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Implement multitouch sensing in automotive apps

Posted: 13 Feb 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Capacitive sensing? Multi-touch? proximity?

Multi-touch screens used to be limited to Sci-Fi movies. Capacitive sensing technology, however, has changed the way we interact with devices. We no longer just push buttons or throw switches. Rather, we can touch, slide, and pinch the data itself on a touchscreen to interact with it. The far-off interfaces of the movies are not such science fiction anymore. In fact, they are already being adapted and designed into a variety of applications including our cars.

Multi-touch all-point sensing is an extension of capacitive sensing which has made it possible for touch technology to become much more intuitive by sensing the presence of multiple fingers simultaneously and being able to recognize gestures. This article covers the basics of capacitive sensing and the evolution of touch sensing technology in automotive environments. After discussing the construction and internal operation of a multitouch touchscreen/trackpad, it will also address the paradigm changes brought about by multitouch all-point sensing in human machine interfaces (HMIs).

Multi-touch sensing
At the heart any multitouch all-point system is a capacitive sensor composed of a pair of adjacent electrodes. When a conductive object such as a finger comes in proximity to these electrodes, there is additional capacitance between the electrodes (figure 1) that can be measured with the help of a microcontroller. Alternately, capacitive sensing can also be used for proximity sensing where no contact is required between the sensor and the user's body. This can be achieved by increasing the sensitivity of the sensors.

 Capacitive sensing

Figure 1: Capacitive sensing.

Capacitive sensing is increasingly being used to replace mechanical buttons, knobs with touch sensitive buttons, and sliders (figure 2) for in-car systems like infotainment controls, trunk release, Heating Ventilation & Air Conditioning control (HVAC), and Passive Keyless Entry sensors (PKE). The consequent reduction in the number of mechanical components and grooves (which require more complex molds, trap dust, etc.) enhance reliability and reduce system costs.

 Capacitive sensing

Figure 2: Capacitive sensing interfaces.

Touchscreens and trackpads
Touchscreens enable users to directly "touch" the application in a device, thus reducing the dependence on external buttons. Similarly in a trackpad, users can interact with the system using instinctive actions and gestures like touching, tapping, pinching, and dragging. Touchscreens come in one of three main forms: Single-Touch, Multi-Touch Gesture, and Multi-Touch All-Point (figure 3).

Single-touch touchscreens were predominantly based on resistive touchscreen technology, locating both the screen and buttons within the same area. The limitations of resistive single-touch touchscreens are the ability to detect only one finger per screen at a time, limited gesture recognition capabilities, and poor performance due to wear and tear of the sensors.

These limitations led to the development of projected capacitance technology and multitouch gesture touchscreens based on it. Multi-touch gesture touchscreens do not depend on pressure to detect user interaction. They are also able to support simultaneous multitouch recognition and tracking along with gesture detection used to manipulate screen viewing size and orientation of web page views.

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