Chapter 5:
Basics of controllers for capacitive touch sensors
5.1 The role of the controller in a capacitive touch system
Seeing the full picture of capacitive systems
In the previous chapters, we have described the basic principles regarding the design and operation of capacitive touch sensors. Regardless of the complexity of their design and operation principles, these devices are nothing more than their name suggests: sensors. A sensor is simply a device that receives and responds to a signal or stimulus.
In the case of capacitive touch sensors, the device we are trying to build is called to respond to stimuli happening in its environment (i.e. movements of the user’s fingers or gestures). In response it should emit an electrical signal, that as we have seen before happens due to the change in capacitance in the area of the action. So, in essence, a capacitive touch sensor is a device that translates physical movements in the environment into an electrical signal.
Figure 1. The role of the capacitive touch sensor.
So far, we have kept the capacitive touch sensor isolated from all the other components one might encounter, in an attempt to reduce complexity. However, an isolated touch sensor offers little to no value: simply being able to detect a change in capacitance and emitting a signal has little practical use. That is why, capacitive touch sensors are always used as part of large and complex electrical systems. These systems incorporate many more components besides the capacitive sensor, such as memories, microprocessors, and signal conditioners. In these systems, the capacitive touch sensor acts as the input mechanism that interfaces the physical world with the rest of the system: it detects actions in the environment, translates them into electrical signals, which are then passed to the rest of the system to be processed. Perhaps the most important component that has to work in tandem with the capacitive touch sensor in such systems is the touch microcontroller.
What does a controller do in a capacitive touch system?
Capacitive touch microcontrollers, or simply microcontrollers (MCUs) or controllers, are perhaps the most elusive components in the capacitive sensing industry. Even though their importance is undeniable and cannot be overstated, a veil of mystery surrounds their operation principles and tuning process. But before we dive into these deeper topics, let us first brush up the basics.
As we discussed earlier, touch sensors emit an electrical signal following a touch (or gesture) event. This is just the first step towards translating the touch event into actionable data for the system. The signal emitted by the touch sensor is an analogue signal. Unfortunately, analogue signals are not very useful for the rest of the system. On the other hand, digital signals are. So, what we need is a device that transforms the analogue signal emitted by the touch sensor, into a digital one, that can be used by the rest of the system. Ideally, the same component would be able to extract as much information as possible regarding the actions of the user.
That is exactly what a touch controller does. A touch controller receives as input the analogue signal, and transforms it into digital data as output, while extracting information about the user’s actions (e.g. the touch location or its duration). Essentially, the main task a touch controller performs is that of an analogue-to-digital converter (ADC converter).
Figure 2. Capacitive touch sensor and controller cooperation.
Equally important to the ADC conversion, is the extraction of information of the user’s action the controller performs. Anyone who has ever used a smartphone knows that it is not just simple taps that the device recognizes: swipes, long presses, or even gestures in some cases are all made possible by touch microcontrollers. Obviously, the more complex the touch event the controller is called to recognize, the more complex its design needs to be (and in turn, the more expensive the controller itself).
Key takeaways from this section:
Capacitive touch sensors translate movements happening in the physical world to electrical signals, sensing the change in capacitance those movements cause.
Touch systems, as a whole, require digital data to operate. This is the main operation controllers execute: analogue to digital conversion of the signals emitted by the touch sensor.
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