David P. Arnold

Review of Microscale Magnetic Power Generation

This paper discusses the history, current state of the art, and ongoing challenges for compact (less than a few cubic centimeters) magnetic power generation systems in the microwatts to tens of watts power range. These systems are of great interest for powering sensor networks, robotics, wireless communication systems, and other portable electronics. The paper considers the following topics. 1) The theoretical and practical implications of miniaturizing magnetic power generators. 2) The design and performance of previously demonstrated devices, which are summarized and compared. 3) Ongoing challenges for implementation, including integrated high-performance hard magnetic materials, microscale core laminations, low-friction bearings, high-speed rotor dynamics, and compact, high-efficiency power converters.

Permanent Magnets for MEMS

This paper reviews the state of the art for the microfabrication of permanent magnets applicable to microelectromechanical systems (MEMS). Permanent magnets are a key building block for the realization of magnetically based MEMS sensors, actuators, and energy converters. In this paper, the basic theories and operational concepts for permanent magnets are first described. Then, different classes of permanent-magnet materials and associated performance tradeoffs are introduced. Challenges relating to the integration of permanent magnets into MEMS applications are then discussed. Last, a summary and review of previously reported fabrication strategies and material properties is provided.

Spherical, rolling magnet generators for passive energy harvesting from human motion

In this work, non-resonant, vibrational energy harvester architectures intended for human-motion energy scavenging are researched. The basic design employs a spherical, unidirectionally magnetized permanent magnet (NdFeB) ball that is allowed to move arbitrarily in a spherical cavity wrapped with copper coil windings. As the ball rotates and translates within the cage, the time-varying magnetic flux induces a voltage in the coil according to Faradays Law. Devices ranging from 1.5 cm3 to 4 cm3 in size were tested under human activity scenarios—held in the users hand or placed in the users pocket while walking (4 km h−1) and running (14.5 km h−1). These harvesters have demonstrated rms voltages ranging from ~80 mV to 700 mV and time-averaged power densities up to 0.5 mW cm−3.

An Input-Powered Vibrational Energy Harvesting Interface Circuit With Zero Standby Power

This paper presents an input-powered energy-harvesting interface circuit that eliminates standby power consumption by automatically shutting down when the ac input voltage amplitude is too low for successful energy reclamation. This feature eliminates the need for precharging the load and allows for indefinitely long intervals between energy harvesting cycles. The interface comprises two subcircuits: an input-powered ac/dc converter and an input-powered dc/dc boost converter with regulated output. The two subcircuits are separately fabricated in the ON Semi 3M-2P 0.5 μm CMOS process. The entire interface circuit starts up when the ac input amplitude exceeds 1 V and supplies a regulated dc output up to 3 V. When the input amplitude drops below 600 mV, the interface automatically enters standby mode and consumes no power. The system achieves a maximum efficiency of 60% with 1.5-V ac input amplitude and 3 V regulated dc output, delivering 3.9 mW of output power. The interface also functions properly in tests with an electrodynamic (magnetic) vibrational energy harvester.

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.

High-Inductance-Density, Air-Core, Power Inductors, and Transformers Designed for Operation at 100–500 MHz

This paper presents the microfabrication and measurement of high-inductance-density, moderate-Q, air-core inductors, and transformers intended for switch-mode power supplies operating in the 100-500 MHz frequency range. The inductors and transformers were fabricated on Pyrex substrates with four layers of electrodeposited copper with each layer up to 10 ¿m thick. Stacked winding layers allowed for mutual coupling between layers to increase areal inductance density. Inductors of various designs exhibited inductance densities of up to 100 nH/mm 2 and quality factors approaching 20 in the frequency range of interest. Transformers were formed by interleaving primary and secondary coils and were designed with increased inductance in the secondary coil for voltage gain. A fabricated transformer, 1.5 mm × 1.5 mm in area, yielded 46 nH primary inductance and 500 nH secondary inductance with a coupling coefficient of 0.63. Measurements indicated that a maximum transformer efficiency of 78% at 125 MHz would be possible.

A study of scaling and geometry effects on the forces between cuboidal and cylindrical magnets using analytical force solutions

Kelvins formula is used to calculate forces acting on a permanent magnet in the presence of an external magnetic field from a second permanent magnet. This approach is used to derive explicit analytical solutions for the axial and lateral forces between cuboidal and cylindrical permanent magnets as functions of magnet dimensions and separation. While exact solutions can be found for cuboidal magnets, a hypergeometric expansion is used to approximate the elliptic integrals in solving for the fields and forces for the cylindrical magnets. The resulting equations are applied over a range of magnet sizes and geometries to explore scaling laws and other geometrical effects. It is shown that cuboidal magnets provide larger forces than equivalently sized cylindrical magnets. Also, the aspect ratio of the magnets significantly affects the forces. These results are intended to benefit the design and optimization of sensors, actuators and systems that rely on magnetic forces, particularly at the microscale.

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.

Design optimization of an 8 W, microscale, axial-flux, permanent-magnet generator

This paper presents the design optimization and characterization of a microscale, permanent-magnet (PM) generator, capable of supplying 8 W of dc power to a resistive load at a rotational speed of 305 000 rpm. The generator is a three-phase, axial-flux, PM machine, consisting of a stator with Cu surface windings and a multi-pole SmCo PM rotor. Optimization of the machine geometries has enabled a 30% improvement in power density (for the same rotational speed) over a previously reported machine. Furthermore, these design improvements, in combination with higher rotational speeds, have enabled a >7x improvement in total output power and a net power density of 59 W cm -3 .

Wax-bonded NdFeB micromagnets for microelectromechanical systems applications

A microfabrication technique is reported for the fabrication of wax-bonded micromagnets in the 100μm–1mm range. This dimensional size is not typically achievable by thin-film deposition techniques or by conventional macroscale magnet processing, and is of interest for magnetic microelectromechanical systems applications. The magnet fabrication approach incorporates a wax powder binder to “lock” the magnetic microparticles together to improve the coercivity. In practice, a NbFeB powder and wax powder are packed into pre-etched trenches in a silicon wafer and subsequently heated, allowing the wax to melt and bond the magnetic powder. Using this method, 500×500×320μm3 magnets have been demonstrated with a coercivity of 737kA∕m and a maximum energy product of 16.6kJ∕m3.