Stephen G. Burrow

Review of Power Conditioning for Kinetic Energy Harvesting Systems

In this paper, a summary of published techniques for power conditioning within energy harvesting systems is presented. The focus is on low-power systems, e.g, <;10 mW, for kinetic energy harvesting. Published concepts are grouped according to functionality and results contrasted. The various techniques described are considered in terms of complexity, efficiency, quiescent power consumption, startup behavior, and utilization of the harvester compared to an optimum load. This paper concludes with an overview of power management techniques that aim to maximize the extracted power and the utilization of the energy harvester.

A wide-speed-range hybrid variable-reluctance/permanent-magnet generator for future embedded aircraft generation systems

This paper presents recent research into the use of an embedded generation system as an alternative to the emergency ram air turbine on aircraft, the proposal being to utilize the windmill effect of the low-pressure turbine of the main aircraft engine for emergency power generation. A novel topology of a high specific output ac permanent-magnet generator is described that has been designed to be driven directly from the low-pressure turbine via an integral fixed-ratio gearbox, the requirement being to generate a regulated 270-V dc 0-20-kW emergency supply over a 12:1 3000-36000-r/min generator shaft speed range. The methodologies behind the machine design and operation are described together with test results taken from a prototype generator system.

Ultralow Power, Fully Autonomous Boost Rectifier for Electromagnetic Energy Harvesters

In this paper, a complete power conditioning system for a vibration energy harvester is presented that operates at ultralow power levels. The power conditioning system, implemented with discrete components, is self-starting and fully autonomous, and based upon a full-wave boost rectifier topology. The design utilizes the stray inductance of the harvesters coil, eliminating the need for separate inductors, and employs open-loop control that reduces the quiescent power overhead to just 21 μW, while still extracting 84% of the maximum available power from the harvester. The design of the subsystems, including self-start circuitry, is described in detail, and it is shown that careful active device selection is required to minimize losses. It is experimentally demonstrated that the power converter achieves conversion efficiencies of up to 76% at submilliwatt power levels, including quiescent losses. The overall system efficiency peaks at 65% at 0.9 mW, while still achieving 51% at 200 μW. The ability of this system to operate efficiently at ultralow average power levels opens up new possibilities to further miniaturize vibration harvesters and deploy them into environments with lower vibration levels than is currently possible.

Control-based continuation for investigating nonlinear experiments

In this paper we present a systematic experimental study of two one-degree-of-freedom nonlinear devices using the newly introduced control-based continuation method of Sieber and Krauskopf. By considering hardening, softening and bistable spring characteristics, we demonstrate the versatility and power of the control-based continuation method for investigating nonlinear experiments. We show that, using this method, it is possible to track the stable orbits of the devices through a saddle-node bifurcation (fold) where they lose stability and continue them up to the resonance peak where they undergo a second saddle-node bifurcation. For the bistable case, a bifurcation diagram is produced that is strongly reminiscent of the bifurcation diagram produced using the classical harmonic balance solution. A detailed introduction to general continuation methods is included to enable implementation by other experimentalists.

Numerical Continuation in a Physical Experiment: Investigation of a Nonlinear Energy Harvester

In this paper we demonstrate the use of numerical continuation within a physical experiment: a nonlinear energy harvester, which is used to convert vibrational energy into usable electrical energy. To continue a branch of periodic orbits through a saddle-node bifurcation and along the associated branch of unstable periodic orbits, a modified time-delay controller is used. At each step in the continuation the pseudo-arclength equation is appended to a set of equations that ensure that the controller is non-invasive. The resulting nonlinear system is solved using a quasi-Newton iteration, where each evaluation of the nonlinear system requires changing the excitation parameters of the experiment and measuring the response. We present the continuation results for the energy harvester in a number of different configurations.Copyright

On the performance of a dual-mode non-linear vibration energy harvesting device

The research trend for harvesting energy from the ambient vibration sources has moved from using a linear resonant generator to a non-linear generator in order to improve on the performance of a linear generator; for example, the relatively small bandwidth, intolerance to mistune and the suitability of the device for low-frequency applications. This article presents experimental results to illustrate the dynamic behaviour of a dual-mode non-linear energy-harvesting device operating in hardening and bi-stable modes under harmonic excitation. The device is able to change from one mode to another by altering the negative magnetic stiffness by adjusting the separation gap between the magnets and the iron core. Results for the device operating in both modes are presented. They show that there is a larger bandwidth for the device operating in the hardening mode compared to the equivalent linear device. However, the maximum power transfer theory is less applicable for the hardening mode due to occurrence of the maximum power at different frequencies, which depends on the non-linearity and the damping in the system. The results for the bi-stable mode show that the device is insensitive to a range of excitation frequencies depending upon the input level, damping and non-linearity.

Energy Harvesting From Vibrations With a Nonlinear Oscillator

In this paper we present a nonlinear electromagnetic energy harvesting device that has a broadly resonant response. The nonlinearity is generated by a particular arrangement of magnets in conjunction with an iron-cored stator. We show the resonant response of the system to both pure-tone excitation and narrow-band random excitation. In addition to the primary resonance, the super-harmonic resonances of the harvester are also investigated and we show that the corresponding mechanical up-conversion of the excitation frequency may be useful for energy harvesting. The harvester is modeled using a Duffing-type equation and the results compared to the experimental data.Copyright

Analysis of Semipermeable Containment Sleeve Technology for High-Speed Permanent Magnet Machines

This paper presents an alternative permanent magnet rotor containment technology. The use of a semipermeable material in a laminated structure is shown to result in a significant improvement in magnetic loading for a given thickness of containment compared to the use of magnetically inert materials such as carbon fiber or Inconel, while minimizing eddy current losses in the containment layer. An analytical model is presented for determining the air-gap field delivered from a permanent magnet array with a semipermeable containment. The analysis is validated through finite element analysis. The fabrication of a prototype machine is detailed and the results presented show good agreement with the analysis. The validated modeling process is used to assess the potential of the new technology.

Maximum Power Transfer Tracking for Ultralow-Power Electromagnetic Energy Harvesters

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 600 mV rms ac output at 870 μW under 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 V dc output, the total current consumption of the combined ancillary and control circuits is just 22 μA. The power conditioning system is capable of transferring up to 70% of the potentially extractable power to the energy storage.

Switched-Mode Load Impedance Synthesis to Parametrically Tune Electromagnetic Vibration Energy Harvesters

Energy harvesters based upon resonant mass-spring-damper systems can only generate useful power over a narrow range of excitation frequencies. This is a significant limitation in applications where the vibration source frequency changes over time. In this paper, an active electrical load is presented which can overcome the bandwidth limitations by parametrically tuning the overall harvester system. The electrical tuning technique synthesizes an optimum reactive load with high-efficiency switch-mode electronics, which also provides rectification, feeding the energy harvested into a dc store. The method is shown to be effective at increasing the power frequency bandwidth of resonant type harvesters and offers the capability of autonomous operation. The theoretical basis for the technique is presented and verified with experiment results. The paper illustrates the challenges of implementing the power electronic converter for a low-quiescent power overhead and in choosing the control architecture and tuning algorithms.