Electrostatic Energy-Harvesting And

1938 IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 56, NO. 9, SEPTEMBER 2009

Electrostatic Energy-Harvesting andBattery-Charging CMOS System PrototypeErick O. Torres, Student Member, IEEE, and Gabriel A. Rincón-Mora, Senior Member, IEEE

Abstract—The self-powering, long-lasting, and functionalfeatures of embedded wireless microsensors appeal to an ever-ex-panding application space in monitoring, control, and diagnosisfor military, commercial, industrial, space, and biomedical appli-cations. Extended operational life, however, is difficult to achievewhen power-intensive functions like telemetry draw whatever littleenergy is available from energy-storage microdevices like thin-filmlithium-ion batteries and/or microscale fuel cells. Harvesting am-bient energy overcomes this deficit by continually replenishingthe energy reservoir and indefinitely extending system lifetime.In this paper, a prototyped circuit that precharges, detects, andsynchronizes to a variable voltage-constrained capacitor verifiesexperimentally that harvesting energy electrostatically from vi-brations is possible. Experimental results show that, on average(excluding gate-drive and control losses), the system harvests 9.7nJ/cycle by investing 1.7 nJ/cycle, yielding a net energy gain ofapproximately 8 nJ/cycle at an average of 1.6 W (in typicalapplications) for every 200 pF variation. Projecting and includingreasonable gate-drive and controller losses reduces the net energygain to 6.9 nJ/cycle at 1.38 W.

Index Terms—Batteries, energy harvesting, microsensors, self-powered, self-sustaining, vibration energy harvester.

I. ELECTROSTATIC ENERGY HARVESTING

S ELF-POWERED microsystems, such as wireless trans-ceiver microsensors [1], biomedical implants [2]–[4],

military monitoring devices [5], and structure-embeddedinstrumentation [6], support power-hungry functions liketransmission and data conversion from miniaturized sources.This combination, when conformed to microscale dimensions,results in limited and finite operational lifetimes, becausemicroscale energy-storage devices cannot store sufficient en-ergy to sustain various system tasks for long. State-of-the-artthin-film lithium-ion batteries (Li Ion) [7] and direct-methanolproton-exchange membrane (DM-PEM) fuel cells [8] exhibitpromising but finite energy densities that, when coupled withimproved power-efficient designs, low duty-cycle multiplexing,and smart power-aware network protocols [1], help extend life,but only as much as volume allows.

Scavenging ambient energy overcomes this space constraintby restocking the system with energy in situ, within the device,

Manuscript received July 02, 2008; revised September 30, 2008. First pub-lished December 22, 2008; current version published August 27, 2009. Thiswork is supported by the Texas Instruments Analog Fellowship Program. Thispaper was recommended by Associate Editor P. K. T. Mok.

The authors are with the Georgia Tech Analog, Power, and Energy IC Re-search Laboratory, School of Electrical and Computer Engineering, GeorgiaInstitute of Technology, Atlanta, GA 30332-0250 USA (e-mail: [email protected]; [email protected]).

Digital Object Identifier 10.1109/TCSI.2008.2011578

Fig. 1. System-in-package (SiP) wireless microsensor system.

as shown in Fig. 1, from energy that would otherwise be lost[9]–[12]. Concurrently, scavenging and consuming energy canextend operational lifetime indefinitely by automatically replen-ishing what is lost, outperforming both state-of-the-art Li Ionand DM-PEM fuel-cell technologies, and any hybrid combi-nation thereof. Harvesting, however, requires energy and pro-ducing a net gain (i.e., harvested energy minus energy requiredto scavenge) in a practical system is challenging, and researchin this area is still in its infancy.

Nevertheless, of the available energy sources, which in-clude light [13], [14], thermal gradients [15]–[17], and motion[18]–[21], the latter, in the form of vibrations, is arguablymost abundant, stable, and predictable in practical applications.While converting and conditioning this energy to electricalpower is possible by harnessing the damping forces producedby magnetic fields [22]–[26], electric fields [27], [28], andstrain on piezoelectric materials [29]–[34], electrostatic meansare probably most compatible with CMOS integration, becausethe harvesting device is a relatively simple variable plate-dis-tance capacitor built with standard microelectromechanicalsystems technologies.

