Research Activities in the Field of Electromagnetic Energy Harvesting Micro Systems
The use of energy from the ambient shows a great possibility to reduce energy consumption. One point is energy harvesting, which means to obtain electrical energy from available sources like solar, thermal or vibrational energy . Therefore the development of new forms for converting energy is in the focus of researchers. In the field of microsystems the use of vibrational energy seems to be the best convertible energy form. This paper gives a review about the state of the art in the field of electromagnetic energy harvesting systems and especially the research activities at the IMPT
Concepts for electromagnetic harvesting systems
There are three general technical applications for the design of an inductive micro energy harvester system . If a rotational excitation can be used the concept of a dynamo is the best way for energy conversion. For a linear excitation of a mass-spring system, a relative movement between a coil and a hard magnet induces a voltage caused by the change of the magnetic field detected by the coil. The third alternative is the use of an imbalanced pendulum to combine the two possibilities above
Example for thin-film device
One approach is to use the principle of an integrated hallbach array in a linear energy harvester . This array has a special arrangement of permanent magnets (Fig. 1) that doubles the magnetic field on one side while cancelling the field to near zero on the other side. Two thin-film coils are located next to the hallbach array and the movement induces a voltage in the coils.
Fig. 1: Operation of the electromagnetic energy harvester 
Investigated Principles at IMPT
For the integration of energy harvesting devices into smart systems it will be necessary to minimize the volume of such an application. Furthermore the use of an adjustment to avoid the resonant frequency dependencies is important. This device consists of four independent thin-film coil systems . Each consists of a double layered spiral coil with 15 windings in each layer. The total height of this coil is approx. 30µm without the substrate. For inducing a voltage a rotational flying wheel is chosen. On this wheel four permanent magnets with a saturation flux density of 1.3 T are placed to induce a change of the magnetic field. The induced voltage depends on the distance between the coils and the permanent magnets. In our case the distance is about 750 µm. The reason for this air gap is that the electrical contacts are realized by wire bonding. The calculation of the systems predicts a voltage of 2.1 mV for a revolution of 1,000 rpm. The measurement shows an induced voltage of about 2.0 mV and therefore verifies the calculations.
The demonstrator was fabricated using an acrylic glass tube with an outer diameter of 11 mm and an inner diameter of 9 mm . The screw thread for the mounting of the springs was fabricated using stainless steel. A groove was milled into the acrylic glass corpus. The 100 windings of the coil were wound around the corpus into the groove. One great challenge was the connection between the springs and the permanent magnet. In a first approach we tried to bring a drill hole into the small permanent magnet but the material of the sintered magnet was to brittle and the magnet broke. For the connection of these parts glue was used. The resonance frequency of the system is about 42 Hz and the system produces a maximum induced voltage output of about 2 V.
The review shows a lot of possibilities to use the inductive principle to convert mechanical energy into electrical energy.
 Prasad, Ramjee; Dixit, Sudhir; van Nee, Richard and Ojanpera, Tero, Globalization of Mobile and Wireless Communications Today and in 2020, Perspectives on Energy-Harvesting Wireless Sensor Networks, Springer-Verlag Berlin Heidelberg, 2011
 Khaligh, Alireza, Kinetic Energy Harvesting Using Piezoelectric and Electromagnetic Technologies—State of the Art, IEEE March 2010
 Zhu, Dibin, A Planar Electromagnetic Vibration Energy Harvester with a Halbach Array. At PowerMEMS 2011, Seoul, Korea, 15 - 18 Nov 2011
 Wurz, Marc, Construction and Development of a Micro-Coil-Generator for Energy Harvesting, Energy Summit, Dallas 2014