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Actuation of THUNDER Materials by a Microwave

Kyo D. Song, Walter T. Golembiewski, Sang-Hyon Chu+, and Glen King‡, and Sang H. Choi‡

Norfolk State UniversityNorfolk, VA 23504

+ NIA, ‡NASA Langley Research Center

ABSTRACT

An experimental study of THUNDER (Thin Layer Composite Unimorph Ferroelectric Driver and Sensor) materials actuator controlled by a microwave is presented in this paper. A proof-of-concept experiment using a smart material such as THUNDER has been demonstrated control in power-fed using a microwave. Such advance system will produce a revolutionary class of smart devices that integrate sensors, actuators, and smart flight control in space applications as well as biologically-inspired systems. A combination of signal generator and amplifier provided 230 W of microwave power to the Narda horn antenna at a frequency setting of 8.5GHz. The 230 W microwave power irradiated on the JPL (Jet Propulsion Laboratory) 6 x 6 rectenna array. The rectenna, which is a rectifier and an antenna, converts the microwave power into DC power. The THUNDER used for the test rated a 31 Hz resonant frequency, a 595 volts driving voltagegenerated by a microwave, and a 7.62 mm maximum displacement.

The result shows a promising technique to actuate smart actuators by a microwave in real applications. It may be better the system imbed into a thin film microcircuit layer with developed dipole rectenna during the fabrication of the smart material in future work.

INTRODUCTION

A large deployable, flexible membrane structure must have autonomous surface correction and self-adjustable capabilities. Surface tension control of large flexible membranes is complex and needs to be segmented in an array matrix without using hard-wired circuitry. Smart material that enables itself to undergo shape changes in a predictable way [1,2] is used for a large ultra-lightweight membrane structure. This surface compensation technique is crucial to the success of NASA’s future missions [3-6] including the Next Generation Space Telescope (NGST) that will replace the existing Hubble Space Telescope so that deep infrared and visible images of the most distant stars in our universe can be effectively imaged and studied.

Since the rectenna was first introduced by W. C. Brown in the 1960’s [7], it has been used for various applications such as microwave-powered helicopter [8], the Solar Power Satellite (SPS) [9] that converts solar energy to RF and beams down to large 2.45-GHz rectennas on Earth, the 4.5-meter wingspan airplane that was powered only by microwave energy [10], and the microwave-powered balloon with an electronically steerable phased array [11].

The power received by a rectenna patch may not be sufficient for mobilizing an actuator because of either the dispersion of microwave or the extensive power feed required for shape changes. For piezoelectric actuators, the breakdown voltage of Schottky barrier diode used in each individual rectenna is a limiting factor. Low power density

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and/or power demand are typical when microwave is considered. Otherwise, high energy and highfrequency microwave sources that are bulky and heavy, thus inappropriate for space applications, must be used. The issues related to low power density of microwave and power demand can be alleviated by employing the concept of power allocation and distribution (PAD) circuit [12].

THUNDER (Thin Layer Composite Unimorph Ferroelectric Driver and Sensor) is a ferroelectric device made of multiple layers of materials, typically stainless steel, aluminum and PZT (Lead Zirconate Tintanate) piezo-ceramic. These layers of materials are sandwiched together with an adhesive bond. A piezo-ceramic material is composed of randomly oriented crystals or grains. By applying electrodes to the ceramic and a strong DC electric field generated by a microwave, the dipoles will tend to align themselves to the direction of the electric field. By aligning in this manner, the smart material will have a permanent residual polarization. The result of this polarization is a change in the geometric dimension. THUNDER has the capability to expand or contract, based on the polarity of the voltage applied. When the applied voltage is positive, THUNDER will flatten, and if the applied voltage were negative, the THUNDER would arch. The THUNDER used for the test rated a 31 Hz resonant frequency, a 595 volts driving voltagegenerated by a microwave, and a 7.62 mm maximum displacement.

In practical applications, the power received by a single element of patch rectenna may not be sufficient for driving even an actuator element because of the low incidence of microwave power on a patch rectenna and, the high power required for shape changes. The power output from patch rectennas is determined by the power flux density, frequency, and incident angle of incident microwave and the rectifying circuit performance. For high voltage output, the breakdown voltage of Schottky barrier diodes used in each individual rectenna is a limiting factor. Accordingly, microwave-driven smart membrane actuators must meet these power requirements in addition to being structurally lightweight, thin and flexible. Devices operating at a low power level must intelligently manage the power to meet user device requirements. In general, high energy and high frequency microwave sources

are bulky and heavy, and thus inappropriate for space applications. In addition to PAD concept, the output of the rectennas has to be sufficient to control actuator. A 200W of amplifier will be employed to test a maximum output from the single recetenna in practical applications.

