As in the previous section, capacitance is a function of the geometry of the conductors and the dielectric materials around them. Altering any of those will result in a change of capacitance. Capacitive sensing is based on this property. This means that when a pointer (finger or stylus) approaches two metal plates, the mutual capacitance between the plates decreases, as the finger “steals” some electric field lines. On the other hand, the self capacitance of both conductors increases, since in this case the pointer acts as an additional conductor of the system. These changes in capacitance may define a touch event, depending on the operation principle of the sensor, that is, mutual- or self-capacitive.
We’ll talk in more depth about self and mutual capacitance touch sensors in chapter 2.
Picture 1. How the presence of a conductive pointer results in a reduced mutual capacitance.
Let’s take as an example a capacitive touch panel having two electrode layers (“double-layer pattern”). This electrode grid forms multiple nodes, that is, areas where the electrodes overlap with each other:
Picture 2. A simplified representation of the electrode grid of a capacitive touch sensor.
Having in mind the simple case of two parallel plates, we can easily understand that the capacitive coupling between two overlapping electrodes is concentrated mainly at their overlapping area. This means that if we change only the length of these electrodes, the mutual capacitance will remain almost the same (see top picture bellow). However, the self-capacitance will become higher (see left picture bellow), since it is inherently affected by the overall size and dimensions of the conductor. Of course, if the electrodes become wider or generally the overlapping area becomes larger in some way, the mutual capacitance will become higher (see right picture bellow).
Picture 3. Effects of electrode dimensions on mutual and self capacitance.
The case of parallel plates can also help us get an insight on the effect of the vertical distance between two overlapping electrodes. When this distance becomes bigger, that is, the dielectric material between the electrodes becomes thicker, then mutual capacitance is reduced. On the other hand, self capacitance is negligible affected, since the size and dimensions of the conductors remained unchanged.
Picture 4. When the thickness of the dielectric is increased, mutual capacitance is reduced.
In case that the dielectric material is replaced with another one that has a higher dielectric constant, the mutual capacitance will increase (with a slight increase in self capacitance as well). An actual example that takes advantage of this effect is the placement of a material of high dielectric constant as a cover lens, so as to increase the capacitive coupling between the electrodes and the pointer, consequently increasing sensor’s sensitivity to finger touches.
Picture 5. Using as a cover lens a material with higher dielectric constant, results in a higher capacitive coupling between the electrodes and the finger.
Capacitive sensors are driven by ICs (Integrated Circuits), so they must be connected to them through conductive paths, called “traces”, “tracking” or “routing”. Traces are relatively thin lines with a certain width and are usually placed close to each other due to space limitations. Consequently, there is a considerable coupling between two adjacent traces, causing an additional mutual capacitance that must be considered together with the one that exists between the electrodes. This mutual capacitance becomes higher for longer traces and when they are coming closer to each other; this effect can be explained by considering once more the case of parallel plates.
Picture 6. Capacitive coupling between traces is increased when (b) they become longer or (c) they are placed closer to each other.
Key takeaways from this section: