Touch detection in a capacitive touch sensing system
Before diving into the basic principles of each technology, we need to say a few words about how a touch event is actually detected in a capacitive touch sensor system.
The electrodes of the sensor, regardless of the technology, are connected through wiring (usually called “traces”, “routing” or “tracking”) to the controller of the system. Being the “brains” of the whole system, the controller uses an Analog-to-Digital converter (ADC) and its firmware to detect a change in the sensor’s state. Since we are talking about touch sensors, this change is most likely caused by a touch event. The controller’s firmware is also responsible for the action that the touch event triggers (for example communicating with the CPU to start an application). To sum up, a change in capacitance (physics) is translated into a change of a digital signal (controller), which ends up in the user interface display (user experience).
Figure 1. Capacitive touch detection principle.
Self-capacitive touch sensors principle
There are two main types of capacitive touch sensor technology; self-capacitive and mutual-capacitive sensors. A self-capacitive system measures changes in capacitance with respect to earth ground. Considering a parallel-plate model, the electrode forms one plate of a capacitor, with the other plate being either ground or the user’s finger. A touch causes the electrode capacitance to increase, as the human body “adds” capacitance to that of the system.
Self-capacitive measurement employs a single electrode and measures the change in capacitance with respect to ground caused by a typical user’s touch; the finger results in a higher capacitance compared to the baseline measured value . Any parasitic capacitances to ground in the system should be minimized, as they reduce the effect of the user touch and make the touch detection harder.
Self-capacitive electrodes project Electric field lines in all directions as shown in the picture above, so interaction can occur on both sides of the electrode. Ground or driven (active) shielding is often added to limit sensitivity to only the desired direction. The following animation shows the change of the self-capacitance of a touch button with respect to finger’s distance.
Mutual-capacitive touch sensors principle
Mutual-capacitive systems also measure a change in capacitance. In contrast to the single electrode design discussed earlier, this approach uses two electrodes that together represent the two plates of the capacitor. The user’s finger modifies the field between the two electrodes and reduces the capacitive coupling between the electrodes. To make things simpler, the human body now “steals” mutual-capacitance from the system of these two electrodes, thus reducing it.