Chapter 2:
Operational principles & types of capacitive touch sensors
2.2 Capacitive touch sensor applications
Capacitive touch buttons, sliders, wheels (Zero or One dimensional sensors)
These applications use mostly self-capacitive sensors, because they are simpler to be implemented and provide accurate and robust performance regarding touch detection. The basic idea behind this application is that each electrode is scanned separately and the controller provides a simple 1/0 answer to whether the electrode is “touched” or not. Combining a number of independent electrodes close to each other, the system can implement sliders, wheels and more complex functions.
Mutual-capacitive buttons are also available in the market, though. This case requires two discrete sets of electrodes, called Transmitters (Tx) and Receivers (Rx). The controller now tracks the change in the mutual-capacitance between each couple of Transmitter-Receiver and detects the touch event. These applications often include more complex sensor designs and can implement multiple button combinations with fewer trace lines, by “grouping” them. For example, a system comprised by 4 Transmitting lines and 3 Receiving lines, can accommodate 12 discrete buttons. However, the complexity is not only increased in the sensor patterns, but also in the trace routing (which can also require vias or through-holes).
Picture 1. Capacitive touch buttons, slider, wheel (Source).
Capacitive touch screens (Two-dimensional sensors)
Capacitive Touch Screens most commonly utilize the mutual-capacitance principle and there is a very good reason behind this. The touch sensor in a touch screen needs to be able to recognize not only that a touch event is occuring (like before), but also where it occurs. That’s why the idea of electrodes matrix was created. The basic concept behind this is that the touch module consists of the 2 groups of electrodes mentioned before, called Receivers and Transmitters. Depending on their orientation, they are also usually called X or Y electrodes, just for simplicity reasons.
Picture 2. Capacitive touch screen (Source)
These electrodes are usually located in different levels in the Z-Axis (Double Layer implementation), but there are also thousands of different configurations, including also Coplanar designs (that include bridges) and even more complex ones.
Picture 3. Electrode matrix consisting of 3 X electrodes (purple) and 3 Y electrodes (pink).
In a Self-capacitance system, each row and each column is scanned as an independent electrode. Combining the data from all of the row and column electrodes, the controller is able to determine the position of a touch. However, this implementation has one major drawback: ghost touches in multi-finger usage.
For example, in the following picture, when two fingers touch the positions marked with the tick marks, the controller will identify touches in columns X1 and X2 and in rows Y0 and Y3. This will lead to 2 more false finger detections in the positions marked with X. These points are called “ghost points”.
Picture 4. False multi-touch detection in self-capacitance mode (Source)
This problem is easily solved with the introduction of the Mutual-capacitance principle. In such a design, the columns can be configured as Tx electrodes, and the rows as Rx electrodes. Every row/column intersection point then has a unique Tx/Rx combination and a unique mutual capacitance. Since each node is a unique capacitance, multiple finger positions can be recognized simultaneously for multi-touch operation.
Picture 5. Correct multi-touch detection in mutual-capacitance mode (Source)
Key takeaways from this section:
Self-capacitive touch sensors
Simple sensor designSimple trace routing
One controller channel
required for each electrode thus, more channels
Difficult water/liquid handling
Low material costs
Extremely difficult to implement multi-touch (screens)
Mutual-capacitive touch sensors
Complex sensor designComplex trace routing
Electrodes can be grouped, resulting in fewer controller channels
Easy water/liquid handling
High material costs
Easy to implement multi-touch (screens)
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