Variable capacitor based mechanical energy-to-electrical energy converter

Abstract

Today miniature stand-alone systems highly benefit from the improvements in IC technology. Shrinking dimensions to submicron technologies and reduced power consumption in the order of nano-watts open possibilities for new power applications[1–3]. Such systems demand integrated, long lasting micro energy sources, and at that power level, ambient energy scavenging arises as an alternative solution, as energy harvesting units can be integrated conveniently through MEMS(micro electromechanical systems) technology. This thesis offers one such solution. A novel generator design with electrostatic approach is presented. The generator creates new electrical charge, thus it can be used to recharge a reservoir. It is composed of variable capacitors and switches. As it does not employ inductive components, it is suitable for environments in which magnetic fields should be avoided. Throughout the thesis the design is further improved to overcome the restriction on the achievable electrostatic field given the dimensions and voltage level. A third electret with permanent charge is embedded in between the plates of the capacitor creating extra field inside the capacitor at the same potential. On the mechanical side, more work is done against increased electrostatic force; and on the electrical side more charge accumulation and a greater charge gain is achieved. The system is simulated using PSpice and the results are consistent with the theoretical expectations. An experiment utilizing macro elements is also carried out with 81% efficiency; when source voltage is 40V, frequency is 1Hz, and Cmax = 1500pF the power gain is 880nW. For micro applications 1500pF is achievable, but 40V is very high; so lower voltage sources should be used and power gain will be much smaller. The electret idea may solve this problem, and one other considerable solution is to increase the maximum capacitance. For the future, our purpose is to reach higher capacitance in limited volumes through new capacitor designs and making use of microfluidics technology.