Shuo Cheng

Modeling of magnetic vibrational energy harvesters using equivalent circuit representations

This paper develops and analyzes an equivalent circuit model of magnetic energy harvesters using reduced-order lumped element modeling (LEM) methods. This model is intended to enhance the design and analysis of a magnetic energy harvesting system by enabling direct physical insight into the system dynamics and simple circuit analysis techniques to extract all relevant performance parameters. Moreover, the model provides the ability to use circuit simulation software (e.g. PSPICE) to model the entire system in conjunction with nonlinear and/or active power electronic circuits. The circuit model is experimentally validated through electrical and mechanical measurements on a prototypical electromagnetic energy harvester.

An Active Voltage Doubling AC/DC Converter for Low-Voltage Energy Harvesting Applications

This paper theoretically and experimentally investigates an ac/dc converter for low-voltage vibrational energy harvesting systems. The circuit employs an active-diode-based voltage doubler, where the output is a dc voltage that is twice the amplitude of the input ac voltage. Analytical solutions for the steady-state open-circuit voltage are derived, capturing the effects of the active-diode comparator hysteresis. It is shown that the hysteresis plays an important role in the rectification characteristics, circuit stability, and overall efficiency. Experimentally, the circuit is able to rectify input voltage amplitude as low as 5 mV and operates over a frequency range of 1 to 500 Hz, which spans most common mechanical vibrations. For input voltage amplitudes 250 mV or higher, the circuit exhibits >;80% efficiency for a range of load resistances, delivering 0.1-10 mW of power. Additionally, the circuit successfully rectifies the voltage from a vibrational energy harvester having a highly irregular and time-varying voltage waveform.

A Voltage-Multiplying Self-Powered AC/DC Converter with 0.35-V Minimum Input Voltage for Energy Harvesting Applications

This paper demonstrates a highly efficient, low-voltage ac/dc converter using a voltage multiplier (octupler) circuit architecture intended for vibrational energy harvesting applications where a low-voltage ac waveform is used to charge a battery. The circuit employs output-powered active diodes and does not require any external power supply or startup circuitry. The circuit rectifies and boosts input ac voltages in the range of 0.35–2 V and 20–500 Hz to a dc voltage output that is ∼8 times higher than the input amplitude. The circuit can cold-start from an input voltage of 0.5 V or higher, providing an output voltage sufficient to charge a 3.7 V lithium ion battery. Once started, the circuit can maintain operation at input voltage amplitudes as low as 0.35 V. Over 80% efficiency is achieved from 20 Hz to 100 Hz, with output power ranging from a few microwatts to one milliwatt. Furthermore, in testing with an actual electrodynamic (magnetic) vibrational energy harvester that generates >0.5 V ac output, the circuit delivers power to a lithium-ion battery with an efficiency of >80%.

A study of a multi-pole magnetic generator for low-frequency vibrational energy harvesting

This paper theoretically and experimentally investigates voltage enhancement effects using a multi-pole magnet array in a magnetic energy harvester. The use of multiple magnetic poles is shown to increase the frequency and amplitude of the output voltage, addressing one of the primary disadvantages of magnetic transduction schemes, especially for low-frequency vibrations. Experimental results from a four-pole prototype demonstrate a three times improvement in rms voltage as compared to conventional single-magnet designs along with frequency up-conversion where the frequency content of the electrical output occurs at harmonics of the mechanical excitation frequency. The magnetic multi-pole approach is also further studied through simulation-based parametric analysis.

An energy harvesting system for passively generating power from human activities

This paper presents a complete, self-contained energy harvesting system composed of a magnetic energy harvester, an input-powered interface circuit and a rechargeable battery. The system converts motion from daily human activities such as walking, jogging, and cycling into usable electrical energy. By using an input-powered interface circuit, the system requires no external power supplies and features zero standby power when the input motion is too small for successful energy reclamation. When attached to a persons ankle during walking, the 100 cm3 system prototype is shown to charge a 3.7 V, 65 mAh lithium-ion polymer battery at an average power of 300 µW. The design and testing of the system under other operating conditions are presented herein.

Optimization of Permanent Magnet Assemblies Using Genetic Algorithms

This paper describes the utilization of genetic algorithms to optimize the topology of assemblies of permanent magnets. Conventional genetic algorithms are modified to employ pixel refinement to improve the computational efficiency of the optimization process. An example case is demonstrated for optimizing the figure of merit of a permanent magnet dipole. By using only four magnetization directions, a figure of merit value of 0.11 is achieved using the proposed optimization method.

Experimental Demonstration of an Electrodynamic Transformer

This paper explores a passive, two-port electromechanical device that employs electrodynamic transduction to yield transformer-like electrical behavior. The device-coined an “electrodynamic transformer” (ET)-comprises two coils that are coupled by their electrodynamic interaction with a strong, static magnet field, supplied by permanent magnets. A prototype structure is built and tested to explore the concept. Operating at 22 Hz, the proof-of-concept device provides a peak open-circuit voltage gain of 12, and a peak efficiency of 39%. The voltage gain is also electrically tunable within the range of ±5%.

Defining the coupling coefficient for electrodynamic transducers

This paper provides a simple, practical definition of the coupling coefficient for electrodynamic transducers. Comparing to efforts made in previous works that assumed a lossless spring-inductor model, the definition presented here is based on a lossy mass-inductor model. Time-harmonic analysis is used to model the energy flow in the transducer. Both energy storage and energy dissipation are included in the electrodynamic coupling coefficient definition. An in-depth discussion is provided to explain and justify the derivation and overall methodology. This definition is expected to provide a useful and practical measure of the electromechanical energy conversion performance of electrodynamic transducers, both actuators and generators.

A voltage-multiplying self-powered ac/dc converter with 0.35 V minimum input voltage for energy harvesting applications

This paper demonstrates a highly efficient, low-voltage ac/dc converter using a voltage multiplier (octupler) circuit architecture intended for vibrational energy harvesting applications where a low-voltage ac waveform is used to charge a battery. The circuit employs output-powered active diodes and does not require any external power supply or startup circuitry. The circuit rectifies and boosts input ac voltages in the range of 0.35-2 V and 20-500 Hz to a dc voltage output, that is, ~8 times higher than the input amplitude. The circuit can cold start from an input voltage of 0.5 V or higher, providing an output voltage sufficient to charge a 3.7-V lithium ion battery. Once started, the circuit can maintain operation at input voltage amplitudes as low as 0.35 V. Over 80% efficiency is achieved from 20 to 100 Hz, with output power ranging from a few microwatts to 1 mW. Furthermore, in testing with an actual electrodynamic (magnetic) vibrational energy harvester that generates >;0.5-V ac output, the circuit delivers power to a lithium ion battery with an efficiency of >;80%.