Existing design criteria for vibration energy harvesting systems provide guidance on the appropriate selection of the seismic mass and load resistance. To harvest maximum power in resonant devices, the mass needs to be as large as possible and the load resistance needs to be equal to the sum of the internal resistance of the generator and the mechanical damping equivalent resistance. However, it is shown in this paper that these rules produce suboptimum results for applications where there is a constraint on the relative displacement of the seismic mass, which is often the case. When the displacement is constrained, increasing the mass beyond a certain limit reduces the amount of harvested power. The optimum load resistance in this case is shown to be equal to the generator's internal resistance. These criteria are extended to those devices that harvest energy from a low-frequency vibration by utilizing an interface that transforms the input motion to higher frequencies. For such cases, the optimum load resistance and the corresponding transmission ratio are derived.

The paper introduces a permanent magnet rotary-to-linear magnetic gear whose topology is derived based on the operating principle of a transverse flux VRPM machine. It is shown that when the number of poles per unit length on all three members, rotor, translator and stationary flux modulating ferromagnetic pieces, are the same, the proposed gear is essentially similar to a magnetic screw gear with discretised helical magnets. Using different number of poles and iron pieces per unit length provides alternative means for adjusting the gear ratio. The paper describes the design and construction of a demonstrator gear whose dimensions were optimised using finite element analysis. Performance results obtained from finite element analysis and experiment are presented and discussed.

This paper explores implementing active resonance frequency tuning of adaptive tuned vibration absorbers (ATVA) by coupling a vibration absorber comprising a voice coil actuator (VCA) and a sprung inertial mass to a solenoid actuator (SA). In this device, the DC current flowing through the SA generates negative magnetic stiffness and varies the equilibrium position of the nonlinear spring of the VCA, thus changing the effective stiffness of the system and its resonant frequency. The paper details the analytical design steps and the parametric sizing process of designing the SA to maximize its force density within defined magnetic, dimensions and thermal constraints. The design of the SA was verified using finite element analysis (FEA), manufactured and coupled to a VCA to make the complete ATVA system. The experimental results demonstrate the tunability of the resonance frequency of the ATVA by controlling the SA’s DC current. It is shown that the system’s resonance frequency can be shifted by up to 23% of the fundamental resonance frequency, which improves the effectiveness of the ATVA at damping a broader range of excitation frequencies.

In some energy harvesters, the maximum throw of the seismic mass is limited due to the physical constraints of the device. The shunt load resistance of such a harvester is generally selected based on the allowable throw of the mass when the device is subjected to the maximum level of excitation. However, the energy harvester with this value of shunt resistance does not perform well at lower levels of excitation. In this article, a variable load resistance, scheduled on the excitation level, is introduced to extend the dynamic range of an energy harvester in applications where excitation level varies. This method is applied to the design of an energy harvester, which comprises a sprung mass coupled to an electric motor through a lead screw. The dynamic equation and parameters of the system are introduced and the device is experimentally characterized, by conducting random vibration tests. The harvested power and the relative displacement are then obtained for different sinusoidal base excitation amplitudes when the system is excited at a frequency close to its natural frequency. It is demonstrated that the use of a variable load resistance mechanism can significantly improve the dynamic range and output power of the energy harvester.