Hello everyone, my name is Ryo Takahashi, from the University of Tokyo.
Today, I’ll be presenting Meander Coil++, our work on a safe and efficient textile coil
for body-scale wireless power transmission.
First, I want to thank my co-primary author, Wakako Yukita, and my advisor, Tomoyuki Yokota,
Takao Someya, and Yoshihiro Kawahara.
As you might know, wearable devices like smartwatch and smartglass are promising factors toward
wearable computing for personal healthcare, activity recognition, and HCI.
However, different from smartphones, most wearable devices suffer from low battery capacity
because the wearables must be compact to be seamlessly worn by the user.
This requires the user to take off and charge the wearables every couple of days, which
is really cumbersome.
So, we aim for continuous wearable computing by directly charging the wearables attached
to the body or clothes.
Here, we present Meander Coil++, which enables both body-scale and energy-efficient wireless
power transmission.
The power transmission of Meander Coil++ is safe, which allows us to transmit tens of
watts power.
Also, its transmission efficiency is approximately 40%, enabling relatively effective transmission.
In total, the wearable devices placed throughout the clothes can receive a few watts power,
which is enough for maintaining power-hungry wearable devices for a week.
Some previous researches were trying to realize continuous wearable computing, using energy
harvesting, wired power transfer, and wireless power transfer.
However, none of them support both body-scale and energy-efficient charging system.
For example, energy harvesting like solar power mainly suffers from the inefficient
energy conversion of the harvesters.
For stable charging, wired power transfer throughout the textile seems promising.
But, the wires need to be exposed for electrical connections, which leads to safety issues.
So, we focus on textile-based wireless power transfer which can charge the wearables without
requiring the wearables to be directly connected.
Unfortunately, current textile-based wireless power transfer methods suffer from their low
efficiency owing to the use of flexible but lossy conductive threads.
Let me explain in more detail.
To realize energy-efficient and body-scale wireless charging, we use wireless charging
technique using a coil, where a transmitter coil can send power wirelessly
to a receiver coil, as shown in this slide.
However, to this end, we need to overcome two challenges.
First, the conductive thread compatible with textile is lossy, which decreases the power
transfer efficiency to a few %. Second, standard coil structure, or helix
coil inevitably generates a strong EM field in the body, thus, limiting the amount of
transmission power up to a few watts.
In total, a receiver, or wearable can receive only tens of mW power, which is quite small.
To solve these two challenges, we combined two approaches.
First, as a low-loss conductive thread, we use liquid metal contained in a stretchable
silicone tube.
It is known that liquid metal is liquid at room temperature and its conductivity is quite
high compared to the one of the conductive thread.
So, the efficient textile coil can be realized.
Second, to avoid the EM interaction with the body, we develop a safe and efficient coil
structure called Meander Coil++.
Different from the standard helix coil, Meander Coil++ changes its winding direction for some
turns, inspired by meander coil.
With this winding, Meander Coil++ confines a strong EM field around the clothing, while
canceling the EM penetration into the body.
Let me show you the difference of EM field generated by the standard helix coil and Meander
Coil++.
This is the result of EM simulation, and as you can see, Meander Coil++ confines the strong
EM field around the clothes.
So, Meander Coil++ can transmit ten times power more than the helix coil.
For more detail, please refer to our paper.
Then, let me explain the fabrication process of Meander Coil++.
First, knit the clothes which have the pocket for inserting the tube.
Then, insert a liquid metal, or eGaln into the silicone tube.
Finally, insert the tube into the textile and solder the multiple chip capacitors for
tuning the resonant frequency of the Meander Coil++.
The capacitors also work to weaken the unnecessary capacitive coupling between the body and Meander
Coil++.
This is the prototype of Meander Coil++.
Meander Coil++ can transmit power up to 41% efficiency in AC, and 25% efficiency in DC.
Its weight is approximately 119g, which is similar to iPhone 13 mini.
Then, I’ll show you a power transfer module.
The module is connected to Meander Coil++ and transmits power up to 10W.
Also, the module automatically tunes the resonant frequency of Meander Coil++ because Meander
Coil++ changes its inductance according to the user style.
Finally, the module transmits power only when detecting the RX coil for saving power.
As application examples of Meander Coil++, we show four demos.
Each demo targets power-hungry on-body devices like smartphone, smartwatch, neck fan, and
sensors like Arduino.
We envision that Meander Coil++ enables a wearable device that can maintain high performance
over a long period because Meander Coil++ serves as a large battery pack for compact
wearable devices.
Let me summarize our work.
As a body-scale wireless charger, we proposed Meander Coil++.
Meander Coil++ employs a safe coil structure with a low-loss liquid-metal-based conductive
cord.
Our evaluation showed that Meander Coil++ enables to transmit a few watts of power to
the wearable devices placed throughout the clothing.
As future work, we plan to simplify the fabrication process with inlay knitting technique, which
inserts a thick tube like our conductive cord while knitting textile with a knitting machine.