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1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.60

20

40

60

80

100

120

140

160

180

200

Input DC Voltage [V]

Cu

rre

nt

[µA

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DC Load Characteristic

Active Mode

40 k!

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#$!%$!!& !E&

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1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.60

20

40

60

80

100

120

140

160

180

200

Input DC Voltage [V]

Cu

rre

nt

[µA

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Active Mode

40 k!

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!30 !20 !10 0

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TABLE ISMARTHAT TAG FORWARD LINK BUDGET

PrPt

= GtGr

�λ

4πr

�2

Pt 35 dBmGt 9 dBiGr 2.15 dBiλ 33 cm

r|Pr = -8.48 dBm† 14.1 m

Pr|r = 16.5 m†† -9.8 dBm

†Observed by dipole substitution††Outdoor maximum distance

to differences in multipath between the indoor and outdoorenvironments.

V. SUMMARY AND FUTURE WORK

An initial prototype SmartHat tag and interrogator devicehave been designed and constructed. The SmartHat tag in-cludes a compact printed-circuit vee style antenna, an RF-to-DC power harvesting circuit, and a microprocessor-driven alertspeaker. The tag’s average operating power while deliveringa pulsed alert is 1.8 V at 61 µA, or 110 µW (-9.6 dBm). Itspower-up threshold when not delivering an alert is 1.8 V at≈ 10 µA. We also present a specialized interrogator deviceoperating under FCC Part 18 rules in the 902-928 MHz bandthat is mounted to a piece of construction equipment to powerand communicate with nearby SmartHats. Initial outdoor rangetesting indicates that the SmartHat tag will deliver an audioalert at distances of up to 16.46 m in unobstructed terrain whenusing an interrogator EIRP of +44 dBm, which is producedby an interrogator conducted output power of +35 dBm and ahorizontally polarized Yagi antenna of 9 dBi gain.

More thorough testing will be conducted to understandsystem performance in different environments where increasedmultipath and terrain obstacles are present, and to validate thatthe SmartHat devices will survive the expected temperature,humidity, and vibration extremes expected in a real worksite environment. The presence of severe multipath or terrainobstacles are the most likely impediments to the feasibility ofa passively powered system, and will determine whether thistype of system can achieve the high level of reliability requiredfor a safety alert device. To mitigate adverse propagationeffects, as well as improve alert reliability as a function ofhard hat pose with respect to the heavy equipment, sometype of local reservoir capacitance or a long life rechargeablebattery could be considered for future SmartHat prototypes.For example, some types of lithium pentoxide rechargeablebatteries have excellent performance over temperature andhave a low self-discharge rate. Because alerts would beinfrequent, the energy harvesting circuit would recharge theSmartHat’s reservoir capacitor or battery in order to improvealert accuracy when sufficient link margin is present to receivethe interrogator’s messages even though insufficient forward-link power is present to operate the tag in alert mode. It willbe necessary to evaluate the lifetime of any battery or reservoircapacitor chemistry under large numbers of charge/discharge

cycles and over extremes of temperature, humidity, and vibra-tion to ensure that the reliability of the alert system could bemaintained.

VI. ACKNOWLEDGMENTS

This material is based in part upon work supported by theNational Science Foundation under Grant Numbers CBET-0931924 and CMMI-0800858.

REFERENCES

[1] BodyGuard proximity warning system. Orbit Communications Pty Ltd.[Online]. Available: http://www.orbitcoms.com/BodyGuard.php

[2] W. H. Schiffbauer and G. L. Mowrey, “An environmentally robustproximity warning system for hazardous areas,” in Proceedings of the

ISA Emerging Technologies Conference 2001. Instrument, Systems,and Automation Society, 2001, pp. 1–10.

[3] ASE model 2200 proximity warning device. Allied Safety EngineeringInc. [Online]. Available: http://www.alliedsafetyeng.com/proximitywarning device.html

[4] Energizer EN91 product datasheet. Energizer Holdings Inc. [Online].Available: http://data.energizer.com/PDFs/EN91.pdf

[5] Energizer L91 product datasheet. Energizer Holdings Inc. [Online].Available: http://data.energizer.com/PDFs/l91.pdf

[6] A. Sample, D. Yeager, P. Powledge, A. Mamishev, and J. Smith, “Designof an RFID-based battery-free programmable sensing platform,” IEEE

Transactions on Instrumentation and Measurement, vol. 57, no. 11, pp.2608–2615, Nov. 2008.

[7] D. Yeager, P. Powledge, R. Prasad, D. Wetherall, and J. Smith,“Wirelessly-charged UHF tags for sensor data collection,” in Proceed-

ings of 2008 IEEE International Conference on RFID, April 2008, pp.320–327.

[8] U. Karthaus and M. Fischer, “Fully integrated passive UHF RFIDtransponder IC with 16.7-µW minimum RF input power,” IEEE Journal

of Solid-State Circuits, vol. 38, no. 10, pp. 1602–1608, October 2003.[9] G. De Vita and G. Iannaccone, “Design criteria for the RF section of

UHF and microwave passive RFID transponders,” IEEE Transactions

on Microwave Theory and Techniques, vol. 53, no. 9, pp. 2978 – 2990,September 2005.

[10] K. Seemann, F. Cilek, M. Schmidt, and R. Weigel, “RFID at UHF fre-quencies: The passive transponder frontend approach,” in International

Conference on Microwaves, Radar & Wireless Communications, 2006.

MIKON 2006., May 2006, pp. 657–661.

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20

40

60

80

100

120

140

160

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200

Input DC Voltage [V]

Cu

rre

nt

[µA

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DC Load Characteristic

Active Mode

40 k!

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