of the batteries, available on the market, reveals a real problem of autonomy of these systems. A promising alternative is to scavenge the ambient energy such as the mechanical one. Up to now, few scavenging structures have operating frequencies compatible with ambient one. And, most of the developed structures are rigid and use vibration as mechanical source. For these reasons, we developed a scavenger that operates in a large frequency spectrum from quasi-static to dynamic range. This generator is fully flexible, light and does not hamper the human motion. Thus, we report in this paper an analytical model for dielectric generator with news electrical and mechanical characterization, and the development of an innovating application: scavenging energy from human motion. The generator is located on the knee and design to scavenge 0.1mJ per scavenging cycle at a frequency of 1Hz, enough to supply a low consumption system and with a poling voltage as low as possible to facilitate the power management. Our first prototype is a membrane with an area of 5*3cm and 31µm in thickness which scavenge 0.1mJ under 170V at constant charge Q.

High damping electrostatic system for vibration energy scavenging

Advances in low power electronics and microsystems design open up the possibility to power small wireless sensor nodes thanks to energy scavenging techniques. Among the potential energy sources, we have focused on mechanical surrounding vibrations. To convert vibrations into electrical power we have chosen mechanical structures based on electrostatic transduction. Thanks to measurements and in agreement with recent studies [1], we have observed that most of surrounding mechanical vibrations occurs at frequencies below 100 Hz. We report here global simulations and designs of mechanical structures able to recover power over a large spectrum below 100 Hz. Contrary to existing structures tuned on a particular frequency [2], we have investigated conversion structures with a high electrical damping. Mathematica analytical models have been performed to determine the mechanical and electrical parameters that maximize the scavenged power for a wide number of applications. Two prototypes of mechanical structures have been designed.

Comparison of electroactive polymers for energy scavenging applications

Electroactive polymers, generally used as actuators, offer very promising levels of performance in sensor mode, particularly in scavenging ambient mechanical energy applications. New, innovative, high performance, flexible generators can be developed to supply low consumption systems. This technology has the potential to be an alternative to traditional solutions based on electromagnetism, electrostatic or piezoelectricity, which are rigid solutions, mostly used in the high frequency range. This paper reports an extensive investigation of electroactive polymers for energy scavenging applications. The operating principle, properties and analytic models of six polymers are presented and compared in terms of scavenging energy density, operating area, advantages and inconveniences.

Dielectric properties of polyacrylate thick films used in sensors and actuators

Dielectric polymers are emerging electro-active materials used in high performance applications such as micropumps, robots and artificial muscles. The development of such applications requires the use of models taking into account the electrical parameters of the material. However, there is still some controversy over the dielectric constant of the most widely used dielectric polymer (VHB 4910, 3M, USA). In this paper, we present an exhaustive study relating to changes in the dielectric constant of VHB 4910 over wide frequency and temperature ranges. We found that the permittivity was a function of: frequency, temperature, the nature of the electrodes and the pre-stress applied to material. Mechanisms of dielectric polarization (β-relaxation) explain the behaviour in temperature and frequency of this parameter. The use of silver grease-compliant electrodes induces an increase in the dielectric constant which moves to a value of 5.4 (against 4.7 with gold electrodes). A pre-strain applied to the material shows a reduction up to 15% in the value of the dielectric constant. Short-range dipolar relaxation, local mechanical constraints in the material and a possible crystallization of material induced by the stretching are suggested to explain these behaviours. Analytic equations of the dielectric constant according to the temperature and pre-strain are then proposed and used to validate the behaviour of these materials for actuator and scavenger devices.

Dynamic determination of Young's modulus of electroplated nickel used in LIGA technique

Mechanical properties of materials involved in the fabrication of new microactuators have to be well characterized in order to be used in CAD and for the simulation of Microsystems. To achieve that goal, we present a study of the Youngs modulus E of electroplated nickel used in the LIGA technique. This mechanical parameter was obtained by the analysis of vibration frequencies of free-clamped microcantilevers. The resonant frequencies of in-plane and out-of-plane flexural modes were measured with an optical bench. The experimental results are compared to the frequencies derived from a pure elastic finite element model. The variation of the boundary conditions, in particular the description of the clamped part of the devices, leads to a good agreement between the experimental and the simulation. The correlation between these two methods leads to the determination of the Youngs modulus of the device. First results lead to an average value of 195 GPa, which is lower than the data reported for the bulk material. These results are in good agreement with our previous values obtained by steady-state bending tests and other works reported in the literature. E is insensitive to the direction of the excited mode which is characteristic of an isotropic behaviour of the electroplated metal.

Microgrippers fabricated by the LIGA technique

A study devoted to the design and fabrication of microgrippers using the LIGA technique is described in this paper. The design method is presented and validated by the use of finite-element analysis. Technological topics are detailed to illustrate the fabrication process. Also, experimental data concerning the mechanical behaviour of one of the microgrippers are reported. These data are used to improve the previous finite-element analysis. Finally, a comparison between experiments and theoretical predictions is discussed.

Deformable magnetic mirror for adaptive optics: technological aspects

Ground-based telescopes suffer from atmospheric turbulences which perturb the quality of the light arriving from space. Astronomers use adaptive optics in order to correct the wavefront of oncoming light. In this context, a prototype of electromagnetic miniature (O 50 mm) deformable mirror is developed, using available microtechnologies. The mirror is composed of a thin polymer membrane (2–5 μm) covered with a matrix of permanent magnets, and of an array of planar microcoils on a O 50 mm substrate. The paper presents the various technologies used to build the actuator (membranes, coils, magnets). The mechanical behaviour of the mirror is tested. Deformations of up to 20 μm are achieved with currents in the range 1–3 A (6.6 μm/A). Resonance occurs at 485 Hz, with good linearity up to 200 Hz.

Modelling of dielectric polymers for energy scavenging applications

An increasing number of scavenging applications use dielectric polymers: for instance, on the heel of a shoe, behind the knee, on a navy buoy, etc. This emerging technology has the potential to be an alternative to traditional, well-known solutions using piezoelectricity or electromagnetism. Indeed, dielectric polymers are suitable for creating flexible and innovative structures working in a quasi-static range. Nevertheless, current analytical models of dielectric polymers in generator mode are too simple and not sufficiently predictive. This paper reports a more reliable method for modelling dielectric generators. This method is a tool for designing any plane structure. It can be used to calculate performance or to optimize a given structure. Moreover, it is modular and can be adapted to any kind of dielectric material and any plane structure. The method is illustrated on a biaxial plane generator comprising 3Ms VHB 4910 polymer and conductive silver grease electrodes. Experiment data are provided to validate the analytical model and thus the whole method.