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One of the main challenges in the field of vibration energy harvesting is to extract energy from the low-frequency ambient vibrations. Since the voltage and power production in vibration energy harvesters depends on the relative velocity (frequency), therefore, comparatively high power levels are produced at a high frequency of vibrations. For low-frequency excitation of the base, technique such as frequency up-conversion (FUC) is widely utilised. In FUC method, a very low-frequency vibration is efficiently harvested using the plucking mechanism. FUC harvesters are comprised of a low-frequency mechanism (primary system) and a high-frequency mechanism (secondary system), which are either coupled operationally through freely moving mass, mechanical plucking, magnetic plucking or mechanical impact. The mechanical contact plucking or magnetic plucking mechanism is used for exciting a secondary system that has a much higher frequency for efficiently harvesting more energy. This work provides a detailed review of the FUC energy harvesters that are utilising freely moving mass, plucking mechanism and mechanical impact techniques reported in the literature. Information about the harvesters' architecture, working, fabrication and output performance is performed with an emphasis on input low frequency, input acceleration, converted high frequency, voltage generation, power levels and power density values reported in the literature. Moreover, all techniques developed for low-frequency vibration applications are compared and the effectiveness of each mechanism used for low-frequency energy harvesting is discussed.

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