Fundamentally, electrostatic harvesters harness the workambient vibrations exert on the electrostatic force of a variablecapacitor (i.e., varactor). In more physical terms, vibrationscause the gap distance and/or overlap area of a parallel-platecapacitor to vary [11] with a net effect, under constantcharge or voltage conditions, of producing electrical energy[27]. When constraining charge by keeping the capacitoropen circuited, voltage increases with decreasing capacitance

, increasing the potential energystored in the capacitor; the increasing squared effects of voltageon energy offset the decreasing linear effects of capacitance(i.e., ). Similarly, by constraining voltage,the mechanical energy moving the capacitor plates drives

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TORRES AND RINCÓN-MORA: BATTERY-CHARGING CMOS SYSTEM PROTOTYPE 1939

charge out of the capacitor, yielding a net harvesting current

(1)

The maximum voltages charge-constrained systems produce,however, surpass the breakdown limits of most modern CMOStechnologies by a considerable margin. A 1–200 pF varia-tion, for instance, amplifies the initial voltage acrossby a factor of 200, by its maximum–minimum capacitanceratio [35]–[38], producing peak voltages of roughly 25–200V from inputs as low as 125 mV to 1 V. More costly andspecialized technologies, such as silicon-on-insulator CMOSprocesses [37], [38], can sustain these voltage extremes, buttheir increased costs limit the extent to which the market willadopt them, especially in wireless microsensors where volumeproduction and low cost are driving factors.

Constraining the voltage may keep voltage excursions withintolerable levels but also requires an additional voltage source[11], [27], which conflicts with integration. Another possibilityis to embed a material that has permanent charge separation ina dielectric and thus a constant voltage, as in the case of elec-trets and charged electrodes [39]–[42], except they require ei-ther a complicated assembly process to integrate two separatesubstrates, one being the electret, or additional fabrication se-quences to charge the material via electron tunneling. Refer-ences [43] and [44] use a storage capacitor to constrain voltage,but the capacitor is not a true low-impedance source; so itsvoltage changes, which is why the capacitor undergoes a charge-constrained phase that leaves otherwise useful energy unhar-vested in the variable capacitor, as only a small fraction of thefull capacitance variation is now harvested.

In the proposed system, the voltage across the capacitor isheld constant by the already-existing energy-storage device (i.e.,the rechargeable battery), the one ultimately receiving the har-vested energy. In this way, the harvester avoids the use of addi-tional voltage sources, as mentioned in [11] and [27]. The keycontribution of this paper is how the proposed and prototypedcircuit (which precharges, detects, and synchronizes to a vari-able voltage-constrained capacitor) verifies experimentally thatharvesting energy electrostatically from vibrations is possible.Sections II and III describe the basic concept, design, and im-plementation of the proposed scheme. Section IV shows experi-mental results, and Section V discusses the impact and meaningof the results. Section VI then draws relevant conclusions.

II. VOLTAGE-CONSTRAINED ENERGY-HARVESTING SCHEME

The proposed voltage-constrained energy-harvesting systemfeatures a battery that both clamps the variable capacitor voltageand stores the harvested energy [45]. The process operates inthree separate steps, as illustrated in Fig. 2. First, the batteryinvests an initial amount of energy to precharge the capacitorto the battery voltage when its capacitance is highest .This investment constitutes an energy loss in the system, whichassuming no other losses exist in the transfer, is

(2)

Fig. 2. Voltage-constrained energy-harvesting steps: (a) precharge, (b) harvest,and (c) reset.

Mechanical forces from ambient vibrations then work againstthe capacitor’s established electrostatic force and cause capaci-tance to decrease, converting mechanical energy to elec-trical in the process. The converted mechanical energy plus theelectrical energy removed from the capacitor (as charge is drivenout of ) charge the battery in the form of , whichnow produces a harvest of

(3)

Ideally, once minimum capacitance is reached, thesame precharge block transfers the energy that remains back tothe battery, effectively resulting in another gain of

(4)

and yielding a net gain for the system of

(5)

The process of transferring energy, however, is not losslessand the system therefore loses some energy in all three steps,which is one of the fundamental challenges of harvesters. In fact,if the energy available for recovery is less thanthe energy losses incurred during the recovery process, it is moreefficient to omit the step altogether, as in the foregoing system.To be more specific, after harvesting, is left open cir-cuited, in other words, under charge-constrained conditions. Asa result, as vibrations force to increase again, its voltagenaturally resets (i.e., decreases) to a substantially lower voltage



Fig. 3. Proposed energy-harvesting and battery-charging system.

