Nonlinear damping in energy harvesters

Abstract

Energy harvesting from ambient vibration has become an attractive topic in the recent years. Initial studies aimed to maximise the performance of small linear device for different excitation scenarios. These devices were assumed to be located in hostile and inaccessible environments and be able to provide energy for low powered sensors. Due to the limited size of the energy harvesters, however, the amount of power produced was small.

More recently, many researchers have considered using nonlinear stiffness to improve the performance of these devices. This thesis, however, focuses on the use of nonlinear damping in energy harvesters. Nonlinear damping may be unwanted and introduced as the mechanism of the harvester, but can also be deliberately introduced to improve the dynamic range of the harvester.

Typically ambient vibration generates a relative displacement between the suspended mass and the base in an energy harvester, which induces an electromotive force (EMF) in a circuit that is used to harvest electrical energy. It is possible to introduce nonlinear mechanical damping by having a circuit with a nonlinear resistance. Specifically, a load, in which the current is a third-power function of the voltage is compared with an equivalent linear load for three kinds of excitation such as harmonic, random white noise and random bandlimited noise. According to the numerical and analytical results, the cubic load provides more harvested power at resonance at low levels when compared to an equivalent linear load at the same level of excitation. As the frequency bandwidth of a random excitation becomes wider, to the limit of white noise, as the power generated by the cubic converges to the linear case. Electromagnetic transducer energy harvesters usually adopt a conversion mechanism of motion, such as ball screw or rack and pinion, which introduce a source of loss such as friction. Static friction is then added to the model and this is shown to affect the harvested power at low input levels.

Another proposed strategy consists of adjusting the electric load according to the input level, which can also enlarge the dynamic range of performance of energy harvested compared to a device with constant load.

To demonstrate the effectiveness of the level-dependent load, an energy harvesting device was designed and manufactured, which comprised of an oscillating beam sprung to the base, and attached to a generator. Across the terminals of the generator, an electric resistance is mounted and the voltage measured is used to compute the harvested power.

Experiments are conducted by exciting the harvested with a harmonic input at resonance via a shaker. A level-dependent load and a constant load were separately tested; with results that 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.

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