Luc G. Fréchette

Power Sources for Wireless Sensor Networks

Wireless sensor networks are poised to become a very significant enabling technology in many sectors. Already a few very low power wireless sensor platforms have entered the marketplace. Almost all of these platforms are designed to run on batteries that have a very limited lifetime. In order for wireless sensor networks to become a ubiquitous part of our environment, alternative power sources must be employed. This paper reviews many potential power sources for wireless sensor nodes. Well established power sources, such as batteries, are reviewed along with emerging technologies and currently untapped sources. Power sources are classified as energy reservoirs, power distribution methods, or power scavenging methods, which enable wireless nodes to be completely self-sustaining. Several sources capable of providing power on the order of 100 μW/cm3 for very long lifetimes are feasible. It is the authors’ opinion that no single power source will suffice for all applications, and that the choice of a power source needs to be considered on an application-by-application basis.

A Low-Power Stand-Alone Adaptive Circuit for Harvesting Energy From a Piezoelectric Micropower Generator

An adaptive energy-harvesting circuit with low power dissipation is presented and demonstrated, which is useful for efficient ac/dc voltage conversion of a piezoelectric micropower generator. The circuit operates stand-alone, and it extracts the piezoelectric strain energy independent of the load and piezoelectric parameters without using any external sensor. The circuit consists of a voltage-doubler rectifier, a step-down switching converter, and an analog controller operating with a single supply voltage in the range of 2.5-15 V. The controller uses the piezoelectric voltage as a feedback and regulates the rectified voltage to adaptively improve the extracted power. The nonscalable power dissipation of the controller unit is less than 0.05 mW, and the efficiency of the circuit is about 60% for output power levels above 0.5 mW. Experimental verifications of the circuit show the following: 1) the circuit notably increases the extracted power from a piezoelectric element compared to a simple full-bridge diode rectifier without control circuitry, and 2) the efficiency of the circuit is dominantly determined by its switching converter. The simplicity of the circuit facilitates the development of efficient piezoelectric energy harvesters for low-power applications such as wireless sensors and portable devices.

1997 Best Paper Award—Controls and Diagnostics Committee: Active Stabilization of Rotating Stall and Surge in a Transonic Single-Stage Axial Compressor

Rotating stall and surge have been stabilized in a transonic single-stage axial compressor using active feedback control. The control strategy is to sense upstream wall static pressure patterns and feed back the signal to an annular array of twelve separately modulated air injectors. At tip relative Mach numbers of 1.0 and 1.5 the control achieved 11 and 3.5 percent reductions in stalling mass flow, respectively, with injection adding 3.6 percent of the design compressor mass flow. The aerodynamic effects of the injection have also been examined. At a tip Mach number, M tip , of 1.0, the stall inception dynamics and effective active control strategies are similar to results for low-speed axial compressors. The range extension was achieved by individually damping the first and second spatial harmonics of the prestall perturbations using constant gain feedback. At a M tip of 1.5 (design rotor speed), the prestall dynamics are different than at the lower speed. Both one-dimensional (surge) and two-dimensional (rotating stall) perturbations needed to be stabilized to increase the compressor operating range. At design speed, the instability was initiated by approximately ten rotor revolutions of rotating stall followed by classic surge cycles. In accord with the results from a compressible stall inception analysis, the zeroth, first, and second spatial harmonics each include more than one lightly damped mode, which can grow into the large amplitude instability. Forced response testing identified several modes traveling up to 150 percent of rotor speed for the first three spatial harmonics; simple constant gain control cannot damp all of these modes and thus cannot stabilize the compressor a this speed. A dynamic, model-based robust controller was therefore us to stabilize the multiple modes that co prise the first three harmonic perturbations in this transonic region of operation.

High-speed microfabricated silicon turbomachinery and fluid film bearings

A single-crystal silicon micromachined air turbine supported on gas-lubricated bearings has been operated in a controlled and sustained manner at rotational speeds greater than 1 million revolutions per minute, with mechanical power levels approaching 5 W. The device is formed from a fusion bonded stack of five silicon wafers individually patterned on both sides using deep reactive ion etching (DRIE). It consists of a single stage radial inflow turbine on a 4.2-mm diameter rotor that is supported on externally pressurized hydrostatic journal and thrust bearings. This work presents the design, fabrication, and testing of the first microfabricated rotors to operate at circumferential tip speeds up to 300 m/s, on the order of conventional high performance turbomachinery. Successful operation of this device motivates the use of silicon micromachined high-speed rotating machinery for power microelectromechanical systems (MEMS) applications such as portable energy conversion, micropropulsion, and microfluidic pumping and cooling.

An electrostatic induction micromotor supported on gas-lubricated bearings

This paper reports the first successful fabrication and demonstration of an electrostatic induction micromotor supported on gas-lubricated bearings for electrical-to-mechanical energy conversion. The device consists of a stack of five (5) deep reactive ion etched (DRIE) fusion bonded silicon wafers, with an enclosed 4.2 mm diameter rotor driven by a high-voltage, high-frequency thin-film stator. Testing has demonstrated a torque of 0.3 /spl mu/Nm at a rotation rate of 15,000 revolutions per minute, corresponding to a shaft power of 0.5 mW. This development effort serves to support the creation of a wide array of power MEMS devices such as micro-scale pumps, compressors, generators, and coolers.