(near zero), that is, to times minimum–maximum capac-itance ratio . Nevertheless, the energy harvestedstill exceeds the investment, leaving a theoretical gain of

- (6)

where energy-transfer losses account for lower values.

III. PROPOSED ENERGY-HARVESTING SYSTEM

A. Topology

Since inductors are quasi-lossless devices, to maximize en-ergy gain, an inductor-based precharger that transfers energyfrom the battery to variable capacitor , as seen in Fig. 3, isimplemented. Inductor L is first energized by imposing batteryvoltage across it with switches and . Inductor cur-rent consequently increases linearly until sufficient energy isstored, at which point switches and open and andclose and channel the stored energy to . This prechargestep occurs at maximum capacitance, just before the onset ofthe harvesting phase.

Once is precharged, as capacitance decreases, batteryclamps capacitor voltage via either a syn-

chronous switch or an asynchronous diode and the resultingharvesting current charges the battery. Note that, while a syn-chronous switch may dissipate lower Ohmic losses (because thevoltage across its terminals is low), the energy used in the addi-tional circuitry required to synchronize it (i.e., prevent reversecurrent flow) offsets some, if not all, of those gains. A diode, onthe other hand, naturally conducts current only in one direction(towards the battery) so it only dissipates Ohmic losses, which,given the current levels, are substantially low to begin with. Asa result, considering the power, risk, and complexity associatedwith the synchronous switch, the asynchronous diode offers amore appealing proposition.

The precharge control block illustrated in Fig. 3 senseswhen to precharge (at maximum capacitance) andapplies the proper gate-drive signal configuration for a pre-determined inductor-energizing time . To

determine when to precharge , instead of sensing capac-itance directly, which might require ac currents or voltages,the precharger detects when capacitance reachesand starts to decrease by monitoring the capacitor voltage

. Since is charge constrained (i.e., open cir-cuit: ) during its reset phase,

increases as soon as starts to decrease fromits maximum value of . At this point, if is lessthan , precharge commences, and because the prechargetime is on the order of nanoseconds and therefore substantiallyshorter than the vibration period, which is in milliseconds,capacitance remains close to its maximum value. Note thatdetecting the state of also conveys motion informationso the system can also double as a vibration sensor.

Precharge time is preset to energize the in-ductor with sufficient energy to subsequently charge the capac-itor to . During this time, inductor current increases lin-early to a maximum value of

(7)

As a result, the amount of energy transferred to inductor L is

(8)

and equating this to the invested energy required to precharge( or ), as derived in (2), yields a

precharge time of

(9)

which is independent of , assuming quasi-lossless energytransfers. Consequently, even as changes (Li Ion spans2.7–4.2 V across its state of charge), a constanttransfers sufficient energy to . In practice,is set slightly higher to offset the energy losses associated withthe transfer.

Precharge ends as soon as capacitor voltage equalsor surpasses battery voltage , when all switches turn OFF.Excess energy in the inductor returns to the battery through theharvesting diode by charging a diode voltage aboveand subsequently transferring the remainder through the nowforward-biased diode. It is best to minimize this extra energyto reduce the losses associated with transferring energy throughthe system.

B. Circuitry

The proposed energy-harvesting power-train circuit, whichis comprised of the harvesting diode and precharge CMOSswitches , and transmission gate ,as illustrated in Fig. 4, was fabricated with the 1.5 m CMOSprocess technology available from AMI Semiconductorfoundry. The control electronics were kept off chip for experi-mental flexibility and ease of reach.

The system energizes inductor L when transistors andare ON, and subsequently channels the stored energy to

variable capacitor when and transmission gateconduct, after and are OFF. Comple-

mentary switches realize switch because



Fig. 4. Complete energy-harvesting and battery-charging system prototype implementation (dimensions in �m).

alone lacks gate drive to conduct enough current when the ca-pacitor is initially discharged. Individual on-chip buffers driveeach power switch, except for , which shares its drivingsignal with . The harvesting diode connecting the capac-itor to the battery is a diode-connected NPN transistor—AMI’s1.5 m CMOS technology offers vertical n-type BJTs.

