Young-Sik Seo, Zachariah Hughes, Matt Hoang, Deena Isom,. Minh Nguyen, Smitha Rao ... operation mechanism and availability of affordable electronic parts.
APMC 2012, Asia-Pacific Microwave Conference, Kaohsiung, Taiwan, Dec. 4-7 2012
Investigation of Wireless Power Transfer in Through-wall Applications Young-Sik Seo, Zachariah Hughes, Matt Hoang, Deena Isom, Minh Nguyen, Smitha Rao*, and J.-C. Chiao The University of Texas at Arlington, *Med-Worx, USA Abstract — In this work, we proposed a through-wall wireless power transfer system and investigated effects of various wall materials. The power transfer system was based on inductive coupling of metal coils at 1.3-MHz resonance. Softwood lumber, concrete brick and drywall with insulation filling were tested at two different thicknesses. Two sets of coils, each set consisting of two coils with identical dimensions, having radii of 5 and 15 cm were utilized. Each experiment was conducted with sequential tuning in receiver circuit, operating frequency and in transmitter circuit to reach maximum output power or maximum power transfer efficiency. The output power and transfer efficiency as well as their changes were obtained before and after tuning for different media and thicknesses. It is concluded that the power attenuation with spacing distance dominates the output power and transfer efficiency, while tuning could counteract the parasitic effects in the material and recover the power lost in deviation from resonance. The power attenuation with distance requires design considerations in coil dimensions. With larger coils, more power can be collected through thicker walls and the system tolerates more variation in wall thickness. Tests on randomly chosen walls in our laboratory building were conducted to validate the system performance. Index Terms — Inductive coupling, wireless power transfer, power efficiency and transfer.
I. INTRODUCTION Wireless power transfer has been utilized for lower energy level applications such as passive RFID systems [1] and sensor transponder [2], and higher energy level ones such as charging of portable electronic devices [3]. Inductive coupling has been utilized due to its relatively simple operation mechanism and availability of affordable electronic parts. The technique provides the ability to transfer high amounts of power, in the order of tens of watts easily without needs for special electrical components. Generally speaking, the operating limitation for inductive coupling is on the attenuation of electromagnetic energy with the distance between transmitter and receiver coils, reducing the power delivered and efficiency. Sufficient energy transfer for nearfield coupling can be achieved for specific applications. In this work, we proposed a through-wall inductive coupling system to transfer electric energy from outdoor energy-harvesting sources, such as solar panels, wind turbines, and hydraulic sources, to indoor energy-consuming systems, such as solid-state lighting, sensors or small electrical appliances, without power line connection. The configuration is shown in Fig. 1. The system targets easy installation of such an energy harvesting and transferring system while avoiding the possibility of compromising structure integrity in altering or modifying buildings.
Several factors in designs affect the maximum power transfer, maximum transfer efficiency, and maximum distance allowed for specific required power, such as the turn numbers, dimensions, metal pattern arrangement and shapes of inductive coils. These factors determine the self inductances, mutual inductance, parasitic capacitances, resistances, and electromagnetic field distribution. Along with physical and material properties of the metal wire used for coils, they determine the quality factor of resonance and coupling coefficient. In our design, the energy source drives an oscillator and a class-E amplifier to produce the RF carrier with large voltage swing amplitudes. The impedance matching between the resonant circuits and class-E amplifier determine the power transfer output. II. METHOD In our previous work [4], we have demonstrated a wireless power transfer system that transmits RF energy through human tissues in the abdominal region. The system was designed for optimal efficiency at required power to operate a stimulator inside the stomach. In the implant application, the maximum efficiency was the main goal due to the limitations of implant size, comfort of the patient and safety issues such as tissue heating by radiated power. In the through-wall energy transfer systems, these constrains are no longer applicable. The system configuration is shown in Fig. 1. The carrier frequency was set at 1.3MHz and the load was 500 Ω. The input signal was a square wave with 6 Vpp amplified by a class-E amplifier consisted of an n-channel power MOSFET (Fairchild, IRF510) with a 50% duty cycle. Output voltage was measured on the load resistor for output power while input voltage and current were measured to obtain input power. The overall power transfer efficiency obtained from the experiments included the class-E amplifier efficiency. Two sets of antennas were utilized in our experiments. In the coil set #1, both coils Sensor had a radius of 5 cm and Solar turn numbers of 17 and Wall 16 for the transmitter panel LED and receiver, respectlighting tively. The measured CC2 RR2 M R1 M R self inductances were 86.11 µH and 76.82 µH. L2L LL 1 CC1 RL R In the coil set #2, both coils had a radius of 15 cm and 10 turns with a Fig. 1. Through-wall wireless power self inductance of 92.87 transfer system configuration. 1
