Long lasting, high power density power sources are important to enable emerging technologies such as wireless sensor networks, robotic platforms, and electronic devices for consumer, military, medical, aerospace and other applications. To meet the energy demands for these applications, devices that scavenge power from the environment (e.g., solar, thermal, vibrations) are of great practical interest. Various energy harvesting and scavenging methods exist for capturing and storing energy from normally occurring environmental sources, such as thermal, solar, or vibrational. For applications on moving platforms, vibrational energy harvesters are advantageous since solar or thermal energy may not be available under all operating conditions.
Current research has focused on a variety of vibrational energy harvesting devices. For example, micromachining and micro-electro-mechanical system (MEMS) technologies have been used to produce sub-millimeter microchip-sized devices, but the power output from these miniaturized devices has been very low (often nW-μW level), which appears to be too small to power many practical devices. The paper entitled “Performance limits of the three MEMS inertial energy generator transduction types,” by P. D. Mitcheson, et al. (J. Micromech. Microeng., vol. 17, S211-S216, 2007) shows that the power density scales unfavorably with length scale.
Most electromagnetic vibrational energy harvesters suffer from weak magnetic fields, which ultimately hampers their performance. For example, in many electromagnetic vibrational energy harvesters, one or two moving magnets are used in an open-field configuration, where soft magnetic cores typically need to be introduced in order to increase the magnetic flux density and, thus, increase the output of power. Unfortunately, the introduction of these soft magnetic cores to the open-field configuration can lead to high attractive magnetic forces, which may negatively impact the motion of the magnet in the configuration. Introduction of soft magnetic cores can also cause the magnet to no longer readily move under external vibration. Practical implementation of these magnetically-based vibrational energy harvesters can also be difficult as the static stray magnetic fields are difficult to shield. Accordingly, magnetically based energy harvesters also tend to be magnetically attracted to ferrous objects external to the harvester, which may interfere with the harvester's operation. Another potential issue with these harvesters is that the magnetic fields can adversely affect surrounding structures. Therefore, there is a need for an electromagnetic vibrational energy harvester that creates a strong magnetic field, where the strong magnetic field is largely, if not entirely, within the device.