This paper describes the design and operation of power conditioning system with maximum power transfer tracking (MPTT) for low power electromagnetic energy harvesters. The system is fully autonomous, starts up from zero stored energy, and actively rectifies and boosts the harvester voltage. The power conditioning system is able to operate the harvester at the maximum power point against varying excitation and load conditions, resulting in significantly increased power generation when the load current waveform has a high peak-to-mean ratio. First, the paper sets out the argument for MPTT, alongside the discussion on the dynamic effects of varying electrical damping on the mechanical structure. With sources featuring stored energy, such as a resonant harvester, maximum power point control can become unstable in certain conditions, and thus, a method to determine the maximum rate of change of electrical damping is presented. The complete power conditioning circuit is tested with an electromagnetic energy harvester that generates 600mVrms ac output at 870μWunder optimum load conditions, at 3.75 m·s−2 excitation. The digital MPTT control circuit is shown to successfully track the optimum operating conditions, responding to changes in both excitation and the load conditions. At 2 Vdc output, the total current consumption of the combined ancillary and control circuits is just 22μA. The power conditioning system is capable of transferring up to 70% of the potentially extractable power to the energy storage.
- The output of small electromagnetic energy harvesters typically requires rectification and boosting in order to produce an output voltage that falls within the allowable operating range of the load electronics. In some applications, there is also a need to buffer energy in high capacity storage elements, such as super capacitors, in order to supply loads with a higher peak demand than the harvester output . Several circuit architectures have been reported in published literature, which meet these requirements, including single-stage ac–dc switch-mode power converters .
- Efficiencies up to 75%–80% at 500μW level have been reported . However, in order to achieve the maximum potential power of an energy harvester, it is important that the power conditioning system provides the optimum load for the generator for the particular input and output conditions.
The work presented in this paper aimed to address the challenges that arise from implementing MPTT for low-power, kinetic electromagnetic energy harvesters. The transient response of the single-degree-of-freedom mechanical system is presented and discussed using experimental results and analytical derivations. A method that aids the design of perturb-and-observe algorithm-based control with discrete perturbations of the control parameter is presented: the minimum time required between perturbations in order to allow the mechanical structure to settle is calculated for highly under damped mass–spring–damper systems under the assumption of a constant, sinusoidal, non-direct excitation that occurs at the natural resonance frequency of the mechanical structure.
Maximum power transfer tracking for Ultra low power electromagnetic energy harvesters
Output voltage and current Screenshot