AC
1
2
1
2
2
L
µH. Resonance was accomplished by matching the series and shunt capacitances throughout the experiments. To study the effect of various wall materials on wireless power transfer, we have selected commonly used construction materials: milled pine lumber panels, concrete bricks, and drywall panel sheets with intervening insulation. Southern Loblolly pine lumber with a thickness of 3.8 cm, a size of 28×28 cm2 and a dryness factor of 90% was obtained. The bricks were composed of drycast concrete and measured Efficiency (4.4cm) Efficiency (8.5cm) Load Power (4.4cm) Load Power (8.5cm)
Efficiency (%)
50
0.5 0.4
40 0.3 30 0.2 20
Load Power (W)
60
0.1
10 0
0 133
138
143 148 153 Matching Capacitance (pF)
158
Fig. 2. Output power and transfer efficiency as a function of matching capacitance at two different distances. 0.5
Load Power (W)
4.4cm
8.5cm
0.4 0.3
1)
2)
3)
4)
0.2 0.1
Air (tuned)
Brick Brick (C1) Brick (Freq.) Brick (C2) (Untuned) Tuning Methods
Fig. 3. The output powers achieved in different tuning steps for the coil set #1 at 4.4- and 8.5-cm spacing. C1 and C2 are the matching capacitors in the receiver and transmitter coils, respectively. The arrows indicate the order of tuning procedures. Output, W Spacing, cm Tuned (Air) Untuned (Medium) Tuned (C1) Tuned (Freq.) Tuned (C2) Output, W Spacing, cm Tuned (Air) Untuned (Medium) Tuned (C1) Tuned (Freq.) Tuned (C2)
Coil Set #1 Brick Wood 4.4 8.5 4.4 8.5
Drywall 4.4 8.5
0.449 0.165 0.392 0.163 0.424 0.157 0.388 0.155 0.428 0.165 0.388 0.161 0.428 0.165 0.388 0.162 0.428 0.165 0.388 0.162 Coil Set #2 Brick Wood 4.4 8.5 4.4 8.5
0.406 0.399 0.399 0.399 0.399
0.354 0.236 0.305 0.306 0.308
0.370 0.339 0.349 0.352 0.352
0.422 0.346 0.371 0.375 0.375
0.305 0.297 0.297 0.299 0.322
0.423 0.388 0.404 0.406 0.405
0.157 0.154 0.157 0.157 0.157
Drywall 4.4 8.5 0.439 0.411 0.415 0.415 0.415
Table I. Comparison of the output powers through air and various media. The matching capacitances in the receiver (C1) and transmitter (C2), and the operating frequency were tuned in each step.
30×30 cm2 with a thickness of 3.9 cm. The drywall wall section was composed of two sheets of standard gypsum boards, each measuring 50×50 cm2 and a thickness of 1.2 cm. Fiberglass acoustical and thermal insulation (Johns-Manville) was stuffed between the drywall sheets. III. MEASUREMENTS AND RESULTS Transfer efficiency was calculated from the output power in the receiver load over the input power in the transmitter. For each set of coils, the following procedures to reach a maximum output power were repeated: The system was first tested with air as the medium. The receiver circuit was tuned with matching capacitance until a maximum voltage was achieved. Then the operating frequency was fine-tuned to increase the load voltage, followed by tuning the transmitter capacitances to reach maximum output voltage. The same tuning procedures were repeated for brick, wood and drywall as the media, each with two different thicknesses of 4.4 cm and 8.5 cm. Since the electromagnetic field attenuation is highly sensitive to separation, the coils were fixed without disturbance when changing materials between them. Then the same tuning procedures with four different media were also repeated for achieving maximum transfer efficiency in each case, in spite of the output power. Figure 2 shows the tuning results with the output power and transfer efficiency as a function of the matching capacitance in the transmitter for the coil set#1. Two distances of 4.4cm and 8.5cm in air were tested. The results indicated that the maximum output power and maximum transfer efficiency were at different matching points, differing by 6 and 9 pF at the 4.4 and 8.5 cm distances, respectively. The maximum efficiency was achieved mainly due to the input power reduction from impedance matching. Dielectric materials of brick, wood and drywall were expected to vary the effective dielectric constant the coils experienced, affecting the parasitic capacitances in both transmitter and receiver coils, and thus changing the resonance. The tuning of matching capacitances was expected to recover the condition for maximum output power. As illustrated in Fig. 3, the maximum output powers in air decreased slightly after inserting the brick due to the parasitic Medium Spacing, cm Maximum Efficiency, % Efficiency, %, at maximum power Maximum Power, W Power, W, at Max. Efficiency ∆ (Efficiency), % ∆(Power), W
Air 4.4
Brick 8.5
4.4
8.5
Wood 4.4
8.5
Drywall 4.4
8.5
45.01 9.48 42.46 8.37 42.99 6.40 41.47 9.01 39.19 8.37 37.84 7.76 36.84 5.77 36.75 8.11 0.475 0.139 0.422 0.123 0.440 0.101 0.423 0.146 0.259 0.022 0.252 0.028 0.157 0.030 0.251 0.028 5.820 1.110 4.620 0.610 6.154 0.626 4.719 0.894 0.216 0.117 0.170 0.095 0.283 0.071 0.172 0.118
Table II. Maximum efficiency and maximum output power through four different media for the coil set #1. ∆(Efficiency) = Maximum efficiency ‒ Efficiency at the maximum output power. ∆(Power)= Maximum output power efficiency ‒ output power at the maximum efficiency.