To detect the state of , and thus determine when toprecharge it, an off-chip control circuit is used. Comparator

detects the first condition required to start theprecharge process, which is to ascertain when fallsbelow . The propagation delay of this comparator shouldbe sufficiently short to ensure the precharge phase stops before

charges above its target value , which could other-wise incur additional losses in the system. Slope detecting com-parator detects whether or not the second conditionis met, that rises (when starts to decrease fromits maximum value of ), by comparing with itsprevious state , a delayed version of itself. Thatway, if increases (or decreases), is lower(or higher) and therefore asserts an enabling (or dis-abling) signal. A 5 M and 4.7 nF RC circuit implements adelay of approximately 20 ms and buffer isolatesand decouples from the RC circuit. The comparators in-clude some hysteresis to desensitize the circuit to glitches andany other extraneous noise present.

When the aforementioned conditions are met, on-chip logiccontrols the precharge switching sequence, including dead timebetween oppositely phased digital signals to avoid transientshoot-through (short-circuit) power losses in the prechargerswitches. Logic gate AND enables a timer with signaland starts the energizing process of inductor L via signal

, while is low.After the timer reaches its preset value, it flags the logic to

stop energizing L with signal , which forcesto drop and, after a dead-time delay, prompts the circuit to

Fig. 5. Energizing timer-circuit schematic (dimensions in �m).

de-energize L via , when it goes high (all switchesare OFF during dead time). The combined propagation delayof gates , and NOR determines this dead time.The de-energizing switches remain ON until the first conditionis no longer true, when is again greater than , atwhich point the logic disables the timer and shuts off all MOSswitches.

The energizing time of the inductor is set with the timercircuit shown in Fig. 5. Once reset and enabled by the logicblock (i.e., is turned off), the circuit triggers acut-off signal when comparator senses that linearlyincreasing ramp voltage surpasses preset referencevoltage , the latter of which effectively sets thetotal energizing time for the inductor. Charging on-chip capac-itor with a constant current-source reference produces

. The current source is realized by forcing referencevoltage across resistance R via the negativefeedback loop comprised of op-amp and transistor

. This reference current is subsequently mirrored and



Fig. 6. Die photograph of the 2.2 mm � 2.2 mm 1.5 �m CMOS energy-har-vesting and battery-charging system prototype.

amplified by a factor of five (i.e., ) before channeling itto

(10)

linearly charging with a slope of

(11)

and defining to

(12)

In practice, parasitic capacitors in parallel to slow therising ramp rate so should be lower.

IV. EXPERIMENTAL RESULTS AND EVALUATION

The power switches, gate drivers, digital logic, diode, andpart of the timer were fabricated on 1.4 mm 1.8 mm of the2.2 mm 2.2 mm die shown in Fig. 6 using AMI’s 1.5 mCMOS process. A 3 V supply emulated a moderately chargedLi Ion ( , whose full range normally spans 2.7–4.2 V.The off-chip surface-mount 4 mm 4 mm 2 mm inductorpackage used had an inductance of roughly 10.72 H with anequivalent series resistance (ESR) of 240 m . Manually turninga trimmer capacitor with a maximum–minimum capacitancerange of approximately 250–60 pF, including parasitic capac-itances present (measured), emulated the harvesting deviceunder vibration conditions. Although continually turning themanual capacitor to charge a battery would have been ideal,the process was impractical because of its nonperiodic natureand the human element of fatigue; however, the objective of thesetup was to test the viability of the harvesting scheme on a percycle basis, not its steady-state behavior, which is the subjectof further research.

A. Harvest With Direct Precharge

To prove harvesting is possible by constraining voltage, thecapacitor is manually precharged and decreased. Momentarilyshorting the variable capacitor to the supply after setting it toits maximum capacitance, manually precharges (i.e., prepares)

the device. The capacitor is subsequently decreased manuallyand its resulting current recorded. Turning the capacitor, how-ever, is a manual process and the device’s response is conse-quently nonlinear, introducing what appears to be noise. The fi-nite delay between the initialization process and actually turningthe device further introduces inaccuracies, allowing parasiticdrain currents to discharge the capacitor slightly from its ini-tial value. Nonetheless, Fig. 7 shows the harvesting currents thevariable capacitor drive through the diode to the battery supplyfor two different sets of measurements. Integrating the powerharvested, which is the product of the measured current and bat-tery voltage , over the cycle time yielded 6.11 and 6.37 nJfor the results shown and an average of 5.82 nJ across eight sep-arate measurements.

