A capacitive touch sensor has a sensor electrode which a finger is on. I analyzed the capacitance of the sensor electrode itself with an electromagnetic simulator. The capacitance depends on the geometric design. Some people complain of its uncertainty, how much it varies. But knowledge of electromagnetics you studied in a high school helps you to grasp how the capacitance varies.
I didn't use an expensive EM simulator here. I used Sonnet Lite of
which we could use for free.
Examples
 |
 |
 |
 |
| 0.770pF |
1.46pF |
6.14pF |
6.43pF |
| Floating electrode |
Electrode surrounded with a GND plane on the same layer |
Electrode on a GND plane with t=1.6mm FR-4 PCB |
Electrode surrounded with a GND plane on the same layer and on a GND plane with t=1.6mm FR-4 PCB |
*A box around an analyzed object is a metal box of which inside Sonnet analyzes. The potential of the box is the GND level.
Contents
An electromagnetic simulator is useful for capacitance simulation of an electrode of a capacitive touch sensor as well as analysis of an RF circuit and a transmission line for high-speed digital signal. Generally an electromagnetic simulator is quite expensive. But Sonnet Software Inc. provides a free version which is limited in analysis features. This article is an example of the application.
I don't intend to seek absolute value of capacitance with the simulation. It is enough for actual electrode design to know how and where the capacitance varies.
The major limitation of Sonnet Lite is that it can not analyze a case which needs more than 16MB memory for analysis. Therefore if you have a large case with Sonnet Lite, you have to split or reduce it.
Analysis of capacitance of a sensor electrode itself is the same thing as analysis of its electric field. We can know how the capacitance varies by reflection of density of electric field lines.
We usually put a GND plane either surrounding or below a sensor electrode, or both. It increases the capacitance. The more a sensor electrode itself has capacitance, the more difficult finger detection is. It is because the capacitance difference by a finger gets smaller.
Afterward I made some kinds of simulation. And shall we see the trend.
(C)KANAYA Hidenori
Hide's Radio Shack
