Wireless power transfer through metal using inductive link

This paper presents a highly efﬁcient power transfer system based on a co-design of a class-E power ampliﬁer (PA) and a pair of inductively coupled Helical coils for through-metal-wall power transfer. Power is transferred wirelessly through a 3.1-mm thick aluminum barrier without any physical penetration and contact. Measurement results show that the class-E PA achieves a peak power gain of 25.2 dB and a maximum collector efﬁciency of 57.3%, all at 200 Hz. The proposed system obtains a maximum power transfer efﬁciency of 9% and it can deliver 5 W power to the receiver side through the aluminum barrier.

INTRODUCTION Design and optimization of wireless power transfer (WPT) systems have been well studied over the last decades to charge cell phones, electric vehicles, and power up sensors over a short distance. In recent years, there has been a great demand for wirelessly powered sensors to support structural health monitoring (SHM) for industrial applications containing metal walls and enclosures (e.g., naval vessel, aircraft, vehicle armor, metal pile and chemical vat). Traditionally, metal wall penetrations are used to feed through wires. However, drilling holes for feeding wires reduces the integrity of the structure and also causes practical issues such as the probability of leakage of toxic chemicals, loss of pressure or vacuum. The SHM systems typically require embedded sensors for data acquisition, wireless communication, and energy harvesting. These sensors need to be powered and controlled wirelessly through a metal wall without physical penetration through mechanical structures.
Several solutions have been proposed to fulfill through-metal-wall power transfer [1]. These solutions can be categorized into two main methods. The first method exploits ultrasound waves including piezoelectric transducer (PZT) [2,3] and electromagnetic acoustic transducer (EMAT) based approach [4]. Ultrasonic waves can propagate easily through a variety of metallic materials. The PZT-based system is by far the most widely used for through-metal-wall power transfer due to the high power transfer efficiency and the possibility of miniaturizing the design. The main limitation of the PZT-based system is that it requires direct coupling of piezoelectric transducers with metal wall to provide a good acoustic transmission path. In addition, the performance of the PZT-based system is highly reliant on the quality of the coupling. Poor coupling will introduce severe impedance mismatch over the acoustic-electric channel and cause the power transfer efficiency to decrease rapidly. EMAT is a non-contact approach but the power transfer efficiency is too low, which makes it unsuitable for highly efficient power transfer through metal wall. The second method is based on electromagnetic coupling involving inductive coupling [4,5], capacitive coupling [6,7] and magnetic resonance coupling [8]. The biggest advantage of electromagnetic coupling method is that there is no need for the transmitter and receiver to contact directly with the metal wall. The inductive power transfer (IPT) is probably the most popular approach based on electromagnetic coupling principles. The inductive coupling approach is suitable for applications with low conductivity, low permeability materials such as aluminum or stainless steel walls. It works by magnetic field coupling using a pair of inductive coils. As the skin depth of the metallic material increases rapidly with the increase of frequency, IPT systems operate at super low frequencies to minimize the influences of the Faraday shielding effects.
In this paper, a fully functional IPT system based on a co-design of a class-E PA and a pair of inductive Helical stacked coils for through-metal-wall power transfer will be presented. Measurement results show that the system obtains a peak power transfer efficiency of 9% and it is able to deliver 5 W power to the receiver side through the 3.1-mm thick aluminum barrier.

2.
SYSTEM OVERVIEW The concept of IPT is similar to the principle of transformers in which an alternative magnetic field in primary coil induces a load voltage on secondary coil when these two coils are tightly coupled [9]. The metallic medium restricts the power transfer due to losses causing by the shielding effect or skin effect. As a result, a specific frequency range is used to overcome the skin-depth limitation of metal thicknesses such as the metal tank layer using 30-300 Hz range and underground pipe using 50 Hz. For a 3.1 mmthick aluminum (σ = 38 MS/m), its skin depth is 2.62 mm and 0.34 mm at 1 kHz and 60 kHz, respectively. Therefore, the super low frequency, f 0 = 200 Hz, has been used as the operating frequency for the proposed system. As shown in Figure 1, the IPT system consists of a class-E PA, a pair of inductive coils, a seriesparallel LC resonant circuit and a rectifier. The power link includes two inductive coils placed on both sides of a aluminum barrier. The transmitting (Tx) and receiving (Rx) coils are represented by the inductors L 1 and L 2 while their parasitic series resistances are R 1 and R 2 , respectively. The system delivers AC power to the Rx coil, which needs to be terminated with an optimal load, R L , to achieve the maximum power transfer. L 2 and C 1 form a series-parallel resonant circuit. When the parallel resonator is used as a receiver, the voltage drop loss in the following rectifier is small because of the high load voltage. The AC voltage is converted into the DC voltage by a full-bridge rectifier GBPC3502W from ON Semiconductor. A smoothing capacitor, C 2 , is used in conjunction with the rectifier. It smooths out the fluctuation in the rectified signal to make a steady DC voltage for sensing and communication operation on the receiver side.

