What is capacitive sensing?

What is capacitive sensing? Capacitive sensing is a form of sensing based on touch operations. As an alternative to traditional mechanical buttons and sliders, capacitive sensing technology can also be used to design touchscreens, touchpads, and proximity-sensing devices. This technology does not sense the specific state of the button, but is used to detect the presence of a conductive object, which in many cases is the user's finger.

  How Capacitive Sensing Works

  So, what does capacitive sensing work? The schematic below shows a cross-section of a capacitive sensing button. As shown, under the cladding material, there are areas of conductive copper blocks and conductive sensors. When the two conductive elements are very close to each other, a capacitance value, marked Cp in this figure, is formed due to the coupling phenomenon between the sensor pad and the ground plate. Cp is a parasitic capacitance, typically on the order of 10pF to 300pF. The proximity of the sensor to the ground plane also creates a fringing electric field that penetrates the overlay. Basically, human tissue is also an electrical conductor. Placing a finger near the fringing electric field increases the conductive surface area of ​​this capacitive system.

capacitive sensing

  However, this additional finger capacitance value, denoted CF in the figure, is on the order of 0.1pF to 10pF. Although the presence of a yoke causes a change in capacitance, the magnitude of the change is quite small compared to the parasitic capacitance. The measured capacitance value of the sensor is called CX. In the absence of fingers, CX is essentially equal to CP. In the presence of a reference, CX is the sum of CP and CF.

  Design of capacitive sensing

  After we understand how capacitive sensing works, how do we start designing the capacitive sensing interface for a particular product? We need to consider the needs of the design solution. Where will this product be used? Will it be used in a harsh environment? Is the most important factor in this design the life of the battery or the durability of the product? Different factors have different effects on the design.

  Depending on the type of product being designed, power consumption may or may not be a critical factor. For example, in battery-operated handheld devices, power consumption is extremely important. And one way to control the
overall average power consumption, or battery life, is to set up 3 different work areas. One working area is the fast response area, and each sensor in this area is scanned every 200 microseconds. The system enters this area while the button and swipe touch are in continuous operation. With little or no operation, the system can enter a slow response zone, reducing the scan frequency to approximately 1 every 100 milliseconds. Finally, if there is no operation for a long time, the system can enter deep sleep mode, thus saving power. By implementing an energy-saving, slow-response mode, the system can consume less than 50µA of average current when the portable handheld device scans 3 buttons every 100 seconds.

  Noise has also become another important consideration in today's electronics world. Various types of induced noise, such as noise from power lines, and radiated noise from mobile phones or fluorescent lamps, are present all the time and must be considered. For effective protection, our goal is to increase the signal-to-noise ratio and eliminate spurious touch responses.

  The choice of cover material and cover thickness has a large impact when designing for signal-to-noise ratio, durability, electrostatic discharge resistance, and accuracy. Also, when considering the type and thickness of materials, there are many compromises that must be made depending on the needs of the product. As the thickness of the overcoat material increases, both signal and noise decrease. However, the thicker the overcoat material, the more resistant it is to electrostatic discharge. Electrostatic voltages on the human body can be as high as 15 KV, and the coating of the capacitive sensing system helps avoidPermanent damage occurs when exposed to this type of electrostatic discharge. Another solution is to use a layer of polyimide (Kapton) tape, a material that works well in applications that require superior ESD protection. Of course, the thicker the overcoat, the less likely it is to crack or be damaged.

Top