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Localization of Touch on Granite Based on
AC Hum Noise
H. Elfekey, S. Okamoto, and Y. Yamada
Department of Mechanical Systems Engineering, Nagoya University
Nagoya, Aichi Prefecture, JapanAbstract—Touch sensors are used as standard human tomachine interfaces in many applications. However, current
technologies that allow everyday objects to be used as touchsensitive surfaces are either expensive in terms of processing orhardware. In this paper, we propose the use of AC hum toimplement a granite touch bar using three electrodes anddemonstrate the localization of touch on the granite bar.

LinecheKeywords—AC hum; touch panel; localization; granite

A/DConverter

I. INTRODUCTION
Touch detection sensors are employed in many applications
including smart phones and augmented reality systems. The
localization of touch has been realized in various ways. For
example, the traditional capacitive touch technique has been
widely used for touch panels. Usually, capacitive touch panels
require a very conductive surface such as indium tin oxide [1].
Touch localization can also be achieved by image processing,which does not require highly conductive materials [2]. Other
techniques leverage infrared photo couplers and the effect ofsound transfer [1]. All of the mentioned techniques use special
surfaces or equipment installed on or around the surface.
Furthermore, touch interfaces based on the abovementioned
techniques typically lack the natural touch sense of materials
because of the smoothly finished material surfaces [2].

GroundFig. 1.

properties. Therefore, the nearby 220/110 V induces a voltage
on the human body. This voltage can be channeled to a device
by electrodes for processing [4].
In the present study, we use granite, a type of natural stone,
as a touch sensitive surface. The relative permittivity of granite
ranges from 4 to 6 [5]. Therefore, AC hum (a 50/60 Hzsinusoidal wave) can travel through granite. In other words, ahuman finger touches the granite, and the electrodes mounted
on the edges form capacitances, as shown in Fig. 1.

We propose the integration of AC hum with everyday
objects. AC hum can localize touch on a broad range ofmaterials because it does not require highly conductivematerials. As an example, we demonstrated the use of AC hum
with modified acrylic paint in previous research [3]. In thispaper, natural stones, such as granite, will be turned into touch
interfaces by the AC hum touch technique. These techniques
further allow indoor objects to be used as touch interfaces,
preserving the haptic feel of their material surfaces.

Although the magnitude of che is dependent on the locationof the touch, it cannot be reliably used to calculate the touch
position. This is because the voltage detected by the electrodedepends on factors other than the touch location. For instance,
the area of touch affects the voltage detected at the electrode.
Therefore, more than one electrode is required to localize thetouch location effectively. With the addition of an electrode,
three more capacitances are added to the model presented inFig. 1. For example, an additional electrode would yield two
more capacitances between both the line and ground and theelectrode and capacitance between it and the finger touching
the surface.

II. THEORY OF OPERATION
The human body is affected by surrounding
electromagnetic fields. This is because the human bodycontains minerals and salts, and thus it possesses conductive

978-1-5090-4045-2/17/$31.00 ©2017 IEEE

Concept of the AC hum granite touch panel.

1632017 IEEE 6th Global Conference on Consumer Electronics (GCCE 2017)

Electrode 1

1Fig. 2.

Electrode 3
2Electrode 2
4536x

Granite touch panel used with mounted electrodes.

Sample number(S)R1Fig. 4.

725k

Input
from thecorresponding
electrodeFig. 3.

OUT+

Output tothecorresponding pin
in the A/DConverterxCurrent-to-voltage converter circuit schematic.

III. DESIGN AND IMPLEMENTATION

Distance(cm)In this paper, we demonstrate the use of surfaces as touch
panels by AC hum. We use a granite bar (7 cm by 45 cm) thatis commonly used for table tops and attach three electrodes to it
as shown in Fig. 2. Each of these electrodes is connected to a
current-to-voltage converter with the circuit shown in Fig. 3.
The resistance R1 was chosen to be 725 kΩ empirically.

Fig. 5.

x = (v1+v3)/( v1+v2+v3),

In this paper, we have presented a methodology based on
AC hum noise to use materials such as granite into touchsensitive surfaces. It can be noted that the AC hum average
amplitude signal fluctuates, which affects the resolution of thesignal adversely. A possible solution is to add electrodes which
will be explored in the future.
ACKNOWLEDGMENT
This study was partly supported by MEXT/JSPS Kakenhi(17H04697).



where v1, v2, and v3 are the voltages detected at electrodes 1, 2,and 3, respectively, as shown in Fig. 2. v1 and v2 are thevoltages of the electrodes at the edges, while v3 is the voltage atthe middle electrode. The voltage at each electrode wasaveraged over 100 ms and used for the localization via theequation above.

REFERENCES
[1]
[2]

[3]

IV. EXPERIMENTATION AND RESULTS[4]In Fig. 2, six red circles indicate the location of the touch. They
are each separated by 8 cm. Fig. 4 shows 50 samples for eachtouch location. A touch location is enumerated in Fig. 2 withthe corresponding samples in Fig. 4. It can be observed that thesamples fluctuate greatly. Fig. 5 shows the interpolated graphbased on the averages of points in Fig. 4. The function
interpolated was rational with a second-degree numerator and athird-degree denominator.

978-1-5090-4045-2/17/$31.00 ©2017 IEEEThe ratio x versus the actual touch distance.

V. CONCLUSION

The signal processing algorithm begins with an analog-todigital converter with a frequency of 50 kS/s for 100 ms orabout 5 cycles of the AC hum. Then, a low-pass digital filter isapplied followed by envelope detection to extract the averagemagnitude of the AC hum signal. Each of the mentioned steps
is applied to each of the three electrode voltages. The following
equation was used to localize the touch in the x-domain only:Average ratio x from multiple touch points.

[5]

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