Level-dependent load in a pendulum like energy harvester

Typically ambient vibration generates a relative displacement between the suspended mass and the base in an energy harvester, and the induced electromotive force (EMF) in an electrical circuit is used to harvest electrical energy. Electromagnetic energy harvesters can operate at an optimum load, which provides the maximum power. However, due to the physical constraints of these devices, such as the limited available volume, the throw is also limited and operating at the optimum load is not feasible. By using a constant load, the optimum conditions cannot be achieved and the harvested power therefore can be reduced dramatically. To overcome this issue, the present paper proposes a new strategy, which consists of adjusting the electric load according to the input level at steady state condition. If the electric load, which is assumed to be a pure electrical resistance, is tuned according to the level of the harmonic excitation, then the harvester can always operate at the optimum conditions. To demonstrate the effectiveness of the level-dependent load, an energy harvesting device in the form of a pendulum is designed and manufactured, which is comprised of an oscillating beam sprung to the base, and attached to a generator. Across the terminals of the generator, an electric resistance (digital potentiometer) is mounted and the voltage produced is converted by Joule effect into thermal power, which is considered here as the harvested power. Experiments are conducted by exciting the harvester with a harmonic input at resonance via a shaker. A level-dependent load and a constant load are separately tested. The experimental results are in good agreement with the simulations. It is shown that by adjusting the load according to the input level, the harvested power is increased compared to a linear constant load.