By Stephan Thaler and Thomas Wenzel
As automotive design engineers integrate more sophisticated touchscreen technologies into their vehicles, they are transforming the driver experience. Resistive touchscreens were once the predominant choice because they are easy to control and relatively inexpensive to manufacture. However, they also support lower light transmission and their surfaces are sensitive to scratching. Capacitive touchscreen technology overcomes many of the problems associated with resistive touchscreens, while bringing the familiar advantages of smartphones and tablets.
When a finger approaches the surface of a capacitive touchscreen, this leads to a slight change in capacities of one or more of the underlying sensors. Self-capacitance and mutual capacitance are two ways to map capacity. The self-capacitance method measures the input signal of a complete row and column of electrodes; however, this method doesn’t always unambiguously classify the position when operated with more than one finger. Mutual capacitance measures every point of intersection in the orthogonal mix. Given this, you can exclude gaps in the finger classification that would be visible on the screen.
Unlike resistive technology, with capacitive technology the user’s finger doesn’t need to exert any pressure on the screen surface in order to be recognized. The precise position of the fingers on the touchscreen is calculated when the measured values of all points of the intersection are evaluated. Sensitive touchscreen controllers, such as Atmel maXTouch devices, can even register the approach of one or more gloved fingers.
Since capacity changes in these applications is very small, it’s critical to minimize the impact of noise and interference. Algorithms in the touchscreen controller can address these issues. For example, maXTouch controllers offer interference suppression that allow you to significantly reduce the number of sensor layers above the LCD screen. Post-processing functions also are integral to reliable operation, especially in different environmental conditions.
To learn more about automotive touch displays, read our full article, Capacitive Touch Technology Opens the Door to a New Generation of Automotive User Interfaces.