Helical stacked coil
The inductive link uses a pair of Helical stacked coils as shown in Figure 2. By co-axially aligning a pair of coils, the wireless IPT channel is formed without any physical penetration through the metal barrier [10]. The Helical circular geometry results in a more uniform magnetic field distribution that significantly improves the efficiency of energy transfer compared to that of the conventional Solenoid counterpart. The Helical stacked coils are constructed using American Wire Gauge (AWG) 17 magnet wire. Both coils use ferrites to increase their magnetic fields. Table 1   Power transfer efficiency (PTE) is determined by the quality factors of the inductive coils and the coupling factor k. The product k 2 Q l Q 2 is desirable to be as high as possible. However, the shielding effect of the aluminum barrier or the metal thickness will severely degrade the coupling coefficient. Whereas, the proximity effect, or air distance from each coil to the metal surface, will reduce the quality factor of the coils. The product k 2 Q l Q 2 experiences a maximum and consequently determines the peak efficiency for an optimal operating frequency range. Therefore, design of a high-quality factor inductive coil in a small form factor is a key aspect in achieving the high efficiency power transfer. The maximum PTE of the system is calculated as follow [11]: where Q 1 = ω 0 L 1 /R 1 and Q 2 = ω 0 L 2 /R 2 are the quality factors of the Tx and Rx coils, and M is their mutual inductance.

200 Hz Class-E Power Amplifier
A class-E PA is co-designed with the inductive link for power transfer. Class-E PAs achieve significantly higher efficiency than conventional class-B or -C counterparts. The efficiency is maximized by minimizing power dissipation, while providing a desired output power. The insulated-gate bipolar transistor (IGBT) STGW15S120DF3 from STMicroelectronics has been used as the power transistor. This IGBT family has been specifically optimized for low switching frequencies. Figure 3 shows the schematic of the 200 Hz class-E PA and its component values are given in Table 2. The PA design is obtained by the well-known Sokal-Raab approach [12,13]. In this design, the IGBT operates as an on/off switch and the load network shapes the voltage and current waveforms to prevent simultaneous high voltage and high current in the transistor. As a result, the power dissipation is minimized, especially during the switching transitions.
where Q L is the loaded quality factor of the load network.

EXPERIMENTAL RESULTS
A complete prototype was built to demonstrate the effectiveness of the proposed system. Figure 4 illustrates the measurement setup for power transfer through a 400 mm×400 mm×3.1 mm aluminum barrier. The Tx coil cannot be seen in the figure since it is covered by the aluminum barrier. Figure 5 shows the measured collector voltage and collector current waveforms in the time domain captured by an oscilloscope. The voltage and current switching transitions are time-displaced from each other. Figure 6 shows a typical measured output signal spectrum of the class-E PA in the frequency domain captured by a vector signal analyzer. The PA exhibits a good linearity when most of the power is at the fundamental frequency. At 200 Hz, the power of transmitted signal is 43.2 dBm. Harmonic output of the the class-E amplifier is similar to that of the class-B counterpart. The strongest harmonic is the second one. Without filtering, the second-harmonic power is -17 dBc (2% of the fundamental power) and the third-harmonic power is -26.4 dBc (0.23% of the fundamental power).    Figure 7 shows the measured gain and output power versus the input powers of the class-E PA. It archives a peak gain of 25.2 dB at 21.7 dBm input power. The output power of the PA increases linearly with the input power. At 24.2 dBm input power, the output power is 48.5 dBm or 70.8 W. The measured collector efficiency as a function of the output power is shown in Figure 8. The PA obtains a peak efficiency of 57.3% at 42.2 W output power. The efficiency decrease slowly with increasing output powers. At 70.8 W output power, the efficiency is 50%. The lower value of PA efficiency can be explained by the reduced influences of ferrites at high output powers. Figure 9 shows the measured PTE and received power versus output power of the proposed IPT system. The power transfer system obtains the peak PTE of 9% and it can deliver 5 W power to the receiver side through the 3.1-mm thick aluminum barrier.   Table 3 summarizes the performance of the proposed IPT system and compares it to other published through-metal-wall power transfer systems based on electromagnetic coupling. Graham et al. [4] transferred power through a 20 mm thick, 130 mm diameter stainless steel disc of a relative permeability of µ r = 1.1. The results showed that an efficiency exceeding 4% can be achievable with an operating frequency of 500 Hz. However, when the relative permeability of the metal material is increased to µ r = 10, the peak achievable efficiency is only 0.2%. Zangl et al. [5] investigated the feasibility of power delivery through a tin container. The experiment results have shown that at least 30 µW of power can be transferred through the 5-mm thick tin container to power the fill level measurement capacitive sensor circuit using a continuous wave carrier signal at 50 Hz. However no information on PTE was presented in the paper. Yamakawa et al. [8] reported their work on wireless power transmission into a space enclosed by metal walls using magnetic resonance coupling. The results have shown that a 1.2 W of electric power can be supplied to LEDs through a stainless steel wall 5-mm thick with an efficiency of 10% over a transmission distance of 10 cm.

CONCLUSIONS
In this paper, we have presented the highly efficient IPT system based on the co-design of the class-E power amplifier and a pair of Helical stacked coils for through-metal-wall power transfer. Measurement results show that the 200-Hz class-E PA achieves the peak power gain of 25.2 dB and the maximum collector efficiency of 57.3%. The proposed IPT system has power delivery of 5 W and peak PTE of 9% for power transfer through the 3.1-mm thick aluminum barrier.