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Wireless Power Transfer (WPT)

Wireless energy transfer or wireless power is the transmission of electrical energy from a power source to an electrical load without interconnecting wires. The Serbian physician and engineer Nicolas Tesla isthe first scientist who started searching and proposing the concept of wireless energy transfer in the year 1899. Nikola Tesla was able to light electric bulbs wirelessly at his Colorado Springs Lab using electrodynamic induction. Tesla had big plans to transmit electricity wirelessly across the Atlantic Ocean which is not yet happened.

Figure 2 - Market trends for wireless power [2]

There are many different types of wireless power transfer; microwave power transmission, inductive-coupling-power transmission and laser-power transmission methods. In this article, the Inductive Power Transfer (IPT) will be examined.

The physical principle of the IPT consists of a transmitter and receiver winding separated by an air gap forming a transformer. The transmitter is supplied through a converter with high frequency currents. The time varying magnetic fields induces an EMF, by Fraday 's law of induction, on the receiver winding. The induced EMF creates the currentswhich get transferred to the connected load directly or via a power system. 

 

Due to its high reliability, efficiency and speed, the IPT can be used in many application fields such as electric vehicles. Electric cars can be charged wirelessly, and those cars can charge smart phones and laptops wirelessly.

 

Also, it is used in medicine for implanted and wearable medical devices in human body such as pacemaker. Wearable and Implantable Medical Devices (WIMD) are gaining prominence and are expected to play a significant role in saving and extending human lives, due to their ability to monitor, stimulate and regulate vital internal organs, and communicate with an external host about the state of health of these internal organs.

Pacemakers are used to regulate the heart beat in case of diseases (Arrhythmia, irregular heartbeat); when the human's heart electrical system does 'not work properly, the battery-powered pacemaker returns the heart beating to its normal rhythm with an electrical impulse. The traditional pacemakersmust be replaced periodically by surgery due to theirnon-rechargeable batteries.the use of pacemakers with rechargeable batteries increased [5], [6] after it was abandoned before.An external coil transmits power to a receiver coil implanted under the skin. The receiver coil will provide energy for the pacemaker battery. More recently, battery-less pacemakers are inventedby the cooperation of Rice University and the Texas Heart Institute [7]. These pacemakers are implanted directly onto the heart because they don't require leads and battery. They are powered by an external RF wireless system. both the last-mentioned pacemaker technologies -with wireless charged battery or RF wireless power system- can offer several advantages like increasing the lifetime and reliability of pacemakers, reducing the maintenance surgeries which can cause medical complications (bleeding, infection), a flexible external control of the pacemaker, etc.

 

 

Analysis of a WPT for pacemaker battery charging using EMS inside SOLIDWORKS

The proposed model [6], is used to recharge a pacemaker battery by inductive coupling. Figure 5 shows the simulated model. The designed model is composed of a transmitter and receiver coil, Two aluminumplates and two ferrite cores. The eddy currents increase at high frequencies because both the electric field and the human body electrical conductivity are proportional with the frequency. Therefore, the studied WPT system operates at a low frequency (20kHz) to keep the Electro-magnetic Fields (EMF) under the EMF standards limits [8]. Operating at a low frequency will decrease the WPT efficiency. To overcome this problem, the aluminum plates and ferrite cores are added to improve the system efficiency. 

 

In this article, the WPT system parameters will be computed and analyzed. AC Magnetic module of EMS coupled to external circuit will be used for this purpose. Table 1 contains the main simulation properties.

Table 1: Main analysis properties

Pacemaker for WPT and Shielding effects

The Figures 7-10 show the magnetic flux density generated by the model for different scenarios. For the model without ferromagnetic cores and aluminum plate, the magnetic flux density is symmetric around the primary coil. An important field is leaked to the air around the transmitter. For the case of model with aluminum plates, a low field is conducted to the receiver (Figure 8). A higher field compared to the previous case is observed when the iron cores are added (Figure 9). For the full, the magnetic flux is much higher than the previous cases. It is following a direct path from the transmitter toward the receiver.  The added iron cores and aluminum plates constitute a shielding for the magnetic flux generated by the WPT system. It reduces the magnetic losses and protect the human body and the electronic devices from the leakage fields. The inductive coupling is about 0.1 for the model without the shielding components while it is about 0.13 for the full model.

 

 

 

 

The circuit parameters of the WPT model are computed using EMS at the frequency of 20kHz. Table 2 summarizes these results. 

Table 2: Results comparison of the studied WPT

	Inductance LTx (uH)	Inductance LRx (uH)	Resistance RTx ()	Resistance RRx ()	Mutual Inductance MTxRx (uH)	Coupling Coefficient
EMS	4.278	3.787	16.05	19.23	51.80	0.128
Ref [3]	4.1685	3.7002	18.78	21.89	52.26	0.133

Influence of the air gap distance on the coupling coefficient

The coupling coefficient formula of the WPT system is the following:  . The WPTefficiency increases with the coupling coefficient [9], [10]. The value of k ranges between 0 and 1 (perfect coupling). In the case of perfect coupling, all the lines of flux of one coil cuts all of the turns of the second coil, that is the two coils are tightly coupled together, the resulting mutual inductance will be equal to the geometric mean of the two individual inductances of the coils and the induced voltages into primary and secondary satisfy the relation .
Figures 11 demonstrates an animation of the magnetic flux density variation versus the air gap distance separating the transmitter and the receiver. A parametric AC Magnetic study is used for this purpose.
As can been seen, the magnetic flux density reaching the secondary coil becomes lower when the air gap distance increases and inversely.

