Harvesting ambient energy will make embedded devices autonomous http://www.embedded.com/shared/printableArticle.jhtml;jsessionid=S... 1 of 4 9/1/2005 9:22 AM Harvesting ambient energy will make embedded devices autonomous By Erick O. Torres and Gabriel A. Rincon-Mora, Courtesy of EE Times Aug 29 2005 (9:00 AM) URL: http://www.embedded.com/showArticle.jhtml?articleID=170102227 Modern electronics continues to push past boundaries of integration and functional density toward the elusive, completely autonomous, self-powered microchip. As systems continue to shrink, however, less energy is available on board, leading to short device lifetime (run-time or battery life). Research continues to develop higher-energy-density batteries, but the amount of energy available is not only finite but also low, limiting the system's life span, which is paramount in portable electronics. Extended life is also particularly advantageous in systems with limited accessibility, such as biomedical implants and structure-embedded micro-sensors. The ultimate long-lasting solution should therefore be independent of the limited energy available during startup. That's where a self-renewing energy source comes in, continually replenishing the energy consumed by the microsystem. State-of-the-art microelectromechanical-system (MEMS) generators and transducers can be such self-renewing sources, extracting energy from vibrations, thermal gradients and light. The energy extracted from these sources is stored in chip-compatible, rechargeable batteries such as thin-film lithium-ion types, which power the loading application (for example, the sensor) via a regulator circuit. Since harvested energy manifests itself in irregular, random, low-energy bursts, a power-efficient, discontinuous, intermittent charger is required to transfer the energy from the sourcing devices to the battery. Energy that is typically lost or dissipated in the environment is therefore recovered and used to power the system, significantly extending its operational lifetime. Energy harvesting is defined as the conversion of ambient energy into usable electrical energy. When compared with the energy stored in common storage elements, such as batteries and the like, the environment represents a relatively inexhaustible source. Consequently, energy-harvesting or -scavenging methods must be characterized by their power density, rather than energy density. Light, for instance, can be a significant source of energy, but it is highly dependent on the application and the exposure to which the device is subjected. Thermal energy, on the other hand, is limited because the temperature differentials across a chip are typically low. Vibration energy is a moderate source, but again dependent on the particular application. Energy extraction from vibrations is based on the movement of a "spring-mounted" mass relative to its support frame. Mechanical acceleration is produced by vibrations that in turn cause the mass component to move and oscillate (kinetic energy). This relative displacement causes opposing frictional and damping forces to be exerted against the mass, thereby reducing and eventually extinguishing the oscillations. The damping forces literally absorb the kinetic energy of the initial vibration. This energy can be converted into electrical energy via an electric field (electrostatic), magnetic field (electromagnetic) or strain on a piezoelectric material. These schemes amount to harvesting energy from vibrations.