0.5 Coil Set #1
Load Power (W)
0.4
Coil Set #2
0.3 0.2 0.1
0 0
10
20 30 Separation Distance (cm)
40
Fig. 4. The maximum output powers achieved after tuning for the coil sets #1 and #2 at different spacing in air.
effect. After tuning the receiver matching capacitance, after retuning the operating frequency, and after tuning the transmitter matching capacitance, the output powers increased also slightly. The same experiments were conducted for three media with coil sets #1 and #2. The results are summarized in Table I. The results indicate the change in effective dielectric constant due to the material was not significant because the material thickness is much smaller than the wavelength at 1.3MHz. The effect of reflection losses at the material interfaces was also not significant due to the same reason. With the tuning procedures, the power transfer could come back higher than the untuned cases owing to impedance matching, especially for the larger coil set. Another experiment was conducted to obtain maximum output power and maximum transfer efficiency for coil set #1, as results summarized in Table II. The variations caused by the wall materials were insignificant, compared to the effect by the spacing distance. The differences in output power when aiming for maximum efficiency or in efficiency when aiming for maximum output power, however, were significant. This indicates that a trade-off strategy should be considered depending on the applications. The goal can be easily achieved with simple tuning. It is clear that the spacing distance plays a more important role than the material properties in terms of transfer power and efficiency. Figure 4 shows the output power as a function of spacing between two coils for both coil sets #1 and #2. With a larger aperture, coil set #2 provides a farther distance with a smoother slope of power attenuation in distance. However, this does not mean a larger coil is a more appropriate choice for the through-wall applications. At a shorter distance of 4.4cm, the output power dropped for coil set #2, possibly due to near-field interference. For practical considerations in allowed space and construction, ones may consider smaller coils for their specific applications. For general purposes, the large coils do provide the flexibility for various thicknesses of walls. We chose several walls in the Nedderman Hall building at UT-Arlington to test our system. The results showed that powers in the range within 200‒400mW were scavenged with the coil set #1 for most of the drywalls. For concrete block walls, coil set #2 was used since coil set #1 could not
collect sufficient Coil taped on wall Brick power. Figure 5 wall Coil shows a typical concrete wall tested with a wall thickness of Circuits 30 cm using the Concrete block wall, painted coil set #2. An Fig. 5. Photos of the through-wall experiment: output power of (left) indoor, (right) outdoor. 25mW with an efficiency of 2% was achieved after tuning, which agreed with the result in Fig. 4. The power was not sufficient for lighting, thus larger coil sets would be tested in the near future. It should also be noted that the through-wall wireless power transfer system was based on our previous 1.3-MHz system, which was the result after trade-offs in component sizes and performance, coil dimensions and resistance, and local field distribution and penetration. Thus, further investigation of the system in other frequency bands to optimize the performance in efficiency and output power is required. V. CONCLUSIONS In this work, we proposed and investigated a through-wall wireless power transfer system that utilizes inductive coupling for energy scavenging from outdoor sources, such as solar panel or wind turbine, for indoor applications, such as lighting or sensing. Different wall materials have been tested with various thicknesses and coil configurations. We concluded that the wall materials do not affect significantly the output power or transfer efficiency due to the long wavelength. The power attenuation with spacing dominates the performance and determines design factors in applications. Tuning the transmitter and receiver circuitry as well as the operating frequency to overcome additional parasitic effects from the wall materials can help in achieving optimal output powers. ACKNOWLEDGEMENTS We appreciated Intel, Texas Instruments and Med-Worx for their support. M. Hoang was supported by NSF EEC1156801, Research Experiences for Undergraduates in Sensors and Applications. REFERENCES [1] J. Curty et al., “Remotely powered addressable UHF RFID integrated system”, IEEE J. Solid-State Circuits, Vol. 40, No. 11, pp. 2193-2202, 2005. [2] H. Cao et al., “Remote Detection of Gastroesophageal Reflux Using an Impedance and pH Sensing Transponder,” 2012 IEEE IMS, Montreal, Canada, June 17-22, 2012. [3] Z. N. Low et al., "Design and Test of a High-Power HighEfficiency Loosely Coupled Planar Wireless Power Transfer System," IEEE Transactions on Industrial Electronics, Vol. 56, No. 5, pp. 1801-1812, May 2009. [4] Y.-S. Seo et al., “Wireless Power Transfer for a Miniature Gastrostimulator,” 2012 EuMC, Amsterdam, Netherlands, Oct. 28 – Nov. 2 2012.