 

Figures 12and 13 show respectively the curves of the mutual inductance and the coupling coefficient versus air gap between primary and secondary coils. Both the mutual inductance and the coupling coefficient decrease with the air gap distance.

 

The induced voltage in the secondary coil is plotted in Figure 14. It has identical behavior with the coupling coefficient. Figure 15 shows the ratio of  .  This ratio gets lower by the increase of the air gap distance because the transferred energy becomes lower when the distance between primary and secondary winding increases.  These results correlate very well with the previous statements.

 

 

Figure 16 - Ratio of the voltages 

WPT operating at the resonance

The WPT operates on its highest efficiency in the resonance. Two resonant capacitances are added to both primary and secondary side. Figure below shows the external circuit coupled to the WPT system inside EMS. The source is considered as ideal (R=0). The windings DC resistance are not modeled in the circuit because they are auto-computed and internally considered inside EMS.
The resonant capacitances values are comptued using the following formula : 

 

Figure 17 shows the curve of both transmitter and receiver currents versus the operating frequency. The maximum generated current is at the resonant frequency (20kHz). 

 

WPT system inside human body

In this section, the case of a receiver implanted inside a human body is analyzed. The transmitter is located outside the human body while the receiver is located few millimeters below the skin. The human body is characterized by its low electrical conductivity which increases with the frequency.  Since the studied inductive coupling system is operating at a low frequency,the generated eddy currents in the human organs are close to zero. Figures 18a) and 18b) contain front and right views of the model meshed. A small mesh size is applied on the aluminum parts to better capture the eddy currents created in the skin depth. EMS mesher has the capability to detect and follow the curvature in the meshed components. This can explain the fine mesh in some zones of the human body.

Figures 19 a) and 19 b) show cross section views of the magnetic flux distribution in the human body. Inside the human body the flux is concentrated around the receiver because of the shielding components. The maximum value of the magnetic field is few microTesla which is less than the standard limits (27 microTesla)published in [11].

 

Conclusion

Due to their several advantages, the use of rechargeable battery pacemakersis becoming more common. EMS, as a powerful numerical simulation software, could help to design and prototype highly efficient wireless power systems to charge pacemaker batteries. It helps also to ensure the safety of the products.  The studied model allows to ensure a good efficiency in a low frequency and prevent exposure to high electromagnetic fields.

References

[1]:https://www.allaboutcircuits.com/technical-articles/introduction-to-wireless-power-transfer-wpt/
[2] : https://www.we-online.com/web/en/passive_components_custom_magnetics/blog_pbcm/blog_detail_electronics_in_action_100414.php
[3] :https://www.edgefxkits.com/blog/wireless-power-transfer/
[4] :https://en.tdk-electronics.tdk.com/tdk-en/373562/tech-library/articles/applications---cases/applications---cases/thin-and-efficient-power-transmission/980554
[5]: Wireless Power Transmission to Charge Pacemaker Battery, Qazi Saeed Ahmad1Tarana A.Chandel2 , Saif Ahmad3 Department of Electronics & Communication Engg.IntegralUniv.Lucknow,India
[6]: Wireless power transfer for a pacemaker application, Vladimir Vulfin, Shai Sayfan-Altman & Reuven Ianconescu, Journal of Medical Engineering & Technology.
[7]:https://www.dailymail.co.uk/sciencetech/article-4575120/Battery-pacemaker-end-battery-replacement-surgery.html
[8]:Human Exposure to Close-Range Resonant Wireless Power Transfer Systems as a Function of Design Parameters, Xi Lin Chen, Aghuinyue E. Umenei,  David W. Baarman, Nicolas Chavannes, Valerio De Santis, Juan R. Mosig, and Niels Kuster, IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY
[9]Design and Evaluation of a Wireless Power Transfer System with Road Embedded Transmitter Coils for Dynamic Charging of Electric Vehicles, KraisornThrongnumchai , Akihiro Hanamura , Yuji Naruse , Kazuhiro Takeda, EV System Lab., Nissan Research Center, Nissan Motor co., ltd., 1-1, Morinosatoaoyama, Atsugi-shi, Kanagawa 243-0123, JAPAN. World Electric Vehicle Journal Vol. 6 - ISSN 2032-6653 - © 2013 WEVA Page
[10]: Wireless (Power Transfer) Transmission of Electrical Energy (Electricity) Intended for Consumer Purposes up to 50 W, Marek PIRI, Pavol SPANIK, Michal FRIVALDSKY, Anna KONDELOVA. POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 | NUMBER: 1 | 2016 | MARCH
 [11]: ICNIRP GUIDELINES FOR LIMITING EXPOSURE TO TIME?VARYING ELECTRIC AND MAGNETIC FIELDS (1HZ – 100 kHZ)