Hello, everyone.
My name is Ryo Takahashi, from the University of Tokyo.
Today, I’d like to talk about this project, Telemetring.
We use passive inductive telemetry to enable a batteryless and wireless ring-shaped keyboard.
This work has been done with Massaki Fukumoto from Microsoft,
and my colleges, Changyo, Takuya, Yoshiaki, and my advisor Yoshihiro Kawahara.
As we all know, wearable devices like smartwatch and smart glass are popular.
They notify the message quickly or track the daily activity constantly.
However, they lack the rich input interface.
Because their touchscreen area is very limited, it is difficult to type text.
So, previous research has explored wearable input interface.
The simple method is to track finger gesture like finger drawing and thumb-tapping.
It is also possible to input text by finger typing like a keyboard.
To detect the typing, a series of FingeRing, it is also called ring-shaped keyboard, has
been proposed.
FingeRing consists of five ring-shaped sensors and one wristband-shaped reader.
Each sensor detects the finger typing and sends the information to the reader, via wire
or wireless channel using body coupling.
It is known that ring-shaped keyboard supports high speed text input with small typing force.
However, the previous wireless ring-shaped keyboard requires batteries to each ring because
the ring needs to empower the components like microcontrollers or wireless communication
modules.
These cause two problems.
First, the battery is bulky, so this makes the users uncomfortable to wear the five rings.
Second, the battery-equipped rings require the users to charge them manually.
So, what can we do to push the boundaries of the wireless ring-shaped keyboard?
The simple answer is to remove the battery from the ring.
Here, I introduce the first batteryless and wireless ring-shaped keyboard named TelemetRing.
TelemetRing consists of five battery less ring-shaped sensor coil and a wristband-shaped
readout coil.
Because the rings and the wristband are coupled inductively, wristband can receive the sensor
information wirelessly.
TelemetRing supports text input by finger typing like this.
By decoding the typing sequence to the characters, we can type text like UIST.
To make the batteryless ring-shaped sensor and communicate with the sensor wirelessly, there are some methods.
Energy harvesting seems to be the intuitive way.
Using harvested energy, the sensor can empower the components like microcontrollers and wireless
communication modules.
However, it is difficult to get enough energy anytime and anywhere.
Although vibration caused by typing seems to be useful energy source, we cannot store
enough energy to empower the components.
Wireless power transfer is also promising way.
By transmitting energy from the wristband to the ring, the sensor attached to the ring
can work without battery.
But, in our case, the distance between the ring and the wristband is too far.
So, the efficiency that we measured is under 1%.
Because of the very low efficiency, we cannot send enough power to the sensors.
Therefore, Telemetring uses passive inductive telemetry.
Passive inductive telemetry is one kind of wireless sensing technology and this mainly
uses coil.
This consists of three components: a passive sensor coil, an external readout coil, and
an impedance measurement circuit connected to the readout coil.
As you can see, the two coils couples inductively.
Because of the inductive coupling, the impedance change of the sensor coil also changes the
input impedance of the readout coil.
So, the impedance measurement circuit can detect the sensor value wirelessly.
Also, this telemetry has two advantages.
First, the sensor coil does not need the battery because the sensor coil can change its impedance
through a passive variable capacitor.
As you know, the variable capacitor can be easily designed using passive materials like
piezo, thermistor, or force-sensitive resistor.
Second, the inductive coupling is robust near human.
So, we use the telemetry toward realizing the battery less and wireless ring-shaped
keyboard.
However, the inductive coupling is very weak because the distance between the ring and
the wristband is very long and the size of the ring is very small.
The coupling strength that I measured is about 0.005, which is about one-tenth compared to
the one of the previous telemetry.
Because of the very weak coupling, the change of the input impedance is extremely small
and the previous telemetry cannot detect the sensor change.
So, we need to develop a new telemetry system that is very high-sensitive.
From here, I will talk about how TelemetRing works.
First, we design a shock-sensistive variable capacitor by combining a piezo and varactors.
By the typing shock, the piezo generates a voltage and the voltage is applied to the
varactors.
Then, varactors change their capacitance value depending on the magnitude of the voltage.
