Energy Scavenging for Wireless Sensor Nodes with a Focus on Vibration to Electricity Conversion by Shadrach Joseph Roundy Doctor of Philosophy in Mechanical Engineering University of California, Berkeley Professor Paul K. Wright, Chair The vast reduction in size and power consumption of CMOS circuitry has led to a large research effort based around the vision of ubiquitous networks of wireless communication nodes. As the networks, which are usually designed to run on batteries, increase in number and the devices decrease in size, the replacement of depleted batteries is not practical. Methods of scavenging ambient power for use by low power wireless electronic devices have been explored in an effort to make the wireless nodes and resulting wireless sensor networks indefinitely self-contained. After a broad survey of potential energy scavenging methods, the conversion of ambient vibrations to electricity was chosen as a method for further research. Converters based on both piezoelectric and electrostatic (capacitive) coupling were pursued. Both types of converters were carefully modeled. Designs were optimized based on the models developed within in total size constraint of 1 cm. Test results from the piezoelectric converters demonstrate power densities of about 200 μW/cm from input vibrations of 2.25 m/s at 120 Hz. Furthermore, test results matched simulated outputs very closely thus verifying the validity of the model as a basis for design. One of

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Energy Scavenging for Wireless Sensor Nodes with a Focus on Vibration-to-Electricity Conversion

Wireless power transfer (WPT) is an emerging technology with an increasing number of potential applications to transfer power from a transmitter to a mobile receiver over a relatively large air gap. However, its widespread application is hampered due to the relatively low efficiency of current Wireless power transfer (WPT) systems. This study presents a concept to maximize the efficiency as well as to increase the amount of extractable power of a WPT system operating in nonresonant operation. The proposed method is based on actively modifying the equivalent secondary-side load impedance by controlling the phase-shift of the active rectifier and its output voltage level. The presented hardware prototype represents a complete wireless charging system, including a dc–dc converter which is used to charge a battery at the output of the system. Experimental results are shown for the proposed concept in comparison to a conventional synchronous rectification approach. The presented optimization method clearly outperforms state-of-the-art solutions in terms of efficiency and extractable power.

A Wireless Charging System Applying Phase-Shift and Amplitude Control to Maximize Efficiency and Extractable Power

The present invention provides wireless charging techniques and devices for charging electronic device(s) within a closed space defined by a substantially hollow housing containing an antenna arrangement and configured to define an inner cavity for propagation of electromagnetic radiation from the antenna arrangement. The housing has an inner surface, encompassing the inner cavity, which geometry and material composition selected to define a general propagation path for the predetermined electromagnetic radiation from the antenna arrangement towards a charging zone. The inner cavity of the housing operates as a waveguide for directionally guiding the electromagnetic radiation to the charging zone and providing substantially maximal intensity of the electromagnetic radiation within the charging zone.

Wireless charging device

An electromagnetic induction power transfer apparatus (100) comprises:an inductive power transmitter (102), comprising:a power supply with an active state and an inactive state, configured to switch between the active state and the inactive state at a selectable duty cycle; and a power-transmitting inductor (102-12) coupled to the power supply; and an inductive power receiver (104), comprising:a power-receiving inductor (104-12), positioned inductively proximate the power-transmitting inductor; a programmable load (104-12); and an impedance controller (104-6) coupled to the power receiving inductor and configured to increase or decrease an electrical impedance of the inductive power receiver in response to a change in a power requirement of the programmable load.

Impedance matching for inductive power transfer systems

Apparatus and methods for envelope tracking are disclosed. In one embodiment, a power amplifier system including a power amplifier and an envelope tracker is provided. The power amplifier is configured to amplify a radio frequency (RF) signal, and the envelope tracker is configured to control a supply voltage of the power amplifier using an envelope of the RF signal. The envelope tracker includes a buck converter for generating a buck voltage from a battery voltage and a digital-to-analog conversion (DAC) module for adjusting the buck voltage based on the envelope of the RF signal to generate the supply voltage for the power amplifier.