DESIGN OF A MICROFABRICATED RANKINE CYCLE STEAM TURBINE FOR POWER GENERATION

This paper presents the system-level and component design of a micro steam turbine power plant-on-a-chip which implements the Rankine cycle for micro power generation. The microfabricated device consists of a steam turbine that drives an integrated micropump and generator. Two-phase flow heat exchangers are also integrated on-chip with the rotating components to form a complete micro heat engine unit, converting heat to electricity. The system-level design includes cycle analysis and overall performance predictions, accounting for the expected performance of miniaturized components, thermal and structural integrity of the microsystem, and system-level trade-offs for optimal overall performance. Operating principles and design studies are also presented for the core component, with emphasis on a multistage, planar, radial microturbine and a spiral groove viscous pump. Design consideration for two-phase flow heat exchangers, microbearings, seals and micro-generators are also presented. Expected power levels range from 1–12 W per chip with energy conversion efficiency in the range of 1–11%. This suggests power density of up to 12 kW/kg for this technology, which is an order of magnitude greater than competing technologies, such as thermoelectrics. This study suggests the viability of a micro Rankine power plant-on-a-chip, but also identifies critical engineering challenges that must be met for practical implementation.Copyright

Active stabilization of rotating stall and surge in a transonic single stage axial compressor

Rotating stall and surge have been stabilized in a transonic single-stage axial compressor using active feedback control. The control strategy is to sense upstream wall static pressure patterns and feed back the signal to an annular array of twelve separately modulated air injectors. At tip relative Mach numbers of 1.0 and 1.5 the control achieved a 11% and 3.5% reduction in stalling mass flow respectively, with injection adding 3.6% of the design compressor mass flow. The aerodynamic effects of the injection have also been examined.At a tip Mach number, Mtip, of 1.0, the stall inception dynamics and effective active control strategies are similar to results for low-speed axial compressors. The range extension was achieved by individually damping the first and second spatial harmonics of the pre-stall perturbations using constant gain feedback.At a Mtip of 1.5 (design rotor speed), the pre-stall dynamics are different than at the lower speed. Both one-dimensional (surge) and two-dimensional (rotating stall) perturbations needed to be stabilized to increase the compressor operating range. At design speed, the instability was initiated by approximately 10 rotor revolutions of rotating stall followed by classic surge cycles. In accord with the results from a compressible stall inception analysis, the zeroth, first, and second spatial harmonics each include more than one lightly damped mode which can grow into the large amplitude instability. Forced response testing identified several modes traveling up to 150% of rotor speed for the first three spatial harmonics; simple constant gain control cannot damp all of these modes and thus cannot stabilize the compressor at this speed. A dynamic, model-based robust controller was therefore used to stabilize the multiple modes which comprise the first three harmonic perturbations in this transonic region of operation.Copyright

An electric induction micromotor

This paper presents the analysis, design, fabrication, and testing of a planar electric induction micromotor. The micromotor is a 6-phase motor with 131 pole pairs distributed on a stator having a 4 mm outer diameter. The axial air gap is 3 /spl mu/m. With a 90 V stator excitation, applied at a 300-kHz slip frequency, the motor produces a torque of 2 /spl mu/N/spl middot/m. Special attention is paid to the limitations that microfabrication places on the design of the motor.

Tailoring etch directionality in a deep reactive ion etching tool

Silicon deep reactive ion etching is a process that produces projected two-dimensional shapes due to the inability to control the direction of the energetic ions arriving at the surface of a wafer. The resulting etched profiles present sidewalls which are nominally 90° to the wafer surface. However, we have developed and demonstrated a new technique that allows us to control the angle that trenches make with respect to the wafer surface. This scheme exploits the charging of buried dielectric layers to achieve ion steering and thus control the direction of the etch. The measured angle variation was controlled between −32° and +32° with respect to a line orthogonal to the wafer surface. We report and describe this new technique to control etch angle.

Micromachined silicon structures for free-convection PEM fuel cells

This paper presents details on the design, fabrication, testing and modeling of micromachined gas diffusion media (GDM) for micro proton exchange membrane (PEM) fuel cell applications. Two-tiered mesh structures were thru-etched into silicon wafers and subsequently assembled with membrane electrode assemblies (MEAs) and tested with hydrogen fuel and ambient air as the oxidizer. These silicon structures doubled as gas diffusion layers and supports for thermally evaporated gold current collection layers that mated with commercially available MEAs of the catalyst-on-membrane variety. In general, the cell V?I performance curves approached that of conventional GDM-based free-convection cells for current densities less than 75 mA cm?2 on an iR-free basis. At higher current densities, the cells operability became less stable as product water flooded the micromesh structures as evidenced by stereoscopic images during cell operation. Single- and two-phase flow modelings of the fuel cell operating in free-convection mode were also developed and the simulations support the experimental results that water accumulation significantly reduces the maximum current density achievable for such micro fuel cells. Improved water management approaches are proposed.