Experimental Results

An experimental was setup to actuate THUNDER actuators using a microwave power as shown in Fig.1. A combination of signal generator and amplifier provided 20 W microwave power to a Narda horn antenna at frequency setting of 8.5 GHz. The 20 W microwave power irradiated a JPL 6 x 6 rectenna array. The horn antenna and the 6 x 6 rectenna array with fixture for the experiment were inside of an anechoic chamber as shown in Fig. 2 (a) and (b). The average power density of a 20W microwave at 8.5 GHz is approximately 5 mW/cm2. Therefore, the effective area exposed to microwave is approximately 2500 cm2 that gives 60% efficiency of the microwave power at the target. The effective area of a single rectenna element is calculated [13] as,

πλ

4re GA = 1)

Where, Gr is the gain of the rectenna, or 8.4 dB at 8.51 GHz. The effective area for a single rectenna is approximately 6.8 cm2 that provides a minimal cell-to cell spacing as to be 2.61 cm for an effective design. However, measured space between cell to cell is approximately 0.6 cm, which gives more conservative design of the rectenna patch. Therefore, we use 9.5 cm by 9.5 cm which is close to an physical size of the rectenna patch as an effective area. The overall area of the patch rectenna is approximately 90.25 cm2 in this configuration. The received power by the patch rectenna could be calculated via multiplying the effective area of the patch rectenna by the power density that was measured (~ 0.45 W) using the microwave probe at a position of the rectenna.

Output Power of 6 x 6 Rectenna - A 6 x 6 patch rectenna was used for the achieving requirement of higher voltage output. The voltage output of patch rectenna is determined by the number of inbedded Schottky barrier diode on a rectifying circuit. Each

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patch has two of Schottky barrier diode embedded in a serial mode that hold off the total of 15 volts. Thus, the 6 x 6 patch rectenna is, theoretically, able to generate the output voltage up to 540 volts. The actual voltage output is lower than the theoretical value and determined by the electrical properties of substrate materials on which a rectifying circuit is built. The input power dependent voltage outputs of the 6 x 6 rectenna were also measured at the distance of 100 cm away from a feed horn as shown in Fig. 3. In fig. 3, the output voltage versus microwave input power are measured and plotted. The peak voltage output was 230 volts at 8 GHz with 20 watts of microwave beam power. The peak voltage output is within the voltage requirement for most piezoelectric actuators.

Actuation of THUNDER Actuators - A THUNDER actuator is used and tested with a 6 x 6 patch rectenna array by a microwave power isshown in Fig. 1. In this experiment, the Narda horn antenna’s 20 W microwave power was converted into a measured 230 V DC by a digital multimeter. The estimated current being produced from the 6 x 6 rectenna array was 0.38mA. Therefore, the output power of the 6 x 6 rectenna is calculated as 0.087 W. As a result, the efficiency of the 6 x 6 rectenna is calculated as close to 20%.

Fig. 4 shows an actuation of THUNDER materials by a microwave with a RC circuit shown in Fig. 5 to demonstrate its motion. This RC circuit that is created by the relay timer is needed to show the pulse operational characteristics of the THUNDER.

The THUNDER used for the test rated a 31 Hz resonant frequency, a 595 volts driving voltage, and a 7.62 mm maximum displacement as denoted as in a squared dot in Fig 6. Fig. 6 shows the displacement levels of THUNDER actuator according to the voltages fed from the 6 x 6 patch rectenna array. A higher voltage output than 230 volts is expected by increasing the beam power. The test of the 6 x 6 patch rectenna was limited by the capacity of the 20 W tunneling wave tube amplifier (TWTA) used.

Currently, a 200 W TWTA is under installation and expected for higher voltage output from the 6 x 6 patch rectenna as predicted in Fig. 7. High voltage

output is, generally, determined by the number density and breakdown voltage of Schottky barrier diodes in a rectenna array, the beam power, the frequency, and the distance, respectively. The number density of Schottky diodes is determined by how patch rectennas are densely packed in an array at a given frequency.

FUTURE DESIGN

This concept eliminates the need for the hard wiring of the smart actuators on the adaptive surfaces of a large space deployable reflectors or inflatable antennas. Hence, it could dramatically reduce the cost of distributed shape-control systems and find applications in many future and technically challenging space missions. The design presented is simple enough to imbed into a thin film microcircuit layer during the fabrication of the smart material. It may be possible to improve the overall design in several ways. In order to applying in practical applications, design of flexible rectenna configuration such as printed dipole patch including higher voltage Schottky diode is essential. Current commercially available Schottky diode is limited by 9.5V as compared 30 V breakdown voltage achieved by JPL diodes with same material. This may also need to improve higher voltage output from rectenna. It is necessary to improve design and integration for packaging circuits, thin film rectenna patch, and flexible membrane actuators will be the future research area. One of the candidates for thin film rectenna (TFR) will be pre-oriented modified carbon nano tubes (CNT) with electro or magneto active polymer film inside of the rectifier as shown in Figs. 8 and 9. In this configuration, as a microwave energy is applied on TFR patch, the CNTs will be oriented along the applied microwave and the surface shape will be changed and controlled as a result.