Apparatus and methods for envelope tracking

We have developed highly sensitive electrometers and electrostatic fieldmeters (EFMs) that make use of micromechanical variable capacitors. Modulation of the input capacitance, a technique used in macroscale instruments such as the vibrating-reed electrometer and the field-mill electrostatic voltmeter (ESV), moves the detection bandwidth away from the 1/f-noise-limited regime, thus improving the signal-to-noise ratio (SNR). The variable capacitors are implemented by electrostatically driven resonators with differential actuation and sensing to reduce drive-signal feedthrough. The resonators in the electrometer utilize a balanced comb structure to implement harmonic sensing. Two fabrication methods were employed - a hybrid technology utilizing fluidically self-assembled JFETs and SOI microstructures, and an integrated process from Analog Devices combining 0.8-/spl mu/m CMOS and 6-/spl mu/m-thick polysilicon microstructures. All devices operate in ambient air at room temperature. Measured data from one electrometer with an input capacitance of 0.7 pF indicates a charge resolution of 4.5 aC rms (28 electrons) in a 0.3 Hz bandwidth. The resolution of this electrometer is unequaled by any known ambient-air-operated instrument over a wide range of source capacitances. The EFM has a resolution of 630 V/m, the best reported figure for a MEMS device.

Electrostatic charge and field sensors based on micromechanical resonators

An integrated circuit is described for providing a power supply to a radio frequency (RF) power amplifier (PA). The integrated circuit includes a low-frequency power supply path including a switching regulator and a high-frequency power supply path arranged to regulate an output voltage of a combined power supply at an output port of the integrated circuit for coupling to a load. The combined power supply is provided by the low-frequency power supply path and high-frequency power supply path. The high-frequency power supply path includes: an amplifier including a voltage feedback and arranged to drive a power supply signal on the high-frequency power supply path; and a capacitor operably coupled to the output of the amplifier and arranged to perform dc level shifting of the power supply signal.

Integrated circuit wireless communication unit and method for providing a power supply

We demonstrate a family of wireless power devices that operate in a variety of operating modes. A multimode wireless power receiver unit (PRU) allows a mobile device to charge from any one of three wireless power standards: Qi, Power Matters Alliance, or Rezence. A coil arrangement and matching network is introduced that is resonant in both 6.78-MHz and 100-kHz bands, allowing the same power conversion circuitry to operate in either mode. The multi-mode PRU converts 5 W of ac power to a regulated 5-V output with an efficiency of up to 83% in Rezence mode and up to 86% in Qi mode. A wireless charging power transfer unit (PTU) using the Rezence specification charges three such PRUs at a power level of 5 W each with a power transfer efficiency of 66%. An in-band communications protocol provides enhanced capabilities to the Rezence system. Multiple layers of encoding ensure reliable delivery of messages under difficult channel conditions. Another PTU design allows the same receiver to charge through a table of 30-mm thickness while maintaining the in-band communications link. We show that a wide variety of multimode devices are possible, enabling widespread adoption of wireless power even in the absence of a single unified standard.

Wireless Power Systems for Mobile Devices Supporting Inductive and Resonant Operating Modes

A dual-mode receiver is provided that includes an electromagnetic resonator having one or more inductive elements that are arranged to form a receiver coil and a network of passive components arranged to form a matching network. The electromagnetic resonator includes a first selective frequency defined in a low frequency range and a second selective frequency defined in a high frequency range allowing for a rectification circuit to operate in both the high frequency range and low frequency range making maximum use of active circuits.

Dual-mode wireless power receiver

High-data-rate wireless technologies such as HSUPA and LTE are power-hungry because of the fundamental correlation between data rate and transmit power. Furthermore, the high peak-to-average power ratio (PAPR) of the modulated signals causes a degradation in PA efficiency, since the supply voltage of the PA must be high enough to provide the peak output voltage without loss of linearity. Envelope tracking modulators have been proposed to improve the efficiency and linearity of transmitters. Modulators using multiple input voltages have been shown to improve efficiency, but generating multiple supplies imposes substantial overhead. Several recent works have demonstrated a hybrid modulator, combining the output current of a buck output stage with that of a linear amplifier. The hybrid modulator has the notable advantage that the low-frequency power (which constitutes the majority of power, even in high-PAPR scenarios) can be provided through the efficient buck stage. The inherently less efficient linear amplifier stage needs only to supply the high-frequency power. The efficiency of hybrid modulators degrades at moderate power levels, when the ac amplitude is much less than the supply voltage of the linear amplifier. At low power levels, the power savings afforded by the modulator do not offset its own quiescent current, resulting in a lower efficiency than can be achieved using a fixed-drain, average-power-tracking (FD-APT) supply.

Patrick Stanley Riehl

Papers

Electrostatic charge and field sensors based on micromechanical resonators

Integrated circuit wireless communication unit and method for providing a power supply

Wireless Power Systems for Mobile Devices Supporting Inductive and Resonant Operating Modes

Dual-mode wireless power receiver

An AC-coupled hybrid envelope modulator for HSUPA transmitters with 80% modulator efficiency