The art of powering wearable electronics
The number of smart devices that we use every day is growing, especially as we approach the ramping up of the IoT and 5G. We all use multiple devices now and they all will mostly need charging and maintenance of their power.
The wearables market currently is composed primarily of wristbands and smart watches. The biggest challenge for wearable devices has been keeping these devices properly and conveniently powered. Present designs of wristbands and watches force the user to remove the device and plug it into a wired charger (That’s why I stopped using my device). Even inductive charging solutions, such as Apple’s iWatch still requires its removal and attaching it a charger.
Until the next phase of human evolution endows us with more arms, fingers, ears, etc.---we will need an unobtrusive means of charging and using such devices---enter the art of powering the wearable with new and imperceptible ways to charge and use these wonderful, evolving technologies.
In this article, I will introduce some of the most recent developments in this field as food for thought. I am hoping this will help enable a designer’s creative process towards some new and innovative solutions which are sorely needed in this industry
Wireless Power (Nikola Tesla would approve)
All of our devices will need individual charging, so wireless power is a sure winner, and my number one choice, in that kind of an application.
Inductive power transfer at the garment level1
Using a wearable garment, that would contain multiple smart devices, as a power distribution backbone1 would make the most sense (More research needs to be done here). In addition, power transfer between multiple garments could use bi-directional inductive power transfer techniques. Reference 1 chooses a circuit based upon an LCL-LCL topology vs. a Series-Series (SS) topology2, since SS topology load current varies with a varying load---such as a battery. (See Figure 1)
Figure 1: Shown here are two of four compensation circuit topologies used in Wireless Power Transfer (WPT) designs: (a) SS topology and (b) LCL-LCL topology. (Image courtesy of Reference 2)
This circuit operated at 99 kHz in order to have a two-way power exchange between smart devices.
The bi-directional power sharing between multiple devices makes sense. One device (e.g. A smart phone) may have a larger capacity battery than some of the smaller devices (e.g. A fitness tracker), so it could be used to supply power to those smaller devices as well. A wearer can then easily charge the smartphone while extending the smaller device usage time.
A key to success here is to craft the wearable vest or garment in the most non-obtrusive manner possible to the user in its design. One good way to achieve this is the use of Feed Coils1 made from flexible materials. See Figures 2 and 3.
Figure 2: The ideal circuit diagram for the feed coil (a) and its equivalent circuit (b) (Image courtesy of Reference 1)
Figure 3: The upper figure shows the flexible form-factor feed coil made from copper tape; below is an earlier prototype of a rigid 24 Gauge Wire feed coil (Image courtesy of Reference 1)
Now the bi-directional power transfer circuit could be constructed based upon an LCL inverter where two-anti-phase square waves enter into input 1 and 2 respectively of the four MOSFET inverters. See Figure 4.
Figure 4: The bi-directional inductive power transfer circuit (Image courtesy of Reference 1)
When the whole system is constructed, the following Figure 5 shows the complete power sharing system.
Figure 5: Here is the schematic of the full power-sharing system. On the left is the transmitter, with the feed coil in the middle, and the receive coil can be seen on the right in the diagram. (Image courtesy of Reference 1)