CONCLUSION

The experimental results indicate that the rectenna array generates sufficient output power to drive presently available actuator like THUNDER. The wireless power transmission by microwave eliminates the need for the hard-wiring of actuator elements on the adaptive surfaces of a large space deployable reflectors or inflatable antennas. Hence, it could dramatically reduce the cost of distributed

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shape-control and find space applications in near future. For higher power applications, high voltage hold-off Schottky barrier diodes is essential for driving actuators with a large displacement requirement in practical applications such as aircraft morphing actuators.

ACKNOWLEDGMENTS

This work was performed partially under the NASA grant (NCC1-280) to Norfolk State University.

REFERENCES

1. 1993 American Heritage College Dictionary,3rd edition (Houghton Mifflin).

2. I. V. Galaev, M. N. Gupta, and B. Mattiasson, “Use Smart Polymers for Biosepartions,” CHEMTECH, No. 12, pp. 19-25, 1996.

3. T. K. Wu, “Multiband FSS,” in T. K. Wu (Ed.), Frequency Selective and Grid Array, John and Wiley & Sons, New York, 1995.

4. W. Schneider, J. Moore, T. Blankney, D. Smith, and J. Vacchione, “An Ultra-Lightweight High Gain Spacecraft Antenna,” IEEE Antennas Propagat. Int. Symp., Seattle, WA, June, pp. 886-889, 1994.

5. T. H. Lee, R. C. Rudduck, T. K. Wu, and C. Chandler, “Structure Scattering Analysis for SeaWinds Scatterometer Reflector Antenna Using Extended Aperture Integration and GTD,” IEEE Antennas propagat. Int. Symp., pp. 890-893, Seattle, WA, June, 1994.

6. Sang H. Choi, Lake, M., and Moore, C., “Microwave-driven Smart Material Actuators.” Patent Pending, NASA Case No. LAR 15754-1, Feb 24, 1998.

7. Sang H. Choi, S. Chu, M. Kwak and A. D. Cutler, “A Study on a Microwave-driven Smart Material Actuator,” SPIE Symposium on Smart Structures and Materials, 1999.

8. W. C. Brown, et al., U.S. Patent 3 434 678, Mar. 25, 1969.

9. W. C. Brown, “Experiments involving a microwave beam to power and position a helicopter,” IEEE Trans. Aerosp. Electron. Syst., Vol. AES-5, No. 5, pp. 692-702, 1969.

10. W. C. Brown, “Solar Power Satellite Program Rev. DOE/NASA Satellite Power System Concept Develop. Evalation Program,” Fianl Proc. Conf. 800491, 1980.

11. J. Schlesak, A. Alden and T. Ohno, “ A microwave powered high altitude platform,” IEEE MTT-S Int. Microwave Symp. Dig., pp. 283-286 1988.

12. Kyo D. Song, Walter Golembiewski, and Sang H. Choi, “Networked Rectenna Array for Smart Materials Actuators,” 35th Intersociety Energy Conversion Engineering Conference, July 24 –28, 2000, Las Vegas, NV (AIAA 2000-3066)

13. NASA JPL, “Patch Rectenna for Converting Microwave to DC Power,” NASA Tech Briefs, Vol. 21, January, p. 40, 1997.

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Figure 2: Inside the Anechoic Chamber: Horn Antenna and 6 x 6 Rectenna Patch.

Figure 1: Conceptual Diagram of Microwave Driven Smart Materials.

HP Signal Generator

Logi_Metrics TWT 20W Amplifier

Anechoic Chamber

THUNDER Actuator

Feed Horn

ll +dl

Array of Rectenna Patch

DisplacementMonitoring System

Input Power (W)

8 10 12 14 16 18 20 22

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put V

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at 8 GHz

Figure 3: Input Power Dependent DC Output Voltage from a 6 x 6 Patch Rectenna.

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Fig. 4. Demonstration of a THUNDER actuator by a microwavePower at NASA Langley Research Center

Figure 5: RC Circuit Diagram for the Actuation.

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Applied Voltage (V)

0 100 200 300 400 500 600 7000

Figure. 6:. Displacement of TTHUNDER Aactuator.

Applied Voltage (V)

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Figure 6: Displacement of THUNDER Actuator.

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Figure. 6:. Displacement of TTHUNDER Aactuator. Applied Voltage (V)

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Figure. 6:. Displacement of TTHUNDER Aactuator.

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Figure. 6:. Displacement of TTHUNDER Aactuator.

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Fig. 8. Future MW-driven Flexible Smart Membrane with Actuator Array

Copper patch, < 35 µm Piezo Polymer

Copper ground plate

Dielectric film

Thin-film Rectenna Circuit

Butterfly filter & Impedance matching circuitry

Schottky Diodes

MW-driven Smart Membrane

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Input Power, watts

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Fig.7. Output Voltage from a 6 x 6 rectenna patch in terms of input power of a microwave

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Power “OFF” Power “ON”

Thin-Film Rectenna(TFR) Patch

Electro or magneto-active polymer Film

Pre-oriented modified CNTTFR Rectifier Layer

Microwave

Tension for Shape Control

Inorganic materialattachment

CNT

Modified CNT

Fig. 9. Future Microwave-driven EM-active Polymer TFR with m-CNT for Shape Control

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