B. Precharge

The precharger was then tested by allowing the system to (1)detect the conditions necessary for a precharge cycle, (2) initiatethe sequence, and (3) charge variable capacitor to batteryvoltage . To this end, was set at its maximumcapacitance point and turned. Fig. 8 shows how both conditionsfor precharge are detected. First, during the reset phase, beforeprecharge, is high because is less than orequal to . As soon as begins to increase, whichindicates that (under charge-constrained conditions)starts to decrease from its maximum value ofswitches to a high state because exceeds its delayedversion , which means the precharge sequenceinitiates.

Fig. 9(a) shows control signals and whensubjected to this test, the former of which instructs the systemto energize inductor L and the latter to release its stored energyto . As L is energized, (3 V) is impressed acrossL [inductor voltage is shown in Fig. 9(b)], forcing an in-ductor current of 17.61 mA (on average) and resulting in an av-erage invested energy per cycle of 1.66 nJ. This energy levelis greater than what is required by because it includestransfer losses that must be surmounted, as analyzed and derivedin [45]. When de-energizing L, is reversed by connecting Lto , gradually releasing energy to and consequentlyincreasing its voltage .

While ’s drain capacitance is completely drained at theend of precharge, just after all switches turn off, remnant (ex-cess) energy in L and ’s charged drain capacitance shiftsback and forth (i.e., resonates) between L and both drain capaci-tances. LC oscillations therefore result until parasitic resistancespresent eventually dampen them completely. This excess energyrepresents an over-investment on the part of the precharge cir-cuit. For context, consider that 10 pF of parasitic capacitanceproduces the oscillations measured when 45 pJ of excess energyis available at 3 V, which means excess energy constitutes onlya small fraction of the average invested energy of 1.66 nJ/cycle.

C. System Test: Precharge and Harvest

After determining the precharger was functional, full system-level experiments were performed, verifying precharge and har-



Fig. 7. (a)-(b) Sample harvest measurements by directly precharging variable capacitor � .

vest automatically cycled with variations in , as designedand shown experimentally in Fig. 10. The average energy percycle directed to the battery over 126 different sets of measure-ments was 9.7 nJ/cycle (Fig. 10(a) is a sample run), giving a netenergy gain (by subtracting the investment from the har-vest) of approximately 8 nJ/cycle. Fig. 10(b) illustrates the re-sults of continually turning the trim capacitor for six consecutivecycles, harvesting a total of 63 nJ over the span of the six cyclesshown (not subtracting the corresponding investment energy).

V. DISCUSSION

A few comments on the results are worth mentioning at thispoint. (1) Because there is no manual (i.e., artificial) delaybetween the precharge phase and capacitor decreasing,more average energy per cycle is harvested than under the direct(i.e., manual) precharge method. (2) Average power in thesemeasurements depends on the vibration frequency (cycles persecond ) of the system and the mechanical design of thevariable capacitor . Considering manyapplications exhibit accelerations in the 1–500 Hz frequency

range, such as a person tapping heels at 1 Hz and a car enginevibrating at 200 Hz [20], the proposed system, when subjectedto vibrations of 200 Hz, could gain 1.6 J every second, that is,1.6 W of average power. (3) Manually turning the trimmingcapacitor is considerably slower than 200 Hz, demanding animpractically large delay that dissipates moreenergy than in actual applications. So, for instance, 12.5 pF and20 M would yield a delay nearing 250 s (i.e., 5 % of thetotal 200 Hz cycle) and require only about 112.5 pJ/cycle. (4)Gate drivers and controller circuit must be optimized for lowenergy.

Gate-Drive and Controller Losses: Since parts of theprecharger could not be optimized for low energy because theirphysical parameters (such as transistor aspect ratios and circuitconfiguration) were preset, they were powered from a separatesupply so that unreasonable power requirements wouldnot otherwise mislead the experimental results obtained. Un-derstanding the impact these requirements have on the energyharvested, however, is nonetheless important. To this end,multiplying the gate-oxide capacitance per unit area



Fig. 8. Detection of precharge conditions: (a) variable capacitor voltage � increases (its delayed version � is lower) and (b) the state detectoroutputs � and � .

of the process (e.g., 1.12 fF/ m ) by the total gate area of thepower switches (Fig. 4) indicates gate-drive losses from a 3 Vsupply are approximately 401.3 pJ/cycle. Similarly, three- andfour-stage gate drivers require roughly 266 pJ/cycle to driveall power transistors. Collectively, gate-drive and driver lossessum to 667.3 pJ/cycle, which means the projected net energygain of the harvester reduces from 8 to 7.33 nJ/cycle.

