Advantages And Disadvantages Of Different Schemes For Capacitive Touch

- Sep 17, 2019-

Capacitive sensing is used in a variety of technologies, from industrial, automotive and medical devices to everyday consumer applications such as smartphones and tablets. The rapid adoption of this technology is largely due to its ability to easily enhance the user experience of the device, allowing manufacturers to abandon traditional switching controls and switch to more attractive touch controls.




The technology also helps reduce the number of mechanical components in the device for longer life and smaller size. These features make capacitive sensing products more attractive to consumers, provided that their design, calibration, and control are in place.




Although capacitive sensing is also widely used to implement touch buttons and sliders, which are common in consumer, commercial, and industrial applications, the most common target applications for this technology are touchpads and touchscreens ( Touchscreen). However, it has proven to be a challenge for most engineers to design low-cost, fast-responding and energy-efficient sensors to ensure reliable operation of the equipment in noisy environments, which is a standard requirement on the market today.




This is especially true for Internet of Things (IoT) and wearable device technologies. In the next few years, this market will grow at an extremely fast rate, and consumers expect these devices to offer the same or even better experience than existing IoT devices. Therefore, engineers need to think carefully about which capacitive sensing method is most effective for their application, because there are some solutions and designs for different purposes on the market, and the differences between them are also great.




touchpad




The most basic touch sensing application for the user interface is probably the projected capacitive touch (PCT) touchpad we are all familiar with. These designs consist of a matrix of rows and columns of layers of conductive material between the glass sheets. Applying a voltage to the grid creates an electric field whose intensity can be measured at each intersection. When a conductive object (such as a human finger) approaches and contacts the PCT panel, the electric field at the contact point changes and can be measured as a difference in capacitance.




Engineers can implement PCT technology in two ways: self-capacitive touch panels and mutual-capacitance touch panels.




The self-capacitance design is located on a printed circuit board (PCB) surrounded by a ground lead. Each sensor on the PCB forms a parasitic capacitance with the surrounding ground lead, and the electric field line is at the top of the sensor. A close finger introduces an extra capacitor that distort the electric field. The main disadvantage of this design is that it can only detect one touch at a time. Although this mode is relatively cost effective, it is only suitable for devices with limited space behind the screen.




However, the mutual capacitance sensing method (referring to the capacitance between any two charged objects) enables simultaneous detection of multiple touches, which is ideal for complex devices with large display screens. During finger touch, the mutual capacitance between the two objects is reduced: the touch controller can detect a decrease in mutual capacitance, thereby recognizing the touch of the finger. It is important that each intersection has a unique mutual capacitance and can be tracked independently.




For a mutual-capacitive touch panel, the touch of a finger causes a decrease in capacitance. Conversely, on a self-capacitive touch panel, the extra capacitance from the finger increases the total capacitance measured by the sensor.