In our case, the capacitance of the varactor lowers approximately 1pF.
Next, we distinguish which sensor coil reacts to the typing shock.
To realize this, we utilize the knowledge that the impedance change of the sensor coil
can be most clearly appeared around the resonant frequency of the sensor coil.
So, we tune the five sensor coils at the different frequencies from f_1 to f_5.
At the f_i, the circuit can be simplified in the way that the only one sensor coil is
coupled with the readout coil because the other four sensor coils are off-resonance.
As I explained before, the change of the input impedance is extremely small because of the
very weak coupling.
To make the change bigger, it seems great to increase the inductance value of both sensor
and readout coil.
However, the number of turns, the total length of the coil is limited to the wavelength.
If the wire of coil is too long at the high frequency, the wavelength is so small that
the coil becomes capacitor.
As a result, we need to decrease the number of turns of coil at a high frequency.
To overcome the challenge, we use distributed reactance compensation technique, simply called
DRC, which can make a coil with a high inductance value at a high frequency.
The idea of DRC is very simple.
To mitigate the limitation of the short wavelength, it inserts the multiple capacitors into the
coil.
Imagine that the multiple capacitors split a single long coil into multiple small coils.
Each coil is so short that the wavelength effect can be ignored.
With this, we can increase the number of turns at a high frequency.
In our case, the sensor coil has two inductance in series by inserting two varactors.
Also, the readout coil has ten inductance in series by inserting ten chip capacitors.
Because of DRC, the input impedance of our readout coil can slightly change when typing
occurs.
Finally, I will talk about how we detect the small change of the input impedance and distinguish
which fingers are typed.
First, we use a balanced bridge circuit.
Using this special circuit, the small change of the input impedance can be appeared through
the change of the output phase.
Based on the output of the bridge circuit, next, we calculate a phase spectrum.
As talked before, we assign the different resonant frequencies to our five rings.
So, the finger typing actions can be detected independently by tracking the phase change
at each resonant frequency.
Finally, we pick up the typing action by detecting the sudden phase change and decode the typing
sequence to the characters.
Here, we explain the working principle of our balanced bridge circuit.
This is a variation of the standard Wheatstone bridge circuit, which detects the impedance
change.
The output can be written using reference load Z_bridge, measurement load Z_in, and
input voltage V_in.
To detect the small impedance change of the readout coil, we need to increase the sensitivity
to the maximum.
So, we match the Z_bridge to the readout coil Z_readout.
With this, V_out can be expressed like this.
Notably, the small impedance change Z_in at f_i can appear clearly through the phase change
of V_out at f_i.
So, we can detect the typing action through the phase change.
This is the prototype of TelemetRing.
The resonant frequency of the readout coil is 13.56 MHz, and the five resonant frequencies
of the sensor coils are assigned around 13.56 MHz.
A USRP emits multiple frequency signals and a PC connected to the USRP calculates the
phase spectrum through the output of the balanced bridge circuit and detects the typing action.
Here, I will show the typing recognition rate about TelemetRing.
I tested 31 typing patterns.
As you can see, the four typing patterns show very bad rate because it is difficult to type
these patterns.
Try and feel it yourself.
Based on the result, I use the patterns except the 4 bad patterns for A to Z character inputs,
as shown in the next slide.
As you can see from this table, A to Z, 1 to 9, and symbols are assigned to the different
typing chords.
Based on the table, the users can input text very fast with small typing force.
Of course, I need to improve the recognition rate over 99% to realize the more robust typing
input.
Finally, I would like to summarize my work.
In this work, I introduce TelemetRing, a batteryless and wireless ring-shaped keyboard.
To realize TelemetRing, I develop very high-sensitive passive inductive telemetry by combining distributed
reactance compensation and the balanced bridge circuit.
As future works, first, we need to improve typing recognition rate.
Also, we need to implement a stand-alone telemetering.
Although the current TelemtRing is connected to the power source, we will re-implement
the stand-alone version as a true wearable device.
Finally, we hope our passive inductive telemetry will be utilized for other wearable devices.
Our telemetry has the potential to make them batteryless and wireless wearable devices.
Thank you and happy to answer your questions.