Reducing the controller’s quiescent energy to pJ levels percycle is possible by resizing and biasing transistors to operatein subthreshold (given high-speed circuits are not necessary toprocess low vibration frequencies) and leveraging and duty-cy-cling already-existing circuit blocks (i.e., operate componentsonly for short spurts and duel them for other functions). Com-parators and (Fig. 4), for one, can bebiased with 5 nA each, dissipating about 150 pJ/cycle at 200 Hzcycles. Precharge, for another, occurs only within a small frac-tion of the entire cycle (e.g., 200 ns of the 5 ms period, assuminga vibration frequency of 200 Hz) so the components used to con-trol it can be disabled during the remainder of the period (e.g.,

OFF for 4.9998 ms of 5 ms). Biasing comparator andamplifier with 25 A each from a 3 V supply onlydraws 30 pJ/cycle when operating for 200 ns of the 5 ms period.The 2.4 mA that flows into the timer circuit (Fig. 5) would dis-sipate about 1.44 nJ/cycle when limited to 200 ns of the 5 msperiod. More importantly, however, further reducing its energyis possible by decreasing to, say, 5 pF and its chargingcurrent to 200 A, reducing 2.4 mA and 1.44 nJ/cycleto 240 A and 144 pJ/cycle. In the end, when considering allmeasured and projected power losses, the proposed converter’snet energy gain can be 6.9 nJ/cycle at 200 Hz.

Battery: The battery’s ESR also dissipates conduction powerduring precharge. Just to cite an example, a commercially avail-able Li-Ion polymer battery (from PowerStream PGE014461)offers 200 mAh of capacity with an ESR of 180 m , which ison the same order as the inductor’s ESR and therefore its im-pact on efficiency is, for all practical purposes, negligible. Theonly other possible loss associated with the battery can be theelectronics used to monitor the Li Ion’s state of charge during



Fig. 9. Precharge waveforms: (a) inductor energizing and de-energizing control signals � and � with corresponding variable capacitor voltage� and (b) inductor voltage � .

the charging process. Fortunately, most of the circuit can be dis-abled, except for a slow and relatively inaccurate voltage de-tector whose purpose is to engage the rest of the circuit whenthe Li Ion voltage is near 4 V. In other words, efficiency remainsunchanged and decreases only when the battery is near its fullycharged state, at which point the system’s need for energy is lessacute.

Voltage-Constrained Harvesters: Unlike in charge-con-strained schemes, voltage-constraining harvesting capacitor

protects the circuit from voltages that exceed thebreakdown limits of standard CMOS technologies [37], [38].Additionally, voltage-constraining with the already-ex-isting battery that is to be charged enhances integration becauseno additional source is required. One drawback is that theenergy harvested is proportional to the battery’s voltage, whichmeans less energy is harvested at lower battery voltages. Whatis more, preferably, the constraining voltage should vary tomatch the harvesting electrostatic force with other mechanicaldamping forces to achieve optimum energy conversion from

vibrations [19], [20]. The precharger and control circuitrycould viably boost this voltage up to the maximum allowedprocess voltage, regardless of battery conditions, except doingso increases complexity and controller losses.

Summary: The purpose of this paper (and contribution) isto show experimentally that the proposed prototyped circuit isable to draw energy from a variable voltage-constrained capac-itor. Although the controller and switches were not optimizedin their present form for power efficiency because function-ality was more important, projections show that a net energygain (after considering all losses) is possible. Note an impor-tant feature (and contribution) of the proposed solution is alsoits ability to automatically detect when to precharge the capac-itor without having to sense or measure capacitance or accu-rately synchronize the system to capacitor variations. Finally,as mentioned earlier in the paper, given the innate nature of theharvester, the presented prototype also doubles as a vibrationsensor, increasing the functional efficiency and the packing den-sity of the final solution.



