3.1 Electric field around an electrode

Here I suppose the electric lines about 4 cases of electrodes.

2.1	Floating electrode		Radial electric lines from the electrode
2.2	Electrode surrounded with a GND plane on the same layer		Electric lines gather in the gap between the electrode and the GND plane. Electric lines to the celing and floor are few.
2.3	Electrode on a GND plane with t=1.6 FR-4 PCB		Electric lines densely gather in the gap between the electrode and the GND plane below. Electric lines to the ceiling and sidewalls are few.
2.4	Electrode surrounded with a GND plane on the same layer and on a GND plane with t=1.6 FR-4 PCB		Electric lines densely gather in the gap between the electrode and the GND plane below. Electric lines to the GND plane on the same layer are not many. Electric lines to the celing are few.

In Case 2.4 the density of electric lines to the GND on the same layer may get bigger than that to the GND below. For example, getting the distance to the GND on the same layer short and getting the distance to the GND plane below long make the field on the same layer strong.

3.2 Expected operation as a touch sensor

I suppose the operation as a touch sensor in using each type of electrode.

2.1	Floating electrode	It is easy to detect a distant finger from the electrode. It is easily influenced by object around the electrode.
2.2	Electrode surrounded with a GND plane on the same layer	A sensor detects a finger in nearer distance than 2.1. It is not easily influenced by object around. Touching at the gap between the electrode and the GND makes the capacitance change big.
2.3	Electrode on a GND plane with t=1.6 FR-4 PCB	A sensor detects a finger in nearer distance than 2.1. It is not easily influenced by object around. Capacitance change in touching gets small because the capacitance of the electrode itself gets bigger.
2.4	Electrode surrounded with a GND plane on the same layer and on a GND plane with t=1.6 FR-4 PCB	Same as 2.3. In addition touching at the gap between the electrode and the GND makes the capacitance change big.

3.2.1 An electrode for a proximity sensor

An electrode like Case 2.1, which has radial and spread electric lines, is good for a proximity sensor. It is good performance to detect a finger in long distance and to be influenced by objects around.
On the other hand electrodes which have a close GND plane has weak electric field in long distance. So the sensitivity is not good as a proximity sensor.
When you use these types of electrodes for both button function and a proximity sensor, you have to increase the sensitivity of the control circuit in the proximity mode.

4. Capacitance change with a finger on

I made a simulation in touching the electrode.
I put a t=1.0mm acrylic board on the electrode as an overlay.
A finger is simulated as a 10mm-by-10mm conductor. The capacitance of human body is supposed 100pF.

Fig. 7 is the 3D drawing of the geometry. In this 3D drawing the line from the pseudo finger plate to the box wall is a capacitor of Sonnet.

Supposition

Human model	capacitance 100pF
Finger touching area	The same area to the sensor electrode
Sensor overlay	t=1.0 Acrylic plate Dielectric constant 2.8
Other	Human body and the sensor unit are grounded.

Fig. 7

I got |Z|=1680 ohms. The calculated capacitance was 9.47pF. The capacitance in open was 6.44pF. Touching the sensor makes change +50% in capacitance.

Result

State	Impedance	Capacitance
Open	2471 ohm	6.44pF
Touching	1680 ohm	9.47pF

5. Conclusion

I introduced how to estimete the capacitance of a capacitive touch sensor electrode with an electromagnetic simulator. All simulations done here were conducted by Sonnet Lite, which is a free version simulator. In case that analysis geometry exceeds the memory limit of Sonnet Lite, I reduced the area of analysis. If a analysis geometry has few electric lines to the Sonnet metal box and you get the box smaller, we can get a result without big error.

End of document.

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