Motion-Based Generators for Industrial Applications

Scaling down of electronic systems has generated a large interest in the research on miniature energy sources. In this paper a closer look is given to the use of vibration based scavengers in industrial environments, where waste energy is abundantly available as engine related vibrations or large amplitude motions. The modeling of mechanical generators resulted in the design and realization of two prototypes, based on electromagnetic and electrostatic conversion of energy. Although the prototypes are not yet optimized against size and efficiency, a power of 0.3 mW has been generated in a 5 Hz motion with a 0.5 meter amplitude. INTRODUCTION Since the presentation of the first transistor at the Bell Laboratories the size of electronic components has decreased according to Moore’s law, resulting in highly functional computers on miniature size silicon dies today. At the same time the integration of systems on chips (SOCs) and the development of systems in packages (SIPs) decrease the overall size of electronic devices, transforming them into small, lightweight and highly functional systems. This miniaturization has triggered new desires: mobility, autonomy and wireless connectivity are key features to be taken into account when designing a new system. These systems also require a wireless, autonomous and mobile energy source. The conventional solution is found in storage systems such as electrochemical batteries. The total amount of energy available depends on the energy density and the volume of the storage system. New storage systems have been developed based on higher energy densities in order to decrease the volume and weight occupied by the energy source (e.g. fuel cells). The volume-dependency of the amount of stored energy is however an inevitable drawback. As a consequence, when system dimension decrease, the smaller volume available for energy storage results in a shortened lifetime. A solution to this contradiction is offered by miniature generators, extracting energy from the environment of the mobile system and converting it efficiently into electrical power. Several literature contributions present optimistic evaluations of the performances of this type of waste energy generators, based on solar, thermal or mechanical energy [1]. Industrial environments require small lightweight sensor devices to monitor the processes and the condition of the machinery. In order to prevent the costs of battery replacements, waste energy generators provide a good solution. An ideal source of energy for these scavengers is offered by the mechanical energy available from the “natural” motion and from vibrations of the machinery. The first category of mechanical energy is the result of robotic movements and is characterized by low frequencies (up to 15 Hz or 900 rpm) and large amplitudes of the order of 1 meter. Machinery vibrations have harmonic frequencies up to 5 kHz but exhibit much smaller amplitudes. MODELING MECHANICAL ENERGY SCAVENGERS In order to extract mechanical energy from a motion, it is necessary to convert this motion into a relative movement across the mechanical port of the generator. This is not a trivial task when the dimensions of the generator (and of its parts) become small. Miniature generators therefore use the inertia of a mass to convert the source motion into a local displacement across the generator. The bigger the mass, the better the motion is coupled onto the generator; yet again the need for miniaturization limits the size of the mass. The motion of the mass is damped by the generator, while it is extracting energy from the motion, but also by unwanted factors such as air damping, squeezed film damping and friction, which convert the kinetic energy into heat. The inertial mass is often suspended to reduce these losses and to improve the efficiency of the device.