Fig. 10. System-level measurements showing variable capacitor � voltage � , harvesting current � , and extrapolated energy profile �during (a) one complete and (b) six maximum–minimum–maximum � cycles.

VI. CONCLUSION

The prototyped circuit (which precharges, detects, and syn-chronizes to a variable voltage-constrained capacitor) and ac-companying experimental results presented show that the pro-posed energy-scavenging, battery-charging system harvests 9.7nJ per cycle from a 200 pF capacitance variation, in other words,that harvesting energy electrostatically from vibrations is pos-sible. The solution requires an investment of 1.7 nJ/cycle andresults in a net energy gain of approximately 8 nJ/cycle with1.6 W at 200 Hz, excluding gate-drive or control-circuit losses(portions of which could not be optimized because they werepreset). Projecting and including practical values for these latterlosses, reduces the net energy gain to 6.9 nJ/cycle.

The underlying feature of the proposed system is using thebattery receiving the energy as both the voltage-constrainingdevice and precharge source. The driving technology is thecircuit, which allows the variable capacitor to drive charge back

to the battery when capacitance decreases, effectively synchro-nizing the circuit to the capacitance variations that result inresponse to ambient vibrations. These results prove that themechanical energy in vibrations, which are naturally abundantin many practical applications, can be scavenged and channeledto a rechargeable energy-storage device (e.g., a battery). Byapplying low-power and duty-cycling techniques to the loadingsystem so that it demands low power, the harvested energycan viably replenish the total energy consumed by the system,potentially extending its operational life indefinitely withoutmanual/external recharge or battery-replacement cycles, whichmay be otherwise prohibitive in applications like remote wire-less microsensors and bioimplantable devices.

ACKNOWLEDGMENT

The authors would like to thank Texas Instruments for theirsponsorship and support of this research.



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Erick O. Torres (S’05) was born and raised in SanJuan, Puerto Rico. He received the B.S. degree fromthe University of Central Florida, Orlando, FL, theM.S.E.E. degree from the Georgia Institute of Tech-nology, Atlanta, GA, in 2003 and 2006, respectively,and also where currently he is working toward thePh.D. degree, all in electrical engineering.

During a six-month co-op assignment in 2006,he worked as a circuit design engineer with TexasInstrument’s Mixed-Signal Automotive group de-signing several analog circuit blocks for various

integrated power control unit projects. Currently, he is a Research Assistantat the Georgia Tech Analog, Power, & Energy IC Design Lab, Georgia Insti-tute of Technology. He has been awarded several fellowships, including theGoizueta Foundation Fellowship, Georgia Tech President’s Fellowship, andTexas Instrument’s Analog Fellowship. His recent research interests includelow-energy analog and power IC design for energy harvesters in microscaleself-sustaining systems.

Gabriel A. Rincón-Mora (S’91–M’97–SM’01)received the B.S., M.S., and Ph.D. degrees in elec-trical engineering.

He worked for Texas Instruments in 1994–2003,was appointed Adjunct Professor for Georgia Tech,Atlanta, GA, in 1999–2001, and became a full-timefaculty member in 2001. He has authored or coau-thored five books and one book chapter, 26 patents,over 100 scientific publications, and 26 commercialpower management chip designs.

Dr. Rincón-Mora is an Associate Editor for theIEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS since2007; Circuit Design Vice Chair for IEEE’s 2008 7th International CaribbeanConference on Devices, Circuits and Systems (ICCDCS); Chairman of At-lanta’s joint IEEE Solid-State Circuits Society (SSCS) and Circuits and SystemsSociety (CASS) since 2005; member of IEEE CAS-S Analog Signal Processing(ASP) Technical Committee since 2003; Steering Committee Member for IEEEMidwest Symposium of Circuits and Systems (MWSCAS) since 2006; Tech-nical Program Chair for IEEE 2007 Joint MWSCAS-NEWCAS in Montreal;Technical Program Co-Chair for IEEE’s 2006 MWSCAS in Puerto Rico; ViceChairman of Atlanta’s IEEE SSCS-CASS in 2004; and Selection CommitteeReview Panelist for the National Science Foundation (NSF) for 2003–2007. Heis a Life Member of the Society of Hispanic Professional Engineers (SHPE),and a Professional Member of IET, U.K. He is also a member of Eta KappaNu, Phi Kappa Phi, and a Life Member of Tau Beta Pi.


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