Analysis of capacitive touch sensing technology in home appliances

Because many of the components needed to build a system are now integrated in a unified chip, SoC can significantly accelerate the product listing process. In addition, the time required to connect multiple devices and use for fault debugging can be greatly reduced.

Capacitive sensing technology

The keys based on capacitive sensing technology make the electrical appliances stylish and beautiful, eliminating wear problems, and are more popular than mechanical keys. Many electrical appliances must be used in close proximity to liquids, so consider waterproofing to avoid erroneous button triggers when water splashes up. Figure 2 shows a typical layout of a capacitive sensor printed circuit board (PCB). Waterproofing can be achieved by connecting protective electrodes instead of grounding the shaded part (blue) around the sensor.

The signal driving this electrode is the same as the signal connected to the sensor. Therefore, when the water drops are dropped on the sensor, the potential difference between the shadow area and the sensor is not formed, and no more capacitance is coupled to the sensor, so the waterproof effect can be achieved. If the sensor is completely immersed in water, the protection mechanism will be activated to disable the sensor.

Since many appliances have to work with large currents, the power supply usually generates huge noise / ripple due to changes in load current and digital switches. Digital switches are also coupled to capacitive sensors, which affects the reliability of the user interface. According to the correct schematic diagram and layout guidelines, we can reduce the ripple in the power supply through techniques such as ground wire layout to ensure that there is no ground bounce. However, it is difficult to completely remove noise. If the reference is driven by the + ve line of the power supply, a false alarm of capacitive sensing may be generated.

By switching the sensor between GND and VREF (internally generated), any change in the power supply will not cause an erroneous trigger as long as it is within the specified product operating limits.

Higher integration, lower cost

SoC has integrated on-chip peripherals and hardware-based features that can help designers implement most or even all electrical functions.

In today's fiercely competitive consumer market, appliances must have a wealth of features. Integrated SoCs can not only integrate more and more application features, but also reduce overall system costs, so they have become the best choice for OEMs. This method is also suitable for small appliances such as induction cookers, microwave ovens, and large appliances such as washing machines and refrigerators (of course, large appliances still require special control for specific functions such as FOC motor control).